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Observations and model studies suggest that front dynamics can enhance phytoplankton productivity. This study tested whether frontal systems also increase the abundance of nitrifying microbes and nitrogen recycling during repeat sampling transects across the Mid‐Atlantic Bight shelfbreak in July 2019. We measured ammonium concentrations, nitrate dual isotopes (δ15N, δ18O), and ammonia monooxygenase subunit A (amoA) genes of ammonia‐oxidizing archaea (AOA) and bacteria (AOB). In subsurface shelf waters, ammonium concentrations exceeded 2 μmol L−1, due to a temporary imbalance in regeneration from sinking particles and subsequent nitrification. The inverse correlation between nitrate δ15N values and ammonium concentrations confirmed nitrate was partially or entirely from local nitrification on the shelf. In contrast, the shelfbreak frontal zone and slope sea subsurface waters had much lower ammonium concentrations (0.1–0.2 μmol L−1) due to tight coupling between ammonium regeneration and nitrification. The deviation of nitrate δ15N and δ18O from algal uptake‐driven 1 : 1 ratio suggests concurrent nitrification in the euphotic zone. The shelfbreak front acted as an ecological boundary where AOA and AOB amoA gene numbers were partitioned, with AOAs abounding in slope waters and AOBs in shelf waters, likely due to ammonium availability. At certain slope stations, deep‐water nutrient inputs via isopycnal lifting induced by Gulf Stream intrusions caused unexpectedly high phytoplankton biomass, which doubled nitrifier abundance and potentially stimulated both ammonium regeneration and nitrification. These findings demonstrate distinct distributions of nitrifying microbes along the salinity gradient from shelf to slope and highlight the significant influence of coastal ocean‐western boundary current interactions on nitrogen biogeochemistry. [ABSTRACT FROM AUTHOR]

Shelf water is influenced by atmospheric forcing, river outflows, and the open ocean. Studying its variability is crucial for understanding anthropogenic impacts on coastal oceans and their transport to the open ocean. In the Middle Atlantic Bight (MAB), the interaction of the Gulf Stream with shelf/slope circulation leads to some of the complex exchanges between the shelf and open ocean along the U.S. East Coast. This study employs a Lagrangian particle tracking approach, grounded in a high‐resolution, data‐assimilative ocean reanalysis, to examine the export pathways of surface shelf water in the MAB. We analyzed over 700 daily images of simulated particle distributions using image clustering techniques. This revealed three distinct export patterns: abrupt entrainment to the Gulf Stream, gradual entrainment, and southern transport. Each pattern was observed roughly equally during the study period from January 2017 to December 2018. The observed export patterns are closely linked to the coastal circulation dynamics near Cape Hatteras. Understanding the timing and duration of these patterns is vital for assessing water quality and predicting the settlement of species that spawn in the region. Our study further underscores the influence of tropical cyclones, including Hurricanes Jose, Maria, and Chris, on these export patterns. These extreme weather events lead to significant shifts in coastal circulation near Cape Hatteras. Plain Language Summary: This study focuses on the movement of ocean water in the Middle Atlantic Bight (MAB), a region along the U.S. east coast. The movement of this coastal water, or "shelf water," is affected by the weather, rivers, and the ocean. A Lagrangian particle tracking method was used to track the movement of water by simulating how particles move in the coastal ocean. Over 700 daily images of particle tracking simulations were obtained. Shelf water moves out of the MAB by three main pathways: abrupt entrainment, gradual entrainment, and southern transport. Each of these pathways happened about equally over 2 years (2017–2018). Understanding these water movements is key for knowing how long water stays in an area, which is important for water quality and for the life cycle of marine species that breed there. The study also highlights how tropical cyclones (like Hurricanes Jose, Maria, and Chris) can dramatically change these water movement patterns, especially near Cape Hatteras. Key Points: Three Middle Atlantic Bight shelf water export patterns linked to specific wind conditions are identifiedLagrangian flow patterns of the MAB Shelf Water were notably impacted by 2017 and 2018 Atlantic Hurricane SeasonsSea Surface Velocity maps and Progressive Vector Diagrams reveal varied estuarine water pathways from Chesapeake and Delaware Bays [ABSTRACT FROM AUTHOR]

One of the most significant features of the Northwest Atlantic, the Gulf Stream influences high magnitude environmental fluctuations in deep habitats across the South Atlantic Bight. Amid this variability, the Blake Plateau harbors extensive reefs formed by cold‐water corals that were previously assumed to rely on narrow ranges of temperature, currents, and particulate supply. A benthic lander collected near‐bed conditions at the Richardson Reef Complex, a cold‐water reef dominated by the scleractinian Desmophyllum pertusum at 830 m within the path of the Gulf Stream. Specific behavior of the Gulf Stream resulted in recurring environmental patterns at depth. During offshore meanders, deep stream components intruded onto the reef and caused rapid (3.74°C per hour) temperature increases up to 10.8°C (> 5°C above the site mean) and increased chlorophyll. Within 2 d of peak temperatures, intrusions were replaced by strong, turbid upwelling currents that rapidly cooled the site to temperature minima (4.13°C). While considerable environmental variability from the Gulf Stream may otherwise implicate a thermally stressful setting for corals, high‐temperature events were likely mitigated by their short duration (< 37.4 h) and physical coupling with enhanced organic material. This hypothesis was supported by high‐density clustering of D. pertusum occurrences within 50 km around the Gulf Stream's position along the South Atlantic Bight. This suggests that cold‐water corals experiencing environmental variability can be sustained by relationships between food supply, temperature, and currents that vary in strength along stochastic time scales, shedding further light on the niche of cold‐water corals. [ABSTRACT FROM AUTHOR]

At mid‐latitudes, a narrow band of near‐surface wind convergence (NSWC) overlies the western boundary currents in long‐term climatology as a response to steep sea surface temperature gradients. The underlying dynamics shaping mean convergence in the mid‐latitude region have been investigated in detail. In polar regions, surface temperature gradients are intense along the sea ice edges. However, literature concerning NSWC near sea ice edges is limited. This study investigates time‐mean NSWC along sea ice edges and its shaping processes, focusing on the Greenland Sea, based on atmospheric reanalysis. In cold‐season climatology, positive NSWC overlies the sea ice edge, resulting in a localized upward motion reaching the free atmosphere. The mean NSWC was insensitive to sea ice thickness and surface roughness in the regional model. This study suggests that, in addition to local atmospheric boundary processes, extreme NSWC events play a vital role in shaping the mean distribution. Although these features are similar to those along the Gulf Stream, atmospheric fronts appear to play a relatively minor role in the Greenland Sea. Instead, the frequent cyclone generation near the sea ice edge and the anticyclonic circulation over Greenland in conjunction with the transient synoptic circulation seem essential. In the warm season, positive NSWC was virtually missing in the Greenland Sea, unlike in the Gulf Stream region, reflecting the shallow virtual temperature response to the surface thermal forcing. This study contributes to understanding the mechanisms by which sea ice variability affects large‐scale atmospheric circulation in remote regions. Plain Language Summary: Previous studies have argued that sea ice variability in Arctic regions could affect large‐scale atmospheric circulation in remote regions. However, the underlying mechanisms remain unclear. A deeper understanding of the impact of sea ice on the local atmosphere will aid in revealing these mechanisms. The present study investigates small‐scale (less than 1,000 km) atmospheric features close to sea‐ice edges, where surface temperature gradients are intense and can exert distinct impacts on the atmosphere. In the winter, the sea‐ice edge distinctly modulates the near‐surface wind, leading to an narrow band of intense upward wind. This upward wind reaches the middle of the atmosphere, suggesting that these features could relate to the effect of sea ice variability on large‐scale atmospheric circulation in remote regions. The physical processes modulating the mean near‐surface winds near the sea‐ice edge are found to be similar to those along the warm currents at the western edge of the basins (such as the Gulf Stream). However, atmospheric fronts appear to play a less important role. Instead, the frequent cyclone generation near the sea‐ice edge and the combination of the clockwise near‐surface wind around Greenland and near‐surface wind blowing from the ocean to sea‐ice regions appear essential. Key Points: A distinct band of mean near‐surface wind convergence overlies the sea ice edge in the Greenland Sea during winterThe mean near‐surface wind convergence along the sea ice edge is insensitive to ice thickness and surface roughness in a numerical modelThe anticyclonic circulation over Greenland and transient synoptic circulation are essential for shaping the time‐mean wind convergence [ABSTRACT FROM AUTHOR]

This study proposes deterministic and stochastic energy‐aware hybrid models that should enable simulations of idealized and primitive‐equations Geophysical Fluid Dynamics (GFD) models at low resolutions without compromising on quality compared with high‐resolution runs. Such hybrid models bridge the data‐driven and physics‐driven modeling paradigms by combining regional stability and classical GFD models at low resolution that cannot reproduce high‐resolution reference flow features (large‐scale flows and small‐scale vortices) which are, however, resolved. Hybrid models use an energy‐aware correction of advection velocity and extra forcing compensating for the drift of the low‐resolution model away from the reference phase space. The main advantages of hybrid models are that they allow for physics‐driven flow recombination within the reference energy band, reproduce resolved reference flow features, and produce more accurate ensemble forecasts than their classical GFD counterparts. Hybrid models offer appealing benefits and flexibility to the modeling and forecasting communities, as they are computationally cheap and can use both numerically‐computed flows and observations from different sources. All these suggest that the hybrid approach has the potential to exploit low‐resolution models for long‐term weather forecasts and climate projections thus offering a new cost effective way of GFD modeling. The proposed hybrid approach has been tested on a three‐layer quasi‐geostrophic model for a beta‐plane Gulf Stream flow configuration. The results show that the low‐resolution hybrid model reproduces the reference flow features that are resolved on the coarse grid and also gives a more accurate ensemble forecast than the physics‐driven model. Plain Language Summary: Reliable weather forecast and climate prediction are crucial for socio‐economic sectors, decision making, and strategic planning for mitigating risks and impacts of natural disasters. In order to forecast weather and climate, observations and numerical models are used. A necessary ingredient for these forecasts is the ocean‐atmospheric model that resolves mesoscale oceanic eddies and can use large ensembles (hundred of members). Supercomputers can run eddy‐resolving simulations but only for single runs or use large ensembles but in non‐eddy‐resolving regimes. Applying the large ensemble approach with eddy‐resolving simulations is far beyond what supercomputers will be able to compute for the next decades, while it is urgently needed. This study offers deterministic and stochastic energy‐aware hybrid models that enable simulations of Geophysical Fluid Dynamics models at low resolutions without compromising on quality compared with high‐resolution runs. The use of low resolutions makes hybrid models much faster than high‐resolution physics‐driven runs. This acceleration can be translated into the use of larger ensembles. Thus, the proposed hybrid approach has the potential to exploit large ensembles of high‐quality solutions for long‐term weather forecasts and climate projections for the very first time. Key Points: Deterministic and stochastic energy‐aware hybrid models have been proposedHybrid models allow low‐resolution simulations without compromising on quality compared with high‐resolution runsHybrid approach produce more accurate ensemble predictions than their classical Geophysical Fluid Dynamics counterparts [ABSTRACT FROM AUTHOR]

The interactions between the internal tide and the mesoscale circulation are studied from the internal tide energy budget perspective. To that end, the modal energy budget of the internal tide is diagnosed using a high resolution numerical simulation covering the North Atlantic. Compared to the topographic contribution, the advection of the internal tide by the low‐frequency flow component and the horizontal and vertical shear are found to be significant at global scale, while the buoyancy contribution is important locally. The advection of the internal tide by the low‐frequency currents is responsible for a net energy transfer from the large scale to smaller scale internal tide, without exchanges with the low‐frequency flow. On the opposite, the shear of the mesoscale circulation and the buoyancy field are responsible for exchanges between the internal tide and the low‐frequency flow. The importance of the shear increases in the northernmost part of the domain, and a partial compensation between the buoyancy and the shear contributions is found in some areas of the North Atlantic, such as in the Gulf Stream region. In addition, the temporal variability of these energy transfers is investigated. In contrast to topographic scattering, for which the spring‐neap cycle is the dominant frequency, the energy transfer terms driven by low‐frequency motions in areas of strong mesoscale activity are also modulated by variations of the low‐frequency current spatial distribution. Plain Language Summary: Internal tides are waves generated when the tidal motions, induced by the Moon and Sun gravitational attraction on the ocean, interact with topographical features. These waves, which are ubiquitous in the ocean, then propagate inside of the ocean interior and interact with other types of currents. This study investigates the wave‐currents interactions and their impact on the energy exchanges in the ocean using a high‐resolution realistic numerical simulation. We find that, although the scattering of internal tides by the topography is dominant in their energy budget, the interaction with the currents are significant at the basin scale. They lead to transfers of energy from large scale internal waves to smaller ones, and between currents and internal tides in the Labrador sea and Faroe islands vicinity. This transfer of energy is likely to impact the ocean mixing driven by the small scale internal tide, and thereby to affect the global ocean circulation. The temporal variability of these current‐internal tides interactions shows the same period as the variability of the semi‐diurnal astronomical tide, with a modulation by the temporal variability of the currents. Key Points: Although topographic scattering is dominant, low‐frequency flow has a strong impact on the internal tide energy cycleThe dominant effect of low‐frequency flow on internal tide is a transfer of energy toward smaller scaleLow‐frequency flow induced interactions show an imprint of the spring neap cycle modulated by variations in the currents [ABSTRACT FROM AUTHOR]

In this study, we explore the impact of oceanic moisture fluxes on atmospheric blocks using the ECMWF IFS. Artificially suppressing surface latent heat flux over the Gulf Stream (GS) region reduces atmospheric blocking frequency across the Northern Hemisphere by up to 30%. Affected blocks show a shorter lifespan (−6%), smaller spatial extent (−10%), and reduced intensity (−0.4%), with an increased number of individual blocking anticyclones (+17%). These findings are robust across various blocking detection thresholds. Analysis reveals a qualitatively consistent response across all resolutions, with Tco639 (∼18 km) showing the largest statistically significant change across all blocking characteristics, although differences between resolutions are not statistically significant. Exploring the broader Rossby wave pattern, we observe that diminished moisture fluxes favor eastward propagation and higher zonal wavenumbers, while air‐sea interactions promote stationary and westward‐propagating waves with zonal wavenumber 3. This study underscores the critical role of the GS in modulating atmospheric blocking. Plain Language Summary: In our study, we investigated how changes in oceanic moisture, specifically from the Gulf Stream (GS), affect atmospheric blocking events using the ECMWF Integrated Forecast System. By artificially reducing the moisture flux from the GS area, we observed a notable decrease in the occurrence of atmospheric blocks across the Northern Hemisphere‐up to 30%. These blocks also showed changes in their characteristics: they had shorter durations by 6%, were 10% smaller in spatial size, and had a slight decrease in intensity, alongside a 17% increase in their detection rates. Our findings were consistent across different methods of identifying blocks. We also discovered that the model's resolution influences the observed changes, with coarser resolutions (larger than approximately 18 km) not showing significant alterations in some blocking traits despite the overall frequency reduction. Further analysis into Rossby wave patterns revealed that reduced moisture flux tends to favor faster eastward‐moving patterns with shorter wavelengths, whereas interactions between the air and sea support more stationary or westward‐moving waves, particularly those with a zonal wavenumber of 3. This highlights the crucial role that moisture from western boundary currents like the GS plays in the development and behavior of atmospheric blocking patterns. Key Points: Gulf Stream (GS) moisture flux suppression reduces atmospheric blocking across the Northern HemisphereGS moisture fluxes generate larger jet stream perturbations, fostering faster westward‐propagating Rossby wavesHigher resolution models enhance signal transport from the boundary layer to the upper troposphere [ABSTRACT FROM AUTHOR]

Satellite altimetry combined with data assimilation and optimal interpolation schemes have deeply renewed our ability to monitor sea surface dynamics. Recently, deep learning schemes have emerged as appealing solutions to address space‐time interpolation problems. However, the training of state‐of‐the‐art neural schemes on real‐world case‐studies is hindered by the sparse space‐time coverage of the sea surface of real altimetry data set. Here, we introduce an innovative approach that leverages state‐of‐the‐art ocean models to train simulation‐based neural schemes for the mapping of sea surface height and demonstrate their performance on real altimetry data sets. We analyze further how the ocean simulation data set used during the training phase impacts this performance. This experimental analysis covers both the resolution from eddy‐present configurations to eddy‐rich ones, forced simulations versus reanalyzes using data assimilation and tide‐free versus tide‐resolving simulations. Our benchmarking framework focuses on a Gulf Stream region for a realistic 5‐altimeter constellation using NEMO ocean simulations and 4DVarNet mapping schemes. All simulation‐based 4DVarNets outperform the operational observation‐driven and reanalysis products, namely DUACS and GLORYS. The more realistic the ocean simulation data set used during the training phase, the better the mapping. The best 4DVarNet mapping was trained from an eddy‐rich and tide‐free simulation data sets. It improves the resolved longitudinal scale from 151 km for DUACS and 241 km for GLORYS to 98 km and reduces the root mean square error by 23% and 61%. These results open research avenues for new synergies between ocean modeling and ocean observation using learning‐based approaches. Plain Language Summary: To train an artificial intelligence (AI) model, one need to describe a task using data and an evaluation procedure. Here we aim at constructing images related to the ocean surface currents. The satellite data we use provide images of the ocean surface with a lot of missing data (around 95% of missing pixels for a given day), and we aim at finding the values of the missing pixels. Because we don't know the full image, it is challenging to train an AI on this task using only the satellite data. However, today's physical knowledge makes it possible to numerically simulate oceans on big computers. For these simulated oceans, we have access to the gap‐free image, so we can train AI models by first hiding some pixels and checking if the model fill the gaps with the correct values. Here, we explore under which conditions AIs trained on simulated oceans are useful for the real ocean. We show that today's simulated oceans work well for training an AI on this task and that training on more realistic simulated oceans improve the performance of the AI! Key Points: We suggest utilizing ocean simulation data sets to train neural schemes for mapping real altimeter dataThe trained neural scheme improves the spatial scales resolved over the operational mapping product on a GulfStream case study by 30%Using more realistic simulation data sets improves the resulting neural mapping scheme by up to 20% in the spatial scales resolved [ABSTRACT FROM AUTHOR]

Canonical understanding based on general circulation models (GCMs) is that the atmospheric circulation response to midlatitude sea‐surface temperature (SST) anomalies is weak compared to the larger influence of tropical SST anomalies. However, the ∼100‐km horizontal resolution of modern GCMs is too coarse to resolve strong updrafts within weather fronts, which could provide a pathway for surface anomalies to be communicated aloft. Here, we investigate the large‐scale atmospheric circulation response to idealized Gulf Stream SST anomalies in Community Atmosphere Model (CAM6) simulations with 14‐km regional grid refinement over the North Atlantic, and compare it to the responses in simulations with 28‐km regional refinement and uniform 111‐km resolution. The highest resolution simulations show a large positive response of the wintertime North Atlantic Oscillation (NAO) to positive SST anomalies in the Gulf Stream, a 0.4‐standard‐deviation anomaly in the seasonal‐mean NAO for 2°C SST anomalies. The lower‐resolution simulations show a weaker response with a different spatial structure. The enhanced large‐scale circulation response results from an increase in resolved vertical motions with resolution and an associated increase in the influence of SST anomalies on transient‐eddy heat and momentum fluxes in the free troposphere. In response to positive SST anomalies, these processes lead to a stronger and less variable North Atlantic jet, as is characteristic of positive NAO anomalies. Our results suggest that the atmosphere responds differently to midlatitude SST anomalies in higher‐resolution models and that regional refinement in key regions offers a potential pathway to improve multi‐year regional climate predictions based on midlatitude SSTs. Plain Language Summary: Variations in the ocean surface temperature (SST) influence the atmospheric circulation and thus climate over land. Canonical understanding is that tropical SSTs are more important than SSTs in midlatitudes. However, this understanding is based on climate models that don't resolve processes at scales less than 100 km. Here, we show that by increasing the atmospheric model resolution to resolve features on smaller scales, such as weather fronts, we find a larger atmospheric circulation response to midlatitude SST anomalies in the North Atlantic. North Atlantic SST anomalies can be predicted multiple years in advance, and a larger atmospheric circulation response to these predictable SST anomalies therefore implies increased predictability of climate over the surrounding land regions. Key Points: There is a large circulation response to idealized Gulf Stream sea‐surface temperature (SST) anomalies in an atmospheric model with 14‐km regional grid refinementThis response is weaker or absent in simulations with 28‐km or coarser resolution, which do not fully resolve mesoscale frontal processesTransient‐eddy fluxes of heat and momentum are modified as fronts pass over warm SSTs, leading to a large‐scale circulation response [ABSTRACT FROM AUTHOR]

A long‐standing hypothesis is that the steady along‐shelf circulation in the Northwest Atlantic (NWA) coastal ocean is driven by buoyancy input from continental freshwater runoff. However, the forcing from the freshwater runoff has not been adequately evaluated and compared with other potential driving mechanisms. This study investigates the roles of both wind stress and freshwater runoff in driving the mean along‐shelf flow in the NWA coastal ocean and examines other potential drivers using a newly developed high‐resolution regional model with realistic forcing conditions. The results reveal that wind stress has a larger impact than freshwater runoff on the overall mean circulation and along‐shelf sea‐level gradient on the NWA shelf. While the continental freshwater input consistently contributes to the equatorward along‐shelf flow and higher sea level along the coast, wind stress is more effective for the setup of the broad‐scale circulation pattern by driving the along‐shelf flow on the Labrador Shelf and opposing the flow in the Mid‐Atlantic Bight and on the Scotian Shelf. In addition to the local wind and continental runoff, the sub‐Arctic inflow from higher latitude is an essential part of the NWA shelf circulation system. This remote driver directly contributes to the along‐shelf flow and insulates the shelf flow from the Gulf Stream on the southern shelves. Plain Language Summary: This study addresses a long‐standing question of what drives the mean equatorward flow along the Northwest Atlantic continental shelf. The results show that the mean along‐shelf flow is influenced more by wind stress than local buoyancy forcing resulting from the freshwater discharges from rivers and glaciers, which has long been considered the dominant driver of the along‐shelf circulation. Remote forcing through inflow from higher latitude is also important in maintaining the broad‐scale mean along‐shelf flow and sea level. This work also provides updated views of the circulation dynamics and momentum balance of the mean along‐shelf circulation, contributing to the understanding of both the current and future states of our ocean and climate systems. Key Points: Wind forcing plays an important role in setting up the mean along‐shelf flow in the Northwest Atlantic (NWA) coastal oceanLocal buoyancy from continental runoff is not as significant as previously hypothesizedRemote subpolar inflow is an essential part of the NWA shelf circulation system [ABSTRACT FROM AUTHOR]

Benthic storms are episodes of intensified near‐bottom currents capable of sediment resuspension in the deep ocean. They typically occur under regions of high surface eddy kinetic energy (EKE), such as the Gulf Stream. Although they have long been observed, the mechanism(s) responsible for their formation and their relationships with salient features of the deep ocean, such as bottom mixed layers (BMLs) and benthic nepheloid layers (BNLs), remain poorly understood. Here we conduct idealized experiments with a primitive‐equation model to explore the impacts of the unforced instability of a surface‐intensified jet on near‐bottom flows of a deep zonal channel. Vertical resolution is increased near the bottom to represent the bottom boundary layer. We find that the unstable near‐surface jet develops meanders and evolves into alternating, deep‐reaching cyclones and anticyclones. Simultaneously, EKE increases near the bottom due to the convergence of vertical eddy pressure fluxes, leading to near‐bottom currents comparable to those observed during benthic storms. These currents in turn form BMLs with thickness of O(100 m) from enhanced velocity shears and turbulence production near the bottom. The deep cyclonic eddies transport fluid particles both laterally and vertically, from near the bottom through the entire BML and may contribute to the formation of the lower part of BNLs. A sloping bottom reduces both the intensity of the near‐bottom currents and the extent of vertical transport. Overall, our study highlights a significant response of the abyssal environment to near‐surface current instability, with potential implications for sediment transport in the deep ocean. Plain Language Summary: Benthic storms are episodes of vigorous near‐bottom currents that are capable of sediment resuspension at abyssal depths in the ocean. They typically occur in regions characterized by strong and unstable currents near the surface, such as the Gulf Stream. In this study, we use an ocean circulation model to simulate the instability of a surface current like the Gulf Stream, and to explore the effects of this instability on the environment near the bottom in an idealized oceanic basin. We find that strong cyclonic and anticyclonic currents are simulated near the bottom in response to the instability of the surface current. The intensity of deep currents varies strongly with distance from the bottom, which creates turbulence and a well‐mixed layer adjacent to the deep seabed. In deep cyclones, particles from near the bottom can be transported upward through the entire well‐mixed layer. A steeper topographic slope reduces both the intensity of the bottom currents and the vertical displacement of particles. Overall, our study shows that the physical environment near the bottom may exhibit a strong response to the instability of surface currents, which may have a potentially significant impact on the transport of fine sediment from the seafloor. Key Points: Bottom currents simulated from the intrinsic instability of a near‐surface jet reach speeds similar to those observed during benthic stormsAbyssal eddy kinetic energy increases, producing deep eddies, strong local turbulence, and bottom mixed layers up to O(100 m) thickParticles near the bottom are advected both laterally and vertically in deep cyclones, with potential implications for sediment transport [ABSTRACT FROM AUTHOR]

A continuous decrease in aerosols over the Western North Atlantic Ocean (WNAO) on a decadal timescale provides a long‐term benchmark to evaluate how various natural and anthropogenic processes affect the manifestation of aerosol‐cloud interactions in this region. Furthermore, the WNAO serves as a natural laboratory with diverse aerosol sources, marine boundary layer clouds more variable than those in marine stratocumulus deck regions, and unique flow regimes established by the Gulf Stream and the semi‐permanent Bermuda High. We investigate how satellite‐retrieved macrophysical and microphysical properties of low clouds and the surface shortwave irradiance changed from 2003 to 2020, in tandem with this aerosol decrease. The decadal changes in large‐scale meteorology related to the North Atlantic Oscillation (NAO) are also examined. We find a reduction in low‐cloud optical thickness, accompanied by fewer and larger cloud droplets, yet observe no significant changes in low‐cloud fraction and liquid water path. Despite the reduction in low‐cloud optical thickness together with aerosol decrease, a corresponding increase in the trends of surface shortwave irradiance, also known as surface brightening, is lacking. This absence of brightening is potentially related to concomitant changes found in large‐scale meteorology associated with NAO—Bermuda High strengthening, sea surface warming, and atmospheric moistening— as well as an increase in high‐level cloud fraction that can counteract the surface brightening. Ultimately, our findings suggest that spatial patterns of decadal meteorological variability introduce complexities in the surface cloud radiative effect over the WNAO, thereby complicating the isolation and examination of aerosol‐cloud interactions. Plain Language Summary: Over recent decades, the Western North Atlantic Ocean has experienced a steady decrease in aerosols—tiny particles in the air. This situation offers a chance to study how natural phenomena and human activities influence cloud‐aerosol interactions. The area is known for its diverse aerosol sources, complex cloud types, and distinct oceanic and atmospheric flow patterns influenced by the Gulf Stream and Bermuda High. By analyzing satellite data from 2003 to 2020, we investigated how low‐lying clouds and sunlight penetration have changed in response to fewer aerosols, and how the North Atlantic Oscillation (NAO) affects weather patterns. Our findings show that while low‐lying clouds have become more transparent with fewer, larger droplets, their coverage and juiciness have not changed significantly. Contrary to expectations, these changes have not lead to an increase in sunlight reaching the Earth's surface, a phenomenon known as "surface brightening." This could be due to NAO‐related warmer sea surfaces, increased moisture, and more high‐level clouds, which might obscure any brightening. Our study shows that weather shifts in the Western North Atlantic Ocean add complexity to understanding how clouds influence sunlight, challenging the study of aerosol‐cloud interactions. Key Points: In tandem with the continuous decrease in aerosol, low‐cloud droplets became fewer and larger over the western North AtlanticDespite concurrent changes in aerosol and low‐cloud optical properties, surface shortwave irradiance lacked reciprocal significant trendsMeteorological fields—temperature and humidity—correlated with the North Atlantic Oscillation had changed, exhibiting tripolar patterns [ABSTRACT FROM AUTHOR]

Satellite altimetry offers a unique approach for direct sea surface current observation, but it is limited to measuring the surface‐constrained geostrophic component. Ageostrophic dynamics, prevalent at horizontal scales below 100 km and time scales below 10 days, are often underestimated by ocean reanalyzes employing data assimilation schemes. To address this limitation, we introduce a novel deep learning scheme, rooted in a variational data assimilation formulation with trainable observations and a priori terms, that harnesses the synergies between satellite‐derived sea surface observations, namely sea surface height (SSH) and sea surface temperature (SST), to enhance sea surface current reconstruction. Numerical experiments, conducted using realistic simulations, in a case study area of the Gulf Stream, demonstrate the potential of the proposed scheme to capture ageostrophic dynamics at time scales of 2.5–3.0 days and horizontal scales of 0.5°–0.7°. The analysis of diverse observation configurations, encompassing nadir along‐track altimetry, wide‐swath SWOT (Surface Water and Ocean Topography) altimetry, and SST data, highlights the pivotal role of SST features in retrieving a significant portion of the ageostrophic dynamics (approximately 47%). These findings underscore the potential of deep learning and 4DVarNet schemes in improving ocean reanalyzes and enhancing our understanding of ocean dynamics. Plain Language Summary: Satellite altimetry provides a unique means for direct observation of sea surface currents, but it is confined to the geostrophic component, limiting the recovery of a substantial portion of mesoscale sea surface currents in operational products. To address this limitation, we present a novel deep learning framework, rooted in a variational data assimilation paradigm, that unlocks new avenues for leveraging the synergistic relationships between satellite‐derived sea surface observations, namely sea surface height and sea surface temperature. This innovative scheme demonstrates its remarkable potential to enhance sea surface current reconstruction and recover a substantial portion of the elusive ageostrophic dynamics. Numerical experiments, employing realistic simulations, in a case study area along the Gulf Stream, underscore the efficacy of our proposed approach. These findings support the pivotal role of physics‐informed deep learning in maximizing the utilization of available multimodal observation data sets and numerical simulations to elucidate partially observed sea surface dynamics. Key Points: We present end‐to‐end deep learning schemes to improve the reconstruction of total sea surface currents from satellite‐derived observationsExperiments in a region of the Gulf Stream support the synergistic analysis of sea surface temperature and sea surface height dataThe strain of sea surface dynamics is a proxy of the uncertainty of the retrieved estimation [ABSTRACT FROM AUTHOR]

The climatological mean barotropic vorticity budget is analyzed to investigate the relative importance of surface wind stress, topography, planetary vorticity advection, and nonlinear advection in dynamical balances in a global ocean simulation. In addition to a pronounced regional variability in vorticity balances, the relative magnitudes of vorticity budget terms strongly depend on the length‐scale of interest. To carry out a length‐scale dependent vorticity analysis in different ocean basins, vorticity budget terms are spatially coarse‐grained. At length‐scales greater than 1,000 km, the dynamics closely follow the Topographic‐Sverdrup balance in which bottom pressure torque, surface wind stress curl and planetary vorticity advection terms are in balance. In contrast, when including all length‐scales resolved by the model, bottom pressure torque and nonlinear advection terms dominate the vorticity budget (Topographic‐Nonlinear balance), which suggests a prominent role of oceanic eddies, which are of O(10–100) $\mathcal{O}(10\mbox{--}100)$ km in size, and the associated bottom pressure anomalies in local vorticity balances at length‐scales smaller than 1,000 km. Overall, there is a transition from the Topographic‐Nonlinear regime at scales smaller than 1,000 km to the Topographic‐Sverdrup regime at length‐scales greater than 1,000 km. These dynamical balances hold across all ocean basins; however, interpretations of the dominant vorticity balances depend on the level of spatial filtering or the effective model resolution. On the other hand, the contribution of bottom and lateral friction terms in the barotropic vorticity budget remains small and is significant only near sea‐land boundaries, where bottom stress and horizontal viscous friction generally peak. Plain Language Summary: Vorticity provides a measure of the local rotation and shear of fluid flow. The analysis of physical processes contributing to ocean vorticity has proven fundamental to our understanding of how those processes drive ocean flows, ranging from large‐scale ocean gyres to boundary currents such as the Gulf Stream, which is tens of km in width. Furthermore, a vorticity analysis can inform us about the relative importance of different physical processes in generating flow structures having different length scales. In the present work, we perform a length‐scale dependent vorticity budget analysis using a coarse‐graining method to remove signals finer than a fixed length scale. We coarse‐grain the climatological mean vorticity budget terms over a range of length scales, and then compare the relative magnitudes to identify the dominant vorticity balances as a function of length scale. We find that the spatial structure of the meridional transport is mainly controlled by atmospheric winds, variations in ocean depth and the momentum transport by ocean currents. However, the relative magnitudes of these factors change drastically at different length scales. We conclude that physical interpretations of the primary vorticity balances are fundamentally dependent on the chosen length scale of the analysis. Key Points: Relative magnitudes of barotropic vorticity budget terms display significant length‐scale dependenceBottom pressure torque and wind stress curl control the depth‐integrated meridional flow at length scales larger than 1,000 kmNonlinear advection and bottom pressure torque dominate the barotropic vorticity budget at smaller length scales [ABSTRACT FROM AUTHOR]

Few observational platforms are able to sustain direct measurements of all the key variables needed in the bulk calculation of air‐sea carbon dioxide (CO2) exchange, a capability newly established for some Uncrewed Surface Vehicles (USVs). Western boundary currents are particularly challenging observational regions due to strong variability and dangerous sea states but are also known hot spots for CO2 uptake, making air‐sea exchange quantification in this region both difficult and important. Here, we present new observations collected by Saildrone USVs in the Gulf Stream during the winters of 2019 and 2022. We compared Saildrone data across co‐located vehicles and against the Pioneer Array moorings to validate the data quality. We explored how CO2 flux estimates differ when all variables needed to calculate fluxes from the bulk formulas are simultaneously measured on the same platform, relative to the situation where in situ observations must be combined with publicly‐available data products. We systematically replaced variables in the bulk formula with those often used for local and regional flux estimates. The analysis revealed that when using the ERA‐5 reanalysis wind speed in place of in situ observations, the ocean uptake of CO2 is underestimated by 8%; this underestimate grows to 9% if the NOAA Marine Boundary Layer atmospheric CO2 product and ERA‐5 significant wave height are also used in place of in situ observations. Overall our findings point to the importance of collecting contemporaneous observations of wind speed and ocean pCO2 to reduce biases in estimates of regional CO2 flux, especially during high wind events. Plain Language Summary: The North Atlantic Ocean absorbs a large amount of carbon dioxide (CO2) from the atmosphere. The ocean CO2 uptake mainly occurs during the winter months in a region where a swift ocean current, called the Gulf Stream, carries warm waters from the tropics to northern latitudes. Understanding how much CO2 this ocean region absorbs is important for constraining global scale assessments of the sources and sinks of CO2. In this paper, we reflect on the methods that are typically used to calculate the exchange of CO2 between the ocean and atmosphere. Using new data collected by uncrewed surface vehicles, that resemble small sailboats, in the North Atlantic Ocean in the vicinity of the Gulf Stream, we find that the estimate of the exchange of CO2 is larger when directly measured wind speeds are used in the calculation in comparison to wind speeds from a widely used data product. This result signifies the importance of co‐located sensors on the same observing platform. Key Points: Compared to Saildrone wind speeds, the ERA‐5 reanalysis appears to underestimate the highest wind speed events (>10 m s−1)Co‐located ocean pCO2 and wind speed observations results in larger uptake of CO2 in this region by 9%Using a sea‐state dependent gas transfer velocity leads to a 28% greater ocean uptake of CO2 compared to using a wind‐only equation [ABSTRACT FROM AUTHOR]

The bay barnacle Amphibalanus improvisus (Darwin, 1854) has been found in Northern Germany in brackish marine sediments of the Holsteinian interglacial. Whereas the cause of the contemporary distribution of this species is anthropogenic and there is evidence that it has resulted mainly from shipping, the fossil findings demonstrate that A. improvisus had also already reached Europe naturally during the Pleistocene in Europe, probably via the Gulf Stream system of the Atlantic. Today, A. improvisus is considered one of the most successful aquatic invasive species worldwide and it is characterized by a high environmental tolerance. The ingression of the Holsteinian Sea, during the Pleistocene, took place during the climate optimum from the North Sea into the 'Elbe Estuary' through the Hamburg region and far to the south‐east into Brandenburg. It has been possible to reconstruct the palaeo‐ecological conditions based in particular on molluscs. Accordingly, environmental conditions ranged from normal marine in the north, via brackish in the transitional area to limnic–fluviatile in the Berlin region. The morphological features of the bay barnacles and their ecological parameters are described here in detail. In addition, we provide a general review of the known Pleistocene Balanidae in northern central Europe. [ABSTRACT FROM AUTHOR]

Extracting balanced geostrophic motions (BM) from sea surface height (SSH) observations obtained by wide‐swath altimetry holds great significance in enhancing our understanding of oceanic dynamic processes at submesoscale wavelength. However, SSH observations derived from wide‐swath altimetry are characterized by high spatial resolution while relatively low temporal resolution, thereby posing challenges to extract the BM from a single SSH snapshot. To address this issue, this paper proposes a deep learning model called the BM‐UBM Network, which takes an instantaneous SSH snapshot as input and outputs the projection corresponding to the BM. Training experiments are conducted both in the Gulf Stream and South China Sea, and three metrics are considered to diagnose model's outputs. The favorable results highlight the potential capability of the BM‐UBM Network to process SSH measurements obtained by wide‐swath altimetry. Plain Language Summary: Oceanic dynamic processes can be classified into two categories: balanced geostrophic motions (BM), including large‐scale circulation, mesoscale and submesoscale eddy turbulence, and unbalanced wave motions (UBM), including barotropic tides, and inertia–gravity waves (IGWs). Both types of motions coexist and have respective contributions to the sea surface height (SSH). How to extract the BM from the total SSH observations obtained by satellite altimetry is the crucial problem to be solved in this paper. To tackle this issue, we propose a deep learning model named the BM‐UBM Network to establish the relationship between the total SSH and the BM component. The BM‐UBM Network can generate SSH estimations for the BM when provided with a well‐resolved SSH snapshot. Key Points: A Deep learning model is developed to extract balanced motions from sea surface height snapshot based on a realistic simulationDiagnostics of three metrics reveal the effectiveness of the model in extracting balanced motionsThe model exhibits remarkable advantages over the Gaussian filter (baseline) in capturing the gradient and Laplacian information [ABSTRACT FROM AUTHOR]

Process modeling of Aerosol‐cloud interaction (ACI) is essential to bridging gaps between observational analysis and climate modeling of aerosol effects in the Earth system and eventually reducing climate projection uncertainties. In this study, we examine ACI in summertime precipitating shallow cumuli observed during the Aerosol Cloud meTeorology Interactions oVer the western ATlantic Experiment (ACTIVATE). Aerosols and precipitating shallow cumuli were extensively observed with in‐situ and remote‐sensing instruments during two research flight cases on 02 June and 07 June, respectively, during the ACTIVATE summer 2021 deployment phase. We perform observational analysis and large‐eddy simulation (LES) of aerosol effect on precipitating cumulus in these two cases. Given the measured aerosol size distributions and meteorological conditions, LES is able to reproduce the observed cloud properties by aircraft such as liquid water content (LWC), cloud droplet number concentration (Nc) and effective radius reff. However, it produces smaller liquid water path (LWP) and larger Nc compared to the satellite retrievals. Both 02 and 07 June cases are over warm waters of the Gulf Stream and have a cloud top height over 3 km, but the 07 June case is more polluted and has larger LWC. We find that the Na‐induced LWP adjustment is dominated by precipitation feedback for the 2 June precipitating case and there is no clear entrainment feedback in both cases. An increase of cloud fraction due to a decrease of aerosol number concentration is also shown in the simulations for the 02 June case. Plain Language Summary: Aerosol‐cloud‐interaction (ACI) regulates the energy budget of the Earth and poses the largest uncertainty in climate projection. Particularly, ACI of low clouds is poorly understood and causes the spread of Earth System Models (ESMs) in predicting cloud and climate responses to aerosol changes. Process studies have shown a nonlinear cloud water amount and cloud fraction adjustments due to aerosol changes via precipitation and cloud top entrainment, which are not often captured correctly in ESMs. This study explores the physical mechanisms of ACI in marine low clouds with a focus on precipitating low clouds using a cloud process model and unprecedented field campaign measurements of meteorology states, cloud properties, and aerosols collected during the Aerosol Cloud meTeorology Interactions oVer the western ATlantic Experiment. We show that the aerosol‐induced cloud water amount adjustment is dominated by changes in precipitation and there is no clear entrainment feedback in both cases. Our findings can help improve the representation of ACI within precipitating marine low clouds in ESMs. Key Points: Aerosol‐cloud interactions in precipitating shallow cumuli are investigated using large‐eddy simulations (LES) and observationsLES show that aerosol‐induced cumulus cloud water adjustment is dominated by precipitation with no clear entrainment feedbackAn increase in cloud fraction in response to aerosol number concentration decrease is shown in the precipitating cumuli [ABSTRACT FROM AUTHOR]

Continental shelves contribute a large fraction of the ocean's new nitrogen (N) via N2 fixation; yet, we know little about how physical processes at the ocean's margins shape diazotroph biogeography and activity. Here, we test the hypothesis that frontal mixing favors N2 fixation at the Mid-Atlantic Bight shelfbreak. Using the 15N2 bubble release method, we measured N2 fixation rates on repeat cross-frontal transects in July 2019. N2 fixation rates in shelf waters (median = 5.42 nmol N L−1 d−1) were higher than offshore (2.48 nmol N L−1 d−1) but did not significantly differ front frontal waters (8.42 nmol N L−1 d−1). However, specific N2 uptake rates, indicative of the relative contribution of diazotroph-derived N to particulate N turnover, were significantly higher in frontal waters, suggesting that diazotroph-derived N is of greater importance in supporting productivity there. This study furthered captured an ephemeral shelf-water streamer, which resulted from the impingement of a warm core ring on the shelf. The streamer transported shelf-water diazotrophs (including UCYN-A and Richelia spp., as assessed by qPCR) offshore with sustained high N2 fixation rates. This feature injected >50 metric tons d−1 of newly fixed N to the Slope Sea—a rate equivalent to ∼4% of the total N flux estimated for the entire Mid-Atlantic Bight. As intrusions of Gulf Stream meanders and eddies onto the shelf are increasing in frequency due to climate change, episodic lateral fluxes of new N into the Slope Sea may become increasingly important to regional budgets and ecosystem productivity. [ABSTRACT FROM AUTHOR]

The Kuroshio‐Oyashio Extension and Gulf Stream oceanic frontal zones are characterized by enhanced activity of synoptic‐scale cyclones and anticyclones and vigorous air‐sea heat and moisture exchange in the cold season. However, the time‐mean air‐sea exchange attributed separately to cyclones and anticyclones has not been assessed. Here we quantify cyclonic and anticyclonic contributions around the frontal zones to surface turbulent heat fluxes, precipitation, and the associated hydrological cycle using atmospheric general circulation model experiments with observed and artificially smoothed sea‐surface temperature gradients. The evaluation reveals that precipitation exceeds evaporation climatologically within cyclonic domains while evaporation dominates within anticyclonic domains. These features as well as the net moisture transport from anticyclonic to cyclonic domains are all enhanced by the sharpness of the frontal zones. Oceanic frontal zones thus climatologically act to strengthen the hydrological cycle. These findings aid our understanding of the relationship between midlatitude air‐sea interactions on synoptic‐ and longer‐time scales. Plain Language Summary: Two regions with pronounced sea surface temperature gradients over the North Pacific and North Atlantic are known as major oceanic frontal zones that are important for air‐sea interactions with vigorous heat and moisture release from the ocean to the atmosphere. Recent studies found that day‐to‐day variations, due to migratory low‐ and high‐pressure systems, are essential for the time‐mean air‐sea interaction over these frontal zones. However, the relative importance of those low‐ and high‐pressure systems has not been quantified. Using atmospheric general circulation model experiments with observed and artificially smoothed sea‐surface temperature gradients, we show that contributions of high‐pressure systems are important for the enhanced heat and moisture supply from the ocean in response to realistic oceanic frontal zones, while contributions of low‐pressure systems are crucial for the changes in rainfall. We further demonstrate that the moisture transport from high‐ to low‐pressure systems is strengthened climatologically with the sharpness of midlatitude oceanic frontal zones, indicative of a strengthening of the hydrological cycle. Our findings indicate that synoptic‐scale migratory low‐ and high‐pressure systems play an important role in midlatitude air‐sea interactions. These results bridge the gap between our understanding of midlatitude air‐sea interactions from day‐to‐day to climatic time scales. Key Points: Cyclonic and anticyclonic contributions to air‐sea heat and moisture exchange are quantified around midlatitude oceanic frontal zonesOceanic frontal zones enhance surface turbulent heat fluxes within anticyclones and precipitation within cyclones, respectivelySharpness of midlatitude oceanic frontal zones strengthens the net moisture transport from anticyclones to cyclones [ABSTRACT FROM AUTHOR]

Understanding the interplay of ocean physics and biology at the submesoscale and below (<30 km) is an ongoing challenge in oceanography. While poorly constrained, these scales may be of critical importance for understanding how changing ocean dynamics will impact marine ecosystems. Fronts in the ocean, regions where two disparate water masses meet and isopycnals become tilted toward vertical, are considered hotspots for biophysical interaction, but there is limited observational evidence at the appropriate scales to assess their importance. Fronts around western boundary currents like the Gulf Stream are of particular interest as these dynamic physical regions are thought to influence both productivity and composition of primary producers; however, how exactly this plays out is not well known. Using satellite data and 2 years of in situ observations across the Gulf Stream front near Cape Hatteras, North Carolina, we investigate how submesoscale frontal dynamics could affect biological communities and generate hotspots of productivity and export. We assess the seasonality and phenology of the region, generalize the kilometer‐scale structure of the front, and analyze 69 transects to assess two physical processes of potential biogeochemical importance: cold shelf filament subduction and high salinity Sargasso Sea obduction. We link these processes observationally to meanders in the Gulf Stream and discuss how cold filament subduction could be exporting carbon and how obduction of high salinity water from depth is connected with high chlorophyll‐a. Finally, we report on phytoplankton community composition in each of these features and integrate these observations into our understanding of frontal submesoscale dynamics. Plain Language Summary: The interplay of physics and biology in determining the biomass and composition of phytoplankton communities is poorly understood and is key to understanding marine ecosystem resilience and structure in a changing ocean. In this work we investigated the impact of physics and biology on phytoplankton across scales focusing on the Gulf Stream front. Fronts in the ocean are where lines of equal density go from being horizontal to having a vertical tile, and because of this can enable nutrients and plankton to move from depth to the surface and vice versa. The objective of this work is to understand how physics might drive important changes in phytoplankton biomass and composition in the Gulf Stream front, which is amongst the sharpest gradients in temperature, density, and current speed in the global ocean. We find two frequent processes at the front, the apparent downward movement of cold filaments along the edge of the Gulf Stream, associated with meander troughs, and upward movement of high salinity Sargasso Sea water into the front linked to meander crests. While ephemeral, these processes are frequent and could have a large impact on local phytoplankton biomass, phytoplankton composition, and the export of organic matter to depth. Key Points: The frontal zone of the Gulf Stream has an interface water mass with different origins and a range of biogeochemical impactsMeanders appear to control the front interface: troughs lead to shelf filament subduction and crests lead to high salinity water obductionSubduction and obduction appear to be common at the front and could lead to ephemeral kilometer‐scale export and productivity [ABSTRACT FROM AUTHOR]

The 1991–2020 climate normals for sea surface temperature (SST) are computed based on the NOAA Daily Optimum Interpolation SST dataset. This is the first time that high‐resolution SST normals with global coverage can be achieved in the satellite SST era. Normals are one of the fundamental parameters in describing and understanding weather and climate and provide decision‐making information to industry, public, and scientific communities. This product suite includes SST mean, standard deviation, count and extreme parameters at daily, monthly, seasonal and annual time scales on 0.25° spatial grids. The main feature of the SST mean state revealed by the normals is that in the Tropics, the Indo‐Pacific Ocean is dominated by the warm pool (SST ≥ 28°C) while the eastern Pacific is characterized by the cold tongue (SST ≤ 24°C); in the midlatitudes, SSTs are in zonal patterns with high meridional gradients. Daily SST standard deviations are generally small (<1.0°C) except in frontal zones (>1.5°C) mostly associated with ocean currents such as the Gulf Stream, Kuroshio and Equatorial Currents. Compared to the 1982–2011 climatology, the 1991–2020 mean SSTs increased over most global areas but obvious cooling is seen in the Southern Ocean, eastern tropical South Pacific Ocean and North Atlantic warming hole. The Indo‐Pacific warm pool (IPWP) is found to have strengthened in both intensity and coverage since 1982–2011. By a count parameter criterion of ≥300 days annually with SST ≥ 28°C, the IPWP coverage increased 33% from 1982–2011 to 1991–2020. The global mean SST of 1991–2020 is warmer than that of 1982–2011, and the warming rate over 1991–2020 doubles that over 1901–2020. [ABSTRACT FROM AUTHOR]

Heterotrophic bacteria hydrolyze high molecular weight (HMW) organic matter extracellularly prior to uptake, resulting in diffusive loss of hydrolysis products. An alternative 'selfish' uptake mechanism that minimises this loss has recently been found to be common in the ocean. We investigated how HMW organic matter addition affects these two processing mechanisms in surface and bottom waters at three stations in the North Atlantic Ocean. A pulse of HMW organic matter increased cell numbers, as well as the rate and spectrum of extracellular enzymatic activities at both depths. The effects on selfish uptake were more differentiated: in Gulf Stream surface waters and productive surface waters south of Newfoundland, selfish uptake of structurally simple polysaccharides increased upon HMW organic matter addition. The number of selfish bacteria taking up structurally complex polysaccharides, however, was largely unchanged. In contrast, in the oligotrophic North Atlantic gyre, despite high external hydrolysis rates, the number of selfish bacteria was unchanged, irrespective of polysaccharide structure. In deep bottom waters (> 4000 m), structurally complex substrates were processed only by selfish bacteria. Mechanisms of substrate processing—and the extent to which hydrolysis products are released to the external environment—depend on substrate structural complexity and the resident bacterial community. [ABSTRACT FROM AUTHOR]

Diatoms are important components of the marine food web and one of the most species‐rich groups of phytoplankton. The diversity and composition of diatoms in eutrophic nearshore habitats have been well documented due to the outsized influence of diatoms on coastal ecosystem functioning. In contrast, patterns of both diatom diversity and community composition in offshore oligotrophic regions where diatom biomass is low have been poorly resolved. To compare the diatom diversity and community composition in oligotrophic and eutrophic waters, diatom communities were sampled along a 1,250 km transect from the oligotrophic Sargasso Sea to the coastal waters of the northeast US shelf. Diatom community composition was determined by amplifying and sequencing the 18S rDNA V4 region. Of the 301 amplicon sequence variants (ASVs) identified along the transect, the majority (70%) were sampled exclusively from oligotrophic waters of the Gulf Stream and Sargasso Sea and included the genera Bacteriastrum, Haslea, Hemiaulus, Pseudo‐nitzschia, and Nitzschia. Diatom ASV richness did not vary along the transect, indicating that the oligotrophic Sargasso Sea and Gulf Stream are occupied by a diverse diatom community. Although ASV richness was similar between oligotrophic and coastal waters, diatom community composition in these regions differed significantly and was correlated with temperature and phosphate, two environmental variables known to influence diatom metabolism and geographic distribution. In sum, oligotrophic waters of the western North Atlantic harbor diverse diatom assemblages that are distinct from coastal regions, and these open ocean diatoms warrant additional study, as they may play critical roles in oligotrophic ecosystems. [ABSTRACT FROM AUTHOR]

The Sargasso Sea, accommodating marine species and mode water, is highly productive relative to other oligotrophic subtropical regions with intense air‐sea carbon and heat exchanges. Turbulent diapycnal mixing regulates these processes and is generally thought to be weakened in a more stratified ocean under anthropogenic forcing. However, by applying a finescale parameterization to long‐term hydrographic profiles collected during the Bermuda Atlantic Time‐Series Study (BATS), we show that its intensity has become stronger from 1994 to 2019 in the permanent thermocline. The enhanced turbulent diapycnal mixing is mainly attributed to the increase of wind power on internal waves along the Gulf Stream extension. Numerical simulations suggest that stronger internal waves excited along the Gulf Stream extension radiate downward and equatorward, causing enhanced turbulent diapycnal mixing in the northern Sargasso Sea including the BATS station. The findings have important implications for understanding responses of mode water and ecosystem in the Sargasso Sea to anthropogenic forcing. Plain Language Summary: Turbulent mixing contributes to the vertical transport of heat, carbon and nutrients in the ocean, exerting significant effects on the ocean circulations and climate. It is thus important to know how turbulent mixing in the ocean will change under anthropogenic forcing. There is a prevailing thought that turbulent mixing should be weakened under anthropogenic forcing, as the enhanced stratification due to the faster warming in the upper than deeper ocean would hinder the processes generating turbulence. Here, using observation data sampled during the Bermuda Atlantic Time‐Series Study (BATS), we find that the turbulent mixing in the permanent thermocline there has increased by 42% over the past three decades. This positive trend is due to stronger wind energy input on internal waves that are gravity waves propagating within the ocean interior rather than on its surface. As these waves radiate downward and equatorward, they transfer their energy into smaller‐scale internal waves and eventually break, leading to enhanced turbulent mixing in the northern Sargasso Sea. Key Points: Thermocline diapycnal mixing inferred from a finescale parameterization in the northern Sargasso Sea is enhanced from 1994 to 2019Enhanced thermocline diapycnal mixing in the northern Sargasso Sea is due to stronger internal waves excited by intensified winds [ABSTRACT FROM AUTHOR]

The use of coarse resolution and strong grid‐scale dissipation has prevented global ocean models from simulating the correct kinetic energy level. Recently parameterizing energy backscatter has been proposed to energize the model simulations. Parameterizing backscatter reduces long‐standing North Atlantic sea surface temperature (SST) and associated surface current biases, but the underlying mechanism remains unclear. Here, we apply backscatter in different geographic regions to distinguish the different physical processes at play. We show that an improved Gulf Stream path is due to backscatter acting north of the Grand Banks to maintain a strong deep western boundary current. An improved North Atlantic Current path is due to backscatter acting around the Flemish Cap, with likely an improved nearby topography‐flow interactions. These results suggest that the SST improvement with backscatter is partly due to the resulted strengthening of resolved currents, whereas the role of improved eddy physics requires further research. Plain Language Summary: Global ocean models often suffer from a lack of kinetic energy, and energy backscatter is a relatively new method designed to put kinetic energy back into the resolved flow field. It is also found to help better simulate the surface temperatures in the North Atlantic Ocean, which strongly affect the weather and climate in eastern North America and Europe but are often poorly represented by numerical simulations. Why models struggle with getting these temperatures right remains unclear, so it is important to determine mechanistic reasons for why backscatter can reduce the problem. In this study, we find that the global impact of backscatter can be separated into individual parts for different ocean regions. This regional separation of the impacts allows us to pin down the key ocean regions that give us the source of improvement and compare the roles of different ocean physical processes emphasized in previous studies. Our results point to the importance of backscatter in strengthening the resolved currents, and a need to better understand the role of eddies and their response to backscatter in the key regions identified. Key Points: Parameterization of kinetic energy backscatter reduces the North Atlantic sea surface temperature (SST) biasesThe SST response to backscatter is attributed to the effect of backscatter in different regionsRoles of backscatter in strengthening resolved currents versus improving eddy physics are accordingly discussed [ABSTRACT FROM AUTHOR]

Understanding the transport of large ocean currents, like the Gulf Stream, has been of interest since the early days of oceanography. There has been less attention on the widths of the currents, although there exist several theoretical predictions. We present a census of time‐averaged jet profiles, using in situ and satellite data. The jets are typically asymmetrical, being narrower on the side with weaker stratification. The half‐widths Lj are correlated with the local deformation radius Ld associated with the first surface mode on either side. The dependence of Lj on Ld is predicted by simple shallow water geostrophic adjustment models, with or without outcropping layers. This implies that potential vorticity is well‐mixed adjacent to the jets, due most likely to mesoscale eddies. The findings suggest that surface jet widths are determined locally, by eddy‐mean flow interactions. Plain Language Summary: Narrow, persistent currents a few tens of kilometers wide are found in several regions of the ocean, like fast‐flowing rivers. The Gulf Stream and Kuroshio are familiar examples. We use different measurements to show that these currents have a generic shape, being asymmetric relative to their point of maximum velocity, that is, they are wider on one side and narrower on the other. The profile has a distinctive shape and results from strong eddy stirring on either side of the jet. The jet widths depend on the local density profile and on the Earth's rotation. Key Points: We present a census of the widths of several surface jets across the global ocean, using in situ and satellite dataMost jets are asymmetrical, with the half‐widths correlated with the first surface radius on each side of the jetThe asymmetry and deformation scale dependence are predicted by simple layered analytical models and reflect active lateral mixing [ABSTRACT FROM AUTHOR]

Subsurface nutrients on the Scotian Shelf, an ocean region at the convergence of the subpolar and subtropical western boundary currents (i.e., Labrador Current (LC) and Gulf Stream (GS)), are chiefly modulated by upstream shelf and slope waters. Yet little is known about long‐term fluctuations in the advective transport of nutrients to the shelf. Here, we synthesized nutrient and hydrographic data from 1975 to 2020 to characterize decadal changes in upper (60–150 m) and lower (150–200 m) subsurface nutrient availability across the shelf. Hydrographic and nutrient anomalies highlight a transition from a warm, saline, high‐nutrient GS‐dominated system in the 1980 to colder, fresher, lower‐nutrient LC‐dominated conditions in the late 1980 and early 1990. In the mid‐1990, an increase in excess silicate and phosphate (relative to nitrate) marks a shift in the LC contribution toward a strengthening of the inner shelf component relative to the outer shelf‐edge branch. Since 2010, rapid increases in subsurface temperature and salinity indicate the transition back to a GS‐dominated system, albeit with a distinct decline in dissolved nutrients relative to the previous warm period, which diverges from the nutrient increase generally associated with GS water. Declines in lower subsurface nutrient concentrations since 2010 are likely driven by a density‐related shift in the GS source water toward a less dense, lower‐nutrient GS contribution. This shift is evidenced by a decrease in excess phosphate amidst minor declines in excess silicate, reflecting the characteristic density‐related patterns of excess nutrients in GS waters. Plain Language Summary: The Northwest Atlantic continental shelf is a highly productive region, sustaining large ecologically and commercially important fisheries from the Grand Banks across the Scotian Shelf into the Gulf of Maine. This high productivity depends on the supply of bioavailable nutrients, which are essential components feeding phytoplankton growth and ecosystem productivity. On the Scotian Shelf, surface productivity is thought to be fueled by the vertical supply of nutrient‐rich subsurface water, deriving from upstream coastal waters, and the onshore transport of deeper slope waters. In this study, we used hydrographic and nutrient data from 1975 to 2020 to investigate long‐term trends in nutrient availability on the shelf. We show that over the past 45 years, subsurface properties have shifted from warm, salty, nutrient‐rich waters at the beginning of the time series to colder, fresher, lower‐nutrient conditions in the 1990/early 2000, and back to warm, salty, yet nutrient‐poor conditions after 2010 due to changes in the dominant source water advected onto the shelf. The significant decline in nutrients since 2010 suggests a shift to a less dense, lower‐nutrient source from the Gulf Stream compared to the previous warm period. Key Points: Subsurface nutrients on the shelf are modulated by advection of upstream coastal water, and offshore Gulf Stream and Labrador Slope WaterNutrient properties indicate a shift in Labrador Current contributions, with a strengthening of the inner versus outer branch in 1995–2010Enhanced contributions from lower‐density Gulf Stream water drive a warming and decrease in subsurface nutrient concentrations since 2010 [ABSTRACT FROM AUTHOR]

The Gulf Stream (GS) is a powerful ocean current that is instrumental in regulating the global climate. While a correct reproduction of GS dynamics is contingent on an appropriate representation of energy dissipation, the specific role of tides in dissipation pathways of the wind‐driven circulation is not well understood. Here, we present evidence, using high‐resolution ocean simulations (∼2 km grid spacing), that ocean tides, by generating internal gravity waves, increase the forward cascade of energy in the GS region. This effect is greatest in summer, when the intensity of internal tides increases. However, the dissipation route associated with the forward energy cascade remains an order of magnitude weaker than frictional dissipation near the surface and bottom boundaries. Plain Language Summary: Understanding the dynamics of the Gulf Stream (GS) is essential because of its influence on global climate and ocean circulation. Previous studies have shown that a realistic representation of this region using oceanic models depends on a correct representation of the energy balance and, in particular, how the system loses energy. Energy can be dissipated at the boundaries (bottom and surface ocean) and in the interior, but how tides affect energy dissipation in the GS is unknown. In this study, we found that the interior dissipation increases when tides are included, but this route remains small compared to the energy dissipated at the boundaries. Key Points: Internal tides modulate the Gulf Stream (GS) turbulent cascade through wave‐flow interactionExternal tides increase bottom drag of subtidal circulation in the deep ocean. intensifies the forward cascadeTide‐induced increased dissipation pathways reduce GS mesoscale activity [ABSTRACT FROM AUTHOR]

The Gulf Stream is a vital limb of the North Atlantic circulation that influences regional climate, sea level, and hurricane activity. Given the Gulf Stream's relevance to weather and climate, many studies have attempted to estimate trends in its volumetric transport from various data sets, but results have been inconclusive, and no consensus has emerged whether it is weakening with climate change. Here we use Bayesian analysis to jointly assimilate multiple observational data sets from the Florida Straits to quantify uncertainty and change in Gulf Stream volume transport since 1982. We find with virtual certainty (probability P > 99%) that Gulf Stream volume transport through the Florida Straits declined by 1.2 ± 1.0 Sv in the past 40 years (95% credible interval). This significant trend has emerged from the data set only over the past ten years, the first unequivocal evidence for a recent multidecadal decline in this climate‐relevant component of ocean circulation. Plain Language Summary: The Gulf Stream is a major ocean current located off the East Coast of the United States. It carries a tremendous amount of seawater and along with it heat, carbon, and other ocean constituents. Because of this, the Gulf Stream plays an important role in weather and climate, influencing phenomena as seemingly unrelated as sea level along coastal Florida and temperature and precipitation over continental Europe. Given how important this ocean current is to science and society, scientists have tried to determine whether the Gulf Stream has undergone significant changes under global warming, but so far, they have not reached a firm conclusion. Here we report our effort to synthesize available Gulf Stream observations from the Florida Straits near Miami, and to assess whether and how the Gulf Stream transport there has changed since 1982. We conclude with a high degree of confidence that Gulf Stream transport has indeed slowed by about 4% in the past 40 years, the first conclusive, unambiguous observational evidence that this ocean current has undergone significant change in the recent past. Future studies should try to identify the cause of this change. Key Points: Gulf Stream volume transport through Florida Straits declined by 1.2 ± 1.0 Sv during the past 40 years (95% credible interval)We find a weakening trend in the Gulf Stream by applying Bayesian methods to synthesize cable, in situ, and satellite data sets congruently [ABSTRACT FROM AUTHOR]

The United States Northern Shortfin squid fishery is known for its large fluctuations in catch at annual scales. In the last 5 years, this fishery has experienced increased availability of Illex illecebrosus along the Northeast US continental shelf (NES), resulting in high catch per unit effort (CPUE) and early fishery closures due to quota exceedance. The fishery occurs within the Northwest Atlantic, whose complex dynamics are set up by the interplay between the large‐scale Gulf Stream, mesoscale eddies, Shelfbreak Jet, and shelf‐slope exchange processes. Our ability to understand and quantify this regional variability is requisite for understanding the availability patterns of Illex, which are largely influenced by oceanographic conditions. In an effort to advance our current understanding of the seasonal and interannual variability in this species' relative abundance on the NES, we used generalized additive models to examine the relationships between the physical environment and hotspots of productivity to changes in CPUE of I. illecebrosus in the Southern stock component, which comprises the US fishery. Specifically, we derived oceanographic indicators by pairing high‐resolution remote sensing data and global ocean reanalysis physical data to high‐resolution fishery catch data. We identified a suite of environmental covariates that were strongly related to instances of higher catch rates. In particular, bottom temperature, warm core rings, subsurface features, and frontal dynamics together serve as indicators of habitat condition and primary productivity hotspots, providing great utility for understanding the distribution of Illex with the potential for forecasting seasonal and interannual availability. [ABSTRACT FROM AUTHOR]

This study investigates Gulf Stream (GS) sea surface temperature (SST) anomalies associated with the extratropical transition (ET) of tropical cyclones (TCs) in the North Atlantic. Composites of western North Atlantic TCs indicate that GS SSTs are warmer, and both large‐ and fine‐scale SST gradients are weaker than average, for TCs that begin the ET process but do not complete it, compared with TCs that do. Further analysis suggests that the associated fine‐scale GS SST gradient anomalies are related to atmospheric processes but not the same as those that are typically associated with the onset of ET. As sensible heat flux gradients and surface diabatic frontogenesis are shown to generally scale with the local SST gradient strength, these results suggest that knowledge of the fine‐scale GS SST gradient in the weeks prior to the arrival of a TC might potentially provide additional information regarding the likelihood of that system completing ET. Plain Language Summary: Tropical cyclones can undergo a process known as extratropical transition as they move toward higher latitudes. The resultant weather systems can cause extensive damage and can be difficult to forecast. This study discusses how knowledge of the Gulf Stream sea surface temperature gradients in the week prior to the passage of a tropical cyclone may provide additional insight into whether that tropical cyclone will complete extratropical transition in the region. In particular, we discuss how fine‐scale sea‐surface temperature gradients can modulate the gradient in heat exchange between the ocean and the atmosphere, and in turn influence how favorable the lower atmosphere is to the development of sharp atmospheric temperature gradients. Key Points: Gulf Stream sea surface temperature anomalies are statistically related to the completion of tropical cyclone extratropical transitionFine‐scale sea surface temperature gradients present do not come from an atmospheric setup conducive to extratropical transition completionThe associated sensible heat flux gradients may potentially influence extratropical transition by impacting frontogenesis in the region [ABSTRACT FROM AUTHOR]

North Atlantic Oscillation (NAO) has a significant impact on surrounding winter weather and climate. However, the causes of its occasional reversal between early and late winter remain unclear. This study proposes a mechanism for the mid‐winter reversal of NAO from the perspective of local midlatitude air‐sea interaction. Strong sea surface temperature (SST) tripole events, which are defined by empirical orthogonal function of winter‐mean interannual North Atlantic SST anomalies, are primarily induced by NAO in early winter and peak in January. In late winter, the persistent SST tripole exerts active feedback on atmosphere through diabatic heat and transient eddy forcing. The resulting atmospheric circulation anomalies exhibit an almost reversed NAO pattern in February, which forms a wavetrain originating above the Gulf Stream and propagating to the Middle East and weakens the former SST tripole. Consequently, significant reversals of air temperature anomalies occur in Europe and the Caspian Sea area between February and December. Plain Language Summary: The North Atlantic Oscillation (NAO) is a crucial atmospheric system that significantly impacts the weather and climate of the surrounding regions during winter. However, it often reverses between early and late winter, and the reasons for its mid‐winter reversal remain unclear. The NAO behavior can be influenced by multiple factors, such as atmospheric internal processes, underlying surface, and remote climate system, which makes the issue more complex. This study highlights the role of underlying sea surface temperature anomalies (SSTAs) in the mid‐winter reversal of NAO. The North Atlantic SSTAs are closely associated with the NAO and exhibit a tripole pattern. For strong sea surface temperature (SST) tripole events, the NAO primarily induces the early winter North Atlantic SST tripole. The SSTAs develop in early winter, peak in January, and feedback on the atmospheric circulation in late winter. The atmospheric circulation anomalies exhibit an almost reversed NAO pattern in February, forming a wavetrain that propagates above the Gulf Stream to the Middle East. As a result, during the warm phase of the SST tripole, Europe experiences colder temperatures, and the Caspian Sea experiences warmer temperatures in February than usual, as opposed to the positive NAO's control over warm Europe and cold Caspian Sea in December. Key Points: In early winter, North Atlantic Oscillation (NAO) forces a North Atlantic sea surface temperature (SST) tripole, which persists and tends to induce a nearly reversed NAO pattern in FebruarySST tripole induces a wavetrain over the Gulf Stream to the Middle East via diabatic heating and transient eddy forcing in FebruaryAs a result, there are remarkable reversals of air temperature anomalies in Europe and Caspian Sea area between February and December [ABSTRACT FROM AUTHOR]

The Kuroshio and Gulf Stream (GS), Northern Hemisphere warm ocean currents, have narrow bands of intense jets and vigorous eddies. While the importance of them in regulating physical and biological ocean systems and climate has been increasingly recognized, the operational predictability of year‐to‐year variations in these jets and eddies has not been quantitatively assessed yet. Through a series of experiments using an eddy‐resolving dynamical ocean forecasting system, here we show that observed year‐to‐year variations in the intensities of the jets and eddies in the Kuroshio Extension and GS regions can be skillfully predicted up to a lead time of about 2 years. Sources of predictability mostly come from initial ocean memories, and variations in local atmospheric conditions play a secondary role. Our results demonstrate that the observed multiyear variations in ocean jets and eddies in these regions are operationally predictable when ocean models are properly initialized with sufficient spatial resolution. Plain Language Summary: The Kuroshio and Gulf Stream (GS), strong warm currents flowing in the North Pacific and Atlantic Ocean, respectively, transport huge amounts of heat from the tropics to higher latitudes. Year‐to‐year changes in the intensities of both currents are known to exert widespread impacts on atmospheric circulation and marine ecosystems in the midlatitude regions and thus directly influence human lives in densely populated areas. Therefore, an accurate monitoring and prediction of spatial/temporal variations in both current fields are of fundamental importance for various economic activities, such as weather forecasting, fisheries, and ship navigations. As a first step to achieve this goal, we have performed a series of operational forecasting experiments of these two warm currents using an ocean monitoring and prediction system that can explicitly resolve oceanic jets and eddies. We found that our newly developed system can skillfully predict the observed year‐to‐year fluctuations in intensities of jets and eddies in the Kuroshio and GS 2 years in advance when initial ocean conditions are properly incorporated into the model. The operational forecasts of these two currents, which are shown to be possible by this work, will be beneficial for various fields that are closely related to our daily life. Key Points: Multiyear variations in speeds of the Kuroshio and Gulf Stream jets are found to be operationally predictable up to 2 yearsInitial memories in interior oceans hold a key for skillful predictions of year‐to‐year variations in speeds of jetsAmplitude modulations in regional mesoscale eddy activity can also be predicted due to their dependence on background circulation [ABSTRACT FROM AUTHOR]

We previously documented geographic distributions of the optically brightest lightning on Earth—known as "superbolts"—using two space‐based instruments: the photodiode detector (PDD) on the Fast On‐orbit Recording of Transient Events (FORTE) satellite and the Geostationary Lightning Mapper (GLM) on NOAA's Geostationary Operational Environmental Satellites. In this study, we further examine the superbolts identified by the PDD and GLM to reconcile the differences between their geographic distributions. We find that both the physical extent of the parent flash and the development speed of its leaders are important for making a superbolt. The oceanic PDD superbolts tend to occur early in flashes that rapidly expand laterally into long horizontal "megaflashes." The top GLM superbolts occur over land at later times in particularly large megaflashes. These land‐based flashes grow more slowly until they extend over multiple hundreds of kilometers. The FORTE PDD missed these delayed superbolts due to limitations in its triggering. Coincident Tropical Rainfall Measuring Mission measurements show that the warm season megaflash superbolts detected by Lightning Imaging Sensor/GLM and wintertime oceanic superbolts observed by the PDD occur in otherwise similar thunderstorm environments. Both are marked by: low storm heights (<10 km), widespread precipitation near the surface, small infrared brightness temperature gradients, and low flash rates. We suggest that the vertically compact, stratiform nature of these clouds provides favorable conditions for superbolt production. Plain Language Summary: The brightest optical pulses from lightning seen from space have been termed "superbolts." Superbolts are incredibly rare and represent flashes of light that are at least 100 times brighter than what we typically see from lightning. Curiously, the locations and times associated with these brightest optical pulses differ based on the instrument and satellite used. Previous assessments with the photodiode detector (PDD) on the Fast On‐orbit Recording of Transient Events (FORTE) satellite identify wintertime lightning around Japan, along the Gulf Stream in the Atlantic Ocean, and in the Mediterranean Sea as frequent sources of superbolts. However, assessments with NOAA's Geostationary Lightning Mapper (GLM) find hotspots in regions with large‐horizontal "megaflashes"—particularly, the La Plata basin in south America and the south‐central United States in North America. In this study, we examine how these differences in optical superbolt detections arise. We find that the differing global distributions can be explained by the FORTE PDD missing superbolts that occur late in the flash due to its trigger count limit—an issue not encountered by GLM. Key Points: The top Fast On‐orbit Recording of Transient Events (FORTE) photodiode detector, Lightning Imaging Sensor (LIS), and Geostationary Lightning Mapper (GLM) detections capture different portions of the population of the top optical lightning eventsThese differences arise from nuances in the design and operation of these sensors that introduce detection biasesThe FORTE superbolt distribution only represents a subset of all superbolts, which should also include the top LIS/GLM events [ABSTRACT FROM AUTHOR]

The Atlantic Meridional Overturning Circulation (AMOC) is frequently used to diagnose the state of the North Atlantic circulation, but as an integrated quantity the AMOC strength does not necessarily mirror changes in the individual circulation components. Here, we investigate future circulation changes in the subtropical North Atlantic (26.5°N) in CMIP6 models, diagnosing the relationship between the Gulf Stream, Deep Western Boundary Current (DWBC), gyre recirculation, and the integrated AMOC response. Under continued high emissions, we find a multi‐model mean Gulf Stream weakening of 29% (11.2 Sv) and a DWBC weakening of 47% (8.5 Sv) by the end of the century. However, 33% (3.7 Sv) of the Gulf Stream weakening is due to changes in wind stress and therefore not simply a compensating effect for reduced high‐latitude water mass transformation and a weaker DWBC. Our findings have implications for how we understand the dynamics of future North Atlantic circulation changes. Plain Language Summary: Climate models show circulation in the North Atlantic Ocean to weaken under continued high greenhouse gas emissions. How this weakening is expressed in the Gulf Stream, Deep Western Boundary Current (DWBC), and subtropical gyre circulation remains little explored. Here, we use models that contributed to the Coupled Model Intercomparison Project Phase 6 to investigate potential future changes in the subtropical North Atlantic Ocean. Averaging over 14 models, we find the Gulf Stream to weaken by 29% and the DWBC to weaken by 47% by the end of the century. As much as 33% of the Gulf Stream weakening is due to changes in the large‐scale winds, while the remaining fraction is compensating for the weakening DWBC. Our findings have implications for understanding the interconnectedness of the different circulation branches and their relationship to the large‐scale Atlantic Meridional Overturning Circulation. Key Points: Future Atlantic Meridional Overturning Circulation decline is deconstructed by quantifying changes in the Gulf Stream, Deep Western Boundary Current (DWBC), and gyre recirculationIn a high emission scenario, Coupled Model Intercomparison Project Phase 6 models show a Gulf Stream weakening of 29% and a DWBC weakening of 47% at 26.5°N by the end of the century33% of the Gulf Stream weakening is due to changes in winds [ABSTRACT FROM AUTHOR]

Air‐sea flux variability has contributions from both ocean and atmosphere at different spatio‐temporal scales. Atmospheric synoptic scales and the air‐sea turbulent heat flux that they drive are well represented in climate models, but ocean mesoscales and their associated variability are often not well resolved due to non‐eddy‐resolving spatial resolutions of current climate models. We deploy a physics‐based stochastic subgrid‐scale parameterization for ocean density, that reinforces the lateral density variations due to oceanic eddies, and examine its effect on air‐sea heat flux variability in a comprehensive coupled climate model. The stochastic parameterization substantially modifies sea surface temperature (SST) and latent heat flux (LHF) variability and their co‐variability, primarily at scales near the resolution of the ocean model grid. Enhancement in the SST‐LHF anomaly covariance, and correlations, indicate that the ocean‐intrinsic component of the air‐sea heat flux variability is more consistent with high‐resolution satellite observations, especially in Gulf Stream region. Plain Language Summary: Variations in air‐sea heat fluxes arise from both ocean and atmosphere at different space and time scales. Studies suggest that at large scales, for example, thousands of kilometers, atmospheric processes drive the ocean variability at the surface, such as sea‐surface temperature. However, at smaller spatial scales, for example, [100−1,000] km, the oceans control the atmosphere variability near the air‐sea interface. These local air‐sea feedbacks influence both oceans and the atmosphere on various levels and are of significant dynamical importance. However, climate models typically use large grid spacing and fail to represent the air‐sea interaction mechanism inherent to these small scales. We address this problem by modifying the ocean density using random noise at multiple places in the model before coupling it to the atmosphere. We chose density because it is used for multiple purposes in ocean models, and imperfections in it arise due to the missing subgrid‐scale effects that can have a major impact all over the oceans, especially the upper ocean which interacts the most with the atmosphere. The proposed approach led to significant improvement in the air‐sea interaction properties at various spatial scales compared to satellite observations. Key Points: The ability of a subgrid‐scale parameterization to improve the ocean‐intrinsic air‐sea flux variability in a climate model is assessedThe parameterization modifies the sea surface temperature and latent heat flux variability at the grid‐scale level and boosts their simultaneous co‐variabilityThe stochastic parameterization improves consistency with the observations of air‐sea interaction [ABSTRACT FROM AUTHOR]

Between Florida and Cape Hatteras, North Carolina, the Gulf Stream carries warm, salty waters poleward along the continental slope. This strong current abuts the edge of the South Atlantic Bight (SAB) continental shelf and is thought to influence exchange of waters between the open ocean and the shelf. Observations from a pair of instruments deployed for 19 months in the northern SAB are used here to examine the processes by which the Gulf Stream can impact this exchange. The instrument deployed on the SAB shelf edge shows that the time‐averaged along‐slope flow is surface‐intensified with only few flow reversals at low frequencies (>40‐day period). Time‐averaged cross‐slope flow is onto the SAB shelf in a lower layer and off‐shelf above. Consistent with Ekman dynamics, the magnitude of lower‐layer on‐shelf flow is correlated with the along‐slope velocity, which is in turn controlled by the position and/or transport of the Gulf Stream that flows poleward along the SAB continental slope. In the frequency band associated with downstream‐propagating wave‐like meanders of the Gulf Stream jet (2‐15 day period), currents at the shelf‐edge are characterized by surface‐intensified flow in the along‐ and cross‐slope directions. Estimates of maximum upwelling velocities associated with cyclonic frontal eddies between meander crests occasionally reach 100 m/day. Plain Language Summary: Along the southeastern US, the Gulf Stream carries warm, salty waters northward. This strong current flows along the South Atlantic Bight (SAB) continental slope and modulates processes that control movement of waters between the continental shelf and open ocean. This two‐way exchange is important for physical oceanography (controlling temperature and salinity of the SAB shelf) and for the shelf's chemical budgets and ecological systems. This study combines observations from two instruments deployed for 19 months near Cape Hatteras on the edge of the shelf and on the upper continental slope to help characterize two important exchange processes. The first is upwelling associated with Gulf Stream meanders, which are small‐scale undulations of the Gulf Stream path along the SAB shelf edge. Upwelling pumps deep nutrient rich waters from the slope onto the neighboring SAB shelf as cyclonic (counter‐clockwise rotating) cells of circulating waters "ride" along the onshore side of the Gulf Stream between meander crests. A second exchange process occurs within a bottom layer where friction causes water to veer onshore (from the open ocean onto the shelf). Variability in Gulf Stream strength and position along the SAB causes variability in the strength of this Ekman flow. Key Points: Gulf Stream variability drives shelf‐ocean exchange mechanisms in the northern South Atlantic Bight (SAB)Transport pulses onto the SAB shelf in a bottom Ekman layer are driven by low frequency variability in Gulf Stream position and/or strengthUpwelling at the SAB shelf edge associated with intense cyclonic frontal eddies between Gulf Stream meander crests can reach 100 m/day [ABSTRACT FROM AUTHOR]

We expand on a recent determination of the first global energy spectrum of the ocean's surface geostrophic circulation (Storer et al., 2022, https://doi.org/10.1038/s41467-022-33031-3) using a coarse‐graining (CG) method. We compare spectra from CG to those from spherical harmonics by treating land in a manner consistent with the boundary conditions. While the two methods yield qualitatively consistent domain‐averaged results, spherical harmonics spectra are too noisy at gyre‐scales (>1,000 km). More importantly, spherical harmonics are inherently global and cannot provide local information connecting scales with currents geographically. CG shows that the extra‐tropics mesoscales (100–500 km) have a root‐mean‐square (rms) velocity of ∼15 cm/s, which increases to ∼30–40 cm/s locally in the Gulf Stream and Kuroshio and to ∼16–28 cm/s in the ACC. There is notable hemispheric asymmetry in mesoscale energy‐per‐area, which is higher in the north due to continental boundaries. We estimate that ≈25%–50% of total geostrophic energy is at scales smaller than 100 km, and is un(der)‐resolved by pre‐SWOT satellite products. Spectra of the time‐mean circulation show that most of its energy (up to 70%) resides in stationary eddies with characteristic scales smaller than (<500 km). This highlights the preponderance of "standing" small‐scale structures in the global ocean due to the temporally coherent forcing by boundaries. By coarse‐graining in space and time, we compute the first spatio‐temporal global spectrum of geostrophic circulation from AVISO and NEMO. These spectra show that every length‐scale evolves over a wide range of time‐scales with a consistent peak at ≈200 km and ≈2–3 weeks. Plain Language Summary: Traditionally, "eddies" are identified as time‐varying features relative to a background time‐mean flow. As such, "mean" does not imply large length‐scale. Standing eddies or meanders due to topography have little time‐variation, but can have significant energy at small length‐scales that are unresolved and need to be parameterized in coarse climate simulations. Similarly, "eddy" or "time‐varying" do not imply small length‐scale, such as large‐scale motions from Rossby waves or fluctuations of the Kuroshio. Another common method is Fourier analysis in "representative" ocean boxes that cannot capture the circulation's planetary scales. We overcome these limitations thanks to recent advances: (a) a method for calculating spectra by coarse‐graining, (b) properly defining convolutions on the sphere, which "blur" oceanic flow in a way that preserves its underlying symmetries, opening the door for global "wavelet" analysis and, more generally, spatial coarse‐graining, and (c) FlowSieve: an efficient parallel code. We employ coarse‐graining in space‐time to gain new insights into the global oceanic circulation, including how much energy resides in its different spatial structures and how they vary in time. Key Points: Coarse‐graining, which disentangles flow concurrently in scale and space, reveals hemispheric asymmetry in mesoscale energy‐per‐area due to boundariesCoarse‐graining spectra of the time‐mean velocity show that most (up to 70%) of its energy resides in "standing" small‐scale eddies <500 kmWe estimate that ≈25%–50% of total geostrophic energy is at scales smaller than 100 km, and is un(der)‐resolved by pre‐SWOT satellite products [ABSTRACT FROM AUTHOR]

The relationship between Gulf Stream (GS) transport and coastal sea level is investigated using monthly GS transport between 1993 and 2019 at Florida Straits and 10 altimeter tracks. The results show that GS transport decorrelates quickly along its path, indicating it is misleading to assume that transport at a particular location represents strength of the GS as a whole. GS transport south of Cape Hatteras is significantly correlated with coastal sea level in South Atlantic Bight from both altimetry and tide gauges. North of Cape Hatteras, sea level changes associated with GS transport decay rapidly away from GS on the onshore side and become negligible approximately 300 km northwest of GS axis. In this region, the correlations between GS transport and sea level are primarily in the deep ocean and rarely on the shelf, indicating that coastal sea level is unlikely to be driven by geostrophic adjustment to changes in GS transport. Plain Language Summary: Sea level on the U.S. East Coast north of Cape Hatteras has risen rapidly in recent decades. Some previous studies attributed this sea‐level rise to a decline in the Gulf Stream (GS). Here, we investigate the relationship between them from in situ and satellite observations. The results show that GS transport changes continuously along its path, suggesting that the GS strength cannot be represented by its transport at a particular location. South of Cape Hatteras, the GS transport can impact neighboring coastal sea levels via oceanic links. However, its influence on coastal sea level is negligible north of Cape Hatteras. The above results imply that changes in GS transport are unlikely to be the direct cause of rapid sea‐level rise at the U.S. East Coast north of Cape Hatteras. Key Points: Gulf Stream (GS) transport decorrelates rapidly with distance downstream of Cape HatterasMonthly sea level variations on the U.S. East Coast are not correlated with GS transport north of Cape HatterasDownstream of Cape Hatteras, onshore‐side sea level changes associated with GS transport decay rapidly away from the axis [ABSTRACT FROM AUTHOR]

Gridded sea surface height (SSH) maps estimated from satellite altimetry are widely used for estimating surface ocean geostrophic currents. Satellite altimeters observe SSH along one‐dimensional tracks widely spaced in space and time, making accurately reconstructing the two‐dimensional (2D) SSH field challenging. Traditionally, SSH is mapped using optimal interpolation (OI). However, OI artificially smooths the SSH field leading to high mapping errors in regions with rapidly‐evolving mesoscale features such as western boundary currents. Motivated by the dynamical relation between SSH and sea surface temperature (SST) and the notion that even the chaotic evolution of mesoscale ocean turbulence may contain repeating patterns, we outline a deep learning (DL) approach where a neural network is trained to reconstruct 2D SSH by synthesizing altimetry and SST observations. In the Gulf Stream Extension region, dominated by mesoscale variability, our DL method substantially improves the SSH reconstruction compared to existing methods. Our SSH map has 17% lower root‐mean‐square error and resolves spatial scales 30% smaller than OI compared against independent altimeter observations. Surface geostrophic currents calculated from our map are closer to surface drifter observations and appear qualitatively more realistic, with stronger currents, a clearer separation between the Gulf Stream and neighboring eddies, and the appearance of smaller coherent eddies missed by other methods. Our map yields significant re‐estimations of important dynamical quantities such as eddy kinetic energy, vorticity, and strain rate. Applying our DL method to produce a global SSH product may provide a more accurate and higher resolution product for studying mesoscale ocean turbulence. Plain Language Summary: Satellites observe small variations in the height of the sea surface but with large gaps in the observations. Having an estimate of the two‐dimensional sea surface height field allows one to estimate surface ocean currents, so filling in the gaps between the observations is an important problem. The traditionally‐used method for filling in the gaps between sea surface height observations struggles when there are lots of small‐scale, rapidly‐interacting ocean currents. We developed a deep learning (DL) model to estimate the sea surface height field more accurately. We achieved this by combining the sea surface height observations with satellite observations of sea surface temperature. The relationship between sea surface temperature and height is non‐trivial, but our DL model learned to use information from the sea surface temperature observations in the places where sea surface height wasn't observed to improve the accuracy of the sea surface height estimate. We applied and tested our method in the Gulf Stream and demonstrated that our sea surface height map is more accurate than that from the traditional method. Our method provides a more accurate sea surface height map which could allow us in future to learn new lessons about small‐scale surface currents in the ocean. Key Points: We developed a deep learning method that significantly improves the accuracy and resolution of gridded sea surface height anomaliesThis data‐driven method takes advantage of combining sea surface temperature and altimetry observationsInferred surface geostrophic currents are quantitatively and qualitatively more realistic than those from existing sea surface height maps [ABSTRACT FROM AUTHOR]

The impacts of interannual oscillations of the Gulf Stream (GS) on oceanic mesoscale variability are investigated using satellite observations of sea surface height (SSH) and sea surface temperature (SST) from 1993 to 2018. We show that variations in GS position, strength, and meandering status are the dominant spatiotemporal modes in regional SSH variability as they explain over 50% of the total variance. In particular, meridional shift of the GS associated with the large‐scale wind variation over the North Atlantic contributes to approximately 30% of SSH variability. We further find that this path displacement mode can drive approximately 15% of regional mesoscale variability in eddy kinetic energy and divergent eddy heat flux. This observational‐based evidence of ocean mesoscale response to GS shift infers a potentially important forcing mechanism that could drive eddy‐scale ocean variability and has far‐reaching implications for regional ocean and ecosystem dynamics in response to climate variation. Plain Language Summary: The Gulf Stream (GS) is one of the most energetic current in the global ocean which plays an important role in transporting warm water from low to high latitudes to maintain earth's climate system. The region near the GS is occupied by vigorous mesoscale eddy activity which is essential for regional heat and nutrient transport. The impact of GS variation on mesoscale field is largely unknown in this important area. In this study, we systematically investigated the year‐to‐year changes of different GS properties and found that the ocean surface variability in the study region is largely driven by GS changes. We further revealed that variation in mesoscale fields is highly related to the shift of the GS path in response to large‐scale atmospheric oscillation. This observed adaptation of mesoscale activities to large‐scale forcing may have important implications for a new mechanism to explain oceanic mesoscale variability, which may shed some light on potential predictability of mesoscale activities from large‐scale observations in the GS region. Key Points: The observed Gulf Stream (GS) properties drive over 50% of ocean surface variability during 1993–2018The GS shift associated with North Atlantic Oscillation dominates regional interannual variabilityMeridional displacement of the GS explains approximately 15% of oceanic mesoscale variation [ABSTRACT FROM AUTHOR]

This study investigated how warm‐core eddies (WCEs) in the Gulf Stream (GS) modulated the intensity of a distant tropical cyclone (TC) approaching the current in October. We performed cloud‐resolving regional simulations including a control run with the observed WCEs and a sensitivity run excluding the WCEs. These simulations found that the WCEs played a favorable role in the development of the distant TC. The WCEs affected the synoptic‐scale thermodynamic environments over the North Atlantic through the enhanced heat and moisture supply from the GS, increasing the moisture imports toward the distant TC. The WCEs‐enhanced moisture influx created very moist environments in the inner core. This inner‐core moistening was favorable for deep eyewall convection and an associated TC secondary circulation, leading to TC development. This result indicates the importance of WCEs in facilitating the remote process leading to TC development that previous studies have proposed. [ABSTRACT FROM AUTHOR]

The Northwest Atlantic Shelf and Slope have warmed dramatically in the past decade, changing marine life and challenging fisheries management. A rapid warming event in 2009/2010, linked to a reduced supply of cold water from the Labrador Sea, pushed this region to a new state of unprecedentedly high temperatures that persists today. However, a mechanistic understanding of how the Labrador Current connectivity is reduced at the Tail of the Grand Banks of Newfoundland has been lacking. Here, we present the results of a 25‐year (1993–2017) Lagrangian analysis using the HYbrid Coordinate Ocean Model. Synthetic particles were released in the vicinity of the Labrador Current upstream of the Grand Banks and tracked in a 2‐D velocity field. We found that the Labrador Current can be completely blocked by Gulf Stream eddies and meanders that impinge on the shelf break along the Grand Banks. This blocking can occur in many different locations at, upstream, or downstream of the Tail of the Grand Banks, since the Labrador Current needs a clear passage over a long distance to continue its path. In the simulation, the Labrador Current has been blocked more often since 2008, which led to the warming of the Northwest Atlantic Shelf and Slope. These results, which are consistent with satellite observations, can provide predictability for the New England and Nova Scotia shelf environments potentially helpful for ecosystem management. Plain Language Summary: The New England and Nova Scotia continental shelves have been warming at a rate much faster than almost anywhere else in the global ocean. Just east of this region, two of the Atlantic's mightiest currents—the warm Gulf Stream and the cold Labrador Current—interact near the edge of an underwater plateau called the Grand Banks of Newfoundland. Previous observational work suggested that the warming of the continental shelf has been caused by the rising proportion of Gulf Stream waters, due to the reduced contribution of Labrador Current as it rounds the Grand Banks. Here, we use an ocean model that simulates circulation in this region with excellent accuracy to take a deeper look at the drivers of this blockage of the Labrador Current. The model suggests that when the swirling Gulf Stream eddies approach the Grand Banks, they prevent the Labrador Current from continuing its journey to the west, ultimately starving the New England and Nova Scotia continental shelves of a source of cold, fresh water. Because the Gulf Stream eddies have a clear sea surface height signature, an indication of these blocking dynamics may be observed by satellite. Key Points: Virtual Lagrangian particles were launched in the Labrador Current upstream of the Tail of the Grand Banks, using an eddy‐resolving modelThe presence of Gulf Stream eddies at the Tail of the Grand Banks strongly modulates westward transport in the Labrador CurrentInterannual variability in the impingement of Gulf Stream eddies has prevented westward penetration of the Labrador Current in recent years [ABSTRACT FROM AUTHOR]

Bottom drag and top drag—associated with the ocean current feedback (CFB) to the atmosphere—are the energy dissipation processes in the oceanic boundary layers. Both are instrumental in regulating western boundary currents, such as the Gulf Stream (GS). However, the sensitivity of model results to bottom drag parameterization has not yet been considered in simulations where top drag is present. So far, in the absence of top drag, the bottom drag coefficient turns out to be a very sensitive parameter of ocean models. Here, we re‐examine this sensitivity when both top and bottom drag are considered in eddy‐rich ocean simulations of the North Atlantic basin. The top drag is addressed with a parameterization that considers the wind adjustment to CFB. In agreement with previous studies, we confirm that top drag has a large control on GS dynamics by reducing mesoscale activity. Next, we show that the energy sink due to bottom drag increases when top drag is neglected, but that this increase only partially compensates for the absence of top drag. As a result, the GS representation is too sensitive to bottom drag. Finally, we propose a simple scale‐aware parameterization of the subgrid‐scale topographic effect on bottom drag. Our results suggest that for a realistic representation of GS dynamics and energy pathways, top and bottom drags must be considered together. Future studies should include processes missing from this study, such as submesoscale dynamics and the surface gravity waves at the ocean‐atmosphere interface. Plain Language Summary: The oceanic boundary layers dissipate energy via top drag (related to wind) and topographic/bottom drag. These two processes have been shown to play a key role in the regulation of western boundary currents, such as the Gulf Stream (GS). However, bottom drag has, until now, only been considered without top drag. In this case, the models show a fairly strong sensitivity to the bottom drop. In this study, we re‐examine this sensitivity by considering both top and bottom drags in eddy‐rich ocean simulations of the North Atlantic basin. In agreement with previous studies, we first confirm that top drag exerts a significant control on the GS dynamics by reducing eddy activity. Then, we show that neglecting top drag is only partially compensated by bottom drag, thus leaving a surplus of energy as well as an exacerbated sensitivity of the GS representation in models to bottom drag friction. Finally, we propose a simple, scale‐aware parameterization of the subgrid‐scale topographic effect on the bottom drag that allows for resolution‐dependent, spatially distributed bottom friction coefficients. Our results suggest that for a realistic representation of GS dynamics and energy pathways, among other processes, top and bottom drags must be considered together and appropriately parameterized. Key Points: Top and bottom friction affect the Gulf Stream dynamics in different ways but top drag reduces the model sensitivity to bottom frictionA stress‐correction approach for top drag is validated for Gulf Stream dynamicsA scale‐aware parameterization of subgrid topography gives encouraging results [ABSTRACT FROM AUTHOR]

The properties and generation mechanisms of the Florida Current on the subseasonal timescale (20–100 days) are evaluated from in‐situ and satellite observations. The Florida Current volume transport estimates from submarine cable measurements reveal that subseasonal variability accounts for 37% of the total transport variance and is most variable from September to November. Here we show that coastal‐trapped waves are the primary driver of the subseasonal variability in the Florida Current transport. The role of local winds and open ocean signals is of secondary importance. The alongshore wind component of the anomalous North Atlantic subtropical anticyclone generates the subseasonal coastal‐trapped waves off the northern South Atlantic Bight coast. The subseasonal coastal‐trapped waves propagate from Cape May to Apalachicola, behaving as Kelvin waves in the Florida Straits and as continental shelf waves along the rest of the waveguide. They affect the Florida Current transport by up to 2.6 Sv, on average. As the subseasonal waves propagate into the Gulf of Mexico, their energy substantially dissipates. The wave amplitude at Port Canaveral of up to 15 cm is three times higher than at Apalachicola. Concurrent subseasonal changes of the meridional overturning circulation in the subtropical North Atlantic are discussed. Plain Language Summary: The Florida Current, an important constituent of the meridional overturning circulation in the subtropical North Atlantic, is crucial for the oceanic heat and freshwater transports and thereby influences the regional weather and climate variations. Past studies show that the Florida Current exhibits changes with time scales that range from days to years. Our study attempts to elucidate the energetic subseasonal variability of the Florida Current with periods ranging from 20 to 100 days, whose characteristics and genesis have remained least understood. Based on the analyses of the most extensive in‐situ and satellite measurements in the Florida Straits, we demonstrate that coastally trapped waves originating from the northern South Atlantic Bight are the predominant driver of the subseasonal variability in the Florida Current. The alongshore wind‐forced subseasonal waves modulate the Florida Current transport substantially. The far‐reaching impacts of the waves on sea level along the United States East and Gulf Coast and concurrent subseasonal changes of the overturning circulation are discussed. Key Points: Subseasonal variability accounts for a substantial fraction of the Florida Current transport varianceCoastal‐trapped waves are the primary source of the subseasonal variability in the Florida CurrentAlongshore wind‐driven Ekman dynamics control the genesis of the subseasonal coastal‐trapped waves [ABSTRACT FROM AUTHOR]

The strong sea‐surface temperature (SST) gradient associated with the Gulf Stream (GS) is widely acknowledged to play an important role in shaping mid‐latitude weather and climate. Despite this, an index for the GS SST gradient has not yet been standardized in the literature. This paper introduces a monthly index for the large‐scale SST gradient across the separated GS based on the time‐varying GS position detected from sea‐surface height. Analysis suggests that the variations in the monthly average SST gradient throughout the year result primarily from SST variability to the north of the GS, with little contribution from SST to the south. The index exhibits a weak periodicity at ∼2 years. Sea level pressure and turbulent heat flux patterns suggest that variability in the large‐scale SST gradient is related to atmospheric (rather than oceanic) forcing. Ocean‐to‐atmosphere feedback does not persist throughout the year, but there is some evidence of wintertime feedback. Plain Language Summary: The Gulf Stream is a narrow and fast‐flowing oceanic boundary current that transports warm tropical water north along the coast of Florida and out into the North Atlantic past Cape Hatteras. It is a region where sea‐surface temperatures (SSTs) change dramatically over very short distances with much cooler temperatures on one side of the Gulf Stream (GS) (to the north) and warmer temperatures on the other side (to the south). This strong change in SST across the GS is important for weather systems that propagate in the Euro‐Atlantic sector. Here, an index that measures how much the SSTs change with distance from month‐to‐month across the GS once it separates past Cape Hatteras is developed for the first time. It is found that variability in the index primarily results from SST variations to the north of the separated GS, which themselves are influenced by the atmosphere. There is evidence however that the SST variability represented by the index can also exert an influence on the atmosphere in wintertime. Additionally, the value of the index at any given time seemingly provides some information about the state of the index several years into the future. Key Points: A monthly index for the large‐scale sea‐surface temperature gradient of the separated Gulf Stream is presentedVariations in the index throughout the year are related to atmospheric forcingEvidence of ocean‐atmosphere feedback in the index is only apparent in wintertime [ABSTRACT FROM AUTHOR]

The El Niño‐Southern Oscillation (ENSO) is one of Earth's main modes of climate variability, having huge impacts on weather, agriculture, and people worldwide. Although these impacts and teleconnections have been studied for decades, the role of atmospheric transport is not completely understood. We analyze the atmospheric transport outgoing from the Equatorial Pacific with the Lagrangian particle dispersion model FLEXPART driven by reanalysis data. Our results demonstrate the interocean‐basin exchange via the atmosphere: anomalously energetic air from the ENSO region mainly remains within the Tropics and Subtropics, while more air is transported toward the east during El Niño. Transport of anomalous moist air can directly be linked to several observed teleconnections, for example, droughts in the Amazon Basin and precipitation in Southeastern U.S. during El Niño. These results show that atmospheric transport plays a role in several ENSO teleconnections. Plain Language Summary: The El Niño Southern Oscillation (ENSO) is a climate variability that is associated with changes in the climate state every 2–7 years. It occurs due to changes in the sea surface temperature (SST) in the Equatorial Pacific (EP) and causes shifts in the atmospheric circulation. Influences on weather, agriculture and thus the lives of many people worldwide makes understanding the role of the atmosphere important. In order to uncover the role of the atmosphere, we model the atmospheric transport with specialized computer code. We find that the air from the EP Ocean, where the largest anomalies in SSTs occur, is relatively warm when arriving over the Atlantic Ocean and releases its heat into the Ocean. Over South Central America, northern South America, and West Africa warmer and drier air is arriving, which leads to droughts associated with the ENSO. Over Mexico, North America's Gulf Coast and along the Gulf Stream wetter air is arriving, thus favoring precipitation during El Niño. Our results show that the transport of temperature and moisture anomalies from the EP via the atmosphere plays an important role for the weather conditions in several regions. Key Points: Transported air from the Equatorial Pacific (EP) shows significant anomalies in moisture and heat compared to its environmentThe transport of anomalously dry or wet air can be linked to several observed atmospheric teleconnections associated with El Niño Southern OscillationDeposition of excess energy into the Atlantic Ocean can directly be linked to energetic air from the EP [ABSTRACT FROM AUTHOR]

The Gulf Stream is the only pathway in the subtropical North Atlantic by which warm water flows poleward. This transport of warm water and return of cold water at depth is called the Atlantic Meridional Overturning Circulation (AMOC). The dynamic method is applied to hydrocasts collected since the 1930s to estimate upper‐ocean transport (0–1,000 m) between the U.S. Continental Slope and Bermuda and separately to Africa with focus on the longest directly observable timescale. Calculating transport between the Slope and Bermuda eliminates the Gulf Stream's northern and southern recirculation gyres, while calculations between the Slope and Africa remove all other recirculating geostrophic flow. The net Slope‐Bermuda upper‐ocean transport is estimated to be 41.1 ± 0.4 Sv, decreasing by 2.0 ± 0.8 Sv between 1930 and 2020. The AMOC contribution is 18.4 ± 0.6 Sv, decreasing by 0.4 ± 0.6 Sv between 1930 and 2020. Plain Language Summary: The Gulf Stream is the only pathway by which the ocean transports warm water to high latitudes in the North Atlantic and Nordic Seas where it cools, sinks and flows back south at intermediate depths from the subpolar North Atlantic and at greater depths from the Nordic Seas. This circulation pattern, the Atlantic Meridional Overturning Circulation (AMOC), plays an important role in Earth's climate. But the Gulf Stream is also the wester boundary current to the horizontal circulation driven by the trade winds and westerlies at low and mid‐latitudes. The Gulf Stream also drives large recirculations to its south and north. In this study we use hydrographic data taken since the 1930s to calculate the poleward flow in the Gulf Stream, while canceling out the recirculation and wind‐driven transports to estimate the strength of the north‐flowing portion of the AMOC. There is evidence for a 2.0 Sv Gulf Stream slow‐down between 1930 and 2020. Whether and to what extent this reflects a slowdown of the AMOC or wind‐driven circulation cannot be established with certainty. Our estimate of a 0.4 Sv AMOC decrease is reported with low confidence. Key Points: Direct measurements along the Oleander line (∼37°N) show that the depth of the Atlantic Meridional Overturning Circulation (AMOC) maximum is close to 1,000 mNet northward transport between North America and Bermuda equals 41.1 ± 0.4 Sv of which the AMOC accounts for about 18.4 ± 0.6 SvNet northward transport NW of Bermuda slowed by 2 ± 0.8 Sv between 1930 and 2020 with 0.4 ± 0.6 Sv attributed to an AMOC decline [ABSTRACT FROM AUTHOR]

Climate change is reorganizing the planet's biodiversity, necessitating proactive management of species and habitats based on spatiotemporal predictions of distributions across climate scenarios. In marine settings, climatic changes will predominantly manifest via warming, ocean acidification, deoxygenation, and changes in hydrodynamics. Lophelia pertusa, the main reef‐forming coral present throughout the deep Atlantic Ocean (>200 m), is particularly sensitive to such stressors with stark reductions in suitable habitat predicted to accrue by 2100 in a business‐as‐usual scenario. However, with new occurrence data for this species along with higher‐resolution bathymetry and climate data, it may be possible to locate further climatic refugia. Here, we synthesize new and published biogeographic, geomorphological, and climatic data to build ensemble, multi‐scale habitat suitability models for L. pertusa on the continental margin of the southeast United States (SEUS). We then project these models in two timepoints (2050, 2100) and four climate change scenarios to characterize the occurrence probability of this critical cold‐water coral (CWC) habitat now and in the future. Our models reveal the extent of reef habitat in the SEUS and corroborate it as the largest currently known essentially continuous CWC reef province on earth, and also predict abundance of L. pertusa to identify key areas, including those outside areas currently protected from bottom‐contact fishing. Drastic reductions in L. pertusa climatic suitability index emerged primarily after 2050 and were concentrated at the shallower end (<~550 m) of the regional distribution under the Gulf Stream main axis. Our results thus suggest a depth‐driven climate refuge effect where deeper, cooler reef sites experience lesser declines. The strength of this effect increases with climate scenario severity. Taken together, our study has implications for the regional and global management of this species, portending changes in the biodiversity reliant on CWC habitats and the critical ecosystem services they provide. [ABSTRACT FROM AUTHOR]