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The mixing of water in the ocean is a key influence on our climate and ecosystems, but its importance is often under-recognized. Mixing in the seas around Antarctica—a key process that redistributes heat, carbon, nutrients, plankton, and all other things in the sea, with profound consequences—also affects the stability of the continent's glaciers and ice sheets, with consequences for sea level rise globally. A recent discovery showed that when the fronts of glaciers disintegrate, they "calve" huge chunks of ice that can cause underwater tsunamis in the ocean, which can spread for many miles and cause strong bursts of mixing when they break. This surprising finding is changing the way we think about mixing close to Antarctica, what causes it, and how it matters. [ABSTRACT FROM AUTHOR]

The first operational version of the coupled Hurricane Analysis and Forecast System (HAFSv1) launched in 2023 consists of the HYbrid Coordinate Ocean Model (HYCOM) and finite-volume cubed-sphere (FV3) dynamic atmosphere model. This system is a product of efforts involving improvements and updates over a 4-year period (2019-2022) through extensive collaborations between the Environmental Modeling Center at the US National Centers for Environmental Prediction (NCEP) and NOAA Atlantic Oceanography and Meteorology Laboratory. To provide two sets of numerical guidance, the initial operational capability of HAFSv1 was configured to two systems -HFSA and HFSB. In this study, we present in-depth analysis of the forecast skills of the upper ocean that was co-evolved by the HFSA and HFSB. We chose hurricane Laura (2020) as an example to demonstrate the interactions between the storm and oceanic mesoscale features. Comparisons performed with the available in situ observations from gliders as well as Argos and National Data Buoy Center moorings show that the HYCOM simulations have better agreement for weak winds than high winds (greater than Category 2). The skill metrics indicate that the model sea-surface temperature (SST) and mixed layer depth (MLD) have a relatively low correlation. The SST, MLD, mixed layer temperature (MLT), and ocean heat content (OHC) are negatively biased. For high winds, SST and MLT are more negative, while MLD is closer to the observations with improvements of about 8%-19%. The OHC discrepancy is proportional to predicted wind intensity. Contrarily, the mixed layer salinity (MLS) uncertainties are smaller and positive for higher winds, probably owing to the higher MLD. The less-negative bias of MLD for high winds implies that the wind-force mixing is less effective owing to the higher MLD and high buoyancy stability (approx. 1.5-1.7 times) than the observations. The heat budget analysis suggests that the maximum heat loss by hurricane Laura was 0(< 3°C per day). The main contributor here is advection, followed by entrainment, which act against or with each other depending on the storm quadrant. We also found relatively large unaccountable heat residuals for the in-storm period, and the residuals notably led the heat tendency, meaning that further improvements of the subscale simulations are warranted. In summary, HYCOM simulations showed no systematic differences forced by either HFSA or HFSB. [ABSTRACT FROM AUTHOR]

The atmosphere‐ocean is a highly coupled system with significant diurnal and hourly variations. However, current coupled models usually lack sub‐diurnal scale processes at the air‐sea interface due to the finite vertical resolution for ocean discretization. Previous modeling studies showed that sub‐diurnal scale air‐sea interaction processes are important for ocean mixing. Here, by designing an integrated sub‐diurnal parameterization (ISDP) scheme which combines different temperature profiling functions, we stress sub‐diurnal air‐sea interactions to better represent the local ocean mixing. This scheme has been implemented into two coupled models which contributed to the Climate Model Intercomparison Project (CMIP), referenced by the Intergovernmental Panel on Climate Change—Community Earth System Model and Coupled Model version 2. The results show that the ISDP scheme improves model simulations with better climatology and more realistic spectra, especially in the tropics and North Pacific Ocean. With the scheme, the tropical cold tongue bias is significantly relaxed by reducing the overestimation of ocean upper mixing, and the cold bias of North Pacific Ocean is reduced due to the improvement on currents and net heat fluxes. Our scheme may help better the simulation and prediction skills of coupled models when their horizontal resolution becomes fine but vertical resolution remains relatively coarse as it describes high‐frequency air‐sea interactions more realistically. Plain Language Summary: The atmosphere and ocean interact with each other. In these interactions, changes may occur over different time periods. Some changes take years or months, while others happen in days or hours. The atmosphere changes quickly throughout the day, such as air temperature, wind, and rain. These quick changes in the atmosphere can affect the ocean rapidly. Similarly, there are quick changes in the ocean. These quick changes also affect the atmosphere in return. Understanding these quick changes is important. However, it is difficult to perfectly capture the quick changes in the climate models because they do not always represent the ocean behavior accurately. In this study, we developed a new scheme (named the sub‐diurnal scale parameterization, ISDP) to better represent these quick changes. We added this new scheme into two widely‐used climate models. These models are important tools for studying climate change. Our new scheme represents better the ocean mixing and interaction based on the local weather conditions. When we use ISDP, the climate models get better at simulating climate. They're more accurate in showing ocean temperatures in the tropics. These models make the tropical oceans seem colder than they really are, but our scheme, to some extent, fixes this problem. The results also show the more accurate ocean temperatures in the North Pacific Ocean. Our new scheme has a great potential to make climate models more accurate. Key Points: An integrated sub‐diurnal scale parameterization (ISDP) scheme has a more appropriate representation for local ocean mixingThe ISDP scheme has been implemented into two models which contributed to the Climate Model Intercomparison Project, referenced by IPCC: Community Earth System Model and Coupled Model version 2The model tropical cold tongue bias with ISDP is relaxed by reducing the overestimation of ocean upper mixing [ABSTRACT FROM AUTHOR]

Recent studies have challenged the validity of the Redfield ratio. It is proposed that physical and biogeochemical processes govern the geographical variations in carbon : nitrogen : phosphorus (C : N : P) ratios. However, this proposal remains to be examined through concurrent observations of C : N : P ratios with physical and biogeochemical processes in various marine reservoirs. Here, we sampled the Arabian Sea for its C, N, and P content in organic and inorganic pools during the winter monsoon. We analyzed the role of convective mixing, eddies, and N2 fixation to explain the variation in observed elemental ratios. Convective mixing injected the cold water and enhanced the supply of N and P nutrients in the top layer (surface to 50–75 m) of the northern Arabian Sea. This led to a decrease in the N : P and C : P ratios in the particulate organic matter in the northern region, but C : N : P increased equatorward, averaging 245 : 32 : 1 in the top layer of the Arabian Sea. The variation in the elemental ratios in the top layer is best explained by the changes in water temperature. N2 fixation contributed negligibly to the N : P ratio of the export flux. The substantial decrease in N : P ratios of nutrients in the subsurface waters is most likely caused by the denitrifying conditions in the Arabian Sea. As the processes of convective mixing and eddies are are prevalent oceanic processes, our observations underpin that the interplay of these processes leads to changes in the elemental ratios globally. [ABSTRACT FROM AUTHOR]

Tropical Instability Waves (TIWs) have been shown to modulate upper ocean mixing. However, previous studies on the modulation of TIW related mixing are based on small numbers of TIWs and have not considered temporal variability, which can lead to discrepancies in the findings. In this study, using a 12‐year simulation carried out with a comprehensive, global, high‐resolution ocean model, we present evidence of seasonally modulated shear instabilities at TIW fronts in the Atlantic Ocean that reach down to the thermocline depth, potentially inducing mixing below the mixed layer depth. We find that, regardless of whether TIWs are present earlier in the year, frontal instabilities and potential mixing primarily occur in boreal summer, coinciding with a vertical shear maximum between the mean zonal currents. We argue that in the Atlantic Ocean, vertical shear at TIW fronts does usually not suffice to cause frontal instabilities below the mixed layer depth. Instead, the background shear needs to be sufficiently large in addition to TIW shear, to overcome the stability. The background shear in turn varies seasonally and is strongly driven by the variability of the northern branch of the South Equatorial Current (nSEC). As such, the variability of the nSEC strongly contributes to the generation and modulation of instabilities at TIW fronts that reach below the mixed layer depth and have the potential to induce mixing. Our results highlight the importance of seasonal variability when studying TIW impacts and their effect on mixing. Plain Language Summary: Tropical Instability Waves (TIWs) create alternating warm and cold patterns at the ocean's surface, leading to sharp temperature fronts. Past studies have shown that mixing can occur along these fronts, but how this mixing changes over time hasn't been explored. Our research focuses on how instabilities below the surface at TIW fronts vary over time. Studying such instabilities helps to better understand the processes that can impact the regional climate. We analyzed over 10 years of data from a detailed ocean model and found that instabilities at TIW fronts in the Atlantic Ocean can extend down to 100 m and follow a clear seasonal cycle. These instabilities are most common in summer when the difference in zonal current speeds is greatest. We discovered that shear at TIW fronts alone isn't enough to cause these deep instabilities; the overall background shear also needs to be strong. This background shear changes with the seasons, mainly influenced by the northern branch of the South Equatorial Current (nSEC). Thus, the nSEC plays a crucial role in modulating instabilities at TIW fronts and potential ocean mixing. Key Points: Tropical instability waves exhibit shear instabilities at the front that reach the thermocline depth and follow a distinct seasonal cycleThe seasonality of frontal shear instabilities results from a superposition of shear from the instability wave and background current shearVariability in the northern South Equatorial Current dominates the modulation of shear instabilities at Tropical Instability Wave fronts [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]

As one kind of submesoscale instabilities, symmetric instability (SI) with spatiotemporal scales of O (100) m–O (1) km and O (1) hour exerts significant effects on vertical material transports and forward energy cascade in the ocean. The potential vorticity (PV) is an important conservative parameter controlling quasi‐geostrophic flows, whose budget can be modulated by SI. However, due to the small spatial scale of SI which is hardly resolved by most current observations and regional models, how SI affects the PV budget and how big the effect is remain unclear. In this work, the effect of SI on the PV budget in the surface mixed layer (SML) of the Kuroshio Extension region is quantitatively analyzed based on high‐resolution simulations by applying an existing SI parameterization scheme. Compared with the case without SI effects, negative PV is found to be eliminated in the SML in the SI‐parameterized case. The negative‐PV likelihood in the SI‐parameterized case is decreased by up to 12% due to SI. Analysis of the PV budget indicates that SI contributes to the PV budget mainly by modulating the friction term. The friction term tends to generate negative PV but its magnitude is decreased by 35% due to SI. Apart from the frictional term, both advection and non‐adiabatic terms are also found to be modulated by SI. This work sheds light on the contribution of SI in the PV budget in the ocean mixed layer and suggests a significant role of SI in quasi‐geostrophic PV dynamics. Plain Language Summary: Potential vorticity (PV) is a conservative but dynamically active tracer and an important indicator to characterize instability in the ocean. Traditionally, PV is applied to quasi‐geophysical dynamics, such as large‐scale circulation (O (1,000) km) and mesoscale currents (O (100) km). As the link between meso‐ and micro‐scales, submesoscale symmetric instability with spatial scales of O (100) m–O (1) km contribute to the PV budget, potentially modulating the quasi‐geostrophic dynamics. This work quantitatively evaluates the effect of SI on the PV budget by applying a parameterization scheme. Key Points: The effect of symmetric instability (SI) on potential vorticity is evaluated by applying a parameterization schemeElimination of negative potential vorticities is mainly caused by the friction effect with magnitude decreased by 35%The advection and non‐adiabatic effect are also found to be modulated by SI [ABSTRACT FROM AUTHOR]

Introduction: The impact of upper-ocean temperature on tropical cyclone (TC) activity is an open issue. Compared to the attention devoted to the effect of seasurface temperature (SST) on TC activities, much less is known about the effect of ocean mixed layer depth (OMLD) on TC activities, which is determined by the ocean temperature below the surface. Methods: In this study, a series of idealized numerical experiments were conducted to investigate the possible responses of TC activities to OMLD. Results: It was found that while OMLD exerts a minor influence on TC track, it evidently affects TC intensity, size, and destructiveness before reaching a certain OMLD threshold (approximately 15 m). Once the OMLD exceeds the threshold, changes in TC intensity, size, and destructiveness become marginal with further increase in OMLD. The threshold of OMLD is largely determined by TC intensity, which in turn is dictated by surface wind speed. Discussion: Specifically, before reaching the threshold of OMLD, the surface wind, namely TC-related surface wind, may bring the cold water from below the OMLD, and effectively decreases the upper ocean temperature (including the SST). As OMLD increases, the effect of surface wind on SST cooling gradually decreases, leading to an increase of SST below the TC. Subsequently, the SST increase leads to more surface enthalpy flux (SEF) input into the TC by increasing air-sea temperature and moisture differences. By altering TC's thermodynamic and dynamic structures, the increase of SEF eventually results in the increase of TC intensity and size, and thus its destructiveness. [ABSTRACT FROM AUTHOR]

Turbulent mixing in the ocean exerts an important control on the rate and structure of the overturning circulation. However, the balance of processes underpinning this mixing is subject to significant uncertainties, limiting our understanding of the overturning's deep upwelling limb. Here, we investigate the hitherto primarily neglected role of tens of thousands of seamounts in sustaining deep-ocean upwelling. Dynamical theory indicates that seamounts may stir and mix deep waters by generating lee waves and topographic wake vortices. At low latitudes, stirring and mixing are predicted to be enhanced by a layered vortex regime in the wakes. Using three realistic regional simulations spanning equatorial to middle latitudes, we show that layered wake vortices and elevated mixing are widespread around seamounts. We identify scalings that relate mixing rate within seamount wakes to topographic and hydrographic parameters. We then apply such scalings to a global seamount dataset and an ocean climatology to show that seamount-generated mixing makes an important contribution to the upwelling of deep waters. Our work thus brings seamounts to the fore of the deep-ocean mixing problem and urges observational, theoretical, and modeling efforts toward incorporating the seamounts' mixing effects in conceptual and numerical ocean circulation models. [ABSTRACT FROM AUTHOR]

Changes in sea ice conditions directly impact the way the wind transfers energy to the Arctic Ocean. The thinning and increasing mobility of sea ice is expected to change the size and speed of ridges on the underside of ice floes, called ice keels, which cause turbulence and impact upper-ocean stratification. However, the effects of changing ice keel characteristics on below-ice mixing are difficult to determine from sparse observations and have not been directly investigated in numerical or laboratory experiments. Here, for the first time, we examine how the size and speed of an ice keel affect the mixing of various upper-ocean stratifications using 16 two-dimensional numerical simulations of a keel moving through a two-layer flow. We find that the irreversible ocean mixing and the characteristic depth over which mixing occurs each vary significantly across a realistic parameter space of keel sizes, keel speeds, and ocean stratifications. Furthermore, we find that mixing does not increase monotonically with ice keel depth and speed but instead depends on the emergence and propagation of vortices and turbulence. These results suggest that changes to ice keel speed and depth may have a significant impact on below-ice mixing across the Arctic Ocean and highlight the need for more realistic numerical simulations and observational estimates of ice keel characteristics. [ABSTRACT FROM AUTHOR]

This study represents the first use of the artificial radionuclides 129I and 236U, released into the ocean mainly from Nuclear Reprocessing Plants, as a dual tracer in the vicinity of Iceland with novel estimation of ocean circulatory pathways and mixing in the region. Iceland lies at the gateway to the Arctic where warm, saline Atlantic waters interact with waters of Arctic origin in ways that have critical consequences for the strength and stability of the Atlantic Meridional Overturning Circulation. Many of these interactions are not yet fully understood, such as how Atlantic water circulates around the Arctic Ocean and Nordic Seas and the composition and fate of the major overflows of the Greenland‐Scotland Ridge. Using new and previous measurements of 129I and 236U in seawater, we present a new method of appraising water mass provenance and mixing in the form of the 129I–236U dual mixing plot. With this method, we estimate that at least half the Atlantic‐origin water entering the Arctic Ocean circulates around the Canada Basin before exiting at Fram Strait and that this outflow is increased by about 40% by mixing with Return Atlantic Water "short‐circuiting" the Arctic Ocean at Fram Strait. We present tracer‐based evidence that water carried by the East Greenland Current has an unbroken pathway to the Faroe‐Shetland Channel and that Iceland‐Scotland Overflow Water (ISOW) entrains 60% Labrador Sea Water during transit past southeast Iceland. We present an unambiguous way to differentiate ISOW from DSOW after they partially merge in the Irminger Sea. Plain Language Summary: This study is the first to use a pair of man‐made nuclear products to study ocean currents near Iceland. Understanding these currents is important because Iceland sits where warm Atlantic waters meet colder Arctic waters, affecting a key ocean circulation system and the global climate. However, many details about how these waters move and mix are still unclear. Using new and previous measurements of these tracers in seawater, we demonstrate a new method to estimate the origins of currents and how they mix. We estimate that the outflow of Atlantic‐origin water from the Arctic Ocean at Fram Strait is increased by about 40% by Atlantic water bypassing the Arctic Ocean altogether at this location. Some of this Atlantic‐origin water then flows all the way from Iceland to Shetland in an unbroken pathway and the water that spills over the Iceland‐Scotland ridge increases by 60% by mixing with water from the Labrador Sea southeast of Iceland. Finally, we present a new way to distinguish this Iceland‐Scotland Overflow Water from similar water that overflows the ridge at Denmark Strait. Key Points: Recirculating Atlantic Water contributes about 40% to Atlantic layer of the East Greenland CurrentAtlantic‐origin waters can be traced from the East Greenland Current to the Faroe‐Shetland ChannelIceland‐Scotland Overflow Water and Denmark Strait Overflow Water are traceable separately after joining Deep Western Boundary Current [ABSTRACT FROM AUTHOR]

North Atlantic subtropical mode water (NASTMW) serves as a major conduit for dissolved carbon to penetrate into the ocean interior by its wintertime outcropping events. Prior research on NASTMW has concentrated on its physical formation and destruction, as well as Lagrangian pathways and timescales of water into and out of NASTMW. In this study, we examine how dissolved inorganic carbon (DIC) concentrations are modified along Lagrangian pathways of NASTMW on subannual timescales. We introduce Lagrangian parcels into a physical‐biogeochemical model and release these parcels annually over two decades. For different pathways into, out of, and within NASTMW, we calculate changes in DIC concentrations along the path (ΔDIC), distinguishing contributions from vertical mixing and biogeochemical processes. The strongest ΔDIC is during subduction of water parcels (+101 μmol L−1 in 1 year), followed by transport out of NASTMW due to increases in density in water parcels (+10 μmol L−1). While the mean ΔDIC for parcels that persist within NASTMW in 1 year is relatively small at +6 μmol L−1, this masks underlying dynamics: individual parcels undergo interspersed DIC depletion and enrichment, spanning several timescales and magnitudes. Most DIC enrichment and depletion regimes span timescales of weeks, related to phytoplankton blooms. However, mixing and biogeochemical processes often oppose one another at short timescales, so the largest net DIC changes occur at timescales of more than 30 days. Our new Lagrangian approach complements bulk Eulerian approaches, which average out this underlying complexity, and is relevant to other biogeochemical studies, for example, on marine carbon dioxide removal. Plain Language Summary: Mode waters are relatively thick water masses with homogeneous properties, such as temperature and salinity. The North Atlantic subtropical mode water (NASTMW), found in the Sargasso Sea, is one such water mass. Lying underneath the ocean surface, it comes into contact with the atmosphere during winter, when the surface layer is vigorously mixed due to strong winds, causing the mixed layer to connect with NASTMW. This way, NASTMW can buffer atmospheric temperature and carbon anomalies during the summer, when there is no surface connection. It is also a conduit for carbon to penetrate beneath the ocean's upper mixed layer, with the potential to sequester it. We study NASTMW from the viewpoint of a water parcel that moves with the currents and see how carbon concentrations in the water parcels change along different NASTMW pathways. For each pathway, the carbon concentration changes due to an interplay of vertical mixing and biogeochemical processes, for example, related to plankton growth and decay. These processes can unfold over different timescales and may counteract or enhance themselves or one another. The largest change in carbon concentration is found when a parcel moves from the upper ocean mixed layer into NASTMW, mostly due to vertical mixing. Key Points: Carbon transformations along pathways of North Atlantic Subtropical Mode Water are split into mixing and biogeochemical contributionsAlong paths into, within, and out of this mode water, mixing and biogeochemistry alter carbon in water parcels over a range of timescalesEnrichment is highest during mixed layer subduction, which few parcels undergo annually; persistence in mode water is the dominant pathway [ABSTRACT FROM AUTHOR]

The Arctic Ocean and adjacent seas are undergoing increased freshwater influx due to enhanced glacial and sea ice melt, precipitation, and runoff. Accurate delineation of these freshwater sources is vital as they critically modulate ocean composition and circulation with widespread and varied impacts. Despite this, the delineation of freshwater sources using physical oceanographic measurements (e.g., temperature, salinity) alone is challenging and there is a requirement to improve the partitioning of ocean water masses and their mixing relationships. Here, we complement traditional oceanographic measurements with continuous surface seawater isotopic analysis (δ18O and deuterium excess) across a transect extending from coastal Alaska to Baffin Bay and the Labrador Sea conducted from the US Coast Guard Cutter Healy in Autumn 2021. We find that the diverse isotopic signatures of Arctic freshwater sources, coupled with the high freshwater proportion in these marine systems, facilitates detailed fingerprinting and partitioning. We observe the highest freshwater composition in the Beaufort Sea and Amundsen Gulf regions, with heightened freshwater content in eastern Baffin Bay adjacent to West Greenland. We apply isotopic analysis to delineate freshwater sources, revealing that in the Western Arctic freshwater inputs are dominated by meteoric water inputs—specifically the Mackenzie River—with a smaller sea ice meltwater component and in Baffin Bay the primary sources are local precipitation and glacial meltwater discharge. We demonstrate that such freshwater partitioning cannot be achieved using temperature‐salinity relationships alone, and highlight the potential of seawater isotopic tracers to assess the roles and importance of these evolving freshwater sources. Plain Language Summary: Freshwater inputs to the Arctic seas, including glacial and sea ice meltwater, precipitation, and river runoff, are increasing as the Arctic warms. The impacts of these changing freshwater influxes are varied depending on the type of freshwater source, and thus it is important to delineate and trace these different freshwater sources, which represents a significant challenge using only traditional physical oceanographic measurements (e.g., temperature, salinity). In this study, we utilize a new approach to identify and trace freshwater sources using continuous seawater isotopic measurements during a cruise extending from coastal Alaska, through the Canadian Archipelago, and across Baffin Bay and the Labrador Sea. We show that these isotopic measurements, which have been commonly used in other media (e.g., precipitation, water vapor, ice cores), hold important and distinct information about the source and mixing of different freshwater sources. We use these measurements to identify the freshwater sources (e.g., Mackenzie vs. Yukon River) contributing to ocean surface waters across the Arctic region. Key Points: Seawater isotopic measurements (δ18O, δ2H, deuterium excess) show heightened freshwater content in the Beaufort Sea and Baffin BayIsotopic observations enable freshwater source delineation not feasible from traditional physical oceanographic methodsFreshwater source delineation includes the Mackenzie and Yukon Rivers around coastal Alaska and glacial meltwater in Baffin Bay [ABSTRACT FROM AUTHOR]

Small‐scale turbulent mixing supplies potential energy for the upwelling of deep waters in the abyssal ocean, a key component of the global overturning circulation. This process is particularly significant in critical regions such as the Northwestern Pacific where the upwelling structure of deep waters remains poorly understood due to limited knowledge of deep ocean mixing. Here, we investigate the full‐depth spatiotemporal variability of turbulent mixing in the deep Northwestern Pacific based on hydrographic data collected over repeated surveys. Nineteen‐year‐average diapycnal diffusivity of 1.42 × 10−4 m2 s−1 is reveled in the deep Philippine Sea, indicating significantly stronger mixing compared to the stratified ocean interior. Spatially, turbulent mixing strengthens toward the bottom and intensifies westward from the open Pacific to the Philippine Sea due to rough topography. At certain mixing hotspots, enhanced mixing can penetrate up to 2,500 m above the bottom, suggesting a substantial potential for upwelling. Below 2,000 m, turbulent mixing exhibits pronounced seasonal variation that deep mixing is more intense in summer (winter) than in winter (summer) in the West Caroline Basin (the Parece Vela Basin). This spatially varying seasonality may be attributed to the inhomogeneous internal tidal energy dissipation in the Northwestern Pacific. Our study will serve to clarify the modulation of turbulent mixing to deep‐water mass transformation and circulation in the Northwestern Pacific. Plain Language Summary: The global overturning circulation plays an important role in transporting and redistributing climate‐sensitive matters such as oxygen and carbon. This circulation is enclosed by the upwelling of deep waters facilitated by turbulence in the deep ocean, which mixes dense waters with the lighter waters above. In the Northwestern Pacific, a significant portion of deep waters from the Southern Ocean continuously intrudes into the Philippine Sea, rendering it a critical region for global circulation. However, the upwelling process of deep water remains largely unknown due to the limited understanding of turbulent mixing in the deep Northwestern Pacific. To fill this gap, we characterize the spatiotemporal distribution of turbulent mixing in the Northwestern Pacific based on long‐term observations during 2004–2022. We reveal that turbulent mixing here is highly enhanced by rough topography, 10 to 100 times stronger than that in the open ocean, particularly near some key terrains such like Kyushu‐Palau Ridge. Besides, a region‐dependent seasonal variation of deep mixing is also observed. Our study contributes to a further understanding of the abyssal mixing and circulation in the Northwestern Pacific. Key Points: A full‐depth distribution of turbulent mixing in the Northwestern Pacific is obtained based on 19‐year repeated hydrographic observationsRough topography in the Philippine Sea enhances the deep mixing, with diffusivities even up to O (10−2) m2s−1 near the bottomLatitude dependent seasonality of deep ocean turbulent mixing is revealed, which could be related to spatial variations of internal tides [ABSTRACT FROM AUTHOR]

Anthropogenic perturbations from fossil fuel burning, nuclear bomb testing, and chlorofluorocarbon (CFC) use have created useful transient tracers of ocean circulation. The atmospheric 14C/C ratio (∆14C) peaked in the early 1960s and has decreased now to pre‐industrial levels, while atmospheric CFC‐11 and CFC‐12 concentrations peaked in the early 1990s and early 2000s, respectively, and have now decreased by 10%–20%. We present the first analysis of a decade of new observations (2007 to 2018–2019) and give a comprehensive overview of the changes in ocean ∆14C and CFC concentration since the WOCE surveys in the 1990s. Surface ocean ∆14C decreased at a nearly constant rate from the 1990–2010s (20‰/decade). In most of the surface ocean ∆14C is higher than in atmospheric CO2 while in the interior ocean, only a few places are found to have increases in ∆14C, indicating that globally, oceanic bomb 14C uptake has stopped and reversed. Decreases in surface ocean CFC‐11 started between the 1990 and 2000s, and CFC‐12 between the 2000–2010s. Strong coherence in model biases of decadal changes in all tracers in the Southern Ocean suggest ventilation of Antarctic Intermediate Water was enhanced from the 1990 to the 2000s, whereas ventilation of Subantarctic Mode Water was enhanced from the 2000 to the 2010s. The decrease in surface tracers globally between the 2000 and 2010s is consistently stronger in observations than in models, indicating a reduction in vertical transport and mixing due to stratification. Plain Language Summary: The ocean contains many dissolved gases that can be measured by sampling ocean water from ships. Some of these dissolved gases are changing over time because their atmospheric concentrations are changing. By measuring these changes in the ocean, we can learn about ocean transport and mixing, which is an important factor regulating climate change. We describe ocean measurements from the past three decades of radiocarbon in dissolved inorganic carbon, that was affected by nuclear bomb testing, and of chlorofluorocarbon gases, CFCs, that were banned in the 1990s due to their destructive effects on the ozone layer. The measurements show how their oceanic distributions have changed, as a result of their atmospheric histories and the patterns of ocean uptake and transport. By comparing the measurements with ocean models, we can identify biases in the models that affect how well future climate change can be predicted. In the most recent decade, the presence of all three tracers is shown to be decreasing in the upper ocean, a reversal of the preceding decades. Also in the most recent decade, the decrease in surface tracers is stronger in the observations than the models, suggesting a change to upper ocean mixing and stratification. Key Points: Recent trends in upper ocean Δ14C, pCFC‐11, and pCFC‐12 are negative, reflecting their decreasing atmospheric trendsIncreases in ∆14C are only observed in a few places over 2000–2010s, showing oceanic bomb 14C uptake has stopped and reversedModel‐data Δ14C and chlorofluorocarbon differences could be consistent with decadal ventilation changes and large‐scale stratification or model errors [ABSTRACT FROM AUTHOR]

Analysis of dissolved oxygen (O2) in the Arctic's surface ocean provides insights into gas transfer between the atmosphere‐ice‐ocean system, water mass dynamics, and biogeochemical processes. In the Arctic Ocean's Canada Basin mixed layer, higher O2 concentrations are generally observed under sea ice compared to open water regions. Annual cycles of O2 and O2 saturation, increasing from summer through spring and then sharply declining to late summer, are tightly linked to sea ice cover. The primary fluxes that influence seasonal variability of O2 are modeled and compared to Ice‐Tethered Profiler O2 observations to understand the relative role of each flux in the annual cycle. Findings suggest that sea ice melt/growth dominates seasonal variations in mixed layer O2, with minor contributions from vertical entrainment and atmospheric exchange. While the influence of biological activity on O2 variability cannot be directly assessed, indirect evidence suggests relatively minor contributions, although with significant uncertainty. Past studies show that O2 molecules are expelled from sea ice during brine rejection; sea ice cover can then inhibit air‐sea gas exchange resulting in winter mixed layers that are super‐saturated. Decreasing mixed layer O2 concentrations and saturation levels are observed during winter months between 2007 and 2019 in the Canada Basin. Only a minor portion of the decreasing trend in wintertime O2 can be attributed to decreased solubility. This suggests the O2 decline may be linked to more efficient air‐sea exchange associated with increased open water areas in the winter sea ice pack that are not necessarily detectable via satellite observations. Plain Language Summary: Dissolved oxygen (O2) is a valuable ocean property that allows us to better understand the exchange of gases between the different ocean layers, sea ice, and atmosphere, and the physical and biological processes that control its variability. Understanding how and why O2 concentrations in the Arctic Ocean mixed layer vary spatially and seasonally is crucial for interpreting its evolution over timescales of years to decades that are influenced by global warming. We use physical and thermodynamical relationships to model the main factors that influence O2 concentrations in the mixed layer of the Arctic Ocean's Canada Basin, which we compare to observations made by Ice‐Tethered Profilers. Model results indicate that seasonal variations in O2 concentrations are dominated by the effects of sea ice growth and melt. Other processes that modulate mixed layer O2, including air‐sea exchange and ocean mixing, have a lesser influence. Between 2007 and 2019, mixed‐layer O2 has decreased in winter months, which we attribute to more openings in the sea‐ice pack during wintertime in the Canada Basin. Key Points: Spatial and seasonal distributions of O2 concentrations in the Canada Basin mixed layer are linked to the seasonal evolution of sea iceModeled fluxes suggest brine rejection and meltwater dilution during sea ice melt/formation dominate seasonal variability of mixed layer O2Decreases in mixed‐layer O2 during winter (over 2007–2019) suggest outgassing, likely driven by changes in the wintertime sea ice pack [ABSTRACT FROM AUTHOR]

Diapycnal mixing in the ocean interior is largely fueled by internal tides. Mixing schemes that represent the breaking of internal tides are now routinely included in ocean and earth system models applied to the modern and future. However, this is more rarely the case in climate simulations of deep‐time intervals of the Earth, for which estimates of the energy dissipated by the tides are not always available. Here, we present and analyze two IPSL‐CM5A2 earth system model simulations of the Early Eocene made under the framework of DeepMIP. One simulation includes mixing by locally dissipating internal tides, while the other does not. We show how the inclusion of tidal mixing alters the shape of the deep ocean circulation, and thereby of large‐scale biogeochemical patterns, in particular oxygen distributions. In our simulations, the absence of tidal mixing leads to a relatively stagnant and poorly ventilated deep ocean in the North Atlantic, which promotes the development of a basin‐scale pool of oxygen‐deficient waters, at the limit of complete anoxia. The absence of large‐scale anoxic records in the deep ocean after the Cretaceous anoxic events suggests that such an ocean state most likely did not occur at any time across the Paleogene. This highlights how crucial it is for climate models applied to the deep‐time to integrate the spatial variability of tidally driven mixing as well as the potential of using biogeochemical models to exclude aberrant dynamical model states. Key Points: Inclusion of realistic near‐field tidal mixing substantially modifies global deep ocean circulation in the Early EoceneThese tidally driven changes yield significantly different biogeochemical properties of water masses, in particular in the AtlanticThe simulation that includes tidal mixing compares more favorably to inferences from the O2 proxy record [ABSTRACT FROM AUTHOR]

Based on Argo observations and a coupled atmosphere–ocean–wave model, the upper ocean responses to the tropical cyclone (TC) Mekunu (2018) were investigated, and the role of a pre-existing cold eddy in modulating the temperature response to TC Mekunu was quantified by employing numerical experiments. With TC Mekunu's passage, the mixed layer depth (MLD) on both sides of its track significantly deepened. Moreover, two cold patches (<26 °C) occurred, where the maximum cooling of the mixed layer temperature (MLT) reached 6.62 °C and 6.44 °C. Both the MLD and MLT changes exhibited a notable rightward bias. However, the changes in the mixed layer salinity (MLS) were more complex. At the early stage, the MLS on both sides of the track increased by approximately 0.5 psu. When TC Mekunu made landfall, the MLS change around the track was asymmetric. Significantly, a cold eddy pre-existed where the second cold patch emerged, and this eddy was intensified after TC Mekunu's passage, with an average sea surface height reduction of approximately 2.7 cm. By employing the stand-alone ocean model, the numerical experimental results demonstrated that the pre-existing cold eddy enhanced TC-induced MLT cooling by an average of approximately 0.41 °C due to steeper temperature stratification at the base of mixed layer. Moreover, heat budget analysis indicated that the pre-existing cold eddy also enhanced subsurface temperature cooling mainly through zonal advection. [ABSTRACT FROM AUTHOR]

Using 9 years of full‐depth profiles from 55 Deep Argo floats in the Southwest Pacific Basin collected between 2014 and 2023, we find consistent warm anomalies compared to a long‐term climatology below 2,000 m ranging between 11 ± 2 to 34 ± 2 m°C, most pronounced between 3,500 and 5,000 m. Over this period, a cooling trend is found between 2,000 and 4,000 m and a significant warming trend below 4,000 m with a maximum rate of 4.1 ± 0.31 m°C yr−1 near 5,000 m, with a possible acceleration over the second half of the period. The integrated Steric Sea Level expansion below 2,000 m was 7.9 ± 1 mm compared to the climatology with a trend of 1.3 ± 1.6 mm dec−1 over the Deep Argo era, contributing significantly to the local sea level budget. We assess the ability to close a full Sea Level Budget, further demonstrating the value of a full‐depth Argo array. Plain Language Summary: Cold, dense waters formed near polar regions in both hemispheres, sink to great depths and fill‐up the majority of the world's deep ocean. Compilation of sparse observations of temperature from global ship‐based surveys at roughly 10‐year intervals worldwide have shown that sequestration of excess atmospheric heat into the deep ocean has caused these waters to warm steadily since the 1990's into the Present. Not only does this warming have implications for changes in large scale ocean circulation, but is also associated with warming‐induced sea level rise. Using a new data set collected between 2014 and 2023 from 55 freely drifting robotic floats (Deep Argo) which gather crucial bimonthly temperature and salinity data between the surface ocean and the ocean floor, we find the greatest warming trend at a depth of 5,000 m of 4 ± 0.3 m°C yr−1 and an associated sea level rise rate below 2,000 m of 1.3 ± 1.6 mm dec−1. Deep Argo data being collected in ocean basins worldwide are crucial in providing high resolution data of the warming deep ocean and its implications on global sea level, ocean mixing and large‐scale ocean circulation. Key Points: Nine years of Deep Argo data in the S.W. Pacific reveals continued warming in the abyss while the mid‐depths cooledWaters below 4,000 m show an accelerated warming trend with a maximum overall warming rate of 4.1 ± 0.31 m°C yr−1 at 5,000 mDeep ocean steric expansion contributed 1.3 ± 1.6 mm dec−1 to total the local sea level [ABSTRACT FROM AUTHOR]

Amplification of sea surface temperature (SST) seasonality in response to global warming is a robust feature in climate model projections but season‐dependent regional disparities in this amplification and the associated mechanisms are not well addressed. Here, by analyzing large ensemble simulations using Community Earth System Model version 2, we investigate detailed spatiotemporal characteristics of the amplification of SST seasonality focusing on the North Pacific and North Atlantic, where robust changes are projected to emerge around 2050 under SSP3‐7.0 scenario. Our results indicate that atmosphere‐ocean coupled processes shape regional changes in SST seasonality differently between warm (MAMJJAS) and cold seasons (ONDJF). During the warm season, the projected warming tendency is mainly due to increased net surface heat flux and weakening of vertical mixing. On the other hand, in the cold season, the projected cooling tendency is driven by strengthened vertical mixing over the North Pacific associated with the northward shift of storm tracks but weakened horizontal advection and mixing due to changes in ocean currents over the North Atlantic. Plain Language Summary: In addition to the increase in annual mean sea surface temperature (SST), the amplitude of SST seasonal cycle is projected to intensify in response to greenhouse warming over many parts of the global ocean due to more warming during local summer than winter. By analyzing large ensemble simulations, we show how different atmospheric and ocean processes influence spatiotemporal differences in SST seasonality changes in the North Pacific and North Atlantic. Our findings indicate that during local summer, the projected warming tendency intensifies with the increased heat input into the ocean and weakening of ocean mixing. Meanwhile, during local winter, the projected cooling tendency intensifies with the strengthening of vertical mixing associated with the enhanced storm track activities in the North Pacific, but the weakening of ocean horizontal advection and mixing attributable to the weakened ocean circulation over the North Atlantic. They together contribute to the amplification of SST seasonality, which could have implications for marine ecosystems, including the plankton phenology. Key Points: Using CESM2 large ensemble simulations, this study explores the robustness of future sea surface temperature (SST) seasonality changes and the associated atmospheric and surface ocean processesIn the warm season, the future SST warming is mainly driven by the reduced upper ocean mixing and the increased net surface heat influx into the oceanIn the cold season, the future SST cooling is attributable to strengthened vertical mixing over the North Pacific but weakened horizontal advection and mixing in the North Atlantic [ABSTRACT FROM AUTHOR]

Mesoscale eddies are known to enhance diapycnal mixing in the ocean, yet direct observation of this effect remains a significant challenge, especially in the robust Antarctic Circumpolar Current (ACC). To quantify the diapycnal mixing induced by mesoscale eddies in the standing meander of the ACC, satellite altimeter and Argo profile data were combined to composite eddies, where the 1.6 m dynamic height contour was used for the first time instead of the climatological Northern Sub-Antarctic Front (SAFN) to define the northern boundary of the ACC to eliminate the influence of frontal shift. The 3D structures of the composite anticyclonic/cyclonic eddy (CAE/CCE) were obtained. Both the CAE and CCE were similar in shape to Taylor columns, from sea surface to the neutral surface of 28.085 kgm − 3 (1689 ± 66 dbar) for the CAE, and from sea surface to 28.01 kgm − 3 (1491 ± 202 dbar) for the CCE. On the same neutral surface, the diffusivity (κ) inside the CCE was one to two orders of magnitude higher than that inside the CAE. Vertically, the maximum influence depth of the CCE on κ reached 1200 dbar, while for the CAE, it reached 800 dbar, where κ exceeded O (10 − 4)   m 2 s − 1 , and κ gradually decreased from these depths downwards. [ABSTRACT FROM AUTHOR]

The non‐local model of mixing based on internal wave breaking, IDEMIX, is implemented as an enhancement of a turbulent kinetic energy closure model in three non‐eddy resolving general circulation ocean models that differ in the discretization and choice of computational grids. In IDEMIX internal wave energy is generated by an energy flux resulting from near‐inertial waves induced by wind forcing at the surface, and at the bottom, by an energy flux that parameterizes the transfer of energy between baroclinic and barotropic tides. In all model simulations with IDEMIX, the mixing work is increased compared to the reference solutions without IDEMIX, reaching values in better agreement with finestructure observations. Furthermore, the horizontal structure of the mixing work is more realistic as a consequence of the heterogeneous forcing functions. All models with IDEMIX simulate deeper thermocline depths related to stronger shallow overturning cells in the Indo‐Pacific. In the North Atlantic, deeper mixed layers in simulations with IDEMIX are associated with an increased Atlantic overturning circulation and an increase of northward heat transports toward more realistic values. The response of the deep Indo‐Pacific overturning circulation and the weak bottom cell of the Atlantic to the inclusion of IDEMIX is incoherent between the models, suggesting that additional unidentified processes and numerical mixing may confound the analysis. Applying different tidal forcing functions leads to simulation differences that are small compared to differences between the different models or between simulations with IDEMIX and without IDEMIX. Plain Language Summary: Waves in the ocean interior play a fundamental role for ocean dynamics since they can carry energy over long distances and, once they break, lead to turbulent mixing. This turbulent mixing can cause dense water masses to rise from the deep ocean with a direct impact on large‐scale currents. The wave dynamics occur on spatial scales that cannot be resolved in global ocean or climate models. To account for these processes, we apply the new parameterization IDEMIX that describes internal wave generation, propagation, and mixing. Using three different ocean models with and without IDEMIX ensures that we can identify model‐specific effects of the parameterization and discriminate them from those independent of the model. We find that the simulated mixing patterns agree better with observations once IDEMIX is applied. Large‐scale currents and the vertical temperature distribution are substantially affected by the internal wave parameterization. Whether this leads to an improved agreement with observed currents and water mass properties depends on the specific model and on numerical effects. In most cases, simulations with IDEMIX are not very sensitive to details of how the internal wave model is driven by tidal energy input. Key Points: the IDEMIX closure for a consistent representation of internal wave‐induced mixing is evaluated in three state‐of‐the‐art ocean modelsonly in simulations with IDEMIX can the models reproduce the magnitude and spatial variability of the observed mixing workmost changes with IDEMIX can be attributed to stronger mixing, but some effects are confounded by other processes and numerical mixing [ABSTRACT FROM AUTHOR]

Radium‐226(226Ra) and barium (Ba) exhibit similar chemical behaviors and distributions in the marine environment, serving as valuable tracers of water masses, ocean mixing, and productivity. Despite their similar distributions, these elements originate from distinct sources and undergo disparate biogeochemical cycles, which might complicate the use of these tracers. In this study, we investigate these processes by analyzing a full‐depth ocean section of 226Ra activities (T1/2 = 1,600 years) and barium concentrations obtained from samples collected along the US GEOTRACES GP15 Pacific Meridional Transect during September–November 2018, spanning from Alaska to Tahiti. We find that surface waters possess low levels of 226Ra and Ba due to export of sinking particulates, surpassing inputs from the continental margins. In contrast, deep waters have higher 226Ra activities and Ba concentrations due to inputs from particle regeneration and sedimentary sources, with 226Ra inputs primarily resulting from the decay of 230Th in sediments. Further, dissolved 226Ra and Ba exhibit a strong correlation along the GP15 section. To elucidate the drivers of the correlation, we used a water mass analysis, enabling us to quantify the influence of water mass mixing relative to non‐conservative processes. While a significant fraction of each element's distribution can be explained by conservative mixing, a considerable fraction cannot. The balance is driven using non‐conservative processes, such as sedimentary, rivers, or hydrothermal inputs, uptake and export by particles, and particle remineralization. Our study demonstrates the utility of 226Ra and Ba as valuable biogeochemical tracers for understanding ocean processes, while shedding light on conservative and myriad non‐conservative processes that shape their respective distributions. Key Points: Water mass analysis shows significant shallow deficits and deep excesses of dissolved 226Ra and Ba in the Pacific Ocean along the GP15Shallow deficits seem driven by BaSO4 precipitation, deep excesses reflect benthic inputs that are more significant for 226Ra than for BaBudgets suggest that regional barium fluxes nearly balance, and the Pacific is likely a significant source of radium‐226 to the global ocean [ABSTRACT FROM AUTHOR]

Turbulent mixing in the ocean, lakes and reservoirs facilitates the transport of momentum, heat, nutrients, and other passive tracers. Turbulent fluxes are proportional to the rate of turbulent kinetic energy dissipation per unit mass, ε. A common method for ε measurements is using microstructure profilers with shear probes. Such measurements are now widespread, and a non-expert practitioner will benefit from best practice guidelines and benchmark datasets. As a part of the Scientific Committee on Oceanographic Research (SCOR) working group on "Analysing ocean turbulence observations to quantify mixing" (ATOMIX), we compiled a collection of five benchmark data of ε from measurements of turbulence shear using shear probes. The datasets are processed using the ATOMIX recommendations for best practices documented separately. Here, we describe and validate the datasets. The benchmark collection is from different types of instruments and covers a wide range of environmental conditions. These datasets serve to guide the users to test their ε estimation methods and quality-assurance metrics, and to standardize their data for archiving. [ABSTRACT FROM AUTHOR]

Oceanic mesoscale eddy mixing plays a crucial role in Earth's climate system by redistributing heat, salt, and carbon. For many ocean and climate models, mesoscale eddies still need to be parameterized. This is often done via an eddy diffusivity K , which sets the strength of turbulent downgradient tracer fluxes. A well-known effect is the modulation of K in the presence of background potential vorticity (PV) gradients, which suppresses cross-PV gradient mixing. Topographic slopes can induce such suppression through topographic PV gradients. However, this effect has received little attention, and topographic effects are often not included in parameterizations for K. In this study, we show that it is possible to describe the effect of topography on K analytically in a barotropic framework, using a simple stochastic representation of eddy–eddy interactions. We obtain an analytical expression for the depth-averaged K as a function of the bottom slope, which we validate against diagnosed eddy diffusivities from a numerical model. The obtained analytical expression can be generalized to any constant barotropic PV gradient. Moreover, the expression is consistent with empirical parameterizations for eddy diffusivity over topography from previous studies and provides a physical rationalization for these parameterizations. The new expression helps to understand how eddy diffusivities vary across the ocean, and thus how mesoscale eddies impact ocean mixing processes. Significance Statement: Large oceanic "whirls," called eddies, can mix and transport ocean properties such as heat, salt, carbon, and nutrients. Mixing plays an important role for oceanic ecosystems and the climate system. In numerical simulations of Earth's climate, eddy mixing is typically represented using a simplified expression. However, an effect that is often not included is that eddy mixing is weaker over a sloping seafloor. In most areas of the ocean the bottom slope is steep enough for this effect to be significant. In this study we derive an expression for eddy mixing that accounts for oceanic bottom slopes. The present effort provides a physical basis for eddy mixing over oceanic bottom slopes, justifying their use in climate models. [ABSTRACT FROM AUTHOR]

Strongly coupled data assimilation allows observations of one Earth system component (e.g., the ocean) to directly update another component (e.g., the atmosphere). The majority of the information transfer in strongly coupled atmosphere‐ocean systems is passed through vertical correlations between atmospheric boundary layer and ocean mixed layer fields. In this work we use correlations from a global, coupled model to study vertical observation‐space localization techniques for strongly coupled data assimilation. We generate target correlations using a bootstrapping approach from a single 24 hr forecast from a realistic global, weakly coupled atmosphere‐ocean cycling system with an 80‐member ensemble, which is the ensemble size currently used by the NOAA operational global data assimilation system. We compare data assimilation methods with different localization schemes using single‐update, offline experiments. We develop a new strategy for optimal observation space localization, called Empirical Optimal R‐localization (EORL), to give an upper bound on the improvement we can expect with any localization scheme. We then evaluate Gaspari‐Cohn localization, which is a commonly used parametric localization function and review its performance with respect to the optimal localization scheme. We investigate how the performance of these localization strategies changes with increasing ensemble sizes. Our results show that strongly coupled data assimilation has the potential to be an improvement over weakly coupled data assimilation when large ensembles are used. We also show that the Gaspari‐Cohn localization function does not appear to be a particularly good choice for cross‐fluid vertical localization. Plain Language Summary: Accurate Earth system forecasts rely on accurate estimates of the current state of the system. This initial state is estimated through a process called data assimilation, which combines the previous forecast with current observations. This process relies on accurate estimates of uncertainty in both the model and the observations. Inaccurate estimates of model and observation uncertainty can result in a degradation of future forecasts. A technique called localization is widely used to minimize the impact of distant observations due to unreliable uncertainty estimates over long distances. In this work we study localization in the context of a global, coupled atmosphere‐ocean model. In particular, we look at strongly coupled data assimilation, where observations of the ocean are used to update the atmospheric model state and vice versa. This is in contrast to weakly coupled data assimilation, where observations of the ocean are only used to update the ocean model state. We investigate different types of localization and find that a commonly used localization function does not perform particularly well in strongly coupled model initialization. Key Points: We investigate vertical localization for strongly coupled atmosphere‐ocean data assimilation in a realistic global modelStrong coupling can improve data assimilation effectiveness over weak coupling when large ensembles are usedWe present a method for optimal observation space localization, called EORL, and demonstrate its performance in offline experiments [ABSTRACT FROM AUTHOR]

Tropical sea surface temperature (SST) biases can cause atmospheric biases on global scales, hence SST needs to be represented well in climate models. A major source of uncertainties is the representation of turbulent mixing in the oceanic boundary layer, or mixed layer (ML). In the present study we focus on near‐inertial wave (NIW) induced mixing. The performance of two mixing schemes, Turbulent Kinetic Energy and K‐profile parameterization (KPP), is assessed at two sites (11.5°N, 23°W and 15°N, 38°W) in the tropical Atlantic. At 11.5°N, turbulence observations (eddy diffusivities, shear and stratification) are available for comparison. We find that the schemes differ in their representation of NIWs, but both under‐represent the observed enhanced diffusivities below the observed ML. However, we find that the models do mix below the ML at 15°N when a storm passes nearby. The near‐inertial oscillations remain below the ML for the following 10 days. Near‐inertial kinetic energy (NIKE) biases in the models are not directly correlated with the wind speed, the MLD biases, or the stratification at the ML base. Instead, NIKE biases are sensitive to the vertical mixing scheme parameterization. NIKE biases are lowest when the KPP scheme is used. Plain Language Summary: The surface temperature of the ocean is highly dependent on the depth of the mixed layer (ML), the uppermost layer in the water column, where density, temperature and salinity are approximately constant. In climate models, the vertical mixing processes cannot be resolved, and instead they are computed with the use of vertical mixing schemes. We assess how well two of such schemes can represent the mixing induced by a specific type of ocean waves, near‐inertial waves (NIWs). We compare recent observations of turbulent mixing induced by NIWs in the tropical Atlantic with numerical simulations that resolve storms. Our results show that the models are able to reproduce the observed NIWs, but underestimate their mixing and amplitude. Our analysis also shows that NIWs are a driver of mixing below the uppermost ocean layer in the models. The strength of the near‐inertial currents is sensitive to the vertical mixing parameterization. Key Points: Observations of inertial oscillations are used to evaluate the performance of two vertical mixing schemes in two high‐resolution modelsBoth the K‐profile parameterization and the Turbulent Kinetic Energy closure underestimate the NIW‐induced mixingNear‐inertial kinetic energy biases are sensitive to the vertical mixing parameterization [ABSTRACT FROM AUTHOR]

The Gregg‐Henyey‐Polzin (GHP) finescale parameterization is widely employed to infer turbulent mixing when microstructure measurements are unavailable. However, the strain‐only GHP scaling is the only option when shear information is lacking, in which case the shear‐to‐strain ratio (Rω) is commonly treated as a constant. Since Rω has been reported to vary spatiotemporally, using a fixed value of Rω might result in a significant bias in inferring turbulent mixing. In this study, we present a global map of Rω, based on the microstructure database contributed by the Climate Process Team on internal wave‐driven ocean mixing, which covers the Indian, Pacific, and Atlantic oceans across both high and low latitudes. Then, we propose a parameterization of Rω by considering buoyancy frequency, Coriolis frequency, and topographic features. Compared to the GHP scaling with constant Rω, the turbulent dissipation rate values inferred from the GHP scaling with parameterized Rω are more accurate. In this manner, this study provides a reference for choosing optimal Rω when using the strain‐only GHP scaling to explore turbulent mixing in the open ocean. Plain Language Summary: Although diapycnal mixing is the smallest‐scale motion in the ocean, understanding its global distribution is crucial because it can modulate large‐scale circulation and hence influence climate change. However, field observations of ocean mixing are costly and therefore limited in a global sense. Given that internal wave breaking is the dominant energy source for ocean mixing, estimating internal wave‐driven mixing using easily available temperature and salinity data becomes an effective way to understand its spatiotemporal variability. Here, we modify an essential parameter in such an estimation method by changing the constant to a variable that depends on stratification, latitude, and topography. This modification significantly improves the accuracy of the method for estimating internal wave‐driven mixing, making it more consistent with observations compared to using a constant. Key Points: Shear‐to‐strain ratio varies significantly with spaceA parameterization of shear‐to‐strain ratio is proposed based on buoyancy frequency, Coriolis frequency, and topographic featuresBy using a parameterized shear‐to‐strain ratio, instead of a fixed value, we show the finescale parameterization performs better [ABSTRACT FROM AUTHOR]

We show that steric sea‐level varies with a period of 18.6 years along the western European coast. We hypothesize that this variation originates from the modulation of semidiurnal tides by the lunar nodal cycle and associated changes in ocean mixing. Accounting for the steric sea level changes in the upper 400 m of the ocean solves the discrepancy between the nodal cycle in mean sea level observed by tide gauges and the theoretical equilibrium nodal tide. Namely, by combining the equilibrium tide with the nodal modulation of steric sea level, we close the gap with the observations. This result supports earlier findings that the observed phase and amplitude of the 18.6‐year cycle do not always correspond to the equilibrium nodal tide. Plain Language Summary: The orbital position of the moon and the gravity pull it exerts on the earth varies with a period of 18.6 years. This cycle is called the lunar nodal cycle and it results in small variations of yearly averaged sea level (∼1–2 cm). Understanding this variability is important because it allows, for example, to quickly detect an acceleration in local sea‐level rise due to global warming. Here we show that the lunar nodal cycle also has an influence on the temperature and salinity in the surface 400m of the ocean. As a result, the ocean density changes and amplifies sea level variations along the western European coast. We make the hypothesis that since the lunar nodal cycle also influences the amplitude of the semidiurnal tides, and since those tides are known to be responsible for a large part of ocean mixing, a change in ocean mixing could be the cause of the ocean density variability that we observe. Key Points: Steric sea level changes are influenced by the 18.6‐year lunar nodal cycle along the western European coastThis influence could result from the modulation of semidiurnal tides by the lunar nodal cycle and the associated change in ocean mixingThis finding is a step toward resolving the long‐standing discrepancy between the theoretical long‐period nodal tide and observed signal [ABSTRACT FROM AUTHOR]

The main sources of redox gradients supporting high-productivity life in the Europan and other icy ocean world oceans were proposed to be photolytically derived oxidants, such as reactive oxygen species (ROS) from the icy shell, and reductants (Fe(II), S(-II), CH4, H2) from bottom waters reacting with a (ultra)mafic seafloor. Important roadblocks to maintaining life, however, are that the degree of ocean mixing to combine redox species is unknown, and ROS damage biomolecules. Here, we envisage a unique solution using an acid mine drainage (AMD)-filled pit lakes analog system for the Europan ocean, which previous models predicted to be acidic. We hypothesize that surface-generated ROS oxidize dissolved Fe(II) resulting in Fe(III) (hydr)oxide precipitates, that settle to the seafloor as "iron snow." The iron snow provides a respiratory substrate for anaerobic microorganisms ("breathing iron"), and limits harmful ROS exposure since they are now neutralized at the ice-water interface. Based on this scenario, we calculated Gibbs energies and maximal biomass productivities of various anaerobic metabolisms for a range of pH, temperatures, and H2 fluxes. Productivity by iron reducers was greater for most environmental conditions considered, whereas sulfate reducers and methanogens were more favored at high pH. Participation of Fe in the metabolic redox processes is largely neglected in most models of Europan biogeochemistry. Our model overcomes important conceptual roadblocks to life in icy ocean worlds and broadens the potential metabolic diversity, thus increasing total primary productivity, the diversity and volume of habitable environmental niches and, ultimately, the probability of biosignature detection. [ABSTRACT FROM AUTHOR]

Temporal and spatial variations in the ocean surface mixed layer are important for the climate and ecological systems. During 1980–2019, the Southern Indian Ocean (SIO) mixed layer depth (MLD) displays a basin‐wide shoaling trend that is absent in the other basins within 40°S–40°N. The SIO MLD shoaling is mostly prominent in austral winter with deep climatology MLD, substantially weakening the MLD seasonality. Moreover, the SIO MLD changes are primarily caused by a southward shift of the subtropical anticyclonic winds and hence ocean gyre, associated with a strengthening of the Southern Annular Mode, in recent decades for both winter and summer. However, the poleward‐shifted subtropical ocean circulation preferentially shoals the SIO MLD in winter when the meridional MLD gradient is sharp but not in summer when the gradient is flat. This highlights the distinct subtropical MLD response to meridional mitigation in winds due to different background oceanic conditions across seasons. Plain Language Summary: The ocean surface mixed layer (ML) is a well‐mixed layer with uniform physical and chemical properties and is key for the ocean in exchanging materials and energy with the atmosphere. The present study shows that during 1980–2019, the Southern Indian Ocean (SIO) ML depth (MLD) displays a basin‐wide decreasing trend, which is absent in the other basins within 40°S–40°N. The SIO MLD shoaling primarily appears in austral winter when the climatology ML is deep but is insignificant in summer, substantially weakening the MLD seasonality. The SIO MLD changes are primarily explained by the ocean dynamical adjustment driven by the surface zonal wind changes. Specifically, the strengthened Southern Annular Mode in recent decades drives southward shifts of the subtropical anticyclonic winds and ocean gyre year‐round. However, the poleward‐shifted ocean gyre preferentially decreases the SIO winter MLD as the meridional MLD gradient is sharp and thus efficiently reduces the deep ML water converging from the Southern Ocean into the SIO. In contrast, the SIO MLD displays negligible change in summer when its meridional gradient is flat. The results highlight that despite under nearly identical southward‐shifted subtropical winds, the winter and summer MLD responses are distinct due to different background oceanic conditions. Key Points: The seasonality of the Southern Indian Ocean surface mixed layer (ML) depth prominently weakens during 1980–2019The weakened seasonality mainly results from a pronounced winter ML shoalingThe southward shift of the subtropical ocean gyre driven by the strengthened Southern Annular Mode dominates the ML shoaling [ABSTRACT FROM AUTHOR]

Through a combination of laboratory experiments and theoretical models, we investigate the interaction of a mean upwelling through a closed basin with a vertical buoyancy flux. The fluid is mixed by a horizontally oscillating rake, which either traverses the whole basin or which oscillates just near one vertical boundary. We first review the steady state and demonstrate that, in both mixing regimes, the vertical density profile across the basin is controlled by the steady-state balance between the upward advective and diffusive fluxes of salinity as described by the classical model introduced by Munk (Deep-Sea Res. , vol. 13, issue 4, 1966, pp. 707–730). However, with boundary mixing, we show that both the upwelling and the buoyancy transport are localised to the mixing zone near the boundary, and the interior fluid is stagnant. We then develop a model to describe the transient evolution of the system if there is either a discrete increase or gradual decrease to the buoyancy flux. In the boundary mixing case, the change in the buoyancy flux at the lower boundary leads to a change in the buoyancy of the fluid in the boundary mixing region, and this induces a transient, buoyancy-driven flow in the boundary region in addition to the steady upwelling. In turn, an equal and opposite vertical flow develops in the interior, and this leads to a change in the density stratification of the interior fluid as the system adjusts to a new equilibrium. However, in our experiments, there is no vertical mixing in the interior and interior fluid may upwell or downwell dependent on the change to the buoyancy forcing. We discuss the implications of our results for the transport and mixing in the deep ocean, and the associated interpretation of field experiments. [ABSTRACT FROM AUTHOR]

The impacts of Stokes drift and sea-state-dependent Langmuir turbulence (LT) on the three-dimensional ocean response to a tropical cyclone in the Bohai Sea are studied through two-way coupled wave-current simulations. The Stokes drift is calculated from the simulated wave spectrum of the wave model, Simulating Waves Nearshore (SWAN), and then input to the Princeton Ocean Model with the generalized coordinate system (POMgcs) to represent the Langmuir effect. The Langmuir circulation is included in the vertical mixing of the ocean model by adding the Stokes drift to the shear of the vertical mean current and by including LT enhancements to the Mellor-Yamada 2.5 turbulent closure submodel. Simulations are assessed through the case study of Typhoon Masta in 2005 with a set of diagnostic experiments that incorporated different terms of Stokes production (SP) respectively. It is shown that with the consideration of SP, a deeper mixed layer, an enhanced vertical mixing coefficient KMS, and a more accurate representation of the vertical temperature distribution could be derived. Moreover, the effect of LT in elevating the turbulence mixing is stronger than that of Coriolis Stokes force (CSF) and Craik-Leibovich vortex force (CLVF). LT has a greater influence on the vertical mixing during typhoon than that in normal weather. [ABSTRACT FROM AUTHOR]

The response of the oceanic surface layer to atmospheric shallow convection is explored using realistic atmospheric large eddy simulations coupled with an oceanic 1D model with high vertical resolution. The effects of cloud shading, rain, and enhanced heat loss due to gust fronts on the edge of cold pools and their interactions are investigated in a case study of the Cooperative Indian Ocean Experiment on Intraseasonal Variability/Dynamics of the Madden–Julian Oscillation experiment in the tropical Indian Ocean, during a suppressed phase of the Madden–Julian Oscillation. Conditions of low surface wind and strong solar heating result in diurnal warming of the oceanic surface of 2 °C over a depth of 1 m. Analysis of specific periods covering the diurnal cycle shows the contrasting effects of cloud shading, rain, and turbulent heat fluxes under the cold pools on the sea temperature at the surface and below. On the one hand, decreasing the solar radiation (cloud shading) results in slight cooling extended horizontally and penetrating down to 1–2 m depth, depending on the time of the day. On the other hand, turbulent heat fluxes enhanced up to 300 W·$$ \cdotp $$m −2$$ {}^{-2} $$ by gusts and freshwater lenses due to rain act together and more locally. They isolate and strongly cool a thin inner layer at the surface, which eventually destabilizes the surface layer and propagates the cooling downward. The exact relative part and efficiency of these processes depend on the time evolution of the thermal stratification and vertical turbulent mixing in the oceanic upper layer. Surface cooling of up to −$$ - $$0.5 °C may occur in a few tens of minutes and last for several hours, significantly mitigating the effects of diurnal warming over large extents. [ABSTRACT FROM AUTHOR]

Radiocarbon (14C) in corals can be used as a relatively high-sensitivity indicator of vertical and horizontal advection of water masses, which contributes to the understanding of ocean circulation. In this study, we reconstruct Kuroshio and Ryukyu current transport with a seasonal resolution Δ14C record spanning 1947-2009. This record covers the beginning of the atomic era and was obtained from a coral on Kikai Island in the south of Japan. The Kikai Δ14C curve features a newly discovered Δ14C spike in July 1955, a rapid increase after 1962, and a steady decrease after 1980. The spike in 1955 may directly reflect ocean current transport. The lack of periodicity in the Δ14C record suggests the existence of mesoscale eddies and the complexity of Kuroshio and Ryukyu current transport. In addition, comparing the high-resolution Δ14C of Kikai and Ishigaki islands, both situated along the path of the Kuroshio, reveals the influence of Pacific Decadal Oscillation and El Niño-Southern Oscillation on the Kuroshio and Ryukyu currents. This suggests that seasonally resolved Δ14C in corals along an ocean current can produce a long-term record of ocean mixing that responds to climate variability. [ABSTRACT FROM AUTHOR]

We evaluate the vertical turbulent kinetic energy (TKE) mixing scheme of the NEMO-SI3 ocean–sea ice model in sea ice-covered regions of the Arctic Ocean. Specifically, we assess the parameters involved in the TKE mixed layer penetration (MLP) parameterization. This ad-hoc parameterization aims to capture processes like near-inertial oscillations, ocean swells, and waves that impact the ocean surface boundary layer, often not well-represented in the default TKE scheme. We evaluate this parameterization for the first time in three regions of the Arctic Ocean: the Makarov, Eurasian, and Canada Basins. We demonstrate the strong effect of the scaling parameter that accounts for the presence of sea ice. Our results confirm that the TKE MLP must be scaled down below sea ice to avoid unrealistic deep mixed layers. The other parameters evaluated are the percentage of energy penetrating below the mixed layer and the length scale of its decay with depth. All these parameters affect the mixed layer depth and its seasonal cycle, the surface temperature and salinity, as well as the underlying stratification. Shallow mixed layers are associated with stronger stratification and fresh surface anomalies, and deeper mixed layers correspond to weaker stratification and salty surface anomalies. Notably, we observe significant impacts on sea ice thickness across the Arctic Ocean in two scenarios: when the scaling parameter due to sea ice is absent and when the TKE mixed layer penetration process vanishes. In the former case, we observe an increase of several meters in the mixed layer depth together with a reduction in sea ice thickness ranging from 30 to 40 centimeters, reflecting the impact of stronger mixing. Conversely, in the latter case, we notice that a smaller mixed layer depth is accompanied by a moderate increase in sea ice thickness, ranging from 10 to 20 centimeters, as expected from a weaker mixing. Additionally, inter-annual variability suggests that experiments incorporating a scaling parameter based on sea ice concentration display an increased mixed layer depth during periods of reduced sea ice, which is consistent with observed trends. These findings underscore the influence, through specific parameterizations, of enhanced ocean mixing on the physical properties of the upper ocean and sea ice. [ABSTRACT FROM AUTHOR]

Marine heatwaves (MHWs) are a type of widespread, persistent, and extreme marine warming event that can cause serious harm to the global marine ecology and economy. This study provides a systematic analysis of the long-term trends of MHWs in the Eastern China Marginal Seas (ECMS) during summer spanning from 1982 to 2022, and occurrence mechanisms of extreme MHW events. The findings show that in the context of global warming, the frequency of summer MHWs in the ECMS has increased across most regions, with a higher rate along the coast of China. Areas exhibiting a rapid surge in duration predominantly reside in the southern Yellow Sea (SYS) and southern East China Sea (ECS, south of 28°N). In contrast, the long-term trends of mean and maximum intensities exhibit both increases and decreases: Rising trends primarily occur in the Bohai Sea (BS) and Yellow Sea (YS), whereas descending trends are detected in the northern ECS (north of 28°N). Influenced jointly by duration and mean intensity, cumulative intensity (CumInt) exhibits a notable positive growth off the Yangtze River Estuary, in the SYS and southern ECS. By employing the empirical orthogonal function, the spatio-temporal features of the first two modes of CumInt and their correlation with summer mean sea surface temperature (SST) and SST variance are further examined. The first mode of CumInt displays a positive anomalous pattern throughout the ECMS, with notable upward trend in the corresponding time series, and the rising trend is primarily influenced by summer mean SST warming. Moreover, both of the first two modes show notable interannual variability. Extreme MHW events in the SYS in 2016 and 2018 are examined using the mixed layer temperature equation. The results suggest that these extreme MHW events originate primarily from anomalous atmospheric forcing and oceanic vertical mixing. These processes involve an anomalous highpressure system over the SYS splitting from the western Pacific subtropical high, augmented atmospheric stability, diminished wind speeds, intensified solar radiation, and reduced oceanic mixing, thereby leading to the accumulation of more heat near the sea surface and forming extreme MHW events. [ABSTRACT FROM AUTHOR]

Oceanic mixing, mostly driven by the breaking of internal waves at small scales in the ocean interior, is of major importance for ocean circulation and the ocean response to future climate scenarios. Understanding how internal waves transfer their energy to smaller scales from their generation to their dissipation is therefore an important step for improving the representation of ocean mixing in climate models. In this study, the processes leading to cross-scale energy fluxes in the internal wave field are quantified using an original decomposition approach in a realistic numerical simulation of the California Current. We quantify the relative contribution of eddy–internal wave interactions and wave–wave interactions to these fluxes and show that eddy–internal wave interactions are more efficient than wave–wave interactions in the formation of the internal wave continuum spectrum. Carrying out twin numerical simulations, where we successively activate or deactivate one of the main internal wave forcing, we also show that eddy–near-inertial internal wave interactions are more efficient in the cross-scale energy transfer than eddy–tidal internal wave interactions. This results in the dissipation being dominated by the near-inertial internal waves over tidal internal waves. A companion study focuses on the role of stimulated cascade on the energy and enstrophy fluxes. [ABSTRACT FROM AUTHOR]

Through an expansive series of simulations, we investigate the effects of spatially uniform shear on the transport, structure, and dynamics of salt fingers. The simulations reveal that shear adversely affects the heat and salt fluxes of the system, reducing them by up to an order of magnitude. We characterize this in detail across a broad range of Richardson numbers and density ratios. We demonstrate that the density ratio is strongly related to the amount of shear required to disrupt fingers, with larger density ratio systems being more susceptible to disruption. An empirical relationship is proposed that captures this behavior that could be implemented into global ocean models. The results of these simulations accurately reproduce the microstructure measurements from North Atlantic Tracer Release Experiment (NATRE) observations. This work suggests that typical salt finger fluxes in the ocean will likely be a factor of 2–3 less than predicted by models not taking the effects of shear on double-diffusive systems into account. Significance Statement: The purpose of this work is to measure how large-scale (>1 m) motion affects specific small-scale (∼1 cm) mixing in the ocean. We approach this topic using simulations of meter-scale boxes of fluid containing both temperature and salt concentration with an applied background flow. We measure how this background flow changes the mixing of temperature and salt as the strength of the applied flow increases. We find that current estimates may overpredict mixing at this scale throughout the World Ocean by about a factor of 2–3, on average. This could have consequences for estimates of climate in addition to heat, salt, nutrient, and pollutant transport. [ABSTRACT FROM AUTHOR]

Breaking of internal lee waves generated by flow‐topography interaction is an important driving mechanism for abyssal mixing. By assuming that lee‐wave‐driven mixing is a local process, direct measurements in the past mainly focused on the water column over rough topography. In this study, a non‐local role of lee waves on remote mixing is investigated by using mooring observations in the northwestern Philippine Basin and a series of 2‐dimensional high‐resolution numerical simulations. We find that the breaking of lee waves can generate near‐inertial waves (NIWs) which can propagate away from their generation sites. The NIWs have strong vertical shear which can significantly elevate turbulent dissipation and mixing over remote uneven seafloor. Based on the topography perturbations numerical experiments, we find that for the remote gentle topography with inverse Froude number (Fr−1) between 0 and 0.5, the arrival of upstream NIWs can elevate the near‐bottom layer (0–1,000 m above bottom) dissipation rate and vertical diffusivity by 9.2 and 10.4 times, respectively at 30–50 km away from the NIWs source. For the remote rough topography with Fr−1 between 0 and 4, the corresponding increases are 3.5 and 4.4 times, respectively. The mechanism of upstream NIWs to elevate the downstream dissipation and mixing is through strengthening the shear instability over the far‐field topography, which is associated with NIWs' interactions with the topography and the generated lee waves therein. The results of this study will provide a foundation for improving the mixing parameterizations associated with current‐topography interactions. Plain Language Summary: Lee waves are waves inside the ocean that are generated when a strong current goes over rough seafloor. The breaking of lee waves can cause a lot of turbulence and stir up the seawater significantly (i.e., ocean mixing process) near the seafloor. Directed by linear lee wave theory, existing measurements mainly focus on the water column over rough topography, and researchers rarely address the impact of lee waves to remote areas. Here, we explore the remote impact of lee waves in the northwestern Philippine Basin using in situ observations and two‐dimensional high‐resolution numerical experiments. We find that when the strong background flow goes over rough seamounts, the breaking of lee waves not only causes turbulence locally but also excites a unique type of internal waves called near‐inertial waves (NIWs). The NIWs have large vertical shear, which can travel away and cause a more dissipative waves field over remote rugged seafloor. Further analysis suggests that the NIWs interact with the remote seafloor and the generated lee waves therein, which strengthen the shear instability of internal waves is the reason why the remote dissipation and mixing field is enhanced. Key Points: Generation of near‐inertial waves (NIWs) through interaction between bottom‐reaching flow and rough topography is observedThe NIWs are generated by lee wave breaking which can leave the topography after being generatedThe NIWs have large vertical shear which can enhance mixing remotely by inducing shear instability [ABSTRACT FROM AUTHOR]

Subpolar Mode Water (SPMW) is an important water mass originating in the eastern North Atlantic. Its formation, subject to modification through oceanic interior mixing, can directly influence the volume of water contributing to the Atlantic meridional overturning circulation. Utilizing observation‐based data sets spanning from 1993 to 2018, we estimated the formation rates and volume of SPMW within isopycnal layers and examined its temporal variability. Two complementary approaches were used to estimate the formation rate: a thermodynamic approach focusing on the air‐sea interactions and a kinematic approach involving volume transport from the mixed layer to the ocean's interior, including the entrainment/detrainment of the mixed layer itself. This is the first time that thermodynamic and kinematic approaches are applied to observation‐based data in the North Atlantic. Our results suggest a substantial role of diapycnal mixing in diluting the dense waters formed by air‐sea fluxes toward the range of SPMW densities. The study reveals a complex interplay of processes, with entrainment being the primary driver of subduction/obduction rates, while advection contributes to the overall small‐scale dynamics. Variations in the volume and location of SPMW formation are observed from year to year. Notably, when SPMW forms extensively in lighter isopycnal layers, the volume occupied by denser isopycnals decreases and vice versa. We attributed this compensation effect to a propagation signal, where formation in the lightest isopycnal bins influences the formation in denser isopycnal bins with a delay of a few years, emphasizing the circulation's role in shaping the SPMW distribution. Plain Language Summary: We present an analysis of one important type of water mass, the Subpolar Mode Water (SPMW) located in the North Atlantic Ocean. We looked at data collected over a period of 26 years from 1993 to 2018, to understand how SPMW forms and how its volume changes over time. We used two different methods to calculate how much SPMW forms each year to have the best possible picture. One method looked at air‐water interactions, and the other at how the water moves from the top layer of the ocean to deeper parts. This is the first time these methods are applied to observation‐based data in the North Atlantic. Our findings highlight the important role of water mixing in changing the density of SPMW. We also found that one of the main reasons how the SPMW leaves the top layer of the ocean is through the shoaling/deepening of the mixed layer itself over time, modulated horizontally by the small‐scale dynamics of the ocean. Additionally, the volume of SPMW and location where it forms can vary from year to year. Notably, when SPMW forms in larger amounts in the upper part of the ocean, there is less SPMW in the denser water. Key Points: Subduction of water below the mixed layer is driven by entrainment with advection modulating the small‐scale dynamicsVolume and formation of Subpolar Mode Water changes every year in size and locationThe difference between the formation rate from the thermodynamic and kinematic approaches suggests a large role of diapycnal mixing [ABSTRACT FROM AUTHOR]

The half‐life of 228Ra (5.7 years) aligns well with near‐surface and near‐bottom ocean mixing timescales. Because 228Ra is sourced from sediments, regions of enhanced activity represent water that has recently interacted with sediments on the continental margin or seabed. The GP15 meridional transect from Alaska to Tahiti along152°W encountered several regions in the upper ocean where 228Ra was enriched. These enrichments follow surface and subsurface ocean current patterns and pair with earlier measurements of 228Ra and transient radionuclides to reveal the origins of these enriched regions. An enriched region at Alaska margin stations 1–3 was sourced locally but did not extend to the Alaskan trench at station 4. A large shallow region between 47° and 32°N. was sourced from the west by the North Pacific Current; another shallow enriched region between 11° and 5° N was also sourced from the west by the North Equatorial Countercurrent. Subsurface enrichments (100–400 m) between 18 and 47°N were associated with Central Mode Water and North Pacific Intermediate Water. The 228Ra activities in the upper Pacific were six times lower than activities in the Atlantic. In deep waters the primary enrichment was 27°–47°N. Two stations (32° and 37°N) were especially enriched, having near‐bottom inventories several times greater than other stations. With these two exceptions the remaining Pacific stations exhibited averaged inventories lower than those in the Atlantic. There was one region of enriched 223Ra (half‐life = 11 days) above the Puna Ridge near Hawaii. Plain Language Summary: The international GEOTRACES program aims to identify processes and quantify fluxes that control the distribution of trace elements and isotopes (TEIs) in the ocean. Some TEIs are important micronutrients, which may control biological productivity. Others may become concentrated as they pass up the food web and reach potentially harmful levels in some seafood. But measurements of these TEIs are not adequate to determine controlling processes or quantify their fluxes in the marine environment. Radionuclide tracers provide a means to link TEI measurements with fluxes and processes. Here we employ measurements of 228Radium (half‐life = 5.75 years) to trace water movements and current speeds in the upper Pacific and to determine regions near the seabed where sediment‐water interactions are most intense. The upper ocean measurements confirmed a broad circulation pattern that brought water from the Asian margin to the northeast Pacific at speeds of 2–8 cm/sec, in agreement with other estimates of these current speeds. Near the seabed we discovered a region of enhanced 228Ra activity indicating intense sediment‐water interactions. Such regions may control the release of TEIs from the seabed to the water column. We anticipate others will utilize these data to elucidate aspects of other TEI distributions. Key Points: Several open‐ocean regions of enriched 228Ra were documented in the North and Equatorial Pacific at 152°WThe upper ocean 228Ra‐enriched regions correspond to major surface and subsurface currents transporting water from the Asian marginOne seabed region near 32°N was especially enriched in 228Ra [ABSTRACT FROM AUTHOR]

This study examines the influence of Hallasan Mountain (Hallasan) on the track and intensity of two Typhoons, Soulik in 2018 and Chaba in 2016, which passed to the left and right of Hallasan, respectively, using a coupled ocean-atmosphere model. We designed three experiments: one with Hallasan's actual altitude, another with the mountain removed, and a third where Hallasan's altitude was doubled. Results showed that Hallasan had a negligible impact on the tracks of both typhoons. Regarding intensity, however, the central pressure of both typhoons increased (indicating weakening) by up to 2 hPa due to Hallasan; the maximum wind speeds initially increased (Soulik by 1 m/s, Chaba by 3 m/s) and then decreased (Soulik by 1 m/s, Chaba by 5 m/s). These results show that Hallasan does not significantly weaken the intensity of typhoons approaching the Korean Peninsula, but considering the average intensity change (-3.45 hPa) of past typhoons that passed to the left of Jeju Island in terms of central pressure, Hallasan makes a noteworthy contribution. Additionally, this study reveals that changes in typhoon winds due to the wind convergence caused by Hallasan's topography can alter ocean vertical mixing and sea surface cooling, further impacting typhoon intensity. This finding underscores the importance of using a coupled ocean-atmosphere model when studying the impact of topography on typhoons. [ABSTRACT FROM AUTHOR]

The Southern Ocean hosts a winter deep mixing band (DMB) near the Antarctic Circumpolar Current's (ACC) northern boundary, playing a pivotal role in Subantarctic Mode Water formation. Here, we investigate what controls the presence and geographical extent of the DMB. Using observational data, we construct seasonal climatologies of surface buoyancy fluxes, Ekman buoyancy transport, and upper stratification. The strength of the upper-ocean stratification is determined using the columnar buoyancy index, defined as the buoyancy input necessary to produce a 250 m deep mixed layer. It is found that the DMB lies precisely where the autumn–winter buoyancy loss exceeds the columnar buoyancy found in late summer. The buoyancy loss decreases towards the south, while in the north the stratification is too strong to produce deep mixed layers. Although this threshold is also crossed in the Agulhas Current and East Australian Current regions, advection of buoyancy is able to stabilise the stratification. The Ekman buoyancy transport has a secondary impact on the DMB extent due to the compensating effects of temperature and salinity transports on buoyancy. Changes in surface temperature drive spatial variations in the thermal expansion coefficient (TEC). These TEC variations are necessary to explain the limited meridional extent of the DMB. We demonstrate this by comparing buoyancy budgets derived using varying TEC values with those derived using a constant TEC value. Reduced TEC in colder waters leads to decreased winter buoyancy loss south of the DMB, yet substantial heat loss persists. Lower TEC values also weaken the effect of temperature stratification, partially compensating for the effect of buoyancy loss damping. TEC modulation impacts both the DMB characteristics and its meridional extent. [ABSTRACT FROM AUTHOR]

Internal tides are known to be an important source of mixing in the oceans, especially in the bottom boundary layer. The depth of internal-tide generation therefore seems important for benthic life and the formation of cold-water coral mounds, but internal-tide conversion is generally investigated in a depth-integrated sense. Using both idealized and realistic simulations on continental slopes, we found that the depth of internal-tide generation increases with increasing slope steepness and decreases with intensified shallow stratification. The depth of internal-tide generation also shows a typical latitudinal dependency related to Coriolis effects. Using a global database of cold-water corals, we found that, especially in Northern Hemisphere autumn and winter, the global depth pattern of internal-tide generation correlates (rautumn = 0.70, rwinter = 0.65, p < 0.01) with that of cold-water corals: shallowest near the poles and deepest around the Equator, with a decrease in depth around 25° S and N, and shallower north of the Equator than south. We further found that cold-water corals are situated significantly more often on topography that is steeper than the internal-tide beam (i.e. where supercritical reflection of internal tides occurs) than would be expected from a random distribution: in our study, in 66.9 % of all cases, cold-water corals occurred on a topography that is supercritical to the M2 tide whereas globally only 9.4 % of all topography is supercritical. Our findings underline internal-tide generation and the occurrence of supercritical reflection of internal tides as globally important for cold-water coral growth. The energetic dynamics associated with internal-tide generation and the supercritical reflection of internal tides likely increase the food supply towards the reefs in food-limited winter months. With climate change, stratification is expected to increase. Based on our results, this would decrease the depth of internal-tide generation, possibly creating new suitable habitat for cold-water corals shallower on continental slopes. [ABSTRACT FROM AUTHOR]

Internal tides (ITs) play a critical role in ocean mixing, and have strong signatures in ocean observations. Here, global IT sea surface height (SSH) in nadir altimetry is compared with an ocean forecast model that assimilates de‐tided SSH from nadir altimetry. The forecast model removes IT SSH variance from nadir altimetry at skill levels comparable to those achieved with empirical analysis of nadir altimetry. Accurate removal of IT SSH is needed to fully reveal lower‐frequency mesoscale eddies and currents in altimeter data. Analysis windows of order 30–120 days, made possible by the frequent (hourly) outputs of the forecast model, remove more IT SSH variance than longer windows. Forecast models offer a promising new approach for global internal tide mapping and altimetry correction. Because they provide information on the full water column, forecast models can also help to improve understanding of the underlying dynamics of ITs. Plain Language Summary: Tidal flow over topographic features on the seafloor generates vertical displacements along the interfaces of ocean layers that have different densities. These vertical displacements at tidal frequencies are known as internal tides. Internal tide displacements are largest well below the sea surface, but also display a sea surface height (SSH) signature that is large enough to be measured by satellite altimeters. Removing internal tide signals from satellite altimeter SSH allows for a more accurate accounting of non‐tidal features, including slowly evolving ocean currents and eddies, that are also measured by altimeters. Here, we show that supercomputer ocean forecast simulations of the global internal tide field are able to remove internal tide SSH from satellite altimeter measurements with a skill level that is comparable to the skill of internal tide SSH removal based upon analysis of the satellite altimeter data itself. Thus, forecast models offer a complementary method for this important task. In addition, forecast models provide information on the entire ocean water column, not just the sea surface. Finally, the hourly outputs of forecast models allow for a greater variety of tidal analysis record lengths than can be achieved with altimeter outputs, which report sea surface height fields much less frequently. Key Points: Global ocean forecast models can accurately simulate both long‐term (phase‐locked) internal tides and their short‐term modulationsOcean forecast models offer a useful complement to empirical models for mapping internal tides and correcting altimetry for internal tidesIn regions of strong internal tides, optimal variance reduction in nadir altimetry is attained through short‐term tidal analyses (∼60 days) [ABSTRACT FROM AUTHOR]

Ocean ventilation translates atmospheric forcing into the ocean interior. The Southern Ocean is an important ventilation site for heat and carbon and is likely to influence the outcome of anthropogenic climate change. We conduct an extensive backwards‐in‐time trajectory experiment to identify spatial and temporal patterns of ventilation. Temporally, almost all ventilation occurs between August and November. Spatially, "hotspots" of ventilation account for 60% of open‐ocean ventilation on a 30 years timescale; the remaining 40% ventilates in a circumpolar pattern. The densest waters ventilate on the Antarctic shelf, primarily near the Antarctic Peninsula (40%) and the west Ross sea (20%); the remaining 40% is distributed across East Antarctica. Shelf‐ventilated waters experience significant densification outside of the mixed layer. Plain Language Summary: Only a small fraction of the ocean is interacting with the atmosphere at any given time. This water is definitively found in the upper mixing layer of the ocean. When this water leaves the mixing layer and enters the ocean interior, it has "ventilated" (this term arising from the abundance of oxygen in newly ventilated water). The Southern Ocean is an important region for ventilation, and the uptake of heat and carbon dioxide there is likely to influence the limits and timescales of climate change. By calculating the 30 year history of over 480 million fluid parcels, we find that 60% of open‐ocean ventilation occurs in certain "hotspots" and almost exclusively between August and November. Key Points: Extensive backwards‐in‐time trajectories are used to explore the 30 year history of ventilation in a simulated Southern OceanLocal hotspots only account for a fraction of Southern Ocean ventilation (60%), the remaining ventilation occurs in a circumpolar patternAlmost all Southern Ocean ventilation occurs between August and November while the mixed layer is shoaling [ABSTRACT FROM AUTHOR]

Sea surface temperature (SST) is a pivotal parameter in climate, weather, and ocean sciences because it plays a decisive role in ocean-atmosphere interactions. Identifying deficiencies in the ERA5 reanalysis of ocean skin temperature, we revisited the ocean mixed layer (OML) model used at ECMWF and identified errors in the model, and revised it accordingly. Validation of the revised model was conducted through comparisons of simulated temperatures at the skin layer and 1-m depth with observations from ship-board infrared interferometers and buoy-mounted thermistors. These comparisons revealed a strong correlation, with an absolute mean deviation of less than 0.1 K and a standard deviation under 0.5 K. We concluded that the temperature at the ocean-atmosphere interface possesses the same degree of accuracy. Moreover, the results closely align with anticipated distributions of solar radiation. Consequently, the revised OML model shows promising potential for improving the simulation of diurnal interface SST variations in weather and climate models. [ABSTRACT FROM AUTHOR]

The spring warm pool (SWP) in the South China Sea (SCS) is a region west of Luzon Island with a sea surface temperature nearing 30°C in late spring, significantly higher than the surrounding waters. Understanding its formation aids in reliable prediction of the summer monsoon onset in the SCS. The current explanation for the SWP formation is the wake effect, which claims that the orographic blocking of Luzon Island forms a wake zone west of Luzon Island and so considerably reduces sea surface latent heat (LH) flux release during the winter monsoon season in comparison to adjacent waters. In this study, we enhance this explanation by incorporating the foehn clearance effect. Our statistical analysis indicates that the SWP evolution is strongly associated with frequent foehn clearance west of Luzon Island and that, in the SWP domain relative to its adjacent domains, the increase of surface short‐wave (SW) radiation induced by the foehn clearance effect is almost equal to the decrease of surface LH loss resulting from the wake effect. The ocean mixed layer heat budget analyses not only confirm that the zonal difference of surface heat flux dominates the SWP formation, but also demonstrate that this zonal difference is largely contributed by the zonal difference of SW radiation caused by the foehn clearance effect. Furthermore, sensitivity assessments demonstrate that the SWP would not emerge if the foehn clearance effect was excluded, indicating that both the wake effect and the foehn clearance effect contribute jointly to the SWP formation. Plain Language Summary: The largest island in the Philippines is Luzon Island, which is located in the western rim of the Pacific Ocean. The existence of north‐south mountain ranges on Luzon Island results in the occurrence of blocking effects, such as wake and foehn effects. This leads to frequent leeward wake and foehn phenomenon to the west of Luzon Island when easterly winds prevail, especially during the winter monsoon season. One of the foehn effects is foehn clearance effect, which is characterized by a cloudless sky and intense solar short‐wave (SW) radiation. Foehn clearance is commonly observed on the leeward side of Luzon Island during episodes of foehn phenomena, as evidenced by satellite photography. The spring warm pool (SWP) formation mechanism has been investigated predominantly from an oceanic perspective in previous studies. However, the role of SW radiation and its zonal difference on the SWP formation is poorly understood. We recognize that the cloudless situation of foehn clearance can promote a relative increase in SW radiation to the west of Luzon Island, consequently contributing to the SWP formation. Following this idea, we do a number of investigations and subsequently verify that the foehn clearance effect plays a crucial role in the formation of the SWP. Key Points: The foehn clearance effect is proposed to enhance the current explanation for the spring warm pool (SWP) formation mainly through the wake effectThe zonal difference of surface heat flux is driven largely by the short‐wave radiation difference caused by the foehn clearance effectThe SWP would not emerge if the foehn clearance effect was eliminated during the winter monsoon season [ABSTRACT FROM AUTHOR]