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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]

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]

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]

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]

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]

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]

Tides in the Arctic Ocean affect ocean circulation and mixing, and sea ice dynamics and thermodynamics. However, there is a limited network of available in situ tidal coefficient data for understanding tidal variability in the Arctic Ocean; e.g., the global TICON-3 database contains only 111 sites above 60°N and 21 above 70°N. At the same time, the presence of sea ice and latitude limits of satellite altimetry complicate altimetry-based retrievals of Arctic tidal coefficients. This leads to a reliance on ocean tide models whose accuracy depend on having sufficient in situ data for validation and assimilation. Here, we present a comprehensive new dataset of tidal constituents in the Arctic region, combining analyses of in situ measurements from tide gauges, ocean bottom pressure sensors and GNSS interferometric reflectometry. The new dataset contains 914 measurement sites above 60°N and 399 above 70°N, with each site being quality-assessed and expert guidance provided to help maximise the usage of the dataset. We also compare the dataset to recent tide models. [ABSTRACT FROM AUTHOR]

Scattered outcrops of various lithologies such as limestone, chert, sandstone, mudstone or shale, and microgabbro have been recently exposed in the Hariphunchai Education Center of Chiang Mai University (HECCMU), the northern Lamphun area of the northern Thailand. The outcrops are located in the Inthanon Zone where Palaeo-Tethyan ocean plate stratigraphy is well known. Field and microfacies analysis of limestone blocks recognizes four microfacies interpreted as low-energy lagoon or platform interior and reef flank or shoal of fore-reef. Foraminiferas from the limestone are the Sakmarian (lower Cisuralian) to Kungurian (upper Cisuralian) of the early Permian in age. Field and petrographic analysis of co-occurring radiolarian chert, microgabbro, pelagic mudstone, and turbidites suggest that the limestone blocks were mid-oceanic carbonate build-ups on seamount which were chaotically mixed with oceanic rocks such as chert and gabbro and then embedded in shale matrix. It is concluded that the outcrops are part of mélange formed during the Palaeo-Tethys closure and are significant in representing the Cisuralian (lower Permian) carbonate remnant in the mélange of the Inthanon Zone. [ABSTRACT FROM AUTHOR]

Mesoscale eddies play an important role in transporting water properties, enhancing air‐sea interactions, and promoting large‐scale mixing of the ocean. They are generally referred to as "coherent" structures because they are organized, rotating fluid elements that propagate within the ocean and have long lifetimes (months or even years). Eddies have been sampled by sparse in‐situ vertical profiles, but because in‐situ ocean observations are limited, they have been characterized primarily from satellite observations, numerical simulations, or relatively idealized geophysical fluid dynamics methods. However, each of these approaches has its limitations. Many questions about the general structure and "coherence" of ocean eddies remain unanswered. In this study, we investigate the properties of seven mesoscale eddies sampled with relative accuracy during four different field experiments in the Atlantic. Our results suggest that the Ertel Potential Vorticity (EPV) is a suitable parameter to isolate and characterize the eddy cores and their boundaries. The latter appear as regions of finite horizontal extent, characterized by a local extremum of the vertical and horizontal components of the EPV. These are found to be closely related to the presence of a different water mass in the core (relative to the background) and the steepening of the isopycnals due to eddy occurrence and dynamics. Based on these results, we propose a new criterion for defining eddies at the mesoscale. We test our approach using a theoretical framework and explore the possible magnitude of this new criterion, including its upper bound. Plain Language Summary: Mesoscale eddies are ubiquitous rotating currents in the ocean. They are considered as one of the most important sources of ocean variability because they can live for months and transport and mix heat, salt, and other properties within and between ocean basins. They have been studied extensively from satellite observations because they are often at or near the ocean surface. However, observations of their 3D structure are rare, and calculations of eddy transport are often approximated without precise knowledge of their true vertical extent. In addition, recent studies suggest the existence of subsurface eddies that are not detectable from satellite observations. Here, we characterize and attempt to generalize 3D eddy properties by analyzing observations collected during specific high‐resolution field experiments in the Atlantic Ocean. We also propose a criterion, based on geophysical fluid dynamics theory, that defines the lateral and vertical eddy boundaries. This criterion can be applied broadly to assess eddy structure, volume, transport, and evolution more quantitatively than in previous studies. We also provide insight into why these boundaries are substantial, which may explain why oceanic eddies are coherent structures that can span long distances and have long lifetimes. Key Points: A new criterion that compare the horizontal and vertical components of Ertel PV is introduced to characterize eddies boundariesEddy boundaries behave like a frontThe Rossby number and the vertical stratification anomaly drive eddies boundaries intensity [ABSTRACT FROM AUTHOR]

The exchange of heat and other tracers between the ocean interior and sea ice cover depends on vertical diffusion through the thin and strongly stratified Arctic Ocean transition layer and remnant transition layer. Ice‐Tethered Profiler observations during 2004–2021 were used to characterize and investigate the active and remnant transition layers in the Beaufort Gyre region. Transition layers evolved seasonally, with the deepest, densest, and warmest transition layers observed in spring. In a composite view, the summer mixed layer shoaled over a timescale of approximately 2 weeks, leaving behind a remnant transition layer whose stratification had already begun to weaken. The stratification maximum of the remnant transition layer continued to weaken and broaden throughout the summer, including changes associated with storm events. The weakening stratification was used to estimate an effective vertical turbulent diffusivity which ranged from 10−8 to 10−6 m2/s for individual ITP records; a value of 6 × 10−7 m2/s is representative of the Beaufort Gyre region. Vertical heat fluxes through the active and remnant transition layers were estimated over 90‐day timescales. Springtime vertical heat fluxes increased from 0.3 ± 0.3 W/m2 during 2006–2011 to 1.1 ± 0.7 W/m2 during 2017–2021. Summertime vertical heat fluxes through the remnant transition layer were somewhat less, but also increased interannually. Vertical gradients of density during spring and summer, and so diffusion of density through the active and remnant transition layers, did not increase interannually. Pacific Summer Water warming has increased ocean to ice heat fluxes, which may continue to become more significant in the future. Plain Language Summary: Heat is stored in the Arctic Ocean tens to hundreds of meters beneath the sea ice cover. A small fraction of this heat can gradually or intermittently reach the sea ice and contribute to its melt in spring or delayed growth in fall. We use observations during 2004–2021 from the most strongly stratified portion of the ocean to investigate the energy barrier that isolates heat at depth. Ocean stratification was seasonally and interannually variable, and weakened in particular over the summer season, including due to storm events. Observations of this weakening are used to derive estimates of the turbulent mixing intensity, which are consistent with other estimates for the Arctic Ocean. Even with no interannual variations in the intensity of ocean mixing, vertical heat transport has increased due to changes in subsurface temperature. Our inferred heat transport in springtime has increased from enough heat to melt 1 cm of ice during 2006–2011 to 3 cm of ice during 2017–2021. Estimated vertical heat transport during summer was somewhat less, but also increased interannually. The role of stored ocean heat on the sea ice cover may become more significant in the future. Key Points: The transition layer varied seasonally, and the remnant transition layer evolved throughout the summer season, including due to storm eventsA representative effective vertical diffusivity for the transition layer and remnant transition layer is estimated to be 6 × 10−7 m2/sDiffusion of heat has increased interannually to values of 1 W/m2 or larger, while diffusion of density has remained constant [ABSTRACT FROM AUTHOR]

Internal waves drive ocean mixing and enhance the transport of heat, momentum and other tracers in shelf seas. We collected observations of mixing over a 30‐day period from three cross‐shore moorings placed on the 330, 200 and 150 m isobaths on the offshore side of a pelagic ridge on the Australian North West Shelf. The region is forced by energetic surface and internal tides, exhibits non‐linear internal waves, experiences flow‐topography interactions, and is subject to episodic intense wind events. This complex forcing drove energetic diapycnal mixing at all sites. We identified five dominant internal wave forcing categories: mode‐1 waves at low‐frequency (time scales from double the buoyancy period to 4 hr), mode‐1 waves at high‐frequency (HF) (time scales between the buoyancy period and double the buoyancy period), mode‐2 waves, internal bores, and internal hydraulic jumps. Overall, just 15% of mixing events accounted for 90% of the total observed heat flux over the record. Mixing during internal wave events accounted for as much as 50% of the total heat flux in some locations. Of the internal wave categories, HF mode‐1 waves were the most significant contributors to the total heat flux at all sites (∼20%). On the other hand, internal bores made significant contributions to mixing only at the 200 and 150 m moorings; they made no contribution to mixing at the 330 m mooring. At the shallowest mooring, the different internal wave categories all made similar contributions to the total flux, indicating an increasingly complicated relationship between the evolving internal wavefield and the mixing. Plain Language Summary: Internal waves propagate along the density gradients found beneath the ocean's surface layer, analogous to surface waves propagating along the sharp density gradient where air and water meet. These waves play an important role in the distribution of nutrients, heat, contaminants, and other tracers in the ocean, especially in coastal regions where the waves break. The density structure of the ocean, the tidal and wind forcing, and the seabed features result in many different types of internal waves, each of which travel and break differently. In this work, we examine the mixing caused by different types of internal waves as they travel up and over a subsurface ridge on the Australian North West Shelf. We found that most of the significant mixing resulted from relatively rare events, which often occurred during internal wave forcing events. The magnitude of mixing increased in shallower waters due to internal waves breaking and causing energetic turbulent flows. However, the wave types that were the most significant for mixing changed depending on the location on the shelf and the depth. High‐frequency internal waves were generally the most significant contributor to mixing. Internal bores, a class of waves moving upslope near the seabed, did dominate the total ocean mixing near‐bottom at some locations. Key Points: Energetic but short‐lived nonlinear internal wave events drove most of the vertical turbulent heat flux over the month‐long recordInternal wave events evolved over relatively short time scales and relatively short spatial scalesEnergetic internal wave events contribute significantly to mixing in shelf seas [ABSTRACT FROM AUTHOR]

Actinium‐227 (227Ac) has been used as a powerful tracer of diapycnal mixing in the ocean, assuming that it is conservative and originates mainly from deep‐sea sediments. However, here we show an unexpectedly large source (continental margin) and sink (scavenging) of 227Ac in the ocean, based on high‐resolution 227Ac distributions obtained for the first time by mooring Mn‐fibers in the East Sea (Japan Sea). Although we expected a decrease in radium‐228 (228Ra) to 227Ac ratios with depth owing to their different half‐lives, the ratios increased with depth in the upper layer, indicating efficient removal of 227Ac by particle scavenging. In addition, unusually high 227Ac activities (∼15 dpm m−3) were observed in the surface layer, likely due to the horizontal transport of 227Ac‐enriched shelf water. Thus, our results suggest refining our understanding of the geochemical cycle and application of 227Ac in the ocean. Plain Language Summary: Distributions of 227Ac provide crucial information for the vertical mixing of the deep ocean on timescales of up to 100 years. However, behaviors of 227Ac in the ocean have not been well understood to date because of its extremely low concentration. In this study, we for the first time determined high‐resolution 227Ac profiles by mooring Mn‐fibers in a marginal sea of the northwestern Pacific Ocean. Our results display that the shelf source inputs as well as efficient removal by particle scavenging have been overlooked so far. In particular, we emphasize that the removal of 227Ac by particle scavenging revealed in this study should be considered when using 227Ac as a tracer of mixing rates in the ocean. Key Points: The high‐resolution measurement of 227Ac with our Mn‐fiber mooring method agrees very well with the onboard Mn‐fiber filtration methodSignificantly high 227Ac activities, which might originate from the 227Ac‐enriched shelf water, are observed in the surface layer (0–100 m)High 227Ac scavenging rates are calculated based on the increasing trend of 228Ra to 227Ac ratio with depth in the upper layer (0–1,000 m) [ABSTRACT FROM AUTHOR]

In situ measurements from an array of pressure-equipped inverted echo sounders (PIESs), numerical simulation results using a non-hydrostatic model (SUNTANS) that does not account for mesoscale variability, and a ray-tracing method are used to investigate the behavior of nonlinear internal waves (NLIWs) in the South China Sea, focusing on period of Typhoon Nanmadol passage. Unusual significant differences in NLIWs arrival time occur between the PIES observation and SUNTANS simulation results after Typhoon Nanmadol passage; i.e., the observed NLIWs show a delay of 0.5–1.5 h at the westernmost PIES sites compared to the simulated ones in the SUNTANS. A data-assimilated ocean model outputs in addition to satellite altimetry reveal that the passage of Typhoon Nanmadol alters the oceanic environments remarkably, creating thermocline shoaling of ~80 m, which can slow down the wave propagation speed. We account for the delay of NLIWs using ray-tracing simulations. Our results demonstrate that typhoon-induced changes in mesoscale structure can significantly impact the fate of NLIWs in the South China Sea, which have potential on ocean mixing and biogeochemical systems. [ABSTRACT FROM AUTHOR]

The impact of interannual variations in sea ice area in the Okhotsk Sea was investigated through a composite analysis of years with extensive and limited sea ice areas (referred to as heavy and light ice years, respectively), using atmospheric and oceanic reanalysis data. The comparison of heavy and light ice-year composites in February revealed a substantial decrease in upward surface turbulent heat flux in the Okhotsk Sea (~-250 W m-2) and a notable increase in a surprisingly extensive region in the western North Pacific (30-120 W m-2), spanning 2300 km from the ice edge. These differences were consistent with the decrease in surface air temperature and specific humidity, suggesting that during heavy ice years, cold and dry air blowing from Siberia to the North Pacific via the Okhotsk Sea undergoes less modification over larger sea ice areas, remaining colder and drier in the North Pacific and thereby enhancing the heat flux. Such advection can be associated with the Asian winter monsoon and migratory cyclones. Cloud cover and surface radiation flux altered consistently with these differences, although longwave and shortwave radiation largely counterbalanced each other. Additionally, the Pacific storm track exhibited variation. In accordance with the heat flux difference, sea surface temperature decreased, and the ocean mixed layer deepened around the subarctic during heavy ice years. These findings suggest that sea ice area in the Okhotsk Sea influences the lower atmosphere and surface ocean in the North Pacific. Such impacts could further affect ocean nutrient circulation, ecosystems, and atmospheric teleconnections. [ABSTRACT FROM AUTHOR]

A stochastic horizontal subgrid‐scale mixing scheme is evaluated in ensemble simulations of a tropical oceanic deep convection case using a horizontal grid spacing (Δh) of 3 km. The stochastic scheme, which perturbs the horizontal mixing coefficient according to a prescribed spatiotemporal autocorrelation scale, is found to generally increase mesoscale organization and convective intensity relative to a non‐stochastic control simulation. Perturbations applied at relatively short autocorrelation scales induce differences relative to the control that are more systematic than those from perturbations applied at relatively long scales that yield more variable outcomes. A simulation with mixing enhanced by a constant factor of 4 significantly increases mesoscale organization and convective intensity, while turning off horizontal subgrid‐scale mixing decreases both. Total rainfall is modulated by a combination of mesoscale organization, areal coverage of convection, and convective intensity. The stochastic simulations tend to behave more similarly to the constant enhanced mixing simulation owing to greater impacts from enhanced mixing as compared to reduced mixing. The impacts of stochastic mixing are robust, ascertained by comparing the stochastic mixing ensembles with a non‐stochastic mixing ensemble that has grid‐scale noise added to the initial thermodynamic field. Compared to radar observations and a higher resolution Δh = 1 km simulation, stochastic mixing seemingly degrades the simulation performance. These results imply that stochastic mixing produces non‐negligible impacts on convective system properties and evolution but does not lead to an improved representation of convective cloud characteristics in the case studied here. Plain Language Summary: Regional weather prediction and climate models commonly have horizontal grid lengths of 2–4 km that cannot resolve mixing of air in cumulonimbus clouds with surrounding cooler, drier environmental air, a key process that modulates cloud and storm properties. This study evaluates a method to represent such mixing in models that induces random variability in the magnitude of mixing for a tropical oceanic deep convection case. This approach is found to alter both the intensity and areal coverage of precipitation. As space and time scales of the variability are increased, changes to precipitation coverage and intensity become greater and less systematic relative to a control simulation. Altering mixing also changes the sizes of convective clouds and the degree to which they cluster around one another. Ultimately, the new mixing approach induces variable responses in simulated convective clouds but generally makes them more intense with wider cloud cores, which does not improve upon the control simulation relative to radar observations and a higher resolution simulation. Key Points: A stochastic subgrid‐scale mixing scheme was evaluated in a tropical oceanic deep convection case using 3‐km horizontal grid spacingStochastic mixing modulates the area and intensity of convection, producing more organization and less dilute updrafts relative to a controlMore systematic impacts on convection properties occur for relatively short spatial and time scale variability of stochastic mixing [ABSTRACT FROM AUTHOR]

While various parameterizations of vertical turbulent fluxes at different levels of complexity have been proposed, each has its own limitations. For example, simple first‐order closure schemes such as the K‐Profile Parameterization (KPP) lack energetic constraints; two‐equation models like k−ɛ $k-\varepsilon $ directly solve an equation for the turbulent kinetic energy but do not account for non‐diffusive fluxes, and high‐order closures that include the high‐order transport terms are computationally expensive. To address these, we extend the Assumed‐Distribution Higher‐Order Closure (ADC) framework originally proposed for the atmospheric boundary layer and apply it to the ocean surface boundary layer. By assuming a probability distribution function relationship between the vertical velocity and tracers, all second‐order and higher‐order moments are exactly constructed and turbulence closure is achieved in the ADC scheme. In addition, this ADC parameterization has full energetic constraints and includes non‐diffusive fluxes without the computational cost of a full higher‐order closure scheme. We have tested the ADC scheme against a combination of large eddy simulation (LES), KPP, and k−ɛ $k-\varepsilon $ for surface buoyancy‐driven convective mixing and found that the ADC scheme is robust with different vertical resolutions and compares well to the LES results. Plain Language Summary: The upper ocean (order of few tens of meters depth from the surface) has a substantial influence on our climate and weather systems. Specifically, upper ocean mixing processes play a key role in modulating global heat budget in the ocean and atmosphere by mixing heat deeper into the ocean or warming the atmosphere above. Accurate representation of the effects of these mixing processes on the global climate and in ocean models is crucial for understanding our current and changing climate. However, current mixing schemes used in these models have shown significant biases. We present a new physically‐motivated mixing scheme for the upper ocean inspired by atmospheric mixing schemes. Results show that the proposed mixing scheme can simulate upper ocean mixing efficiently, suggesting its potential use in climate and ocean models to help reduce model biases. Key Points: A new physically‐motivated, PDF‐based parameterization of ocean surface boundary layer turbulence is presentedThe non‐diffusive fluxes are included naturally and the scheme provides a closed set of equations with realizable closure assumptionsThe mixing scheme accurately predicts the effects of convective turbulence across different vertical resolutions [ABSTRACT FROM AUTHOR]

The impact of internal and barotropic tides on the vertical and horizontal temperature structure off the Amazon River was investigated during two highly contrasted seasons (AMJ: April–May–June; ASO: August–September–October) over a 3-year period from 2013 to 2015. Twin regional simulations, with and without tides, were used to highlight the general effect of tides. The findings reveal that tides have a cooling effect on the ocean from the surface (∼ 0.3 ∘ C) to above the thermocline (∼ 1.2 ∘ C), while warming it up below the thermocline (∼ 1.2 ∘ C). The heat budget analysis indicates that the vertical mixing is the dominant process driving temperature variations within the mixed layer, while it is associated with both horizontal and vertical advection to explain temperature variations below. The increased mixing in the simulations including tides is attributed to breaking of internal tides (ITs) on their generation sites over the shelf break and offshore along their propagation pathways. Over the shelf, mixing is driven by the dissipation of the barotropic tides. In addition, the vertical terms of the heat budget equation exhibit wavelength patterns typical of mode-1 IT. The study highlights the key role of tides and particularly how IT-related vertical mixing shapes the ocean temperature off the Amazon. Furthermore, we found that tides impact the interactions between the upper ocean interface and the overlying atmosphere. They contribute significantly to increasing the net heat flux between the atmosphere and the ocean, with a notable seasonal variation from 33.2 % in AMJ to 7.4 % in ASO seasons. This emphasizes the critical role of tidal dynamics in understanding regional-scale climate. [ABSTRACT FROM AUTHOR]

We quantify the volume transport and watermass transformation rates of the global overturning circulation using the Estimating the Circulation and Climate of the Ocean version 4 release 4 (ECCOv4r4) reanalysis product. The ECCO solution shows large rates of intercell exchange between the mid‐depth and abyssal cells, consistent with other recent inferences. About 10 Sv of North Atlantic deep water enters the abyssal cell in the Southern Ocean and is balanced by a similar amount of apparrent diapycnal upwelling in the Indo‐Pacific. However, much of the upwelling in ECCO's deep ocean is not associated with irreversible watermass transformations, as typically assumed in theoretical models. Instead, a dominant portion of the abyssal circulation in ECCO is associated with isopycnal volume tendencies, reflecting a deep ocean in a state of change and a circulation in which transient tendencies play a leading role in the watermass budget. These volume tendencies are particularly prominent in the Indo‐Pacific, where ECCO depicts a cooling and densifying deep ocean with relatively little mixing‐driven upwelling, in disagreement with recent observations of deep Indo‐Pacific warming trends. Although abyssal ocean observations are insufficient to exclude the trends modeled by ECCO, we note that ECCO's parameterized diapycnal mixing in the abyssal ocean is much smaller than observational studies suggest and may lead to an under‐representation of Antarctic Bottom Water consumption in the abyssal ocean. Whether or not ECCO's tendencies are realistic, they are a key part of its abyssal circulation and hence need to be taken into consideration when interpreting the ECCO solution. Plain Language Summary: We analyze results taken from the Estimating the Circulation and Climate of the Ocean (ECCO) state estimate in order to investigate the internal structure and watermass budget of the global ocean's large‐scale circulation. The ECCO solution supports the modern view of an interconnected global ocean with substantial exchange between the overturning circulation of the Atlantic and that of the Indo‐Pacific via the Southern Ocean. However, our investigation also reveals that the density structure of much of the deep ocean in the ECCO product is in a state of change, and that these changes play a key role in the watermass budget of the circulation. These results reveal disagreement between the model's representation of the deep ocean and the prevailing theoretical depictions of the ocean's large‐scale circulation, which generally assume that the circulation is in a steady state. Disagreement between ECCO's deep ocean mixing rates and independent estimates indicate that the trends in ECCO may be biased, but deep ocean observations are insufficient to conclusively infer the true trends. Key Points: The meridional overturning circulation (MOC) in Estimating the Circulation and Climate of the Ocean version 4 release 4 (ECCOv4r4) exhibits substantial linkage between the mid‐depth and abyssal cellsTransient isopycnal volume change plays a key role in the watermass budget of the MOC in ECCOECCO's transient interior state must be taken into account when interpreting its climatological state [ABSTRACT FROM AUTHOR]

In coastal regions and marginal bodies of water, the increase in partial pressure of carbon dioxide (pCO2) in many instances is greater than that of the open ocean due to terrestrial (river, estuarine, and wetland) influences, decreasing buffering capacity and/or increasing water temperatures. Coastal oceans receive freshwater from rivers and groundwater as well as terrestrial-derived organic matter, both of which have a direct influence on coastal carbonate chemistry. The objective of this research is to determine if coastal marshes in Georgia, USA, may be "hot-spots" for acidification due to enhanced inorganic carbon sources and if there is terrestrial influence on offshore acidification in the South Atlantic Bight (SAB). The results of this study show that dissolved inorganic carbon (DIC) and total alkalinity (TA) are elevated in the marshes compared to predictions from conservative mixing of the freshwater and oceanic end-members, with accompanying pH around 7.2 to 7.6 within the marshes and aragonite saturation states (ΩAr) <1. In the marshes, there is a strong relationship between the terrestrial/estuarine-derived organic and inorganic carbon and acidification. Comparisons of pH, TA, and DIC to terrestrial organic material markers, however, show that there is little influence of terrestrial-derived organic matter on shelf acidification during this period in 2014. In addition, ΩAr increases rapidly offshore, especially in drier months (July). River stream flow during 2014 was anomalously low compared to climatological means; therefore, offshore influences from terrestrial carbon could also be decreased. The SAB shelf may not be strongly influenced by terrestrial inputs to acidification during drier than normal periods; conversely, shelf waters that are well-buffered against acidification may not play a significant role in mitigating acidification within the Georgia marshes. [ABSTRACT FROM AUTHOR]

The near-equatorial ocean experiences particular dynamics because the Coriolis force is weak. One modelled effect of these dynamics is strong reduction of turbulent mixing in the ocean interior. Unknowns are effects on internal wave breaking and associated turbulent mixing above steeply sloping topography. In this paper, high-resolution temperature observations are analyzed from sensors that were moored near the top of a deep Ceará Basin seamount for one week. A vertical string held sensors between 0.4 and 56.4 meters above the seafloor. The observations show common semidiurnal-periodic internal wave breaking, with tidal- and 56-m mean turbulence values that are not significantly different from those observed near the top of 1000-m shallower mid-latitude Great Meteor Seamount, despite the twice lower vertical density stratification. Profiles of 6-day mean turbulence values yield vertically uniform values except for a small decrease in the lower 2 m above the seafloor. The lower 2-m show a distinct departure from turbulent inertial subrange in temperature variance spectra. In 10-m higher-up, spectral slopes indicate dominant turbulent convection with reduced flow and turbulence, except when a primary tidal bore is present. Further-up than 15 m, shear dominates (stratified) turbulence. The lack of Coriolis force is not found to be important for internal wave-induced turbulence above steeply sloping topography, except that Kelvin–Helmholtz instabilities seem somewhat less chaotic and more organized roll-up than at mid-latitudes. [ABSTRACT FROM AUTHOR]

Satellite altimetry sea surface height measurements from 1993 to 2022 are used to show the strengthened mode‐1 M2 internal tides in the past 30 years. Two mode‐1 M2 internal tide models M9509 and M1019 are constructed using the data in 1995–2009 and 2010–2019, respectively. The results show that the global mean M2 internal tides strengthened by 6% in energy. However, the internal tide strengthening is spatially inhomogeneous. Significantly strengthened internal tides are observed in a number of regions including the Aleutian Ridge and the Madagascar‐Mascarene region. Weakened internal tides are observed in the central Pacific. On global average, M1019 leads M9509 by about 10° (20 min in time), suggesting that the propagation speed of M2 internal tides increased. M9509 and M1019 are evaluated using independent altimetry data. The results show that M9509 and M1019 perform better for the data in 1993–1994 and 2020–2022, respectively. Plain Language Summary: It is not surprising that ocean stratification increases with global ocean warming, because the upper ocean stores more extra heat and becomes lighter. Given that ocean stratification becomes stronger, it is naturally expected that internal tides should strengthen with global ocean warming as well. However, it is extremely difficult to quantify internal tide strengthening on a global scale. Here we show for the first time that mode‐1 M2 internal tides strengthened in the past 30 years using satellite altimetry data from 1993 to 2022. We further show that the internal tide strengthening is spatially inhomogeneous, which has important implications on the downward heat transport. The strengthened internal tides and eventual ocean mixing may facilitate the vertical transport of tracers and nutrients, compensating partly for the reduction caused by strengthened ocean stratification. The results suggest that the long‐term changes should be taken into account in making internal tide correction for the Surface Water and Ocean Topography (SWOT) mission. Key Points: Mode‐1 M2 internal tide models M9509 and M1019 are constructed using satellite altimetry data in 1995–2009 and 2010–2019, respectivelyMode‐1 M2 internal tides strengthened and their speeds increased; however, the internal tide strengthening is spatially inhomogeneousM9509 (M1019) performs better in making internal tide correction to independent satellite altimetry data in 1993–1994 (2020–2022) [ABSTRACT FROM AUTHOR]

The ocean is the main heat reservoir in Earth's climate system, absorbing most of the top-of-the-atmosphere excess radiation. As the climate warms, anomalously warm and fresh ocean waters in the densest layers formed near Antarctica spread northward through the abyssal ocean, while successions of warming and cooling events are seen in the deep-ocean layers formed near Greenland. The abyssal warming and freshening expands the ocean volume and raises sea level. While temperature and salinity characteristics and large-scale circulation of upper 2000 m ocean waters are well monitored, the present ocean observing network is limited by sparse sampling of the deep ocean below 2000 m. Recently developed autonomous robotic platforms, Deep Argo floats, collect profiles from the surface to the seafloor. These instruments supplement satellite, Core Argo float, and ship-based observations to measure heat and freshwater content in the full ocean volume and close the sea level budget. Here, the value of Deep Argo and planned strategy to implement the global array are described. Additional objectives of Deep Argo may include dissolved oxygen measurements, and testing of ocean mixing and optical scattering sensors. The development of an emerging ocean bathymetry dataset using Deep Argo measurements is also described. [ABSTRACT FROM AUTHOR]

The inversion of remote sensing signatures of internal solitary waves (ISWs) can retrieve dynamic characteristics in the ocean interior. However, the presence of ubiquitous large‐amplitude ISWs poses challenges to the commonly used weakly nonlinear methods for parameter retrieval. Through laboratory experiments, we establish a relationship between surface features and internal characteristics of ISWs by the remote sensing imaging mechanism. The results demonstrate that strong nonlinearity significantly influences the retrieval of ISWs, primarily manifested in the calculation of wave‐induced velocities and the applicability of ISW solutions. A fully nonlinear model Dubreil–Jacotin–Long equation is used in the retrieval and has been tested under different conditions. Mooring observations indicate that the determination of ISW parameters from satellite images is affected by the complexity of in situ stratification, but additional remote sensing information such as surface velocities enables us to perform retrievals even if the real‐time measurement of pycnocline depth is not available. Plain Language Summary: Internal solitary waves (ISWs), as nonlinear internal waves, play an essential role in oceanic human activities and ocean mixing. The surface current induced by ISWs can create rough and smooth regions on the sea surface due to the modulated roughness, hence presenting alternating bright and dark stripes in satellite images. Satellites can observe ISWs over a wide range via surface manifestations, and the internal dynamics can be calculated from surface features using retrieval methods. However, the availability of retrieval methods still needs to be verified, facing the difficulty of matching mooring observations and satellite images of the same ISW in a short time interval. According to the proportional relation of remote sensing signatures and wave‐induced velocities, this study establishes the relationship between surface features and internal characteristics of ISWs in laboratory experiments. Strong nonlinearity significantly influences the retrieval of ISWs and a fully nonlinear model is well applied in retrieval. Then we test the retrieval in oceanic environments, mooring observations show the critical role of stratification in retrieval. This work provides a reliable dynamics model for the inversion of remote sensing signatures of ISWs into characteristics in the ocean interior. Key Points: The relationship between surface features and internal parameters of internal solitary waves is established in laboratory experimentsStrong nonlinearity significantly impacts the determination of wave parameters from the surface. A fully nonlinear model is well appliedAccurate parameter determination is constrained by the complex oceanic stratification, but more remote sensing information can overcome it [ABSTRACT FROM AUTHOR]

A semi-analytical solution to an advection–diffusion equation is coupled with a nonlinear wavemaker model to investigate the effect of strong nonlinearity on wave-induced mixing. The comparisons with weakly nonlinear model predictions reveal that in the case of waves of higher steepness, enhanced mixing affects the subsurface layer of the water column. A fully nonlinear model captures the neglected higher-order terms from a weakly nonlinear solution and provides a reliable estimation of the time-mean velocity field. The corrected wave-induced mass-transport velocity leads to improved estimates of subsurface mixing intensity and ocean surface temperature. [ABSTRACT FROM AUTHOR]

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

We study the spectral energy transfer due to wave–triad interactions in the Garrett–Munk spectrum of internal gravity waves based on a numerical evaluation of the collision integral in the wave kinetic equation. Our numerical evaluation builds on the reduction of the collision integral on the resonant manifold for a horizontally isotropic spectrum. We evaluate directly the downscale energy flux available for ocean mixing, whose value is in close agreement with the finescale parameterization. We further decompose the energy transfer into contributions from different mechanisms, including local interactions and three types of non-local interactions, namely parametric subharmonic instability, elastic scattering (ES) and induced diffusion (ID). Through analysis on the role of each mechanism, we resolve two long-standing paradoxes regarding the mechanism for forward cascade in frequency and zero ID flux for the GM76 spectrum. In addition, our analysis estimates the contribution of each mechanism to the energy transfer in each spectral direction, and reveals new understanding of the importance of local interactions and ES in the energy transfer. [ABSTRACT FROM AUTHOR]

The tides are a major driver of global oceanic mixing. While global tidal elevations are very well observed by satellite altimetry, the global tidal transports are much less well known. For twenty years, magnetic signals induced by the ocean tides have been detectable in satellite magnetometer observations, such as Swarm or CHAMP. Here, we demonstrate how satellite magnetometer observations can be used to directly derive global ocean tidal transports. As an advantage over other tidal transport estimates, our tidal estimates base on very few and very loose constraints from numerical forward models. [ABSTRACT FROM AUTHOR]

The influencing factors of the long-term stagnation of the Baiu front that induced heavy rainfall in July 2020 over Kyushu, Japan, were examined. In July 2020, the position of the Baiu front from a weather chart at 130° E remained stationary over Kyushu for 20 days, and that was the longest from 2002 to 2021. By examining a certain index of the Yellow Sea High (YSH), which is a necessary condition for Baiu front stagnation near Kyushu, it was confirmed that in 2020, a positive high-pressure anomaly persisted over the Yellow Sea until the end of July. The YSH maintenance was due to the significant negative sea surface temperature (SST) anomaly over the Yellow Sea. The significant negative SST anomalies in the Yellow Sea were due to vertical mixing in ocean mixed layer in response to the strong winds of the extratropical cyclones. A rapid SST decrease ranging from 0.5 to 1.5 °C/day was repeatedly occurred by ocean mixed layer deepening in association with the passage of the extratropical cyclones. The anomalous long-term stagnation of YSH due to the negative SST anomalies could have caused the Baiu front to remain stagnant over Kyushu until the end of July. [ABSTRACT FROM AUTHOR]

Neodymium (Nd) isotope ratios are potentially valuable for paleo‐ocean circulation reconstructions because they trace water‐mass transport and mixing in the present‐day oceans. Moreover, the Atlantic and Pacific global end‐member Nd‐isotope values can be constrained through time, and at in‐between sites their proportions vary with past climate changes. However, an important source of uncertainty about Nd‐isotopes' applicability for paleo‐circulation studies arises from the inability to constrain past Nd‐concentrations of the end‐members. Here we address this "paleo‐[Nd] problem" through a Bayesian analysis. Results show that even large variability in end‐member Nd‐concentrations over the Pleistocene is unlikely to significantly impact their concentration ratio, therefore end‐member concentration changes likely have only small impacts on Nd‐isotope ratios at in‐between locations. The results support their applicability to reconstruct past Atlantic paleo‐circulation. In addition, a Nd‐isotope mass balance for Antarctic Bottom Water shows that its present‐day Nd‐isotope values are consistent with the input from its sources. Plain Language Summary: Ocean circulation transfers heat around the Earth and characterizing its past variability is critical for understanding climate changes. Neodymium (Nd) isotopes are often used to trace past ocean circulation because they mimic deep ocean water mass mixing, and the North Atlantic and North Pacific global ocean mixing end‐member water mass compositions can be constrained through time. Therefore, temporal changes in Nd‐isotope ratios at intermediate locations like the Equatorial and South Atlantic are assumed to approximate changes in the proportions contributed by the global end‐members and thus reflect ocean circulation changes. However, Nd‐isotope ratios at intermediate locations are also sensitive to changes in Nd‐concentrations, which cannot be constrained in the global ocean end‐member water masses. This has caused the applicability of Nd‐isotope ratios for tracing paleo‐ocean circulation to be questioned. Our study examines the sensitivity of Nd‐isotope ratios at locations in‐between the global end‐members to changes in their Nd‐concentrations over recent interglacial‐glacial cycles. Results show that even substantial changes in their Nd‐concentrations likely have little impact on the Nd‐isotope ratios at intermediate locations, indicating that given the right intermediate location choices, Nd‐isotope ratios indeed reflect past deep ocean mixing, thus supporting their use to reconstruct past ocean circulation. Key Points: Neodymium (Nd) isotopes trace modern water mass mixing, but unconstrained end‐member Nd concentrations limit their paleocirculation useA Bayesian approach tests effects of Nd‐concentration changes in global water mass end‐members on Nd‐isotopes at intermediate locationsResults predict small impacts on Nd‐isotopes and constrain projected uncertainties in past ocean end‐member mixing reconstructions [ABSTRACT FROM AUTHOR]

Ocean surface waves induced by wind forcing and topographic effects are a crucial physical process at the air–sea interface, which significantly affect typhoon development, ocean mixing, etc. Higher-resolution wave modeling can simulate more accurate wave states but requires a huge number of computational resources, making it difficult for Earth system models to include ocean waves as a fast-response physical process. Given that high-resolution Earth system models are in demand, efficient high-precision wave simulation is necessary and urgent. Based on the wave dispersion relation, we design a new wave modeling framework using a multiscale grid system. It has the fewest number of fine grids and reasonable grid spacing in deep-water areas. We compare the performance of wave simulation using different spatial propagation schemes, reveal the different reasons for wave simulation differences in the westerly zone and the active tropical cyclone region, and quantify the matching of spatial resolutions between wave models and wind forcing. A series of numerical experiments show that this new modeling framework can more precisely simulate wave states in shallow-water areas without losing accuracy in the deep ocean while costing a fraction of the price of traditional simulations with uniform fine-gridding space. With affordable computational expenses, the new ocean surface wave modeling can be implemented into high-resolution Earth system models, which may significantly improve the simulation of the atmospheric planetary boundary layer and upper-ocean mixing. [ABSTRACT FROM AUTHOR]

Intense tropical cyclones (TCs) can cause catastrophic damage to coastal regions after landfall. Recent studies have linked the devastation associated with TCs to climate change, which induces favorable conditions, such as increasing sea-surface temperature, to supercharge storms. Meanwhile, environmental factors, such as atmospheric aerosols, also impact the development and intensity of TCs, but their effects remain poorly understood, particularly coupled with ocean dynamics. Here, we quantitatively assess the aerosol microphysical effects and aerosol-modified ocean feedbacks during Hurricane Katrina using a cloud-resolving atmosphere–ocean coupled model: Weather Research and Forecasting (WRF) in conjunction with the Regional Ocean Model System (ROMS). Our model simulations reveal that an enhanced storm destructive power, as reflected by larger integrated kinetic energy, heavier precipitation, and higher sea-level rise, is linked to the combined effects of aerosols and ocean feedbacks. These effects further result in an expansion of the storm circulation with a reduced intensity because of a decreasing moist static energy supply and enhancing vorticity Rossby wave outward propagation. Both accumulated precipitation and storm surge are enhanced during the mature stage of the TC with elevated aerosol concentrations, implying exacerbated flood damage over the polluted coastal region. The ocean feedback following the aerosol microphysical effects tends to mitigate the vertical mixing cooling in the ocean mixing layer and offsets the aerosol-induced storm weakening by enhancing cloud and precipitation near the eyewall region. Our results highlight the importance of accounting for the effects of aerosol microphysics and ocean-coupling feedbacks to improve the forecast of TC destructiveness, particularly near the heavily polluted coastal regions along the Gulf of Mexico. [ABSTRACT FROM AUTHOR]

Studies of the Last Glacial Maximum (LGM; 26.5–19 ka) tides showed strong enhancements in open ocean tidal amplitudes and dissipation rates; however, changes prior to the LGM remain largely unexplored. Using two different ice sheet and sea level reconstructions, we explicitly simulate the evolution of the leading semi‐diurnal and diurnal tidal constituents (M2, S2, K1, and O1) over the last glacial cycle with a global tide model. Both sets of simulations show that global changes, dominated by the Atlantic, take place for the semi‐diurnal constituents, while changes for the diurnal constituents are mainly regional. Irrespective of the reconstruction, open ocean dissipation peaks during the sea level lowstands of MIS 2 (∼20 ka) and MIS 4 (∼60 ka), although dissipation values prior to MIS 2 are sensitive to differences in reconstructed ice sheet extent. Using the statistically significant relationship between global mean sea level and dissipation, we apply regression analysis to infer open ocean and shelf dissipation, respectively, over the last four glacial cycles back to 430 ka. Our analysis shows that open ocean tidal energy was probably increased for most of this period, peaking during glacial maxima, and returning to near‐present‐day values during interglacials. Due to tidal resonance during glacial phases, small changes in bathymetry could have caused large changes in tidal amplitudes and dissipation, emphasizing the need for accurate ice margin reconstructions. During glacial phases, once global mean sea level decreased by more than ∼100 m, the amount of open ocean tidal energy available for ocean mixing approximately doubled. Key Points: Tides over the last glacial cycle are explicitly modeled, and tidal dissipation ranging back to 430 ka is inferred with regression analysisEnhanced open ocean dissipation (1.8–2.5 × 5 present day) occurred during glacial maxima, and near‐present‐day values during interglacialsPeak glacial M2 tidal dynamics are very sensitive to changes in ice sheet extent, which may influence ocean mixing and glacial climate [ABSTRACT FROM AUTHOR]

The Piacenzian stage (3.6–2.6 Ma) of the Pliocene is the most recent period where Earth experienced sustained intervals of global warmth analogous to predicted near-future climates. Despite considerable efforts to characterize and understand the climate dynamics of the Piacenzian, the deep ocean and its response to this warming remain poorly understood. Here we present new mid-Piacenzian Mg/Ca and Δ47 ("clumped isotope") temperatures from the deep Pacific and North Atlantic oceans. These records cover the transition from Marine Isotope Stage (MIS) M2 – considered the most pronounced "glacial" stage of the Pliocene prior to the intensification of Northern Hemisphere glaciation – to the warm KM5 interglacial. We find that a large (> 4 ∘ C) temperature gradient existed between these two basins throughout that interval, with the deep North Atlantic considerably warmer and likely saltier than at present. We interpret our results to indicate that the deep Pacific and North Atlantic oceans were bathed by water masses with very different physical properties during the mid-Piacenzian, and that only a limited deep oceanic exchange occurred between the two basins. Our results point to a fundamentally different mode of ocean circulation or mixing compared to the present, where heat and salt are distributed from the North Atlantic into the Pacific. The amplitude of cooling observed at both sites during MIS M2 suggests that changes in benthic δ18 O associated with this cold stage were mostly driven by temperature change in the deep ocean rather than by ice volume. [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 250m 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 com pensating effects of temperature and salinity transports on buoyancy. Changes in surface temperature drive spatial variations of 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]

Earth system models indicate that the Subarctic Atlantic Ocean ecosystems are uniquely sensitive to global warming. Nutrients, phytoplankton biomass, and phytoplankton production all decline precipitously in global warming scenarios. Superimposed on this forced response is changing internal variability that is not fully understood. Here, a large ensemble of simulations with the Community Earth System Model is used to quantify how global warming affects the interannual and multidecadal variability of phytoplankton production integrated over the Subarctic Atlantic. Surprisingly, it is found that this variability increases non‐monotonically with global warming. The increased variability of production is caused by an elevated volatility of wintertime surface nutrient concentrations, which is a consequence of a rising sensitivity of these nutrients to winter mixing and overturning fluctuations, which overcomes a reduced amplitude of these physical fluctuations with warming. Future work is needed to fully understand how internal climate variability impacts ocean ecosystems in a warming climate. Plain Language Summary: Marine ecosystems are sensitive to natural interannual to decadal climate variability. For example, fishery yields wax and wane and populations shift with well known climate oscillations like El Niño. However, with the intensification of global warming and other anthropogenic impacts on marine ecosystems, it is necessary to investigate how climate‐driven variability of marine ecosystems is changing. This study uses global Earth system simulations of twenty‐first‐century global warming scenarios to quantify how climate‐driven fluctuations in North Atlantic Ocean ecosystems might change if atmospheric greenhouse gas emissions and global warming continue. The study reveals that global warming intensifies the ecological and biogeochemical volatility at the regionally integrated scale of the Subarctic Atlantic Ocean due to a rising sensitivity to climate variability mediated by ocean circulation and mixing. Key Points: Global warming reduces integrated Subarctic Atlantic biological production while increasing its interannual and multidecadal variabilityProduction variability increases because wintertime surface nutrient variability increasesNutrient variability increases because it becomes more sensitive to variability in meridional overturning and winter mixing [ABSTRACT FROM AUTHOR]

Antarctic sea ice is mostly seasonal. While changes in sea ice seasonality have been observed in recent decades, the lack of process understanding remains a key challenge to interpret these changes. To address this knowledge gap, we investigate the processes driving the ice season onset, known as sea ice advance, using remote sensing and in situ observations. Here, we find that seawater freezing predominantly drives advance in the inner seasonal ice zone. By contrast, in an outer band a few degrees wide, advance is due to the import of drifting ice into warmer waters. We show that advance dates are strongly related to the heat stored in the summer ocean mixed layer. This heat is controlled by the timing of sea ice retreat, explaining the tight link between retreat and advance dates. Such a thermodynamic linkage strongly constrains the climatology and interannual variations, albeit with less influence on the latter. Processes controlling the onset of the Antarctic sea ice season remain unclear. Here, analyses of observations show that ocean solar energy storage and sea ice drift are key drivers, providing insights to understand variations in sea ice season duration. [ABSTRACT FROM AUTHOR]

Climate shifts are particularly conspicuous in glaciated areas. Satellite and terrestrial observations show significant increases in the melting and breakup of tidewater glaciers and their influence on sea level rise and ocean mixing. Increasing melt rates are creating an urgency to better understand the link between atmospheric and oceanic conditions and glacier frontal ablation through iceberg calving and melting. Elucidating this link requires a combination of short- and long-timescale measurements of terminus activity. Recent work has demonstrated the potential of using underwater sound to quantify the time and scale of calving events to yield integrated estimates of ice mass loss. Here, we present estimates of subaerial calving flux using underwater sound recorded at Hansbreen, Svalbard, in September 2013 combined with an algorithm for the automatic detection of calving events. The method is compared with ice calving volumes estimated from geodetic measurements of the movement of the glacier terminus and an analysis of satellite images. The total volume of above-water calving during the 26 d of acoustical observation is estimated to be 1.7±0.7×107 m 3 , whereas the subaerial calving flux estimated by traditional methods is 7±2×106 m 3. The results suggest that passive cryoacoustics is a viable technique for long-term monitoring of mass loss from marine-terminating glaciers. [ABSTRACT FROM AUTHOR]

Vertical mixing parameterizations in ocean models are formulated on the basis of the physical principles that govern turbulent mixing. However, many parameterizations include ad hoc components that are not well constrained by theory or data. One such component is the eddy diffusivity model, where vertical turbulent fluxes of a quantity are parameterized from a variable eddy diffusion coefficient and the mean vertical gradient of the quantity. In this work, we improve a parameterization of vertical mixing in the ocean surface boundary layer by enhancing its eddy diffusivity model using data‐driven methods, specifically neural networks. The neural networks are designed to take extrinsic and intrinsic forcing parameters as input to predict the eddy diffusivity profile and are trained using output data from a second moment closure turbulent mixing scheme. The modified vertical mixing scheme predicts the eddy diffusivity profile through online inference of neural networks and maintains the conservation principles of the standard ocean model equations, which is particularly important for its targeted use in climate simulations. We describe the development and stable implementation of neural networks in an ocean general circulation model and demonstrate that the enhanced scheme outperforms its predecessor by reducing biases in the mixed‐layer depth and upper ocean stratification. Our results demonstrate the potential for data‐driven physics‐aware parameterizations to improve global climate models. Plain Language Summary: The upper region of the ocean is highly energetic and is responsible for transferring mass, energy and biogeochemical tracers between the atmosphere and the deeper regions of the ocean. This transport takes place because of turbulent swirling motions, which are found to be of varying sizes. Climate models cannot represent all of these motions because smaller‐scale swirls are complex and require additional computational resources. As we cannot neglect those small swirls, we try to approximate their effects on larger‐scale motions using mathematical models. These models have a few ad hoc or empirical assumptions that lead to uncertainty when these climate models are used to project the future climate. To reduce this uncertainty, we augment an existing model of turbulent swirling process with machine learning, which replaces some ad hoc approximations with data‐driven neural networks. Neural networks can learn those missing processes more accurately than a traditional physics‐based model. The neural networks are shown to improve physics in climate simulations. Although we only touch on one component in an ocean climate model, this approach can be replicated to improve any other component that was using ad hoc assumptions and replace them with data‐driven models using techniques from machine learning. Key Points: We improve a parameterization of vertical mixing in the ocean surface boundary layer using neural networksNeural networks are trained to predict the diffusivity of second moment closure and maintain energetic constraints of the original parameterizationThe improved scheme reduces biases of mixed layer depth and thermocline in an atmospherically forced ocean model [ABSTRACT FROM AUTHOR]

To determine the emplacement age, petrogenesis, and geodynamic setting of the Xizhelimu diorite in Keyouzhongqi, Inner Mongolia of northeastern China, a detailed study of the petrography, geochronology, and whole-rock geochemistry has been conducted. Geological and petrographic studies show that the Xizhelimu diorite is zoned: the central lithofacies zone is composed of medium-fine-grained monzodiorite and quartz diorite, and the marginal lithofacies zone is fine-grained diorite. The zircon U–Pb dating results show that the ages of the central and marginal facies are 133.5 ± 1.9 and 133.4 ± 1.4 Ma, respectively. The whole-rock rare earth and trace element characteristics of the Xizhelimu diorite show an O-type adakite affinity. Combining the analysis of zircon Hf isotope composition (εHf(t) values of +7.7 to +10.0), the geochemical features of whole rock, and the results of partial melting modeling, we suggest that the parental magma of the Xizhelimu diorite was derived from the partial melting of altered oceanic crust mixing with subducting sediments at shallow depths. In the early stage of early Cretaceous, the Xizhelimu diorite originated in an extensional setting, mainly related to the closure of the western part of the Mongol–Okhotsk Ocean. The upwelling asthenospheric flow in this extensional setting induced partial melting of the paleo-oceanic crust to form the parental magma of the Xizhelimu diorite. [ABSTRACT FROM AUTHOR]

Direct observations of turbulent mixing in energetic ocean currents are fundamentally important to improving the understanding of stratified turbulence and the predictive capability of ocean models. In this study, we have selected an energetic region of the Western Indian Ocean to make simultaneous measurements of temperature and velocity to directly infer the geography of turbulent mixing on the east coast of South Africa. Direct measurements of the rate of turbulent kinetic energy dissipation, ϵ , were recorded by an autonomous underwater vehicle glider platform and vertical microstructure profiler (VMP). The glider was deployed in the Agulhas current in 2015 over a 200 km transect and measured approximately 250 vertical profiles of turbulent microstructure. Additional vertical profiles near the shelf were obtained from approximately 85 VMP casts during 2018 in a region where the powerful Agulhas current is formed and interacts with the shelf. VMP and glider data collected at Sodwana Bay and the Agulhas current showed evidence of elevated dissipation rates 10 - 8 < ϵ < 10 - 6 Wkg - 1 . Our results indicate that in general turbulent mixing in the Agulhas is concentrated spatially in pancake-shaped patches. Ordinary Kriging was used to interpolate between consecutive profiles and showed that the horizontal scale of the turbulent patches was typically O(1 km) while the vertical scales of the patches ranged between 0.01 and 10 m. Article Highlights: Direct measurements of turbulent dissipation in the strong persistent Agulhas current are presented for the first time using sensors fitted on a glider and vertical microstructure profiler. Ordinary Kriging revealed the geography of mixing and that turbulent mixing in the Agulhas is concentrated spatially in pancake-shaped patches. VMP data collected at Sodwana showed evidence of elevated dissipation rates 10 - 8 < ϵ < 10 - 6 Wkg - 1 . [ABSTRACT FROM AUTHOR]

Oceanic general circulation models (OGCMs) are important tools used to investigate mechanisms for ocean climate variability and predict the ocean change in the future. However, in most current ocean models, the impact of sea surface waves as one of the most significant dynamic processes in the upper ocean is absent. In this study, the Stokes drift and the vertical mixing induced by nonbreaking surface waves derived from the wave model (WAVEWATCH III) are incorporated into a Climate System Ocean Model, and their effects on an ocean climate simulation are analyzed. Numerical experiments show that both physical processes can improve the simulation of sea surface temperature (SST) and mixed layer depth (MLD) in the Southern Hemisphere. The introduction of Stokes drift effectively reduces the subsurface warm bias in the equatorial tropics, which is caused by the weakening of vertical mixing in the equatorial region. The nonbreaking surface wave mainly reduces the temperature bias in the Southern Ocean by enhancing mixing in the upper ocean. For the MLD, the Stokes drift mainly improves the simulation of the winter MLD, and the nonbreaking surface wave improves the summer MLD. For MLD south of 40° S in summer, the introduction of nonbreaking surface waves resulted in a reduction of 11.86 m in MLD bias and 7.8 m in root mean square errors (RMSEs), respectively. For winter subtropical MLD in the Southern Hemisphere, considering the Stokes drift, the MLD bias and RMSEs were reduced by 2.49 and 5.39 m, respectively. Adding these two physical processes simultaneously provides the best simulation performance for the structure of the upper layer. The introduction of sea surface waves effectively modulates the vertical mixing of the upper ocean and then improves the simulation of the MLD. Thus, sea surface waves are very important for ocean simulation, so we will further couple a sea waves model in the Chinese Academy of Sciences Earth System Model (CAS-ESM) as part of their default model component. [ABSTRACT FROM AUTHOR]

In recent years, the Arctic Ocean has experienced a dramatic decrease in sea ice cover, leading to enhanced turbulent mixing and eddy activity. Due to the scarcity of high‐resolution observations, our understanding of turbulent mixing in the Arctic is incomplete. Using three seismic reflection transects with high spatial resolution (∼O (10) m) and concurrent ADCP‐derived current velocity, this study identified 11 mesoscale eddies on the ice‐free Chukchi Borderlands of the western Arctic Ocean. 82% of them are intra‐halocline anticyclones. The total length of horizontal scales of these 11 eddies occupies ∼40% of the total length of three seismic lines. Diapycnal diffusivities estimated from seismic data were enhanced and can be up to 10−4 m2 s−1 at the edges of these eddies, compared with the average values (∼10−6 m2 s−1) at the Chukchi Borderlands. Seismic‐estimated diffusivities were validated by fine‐scale parameterization using ADCP‐derived velocity data and historical hydrographic data. Enhanced diffusivities at the edges of eddies may be attributed to shear instabilities at the top and bottom edges and to submesoscale motions at the lateral edges of these eddies. We highlight the enhanced mixing at the edges of eddies in the halocline can increase the upward heat flux. This upward heat flux can transfer the heat of the warm Atlantic water below to the surface, which may further promote the melting of surface sea ice. Plain Language Summary: Turbulence is widely present in the ocean and is crucial for maintaining ocean mixing. In the past, due to sea ice cover, the Arctic Ocean was considered a region of weak turbulent mixing. However, in recent years, the sea ice in the Arctic Ocean has gradually decreased, and the Arctic Ocean is becoming more active. However, due to the lack of high‐resolution observations, our understanding of turbulent mixing in the Arctic Ocean is still insufficient. This paper uses the high‐resolution seismic reflection method, a traditional method in oil exploration, to estimate the turbulent mixing intensity in the western Arctic Ocean. The results indicate that the mixing rate at the edges of the eddies in the halocline is much higher than the background value. The enhanced mixing at the edges of eddies in the halocline can increase the upward heat flux. This upward heat flux can transfer the heat of the warm Atlantic water below to the surface, which may further promote the melting of surface sea ice. Key Points: Seismic and ADCP data reveal 11 mesoscale eddies, mostly intra‐halocline anticyclones, in the western Arctic OceanDiffusivities at the edges of these eddies were generally enhanced and can be up to 10−4 m2 s−1Enhanced diffusivities are attributed to shear instabilities and submesoscale motions [ABSTRACT FROM AUTHOR]

Wind power input to near‐inertial motions is an important energy source for generating diapycnal mixing in the ocean. However, the distribution and long‐term trend of this input over the Southern Ocean have yet to be quantified. In this study, we investigate the near‐inertial wind power input (WPIi) to the Southern Ocean using a global eddy‐permitting coupled ocean‐sea ice model forced by a high‐resolution atmospheric reanalysis product. Our results reveal a zonally asymmetric distribution of WPIi in the Southern Ocean, with the strongest input in the South Indian Ocean and the weakest in the South Pacific. The integrated WPIi between 30°S and 60°S exhibits a significant positive trend over the past four decades due to the intensification of mesoscale weather systems. The surface mixed‐layer depth is found to modulate the spatial pattern and trend of WPIi by altering the surface near‐inertial currents. Plain Language Summary: Wind fluctuations can excite surface ocean currents that oscillate at frequencies close to the inertial frequency. These near‐inertial motions play an important role in generating turbulent mixing in the ocean. However, the distribution and long‐term trend of wind power input to near‐inertial motions in the Southern Ocean remains unquantified. In this study, we investigate this wind power input using a global ocean circulation model driven by a high‐resolution atmospheric reanalysis data set. Our results reveal a pronounced zonal asymmetry in the distribution of near‐inertial wind power input in the Southern Ocean. Furthermore, this power input has increased significantly over the past four decades due to the intensification of mesoscale weather systems. Results from this study have important implications for understanding Southern Ocean mixing and circulation. Key Points: Distribution of near‐inertial wind power input in the Southern Ocean is zonally asymmetricNear‐inertial wind power input in the Southern Ocean has increased significantly in recent decadesThis increase is primarily driven by the intensification of mesoscale weather systems [ABSTRACT FROM AUTHOR]