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

Connecting the Bohai Sea (BS) and Yellow Sea (YS), the Bohai Strait is vital in controlling physical‐biogeochemical conditions in the East Asian marginal seas. In August 2019, Typhoon Lekima moved northward inland into eastern China and caused drastic water exchange through the Bohai Strait. The specific variation and underlying dynamics involved were investigated with observations and high‐resolution numerical simulations. Volume transport featured an intense inflow followed by an intense outflow, with a fluctuation magnitude of 1.88 Sv, which was much greater than the seasonal‐interannual variations. The corresponding current patterns presented three phases: strait‐wide inflow, strait‐wide outflow, and outflow in the south and inflow in the north. The strait‐wide inflow was driven by strong east‐northeasterly winds through local Ekman dynamics. The strait‐wide outflow resulted from the dynamic adjustment of ocean currents accompanying the rapid weakening of easterly winds. The third current pattern was part of a large‐scale cyclonic circulation in the BS and North YS and was governed by different factors in the southern and northern straits. Further comparisons indicated that transport caused by winter storms and other typhoons in the year had opposite phase variations to that caused by Lekima. The underlying dynamics mainly involved local Ekman dynamics and/or remote wind forcing via the propagation of coastal trapped waves but could differ greatly from events. Finally, we found that to a large extent, geostrophic transport derived from the sea surface height difference across the Bohai Strait could be used as actual transport during synoptic weather events. Plain Language Summary: The Bohai Strait is the only passage connecting the Bohai Sea and the Yellow Sea. Water exchange through the Bohai Strait plays a vital role in influencing physical‐biogeochemical conditions in the East Asian marginal seas and thus the socioeconomic development of coastal countries. In August 2019, Typhoon Lekima moved northward inland into eastern China and caused drastic water exchange through the Bohai Strait, which was much stronger than the seasonal‐interannual variations. This is the first time that water exchange variation through the Bohai Strait and underlying dynamics caused by typhoons have been investigated in detail. In addition, the transport fluctuations and underlying dynamics caused by winter storms and other typhoons in the year were compared to provide a comprehensive understanding of this high‐interest research topic. A general conclusion was that during different synoptic weather events, the underlying dynamics mainly involved local Ekman dynamics and/or remote wind forcing via the propagation of coastal trapped waves but could differ significantly from events. Finally, we found that to a large extent, geostrophic transport derived from the sea surface height difference across the Bohai Strait could be used as actual transport during synoptic weather events. Key Points: Typhoon Lekima caused stronger water exchange through the Bohai Strait than winter‐spring storms and seasonal‐interannual variationsCurrent patterns in the Bohai Strait presented three phases controlled by different dynamicsGeostrophic transport derived from the sea level difference across the strait could be used as actual transport during synoptic events [ABSTRACT FROM AUTHOR]

We present a study on the ocean surface currents reconstruction by merging Level-4 (L4, gap-free) altimeter-derived geostrophic currents and satellite sea surface temperature. Building upon past studies on the multi-variate reconstruction of geostrophic currents from satellite observations, we regionalized and optimized an algorithm to improve the altimeter-derived surface circulation estimates in the North Atlantic Ocean. A ten-year-long time series (2010–2019) is presented and validated by means of in situ observations. The newly optimized algorithm allowed us to improve the currents estimate along the main axis of the Gulf Stream and in correspondence of well-known upwelling areas in the North Eastern Atlantic, with percentage improvements of around 15% compared to standard operational altimetry products.

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

The ocean circulation dynamics can be represented as a high‐dimensional multi‐scale nonlinear system with inhomogenous meanfields and topography. Numerical simulations of the ocean dynamics resolve the large scales of motion on a computational grid, with the unresolved subgrid interactions parameterized. These simulations are highly dependent upon the grid resolution, unless the subgrid terms are appropriately parameterized. There are five fundamental classes of subgrid interactions: eddy‐eddy; eddy‐topographic; eddy‐meanfield; meanfield‐meanfield; and meanfield‐topographic. Scale dependent parameterizations representing each of these interaction classes are presented here in oceanic flows for the first time. Subgrid parameterizations are calculated and validated in baroclinic quasi‐geostrophic simulations of idealized Antarctic Circumpolar Current flows with representative mean currents and ocean floor topography. The parameterization coefficients are derived from the coarse grained statistics of high resolution reference simulations in spectral space. Stochastic and deterministic parameterizations are developed for the eddy‐eddy interactions, and deterministic forms for the remaining classes. The kinetic energy spectra and meanfield resulting from large eddy simulations (LES) adopting these coefficients accurately replicate those of the reference simulation. The eddy‐eddy interactions are dominant, but all classes need to be parameterized for best results. For LES where baroclinic instability is explicitly resolved the stochastic variants out‐perform the deterministic ones across all scales. When baroclinic instability is not explicitly resolved, the stochastic variants out‐perform the deterministic cases at the large scales, but introduce some distortions at the smallest resolved scales. This study provides an assessment of the relevant strengths of the subgrid interaction classes in an idealized yet representative ocean. Plain Language Summary: Numerical simulation of oceanic circulations is incredibly challenging due in part to the large range of scales. There are oscillations on the size of entire ocean basins, through to fine scale centimeter sized turbulence, and everything in between. Global simulations of the ocean typically capture the large scale features on a discrete grid. To ensure these simulations are physically representative, the influence of the missing smaller scales needs to be captured accurately. Here we present a first calculation of parameterizations governing the interactions between the resolved dynamics and the missing small scale eddies, meanfield and topographic features. Reduced resolution simulations incorporating these parameterizations without tuning have statistics in very good agreement with the high resolution simulations from which they were derived. Key Points: Subgrid models of all interaction classes between eddies, meanfield and topography are calculated from reference global ocean simulationsInter‐eddy interactions are modeled both stochastically and deterministically, with deterministic models used for the other interactionsCoarse simulations using these subgrid coefficients without tuning are in very good agreement with those of the high‐resolution simulation [ABSTRACT FROM AUTHOR]

The Kuroshio Current in the North Pacific displays path changes on an interannual‐to‐decadal time scale. In an idealized barotropic quasi‐geostrophic model of the double‐gyre wind‐driven circulation under stochastic wind‐stress forcing, such variability can occur due to transitions between different equilibrium states. The high‐dimensionality of the problem makes it challenging to determine the probability of these transitions under the influence of stochastic noise. Here we present a new method to estimate these transition probabilities, using a Dynamical Orthogonal (DO) field approach. In the DO approach, the solution of the stochastic partial differential equations system is decomposed using a Karhunen–Loève expansion and separate problems arise for the ensemble mean state and the so‐called time‐dependent DO modes. The original method is first reformulated in a matrix approach which has much broader application potential to various (geophysical) problems. Using this matrix‐DO approach, we are able to determine transition probabilities in the double‐gyre problem and to identify transition paths between the different states. This analysis also leads to the understanding which conditions are most favorable for transition. Plain Language Summary: In an idealized wind‐driven double‐gyre circulation model of the midlatitude ocean circulation, a northward displaced jet (jet‐up state) and a southward displaced jet (jet‐down state) exist under the same forcing conditions. The existence of these states can help explain behavior of, for example, the Kuroshio Current in the North Pacific, which undergoes path variations on an interannual‐to‐decadal time scale. Under the influence of wind‐stress noise, the model circulation may undergo a transition between the jet‐up state to the jet‐down state and vice versa. Determining transition probabilities between the two states is challenging as the model has many degrees of freedom and transition probabilities are very small when the noise is low. In this paper, we use a new formulation of a model order reduction technique, the so‐called Dynamical Orthogonal (DO) field approach, to determine both transition probabilities and transition paths. Key Points: A matrix version of the Dynamical Orthogonal (DO) field method is presentedDO is used to determine transition probabilities in multi‐stable stochastic dynamical systemsThe method is applied to study transition behavior of ocean western boundary currents [ABSTRACT FROM AUTHOR]

Given the essential impact of mixed-layer temperature (MLT) seasonality in Kuroshio–Oyashio Confluence Region (KOCR) on the variations of climate and marine economic species, the present study investigates causes of seasonal MLT anomaly (MLTA) in KOCR under bimodal states of Kuroshio Extension (KE). The results reveal that KE bimodality plays a modulation role in the seasonal variability of MLTA in KOCR. There is an opposite seasonal variation trend of MLTA under different KE states, and the MLTA amplitude in cold season (winter and spring) is larger than that in warm season (summer and autumn). Such behavior is directly induced by different distribution of climatological MLT and wind field in cold and warm seasons, as well as the opposite anomalous velocity of wind and ocean currents nearby KOCR associated with different KE states through changing sensible and latent heat flux, Ekman and geostrophic advection. The results suggest the dependency of short-term (seasonal) variability of KOCR-MLTA on the long-term (decadal) variations of KE background state. [ABSTRACT FROM AUTHOR]

The contribution that fronts and frontogenesis make to subsurface chlorophyll a (CHL) concentration was investigated near the Kuroshio using data from multiple surveys across the Kuroshio during summer and fall and satellite sea surface height (SSH) data alongside high‐resolution reanalysis data set. An intensive survey was conducted during a period of frontogenesis as seen from the in‐situ and satellite‐derived velocity data as well as the analysis of the reanalysis data set. Physical and biological observation showed shoaling of the nitracline and a subsurface CHL maximum (SCM) near the evolving front. The CHL concentration at the SCM (CHLSCM) was large near the front compared with the concentration in ambient water. Sections of 10 repeat seasonal surveys also captured the frontal structure, the shoaling of the nitracline, and the increase in CHLSCM as well as their interrelationship. The CHLSCM was large, and the depth of the SCM (ZSCM) was shallow above a large horizontal density gradient (i.e., marked mesoscale frontal structure) in the ocean interior. In addition, we demonstrate that the lateral strain rate, which is a measure of frontogenesis obtained from the SSH, can be used to estimate ZSCM and CHLSCM. Frontogenesis, secondary circulation, symmetric instability, and vertical diffusion were considered to cause the nutrient supply to the subsurface layer. The surveys and analysis indicate that fronts and frontogenesis increase CHLSCM, and the physical data obtained from both in‐situ and satellite observations can be used to estimate CHL distribution in the ocean interior. Plain Language Summary: The contribution that oceanic front and its strengthening make to subsurface chlorophyll a (CHL) concentration was investigated near the Kuroshio south of Japan using data from multiple surveys and satellite altimetry alongside high‐resolution reanalysis data set. The Kuroshio front strengthened during an intensive survey across the Kuroshio as seen from in‐situ, satellite‐derived and reanalysis velocity data. Physical and biological observations showed nutrient upwelling and increase in subsurface chlorophyll a concentration near the strengthening front. Sections of 10 repeat surveys during summer and fall showed that the depth where the chlorophyll a concentration increased was shallow and the chlorophyll a concentration was large when the front was strong. In addition, we demonstrate that a measure of strengthening of fronts obtained from the satellite‐derived data is valid to estimate the distribution of subsurface chlorophyll a concentration. Key Points: Frontogenesis supplies nutrients and increases subsurface chlorophyll a concentrationChlorophyll a concentration is positively correlated with the strength of fronts and frontogenesisSubsurface chlorophyll a concentration is estimated using satellite‐derived geostrophic velocity [ABSTRACT FROM AUTHOR]

Residual current analysis of multiple stations' periodic observational data for sea currents, and multiple voyages, multiple seasons from 2018 to 2022, revealed the existence of a strong southwest current zone, 20–30 m underwater and within the coastal current area of Yangjiang, western Guangdong, in the northwest region of the South China Sea. The velocity of the residual currents in the strong–current zone was 38.2–100.3 cm/s. Observational data for wind, sea currents, salinity and tide from multiple coastal stations in the spring (from March 2019 to May 2019) and summer (from June 2019 to August 2019) of 2019 demonstrated the existence of abrupt strong currents in the coastal current sea area of Yangjiang, western Guangdong. Analyses of continuous sequence hydro meteorological data for the Yangjiang area indicated that wind stress was the main factor determining the direction of sea currents; increases in the near-shore water level produce a westward geostrophic current and a wind current in the shallow sea area; their joint effect was the key factor determining the velocity of the coastal current in this sea area. In spring and summer, when Pearl River runoff into the sea reaches a peak, and under the action of the northeast wind, the water level on the west coast of Guangdong rose. This created a barotropic pressure–gradient force from the shore to the outer sea, generating a southwestward geostrophic current in the same direction as the wind-driven current. The joint action of the wind-driven and geostrophic currents then generated a sudden southwestward coastal current. [ABSTRACT FROM AUTHOR]

Over the last 3 decades, satellite altimetry has observed sea surface height variations, providing a regular monitoring of the surface ocean circulation. Altimetry measurements have an intrinsic signal-to-noise ratio that limits the spatial scales of the currents that can be captured. However, the recent progress made on both altimetry sensors and data processing allows us to observe smaller geophysical signals, offering new perspectives in coastal areas where these structures are important. In this methodological study, we assess the ability of three altimeter missions with three different technologies to capture the Northern Current (northwestern Mediterranean Sea) and its variability, namely Jason-2 (Ku-band low-resolution-mode altimeter, launched in 2008), SARAL/AltiKa (Ka-band low-resolution-mode altimeter, launched in 2013) and Sentinel-3A (synthetic aperture radar altimeter, launched in 2016). Therefore, we use a high-resolution regional model as a reference. We focus along the French coast of Provence, where we first show that the model is very close to the observations of high-frequency radars and gliders in terms of surface current estimates. In the model, the Northern Current is observed 15–20 km from the coast on average, with a mean core velocity of 0.39 m s -1. Its signature in terms of sea level consists of a drop whose mean value at 6.14 ∘ E is 6.9 cm, extending over 20 km. These variations show a clear seasonal pattern, but high-frequency signals are also present most of the time. In comparison, in 1 Hz altimetry data, the mean sea level drop associated with the Northern Current is overestimated by 3.0 cm for Jason-2, but this overestimation is significantly less with SARAL/AltiKa and Sentinel-3A (0.3 and 1.4 cm respectively). In terms of corresponding sea level variability, Jason-2 and SARAL altimetry estimates are larger than the model reference (+1.3 and +1 cm respectively), whereas Sentinel-3A shows closer values (-0.4 cm). When we derive geostrophic surface currents from the satellite sea level variations without any data filtering, in comparison to the model, the standard deviations of the velocity values are also very different from one mission to the other (3.7 times too large for Jason-2 but 2.4 and 2.9 times too large for SARAL and Sentinel-3A respectively). When low-pass filtering altimetry sea level data with different cutoff wavelengths, the best agreement between the model and the altimetry distributions of velocity values are obtained with a 60, 30 and 40–50 km cutoff wavelength for Jason-2, SARAL and Sentinel-3A data respectively. This study shows that using a high-resolution model as a reference for altimetry data allows us not only to illustrate how the advances in the performances of altimeters and in the data processing improve the observation of coastal currents but also to quantify the corresponding gain. [ABSTRACT FROM AUTHOR]

A high precision and fine resolution reconstruction of sea surface current is beneficial to the exploration of complicated ocean dynamic processes. Existing studies mainly use satellite sea level and wind stress fields to derive sea surface geostrophic and Ekman currents, and build physical inversion models for global or regional oceans. Despite the obvious success, there are a variety of typical dynamic processes in the ocean such as mesoscale eddies and small-scale waves, and any product of surface current that neglects the contribution of wave motion would be, at best, incomplete. In this context, we present a precise sea surface current product at 15 m depth named GEST (Geostrophic-Ekman-Stokes-Tide) by analyzing the coupling relationship between ocean surface components that correspond to different physical processes and the actual currents as observed by drifting buoys. The GEST was generated based on Ekman, geostrophic currents, and waved-induced Stokes drift and TPXO9 tidal currents. Specifically, in the calculation of Ekman currents, local applicability is taken into account to ensure that the friction layer of wind-driven current can reach where drifters could operate as normal. A comparison proves that combining multi-scale theory and the condition of local applicability improves estimation results by up to 3.6 cm/s compared with OSCAR product, and 0.3 cm/s with GlobCurrent. Furthermore, by comparing the reconstructed products with 1° and 0.25° resolution, we find that the higher resolution not only reveals more details of ocean currents especially mesoscale eddy energy associated with geostrophic currents, but also improves the accuracy by up to 5.62 cm/s. [ABSTRACT FROM AUTHOR]

For over 25 years, satellite altimetry has provided invaluable information about the ocean dynamics at many scales. In particular, gridded Sea Surface Height (SSH) maps allow to estimate the mesoscale geostrophic circulation in the ocean. However, conventional interpolation techniques rely on static optimal interpolation schemes, hence limiting the estimation of non linear dynamics at scales not well sampled by altimetry (i.e. below 150–200 km at mid latitudes). To overcome this limitation in the resolution of small-scale SSH structures (and thus small-scale geostrophic currents), a Back and Forth Nudging algorithm combined with a Quasi-Geostrophic model, a technique called BFN-QG, has been successfully applied on simulated SSH data in Observing System Simulation Experiments (OSSEs), showing a significant reduction in interpolation error and an improvement of space-time resolutions of the experimental gridded product compared to operational products. In this study, we propose to apply the BFN-QG to real altimetric SSH data in a highly turbulent region spanning a part of the Agulhas current. The performances are evaluated within Observing System Experiments (OSEs) that use independent data (such as independent SSH, Sea Surface Temperature and drifter data) as ground-truth. By comparing the mapping performances to the ones obtained by operational products, we show that the BFN-QG improves the mapping of short, energetic mesoscale structures and associated geostrophic currents both in space and time. In particular, the BFN-QG improves (i) the spatial effective resolution of the SSH maps by a factor of 20 %, (ii) the zonal and (especially) the meridional geostrophic currents and (iii) the prediction of Lagrangian transport for lead times up to 10 days. Unlike the results obtained in the OSSEs, the OSEs reveal more contrasting performances in low variability regions that are discussed in the paper. [ABSTRACT FROM AUTHOR]

This study assesses the capability of Surface Quasi-Geostrophy (SQG) to reconstruct the three-dimensional (3D) dynamics in four critical areas of the Arctic Ocean: the Nordic, Barents, East Siberian, and Beaufort Seas. We first reconstruct the upper ocean dynamics from TOPAZ4 reanalysis of sea surface height (SSH), surface buoyancy (SSB), and surface velocities (SSV) and validate the results with the geostrophic and total TOPAZ4 velocities. The reconstruction of upper ocean dynamics using SSH fields is in high agreement with the geostrophic velocities, with correlation coefficients greater than 0.8 for the upper 400 m. SSH reconstructions outperform surface buoyancy reconstructions, even in places near freshwater inputs from river discharges, melting sea ice, and glaciers. Surface buoyancy fails due to the uncorrelation of SSB and subsurface potential vorticity (PV). Reconstruction from surface currents correlates to the total TOPAZ4 velocities with correlation coefficients greater than 0.6 up to 200 m. In the second part, we apply the SQG approach validated with the reanalysis outputs to satellite-derived sea level anomalies and validate the results against in-situ measurements. Due to lower water column stratification, the SQG approach's performance is better in fall and winter than in spring and summer. Our results demonstrate that using surface information from SSH or surface velocities, combined with information on the stratification of the water column, it is possible to effectively reconstruct the upper ocean dynamics in the Arctic and Subarctic Seas up to 400 m. Future remote sensing missions in the Arctic Ocean, such as SWOT, Seastar, WaCM, CIMR, and CRISTAL, will produce enhanced SSH and surface velocity observations, allowing SQG schemes to characterize upper ocean 3D mesoscale dynamics up to 400 m with higher resolutions and lower uncertainties. [ABSTRACT FROM AUTHOR]

An analysis is made of unstable perturbations of a geostrophic current of finite transverse scale with a vertical parabolic velocity profile of a general form (with linear and constant velocity shear) in a vertically bounded layer. The model is based on the potential vortex equation in the quasi-geostrophic approximation taking into account the vertical diffusion of momentum and mass. The equation and boundary conditions are reduced to a spectral eigenvalue problem of the Orr–Sommerfeld type. To search for eigenfunctions and eigenvalues, a high-precision analytic–numerical method was used. Particular attention is paid to the study of unstable perturbations with a phase velocity exceeding the maximum flow velocity. Such instability should be distinguished from baroclinic instability and critical layer instability. It is found that the indicated instability can develop in ocean currents when the problem parameters vary in a wide range of values. It is also obtained that, at an increase in the Prandtl number, the phase velocity of such perturbations increases and can significantly exceed the maximum flow velocity. However, the occurrence of such unstable perturbations is possible only when the maximum flow velocity is located in the inner region of the layer (but not necessarily in its center). It has also been found that narrow currents (the transverse scale is equal to or smaller than the Rossby radius) with a parabolic vertical profile can be unstable. The most unstable perturbations have approximately equal scales along and across the flow; that is, they are circular perturbations. A discussion of various types of geostrophic current instability with a vertical parabolic velocity profile as applied to the ocean is presented. [ABSTRACT FROM AUTHOR]

We study the impact of the Ekman currents and Stokes drift on the surface horizontal transport and mixing properties of the Mediterranean Sea. Finite Size Lyapunov Exponents (FSLE) at the ocean surface are computed over the whole basin using 25 years of satellite altimetry derived geostrophic currents, 10‐m wind velocity and wave fields. We find that the transport pathways unveiled by the geostrophic Lagrangian Coherent Structures (LCS) are significantly modified by the ageostrophic currents (i.e., Ekman and Stokes induced velocities), often leading to a decrease of the retention capacity of the eddies. An exhaustive assessment of the regional and temporal variability of the FSLE shows an increase of the horizontal mixing processes, due to the ageostrophic component, up to 37% in regions such as the Gulf of Lion or the Aegean Sea, during the seasons where wind and waves are intense and persistent. Positive trends in the total FSLE (up to 1.2% of the value of FSLE per year in some regions) suggest that Mediterranean Sea has experienced a significant increase in the intensity of the mixing processes over the last decades. Ageostrophic features are considered to play a role in determining the properties of the relative dispersion. Through the analysis of the Lagrangian Anisotropy Index (LAI) using virtual and real pair of drifters, we observe that the particle dispersion is mainly dominated by the zonal flow, and that the ageostrophic currents induce meridional dispersion, particularly in regions where wind and wave are intensified. Plain Language Summary: The knowledge of surface oceanic transport is crucial to understand the distribution of quantities of physical, biological, and chemical interest in the marine environment. Here, using available data from satellite altimetry and from reanalysis product during 25 years we analyze the impact of the wind and waves on the ocean transport properties. We show that, at the ocean surface, wind and waves modify the transport barriers, with large impact in the retention capacity of eddies and in the particle dispersion. We find that the horizontal mixing processes' intensity in the Mediterranean Sea increased on the last two decades, up to 37% in the Gulf of Lion and the Aegean Sea areas. The predominant direction in the relative dispersion over the whole Mediterranean Sea is zonal, although it can be observed a higher meridional dispersion in areas where intense regional wind and waves are relevant. These transport properties have profound consequences on the regional distribution of drifting organisms, pollutants and, more generally, any tracer that is transported by the flow. Key Points: Ageostrophy modifies drastically the direction of the tracer spreading and the retention capacity of mesoscale geostrophic eddiesThere is an increase in the intensity of the mixing processes in the Mediterranean Sea over the last decades (up to 1.2%/yr in some regions)Mesoscale ageostrophic currents induce meridional dispersion in regions of the Mediterranean where wind and wave are intense [ABSTRACT FROM AUTHOR]

Subsurface eddies are a special type of oceanic eddy that display the maximum velocity in the subsurface layer. Based on field observations, a lens-shaped subsurface anticyclonic eddy (SAE) was detected in the northern South China Sea (SCS) in May 2021. The SAE was located between 20 and 200 m, with a shoaling of the seasonal thermocline and deepening of the main thermocline. Satellite images showed that the SAE exhibited positive sea level anomaly (SLA) and negative sea surface temperature (SST) anomaly. Eddy track indicated that this SAE originated from the Luzon Strait and was generated in the Kuroshio Loop Current (KLC) last winter. The evolution of the SAE was related to the anomalous water properties inside the eddy and the seasonal change of sea surface heat flux. In winter, the continuous surface cooling and Kuroshio intrusion led to a cold, salty core in the upper part of the anticyclonic eddy, which resulted in a subsurface-intensified structure through geostrophic adjustment. As the season changed from winter to spring, sea surface temperature increased. The lens-shaped structure was formed when the seasonal thermocline appeared near the surface that capped the winter well-mixed water inside the eddy. From 1993 to 2021, nearly half of the winter KLC shedding eddies (12/25) survived to late spring and evolved into subsurface lens-shaped structures. This result indicates that the transition of KLC shedding eddy to SAE is a common phenomenon in the northern SCS, which is potentially important for local air–sea interaction, heat–salt balance, and biogeochemical processes. Significance Statement: Subsurface eddies are lens-shaped eddies with anomalous water properties in the subsurface layer. While such eddies have been reported in many regions of the World Ocean, they are poorly investigated in the SCS, especially the periodic subsurface eddies that appear in a fixed time frame with similar patterns and trajectories. This study reported a subsurface anticyclonic eddy (SAE) in the northern SCS and elucidated its generation and evolution processes. Statistical results confirm that this is a periodic SAE, which occurs nearly annually in late spring and evolves from the Kuroshio shedding eddy with seasonal changes. This study provides a new perspective on the evolution of subsurface eddies in the SCS and will benefit targeted observations in the future. [ABSTRACT FROM AUTHOR]

Ergodic properties of a stochastic medium complexity model for atmosphere and ocean dynamics are analyzed. More specifically, a two-layer quasi-geostrophic model for geophysical flows is studied, with the upper layer being perturbed by additive noise. This model is popular in the geosciences, for instance to study the effects of a stochastic wind forcing on the ocean. A rigorous mathematical analysis however meets with the challenge that in the model under study, the noise configuration is spatially degenerate as the stochastic forcing acts only on the top layer. Exponential convergence of solutions laws to the invariant measure is established, implying a spectral gap of the associated Markov semigroup on a space of Hölder continuous functions. The approach provides a general framework for generalized coupling techniques suitable for applications to dissipative SPDEs. In case of the two-layer quasi-geostrophic model, the results require the second layer to obey a certain passivity condition. [ABSTRACT FROM AUTHOR]

The atmosphere of Venus exhibits equatorial planetary-scale waves that are suspected to play an important role in its complex atmospheric circulation. Due to its particularly long sidereal day (243 terrestrial days against 24 h for the Earth), the Venusian waves must be described with the momentum equations for a cyclostrophic regime, but efforts to derive analytical wave solutions have been scarce. Following a classic approach for the terrestrial quasi-geostrophic regime, I present analytical solutions for equatorial waves in the atmosphere of Venus, assuming a single layer of a homogeneous incompressible fluid with a free surface and focusing on two asymptotic cases described by the ratio of their non-dimensional frequency and zonal wavenumber. One of the dispersion relations that has been obtained describes waves on a small spatial scale propagating upstream relative to the zonal flow, which is associated with a Rossby-type wave called "centrifugal". The solutions for the other asymptotic case were interpreted as inertio-surface waves, which describe planetary-scale waves that can propagate "upstream" and "downstream" relative to the zonal winds and have null group velocity. These new wave solutions stress relevant differences between waves in geostrophic and cyclostrophic regimes and may be applicable to Saturn's moon, Titan, and Venus-like exoplanets. [ABSTRACT FROM AUTHOR]

Coastal polynyas in the Ross Sea are important source regions of high-salinity shelf water (HSSW) – the precursor of Antarctic Bottom Water that supplies the lower limb of the thermohaline circulation. Here, the response of sea ice production and HSSW formation to synoptic-scale and mesoscale cyclones was investigated for the Ross Ice Shelf Polynya (RISP) using a coupled ocean–sea ice–ice shelf model targeted on the Ross Sea. When synoptic-scale cyclones prevailed over RISP, sea ice production (SIP) increased rapidly by 20 %–30 % over the entire RISP. During the passage of mesoscale cyclones, SIP increased by about 2 times over the western RISP but decreased over the eastern RISP, resulting respectively from enhancement in the offshore and onshore winds. HSSW formation mainly occurred in the western RISP and was enhanced responding to the SIP increase under both types of cyclones. Promoted HSSW formation could persist for 12–60 h after the decay of the cyclones. The HSSW exports across the Drygalski Trough and the Glomar Challenger Trough were positively correlated with the meridional wind. Such correlations are mainly controlled by variations in geostrophic ocean currents that result from sea surface elevation change and density differences. [ABSTRACT FROM AUTHOR]

The straining regions of the ocean in between mesoscale eddies contain large vertical velocities that may be important in regulating phytoplankton accumulation rates. We analyze time series of variables measured by ocean Bio‐Argo floats (mixed layer depths [MLDs], chlorophyll, and carbon concentrations) in conjunction with variables derived from satellite altimetry (strain rates, Lyapunov exponents, vertical velocities) to determine the evolution of mixed layer phytoplankton biomass in response to straining by the mesoscale geostrophic flow. A Lagrangian (water parcel following) framework is justified by restricting the analysis to profiles whose value of a Quasi‐Planktonic Index—an index quantifying averaged distance between a float trajectory and a surface geostrophic trajectory over three consecutive time steps—is less than 5 km. Bin‐averaged Lagrangian derivatives of phytoplankton biomass and chlorophyll concentration are positive for elevated strain rate and upwelling quasigeostrophic vertical velocities. Lagrangian derivatives of MLD and phytoplankton carbon averaged at straining fronts (in rotated along‐ and across‐front coordinates) have features in common with submesoscale dynamics, including increasing phytoplankton carbon (and chlorophyll) and a shoaling mixed layer over the front. To elucidate a mechanism, we average time derivatives of modeled cell division rates, finding the pattern approximately matches the pattern of phytoplankton accumulation rates and is controlled primarily by the term modulating light stress, suggesting that frontal dynamics cause accelerations of division rates by increasing available light. Regions of increasing chlorophyll are also approximately co‐located with upwelling quasigeostrophic velocity, suggesting non‐Lagrangian behavior of floats causes some imprint of larger scale, more persistent mesoscale signals. Plain Language Summary: When mesoscale eddies (large, swirling flows) interact in the ocean, the currents at their edges strain tracers. Straining sharpens horizontal density gradients, disrupting force balances and causing large vertical velocities that can accelerate division rates of phytoplankton (small, drifting photosynthetic organisms) by supplying them with nutrients or bringing them closer to light. Bio‐Argo floats measure vertical profiles of temperature and salinity (constituents of seawater density) and backscattering (proportional to phytoplankton biomass) as they drift with currents. By evaluating time rates of change of these variables along a float track and collocating float profiles in space and time with straining fronts (identified from satellite altimetry), we obtain a picture of how phytoplankton evolve in response to strain. We find that straining fronts are characterized by increasing density stratification and biomass on the anticyclonic side of, and above, straining fronts, largely consistent with theoretical expectations. Further, by comparing with a model of cell division rates, we find increases in biomass at straining fronts are primarily due to increases in available light. With a larger data set of closely spaced float profiles, our methodology could clarify the importance of straining regions in the carbon cycle. Key Points: Positive phytoplankton accumulation rates are associated with elevated geostrophic strain and quasigeostrophic upwellingAveraged Lagrangian derivatives at straining fronts include positive accumulation rates and a shoaling mixed layer on the less dense sideFrontal dynamics drive increases in phytoplankton biomass by accelerating cell division rates primarily through increased available light [ABSTRACT FROM AUTHOR]

The ocean's sea surface height (SSH) field is a complex mix of motions in geostrophic balance and unbalanced motions including high-frequency tides, internal tides, and internal gravity waves. Barotropic tides are well estimated for altimetric SSH in the open ocean, but the SSH signals of internal tides remain. The transition scale, Lt , at which these unbalanced ageostrophic motions dominate balanced geostrophic motions is estimated for the first time using satellite altimetry. Lt is critical to define the spatial scales above which surface geostrophic currents can be inferred from SSH gradients. We use a statistical approach based on the analysis of 1 Hz altimetric SSH wavenumber spectra to obtain four geophysical parameters that vary regionally and seasonally: the background error, the spectral slope in the mesoscale range, a second spectral slope at smaller scales, and Lt. The mesoscale slope and error levels are similar to previous studies based on satellite altimetry. The break in the wavenumber spectra to a flatter spectral slope can only be estimated in midlatitude regions where the signal exceeds the altimetric noise level. Small values of Lt are observed in regions of energetic mesoscale activity, while larger values are observed towards low latitudes and regions of lower mesoscale activity. These results are consistent with recent analyses of in situ observations and high-resolution models. Limitations of our results and implications for reprocessed nadir and future swath altimetric missions are discussed. [ABSTRACT FROM AUTHOR]

Sea surface slope (SSS) varies in response to a range of physical processes: tides, geostrophic flows, surface and internal waves, etc. We present the sea surface variation in the form of the SSS variability using 30 years of heterogeneous satellite altimetry measurements. We apply band‐pass filters to the along‐track SSS, and derive the mean and seasonal (annual and semi‐annual) components of SSS variability in multiple wavelength sub‐bands from 10 to 1,000 km. We show that the seasonal components are generally small (<10% in amplitude) compared to the mean variability. Through correlation analysis, we show evidence that SSS variability with wavelengths less than 30 km is dominated by wave height noise. At sub‐mesoscale to mesoscale (30–100 km) wavelengths, we identify high variability over western boundary currents and regions of rough topography. In this band, the high‐latitude Northern Hemisphere and the south Indian Ocean are associated with large annual cycles. The variability is higher in local wintertime except for a few regions, for example, the Bay of Bengal, which shows high variability in the boreal spring and fall. Through power spectral density analysis of the seasonal SSS variability, we find that the energy differences between local winter and summer are stronger at smaller scales (<100 km). The Ka‐band radar interferometry instrument on the Surface Water and Ocean Topography satellite mission will allow observation of ocean surface activities down to ∼20 km at submonthly time scales, but wave‐related errors (sea state bias, aliasing, wind‐driven activities, etc.) will still be a major challenge. Plain Language Summary: Many ocean processes have signatures in sea surface slope (SSS) in the 30–100 km wavelength scales, yet SSS in this band is neither well observed nor well studied globally. This paper focuses on seasonal variations of the SSS on length scales from 30 to 100 km, using satellite data collated over the past 30 years. We find high variability over strong currents and regions of rough ocean bottom topography. This variability is related to surface gravity waves and the density changes in the sub‐surface of the ocean. The seasonal changes are small compared to the mean spatial variations in SSS. Key Points: We identify high sub‐mesoscale sea surface slope (SSS) variability over western boundary currents and regions of rough topographySeasonal variations are small (<10% in amplitude) compared to the mean variability on length scales from 30 to 100 kmSSS variation is related to the significant wave height and thermocline depth [ABSTRACT FROM AUTHOR]

Compared to the dynamics of the predominantly geostrophic along-shelf current, our understanding of the cross-shelf dynamics in the Sea of Okhotsk is inadequate despite their importance in water mixing and nutrient entrainment. We investigated the cross-shelf overturning circulation along the East Sakhalin Current, which is a source of nutrients such as iron for the western North Pacific. Here, we reveal that the cross-shelf circulation during winter is characterised by a nearshore upwelling and a shelf-break downwelling under a downwelling-favourable monsoon wind, contrary to a classical Ekman overturning (EOT). This reverse EOT is driven by the internal water stress, which is caused by intensive vertical mixing and geostrophic vertical shear in the shelf-break front produced by riverine discharges from the far-eastern Eurasian Continent. The EOT blocks the Ekman onshore transport from the open ocean, thereby producing a deep mixed layer at the shelf break. Scaling analyses indicate the applicability of this mechanism to various other shelf-break fronts. [ABSTRACT FROM AUTHOR]

We assess the contribution of flow over the Northeast Greenland Shelf (NEGS) to the total freshwater transport (FWT) through the Fram Strait. We analyze available observations since 2013 consisting of hydrographic sections, new mooring data, and surface‐geostrophic velocity and ERA5 winds. We provide first seasonal estimates of the FWT over the entire shelf and find a net southward FWT between 47 and 56 mSv (reference salinity 34.9), or ∼40%–45% of the total FWT through the Fram Strait. In summer, the long‐term mean FWT east of 8°W cumulates to 27 mSv southward, there is no trend toward increasing FWT, and new mooring data show a large seasonal cycle. We show that the NEGS contributes significantly to the Arctic freshwater outflow, however, uncertainty is high. To close the Arctic freshwater budget and provide better FWT estimates to the North Atlantic, sustained year‐round observations on the NEGS are essential. Plain Language Summary: We use new observations since 2013 to calculate transport over the Northeaster Greenland Shelf for the first time, and investigate how much it adds to the transport of the East Greenland Current over the shelf break which is monitored by moorings. Over most of the shelf, direct observations of ocean properties are limited to summer. Thus, we use remote‐sensing and wind reanalysis data, with shelf‐wide and year‐round coverage, but higher uncertainty, to estimate the year‐round transport of low‐salinity water (freshwater, less than 34.9) over the entire shelf. We find that on the shelf, freshwater transport is at least 40% of the total in Fram Strait. Still, uncertainty is high and year‐round, direct observations in the western part of the shelf are required to provide accurate estimates for the Arctic freshwater outflow. Key Points: Summer freshwater transport (FWT) over the shelf cumulates to −27 mSv and shows no trend in timeNew mooring data on the shelf at 10.1°W show a very large seasonal cycleYear‐round FWT over the shelf is estimated to ∼−52 mSv or 43% of the total freshwater outflow from the Fram Strait [ABSTRACT FROM AUTHOR]

Barotropic and baroclinic tides were simulated for the Coral and Tasman Seas off eastern Australia and compared to a simulation without tides. Both simulations included geostrophic currents and replicated a current analogous to the East Australian Current (EAC). Tides and tidal currents in most of this region are weak, generally 1–2 cm s−1, with the exception of the far northwest portion of the Coral Sea. Even these weak tides were found to impact the mean EAC-like current transport, enhancing it by 1–4 Sv in some areas. Southward flow increased over the continental shelf. Tides did not appear to impact eddy formation, size, or rotational speed; however, they affected eddy propagation. Cyclonic eddies propagated northward faster with tides than without tides. Tides impacted cross-shelf transport of colder water, with significantly more on-shore transport occurring with tides, particularly equatorward of the diurnal critical latitudes. Cross-shelf transport of nutrient rich water onto the shelf is important in this oligotrophic region. Although the prime source of vertical shear and mixing were mean currents and eddies, tides played a secondary role. Tides influenced mixing by increasing vertical temperature diffusivities to 10−4 to 10−3 m2 s−1 over portions of the continental slope and over rough topography, particularly in regions near the diurnal critical latitudes (27°–30°). In conclusion, even small tides can significantly impact the circulation through their effects on the mean currents, eddy rotation velocities, eddy propagation, and mixing. [ABSTRACT FROM AUTHOR]

Global ocean surface currents estimated from satellite derived data based on a regular global grid are affected by the grid's shape and placement. Due to different neighbourhood relationships, the rectangular lat/lon grids lose accuracy when interpolating and fitting elevation data. Hexagonal grids have shown to be advantageous due to their isotropic, uniform neighbourhood. Considering these merits, this paper aims to estimate global ocean surface current using a global isotropic hexagonal grid from satellite remote sensing data. First, gridded satellite altimeter data and wind data with different resolutions are interpolated into the centre of the global isotropic hexagonal grid. Then, geostrophic and Ekman currents components are estimated according to the Lagerlof Ocean currents theory. Finally, the inversion results are verified. By analyzing the results, we conclude that the ocean surface currents estimated based on the global isotropic hexagonal grid have considerable accuracy, with improvement over rectangular lat/lon grids. [ABSTRACT FROM AUTHOR]

Satellite altimetry missions flying over the ice-covered Arctic Ocean have opened the possibility of further understanding changes in the ocean beneath the sea ice. This requires complex processing of satellite signals emerging from the sea surface in leads within the sea ice, with efforts to generate consistent Arctic-wide datasets of sea surface height ongoing. The aim of this paper is to provide and assess a novel gridded dataset of sea surface height anomaly and geostrophic velocity, which incorporates both the ice-covered and open ocean areas of the Arctic. Data from the CryoSat-2 mission in the period 2011–2020 were gridded at monthly intervals, up to 88 ∘ N, using the Data-Interpolating Variational Analysis (DIVA) method. To examine the robustness of our results, we compare our dataset to independent satellite data, mooring time series and Arctic-wide hydrographic observations. We find that our dataset is well correlated with independent satellite data at monthly timescales. Comparisons to in situ ocean observations show that our dataset provides reliable information on the variability of sea surface height and surface geostrophic currents over geographically diverse regions of the Arctic Ocean and different dynamical regimes and sea ice states. At all comparison sites we find agreement with in situ observed variability at seasonal to interannual timescales. Furthermore, we find that our geostrophic velocity fields can resolve the variability of boundary currents wider than about 50 km, a result relevant for studies of Arctic Ocean circulation. Additionally, large-scale seasonal features emerge. Sea surface height exhibits a wintertime Arctic-wide maximum, with the highest amplitude over the shelves. Also, we find a basin-wide seasonal acceleration of Arctic slope currents in winter. We suggest that this dataset can be used to study not only the large-scale sea surface height and circulation, but also the regionally confined boundary currents. The dataset is available in netCDF format from PANGAEA at 10.1594/PANGAEA.931869. [ABSTRACT FROM AUTHOR]

We present a new gridded sea surface height and current dataset produced by combining observations from nadir altimeters and drifting buoys. This product is based on a multiscale and multivariate mapping approach that offers the possibility to improve the physical content of gridded products by combining the data from various platforms and resolving a broader spectrum of ocean surface dynamic than in the current operational mapping system. The dataset covers the entire global ocean and spans from 1 July 2016 to 30 June 2020. The multiscale approach decomposes the observed signal into different physical contributions. In the present study, we simultaneously estimate the mesoscale ocean circulations as well as part of the equatorial wave dynamics (e.g. tropical instability and Poincaré waves). The multivariate approach is able to exploit the geostrophic signature resulting from the synergy of altimetry and drifter observations. Sea-level observations in Arctic leads are also used in the merging to improve the surface circulation in this poorly mapped region. A quality assessment of this new product is proposed with regard to an operational product distributed in the Copernicus Marine Service. We show that the multiscale and multivariate mapping approach offers promising perspectives for reconstructing the ocean surface circulation: observations of leads contribute to improvement of the coverage in delivering gap-free maps in the Arctic and observations of drifters help to refine the mapping in regions of intense dynamics where the temporal sampling must be accurate enough to properly map the rapid mesoscale dynamics. Overall, the geostrophic circulation is better mapped in the new product, with mapping errors significantly reduced in regions of high variability and in the equatorial band. The resolved scales of this new product are therefore between 5 % and 10 % finer than the Copernicus product (https://doi.org/10.48670/moi-00148 , Pujol et al., 2022b). [ABSTRACT FROM AUTHOR]

This study focuses on the determination and validation of a new global mean sea surface (MSS) model, named the Shandong University of Science and Technology 2020 (SDUST2020), with a grid size of 1′×1′. This new model was established with a 19-year moving average method and fused multi-satellite altimetry data over a 27-year period (from January 1993 to December 2019). The data of HaiYang-2A, Jason-3, and Sentinel-3A were first ingested in the SDUST2020 MSS but not in any other global MSS model, such as the CLS15 and DTU18 MSS models. Validations, including comparisons with the CLS15 and DTU18 MSS models, GPS-leveled tide gauges, and altimeter data, were performed to evaluate the quality of the SDUST2020 MSS model, all of which showed that the SDUST2020 MSS model is accurate and reliable. The SDUST2020 MSS dataset is freely available at the site (data DOI: 10.5281/zenodo.6555990, Yuan et al., 2022). [ABSTRACT FROM AUTHOR]

• Sea surface height and SSH derived geostrophic current from S3A are evaluated. • Measurable accuracy improvements are observed in DD/SAR compared to LRM. • DD/SAR altimetry can resolve small scale current features in the NW Atlantic shelf. In this study, we evaluate Sentinel-3A satellite synthetic aperture radar (SAR) altimeter observations along the Northwest Atlantic coast, spanning the Nova Scotian Shelf, Gulf of Maine, and Mid-Atlantic Bight. Comparisons are made of altimeter sea surface height (SSH) measurements from three different altimeter data processing approaches: fully-focused synthetic aperture radar (FFSAR), un-focused SAR (UFSAR), and conventional low-resolution mode (LRM). Results show that fully-focused SAR data always outperform LRM data and are comparable or slightly better than the nominal un-focused SAR product. SSH measurement noise in both SAR-mode datasets is significantly reduced compared to LRM. FFSAR SSH 20-Hz noise levels, derived from 80-Hz FFSAR data, are lower than 20-Hz UFSAR SSH with 25% noise reduction offshore of 5 km, and 55–70% within 5 km of the coast. The offshore noise improvement is most likely due to the higher native along-track data posting rate (80 Hz for FFSAR, and 20 Hz for UFSAR), while the large coastal improvement indicates an apparent FFSAR data processing advantage approaching the coastlines. FFSAR-derived geostrophic ocean current estimates exhibit the lowest bias and noise when compared to in situ buoy-measured currents. Assessment at short spatial scales of 5–20 km reveals that Sentinel-3A SAR data provide sharper and more realistic measurement of small-scale sea surface slopes associated with expected nearshore coastal currents and small-scale gyre features that are much less well resolved in conventional altimetric LRM data. [ABSTRACT FROM AUTHOR]

Canada ratified the United Nations Convention on the Law of the Sea (UNCLOS) in 2003. With that ratification is an obligation to submit data and information to the U.N. pertaining to the limits of the country's extended continental shelf (ECS); the portion of the juridical continental shelf that extends beyond 200 nautical miles. A team of Canadian scientists, managers, and legal experts that included representation from three Federal Departments (Natural Resources Canada, Fisheries and Oceans Canada, and Global Affairs Canada) with additional support from other departments, spent 13 years compiling and acquiring data to provide the scientific evidence to support delineation of Canada's seaward most maritime limit. The submission has the potential to provide Canada with 2.4 million km2 of additional submarine landmass in the Atlantic and the Arctic oceans over which Canada exercises sovereign rights for the purpose of exploring and exploiting its natural resources. Specific information such as the tectonic framework of the continental margin, the geomorphology of the margin and in particular the continental slope, the geologic nature of adjoined ridges, rises, and plateaux, and sediment thickness within adjacent basins are examples of fundamental pieces of geoscientific information needed to substantiate Canada's outermost maritime limits. This paper highlights a number of segments of Canada's continental margins to showcase this scientific evidence and how it is applied in the UNCLOS context. In doing so, the paper demonstrates the geologic complexity of Canada's margins as illustrated in scientific publications that have resulted from these new data collections, while at the same time presenting new scientific evidence and interpretations. This collection of data and information provides a wealth of new knowledge in Canada's offshore regions. The massive data compilation in the Atlantic led to conception of continental margins, in a source-to-sink scenario, as having an equilibrium base level or graded form, comparable to river systems. Departures from this shape relate to the interplay of sedimentary processes and in particular to those processes that do not fit the source-to-sink paradigm. For example, a significant part of the Atlantic margin is shown to be heavily influenced by along-slope geostrophic currents that generated massive contourite drift deposits. These deposits reflect lateral transport of sediment that had a significant impact on the morphology of the margin. The role of mass transport processes in shaping continental margins is also highlighted, and in particular the collapses of entire segments of the margin were observed. The prominent role mass failure processes play in delivering sediment to the adjacent abyssal plain is also critical in the ECS context. These observations challenge the entrenched notion of a continental margin comprising a shelf, slope, and rise and in particular the concept of the "continental rise". Prior to 2006, regions of the Arctic Ocean seaward of the Canadian landmass had fewer than 5000 km of seismic reflection data. The massive efforts of Arctic coastal States to map their margins for ECS purposes have led to a leap in technological advances to acquire data in ice-covered seas and have led to a wealth of new geoscientific knowledge. Perhaps foremost amongst this knowledge is demonstration that Canada Basin is indeed a fully developed ocean basin, albeit significantly infilled with sediment. Based on this knowledge and identification of related structures, new realistic tectonic scenarios for opening of the Amerasia Basin are proposed that include a significant component of transform or strike-slip motions. With seismic velocity and rock sample information, the continental nature of Alpha and Mendeleev ridges has been substantiated. Even bathymetric data were lacking in the Arctic and new editions of seafloor maps now support grids of 500 m spacing; although some regions remain sparse. Once thought to be relatively stagnant, sedimentary processes such as found in many ocean basins were discovered in the Arctic Ocean. Evidence of geostrophic currents, sediment mass failures, and deep-sea turbidity current channels were found to be ubiquitous, even in the deepest parts of the Arctic's basins. [ABSTRACT FROM AUTHOR]

Determining an accurate picture of ocean currents is an important societal challenge for oceanographers, aiding our understanding of the vital role currents play in regulating Earth’s climate, and in the dispersal of marine species and pollutants, including microplastics. The geodetic approach, which combines satellite observations of sea level and Earth’s gravity, offers the only means to estimate the dominant geostrophic component of these currents globally. Unfortunately, however, geodetically-determined geostrophic currents suffer from high levels of contamination in the form of geodetic noise. Conventional approaches use isotropic spatial filters to improve the signal-to-noise ratio, though this results in high levels of attenuation. Hence, the use of deep learning to improve the geodetic determination of the ocean currents is investigated. Supervised machine learning typically requires clean targets from which to learn. However, such targets do not exist in this case. Therefore, a training dataset is generated by substituting clean targets with naturally smooth climate model data and generative machine learning networks are employed to replicate geodetic noise, providing noisy input and clean target pairs. Prior knowledge of the geodetic noise is exploited to develop a more realistic training dataset. A convolutional denoising autoencoder (CDAE) is then trained on these pairs. The trained CDAE model is then applied to unseen real geodetic ocean currents. It is demonstrated that our method outperforms conventional isotropic filtering in a case study of four key regions: the Gulf Stream, the Kuroshio Current, the Agulhas Current, and the Brazil-Malvinas Confluence Zone. [ABSTRACT FROM AUTHOR]

Sea surface currents (SSC) derived from the sea surface height anomalies (SSHA) as measured by multi‐satellite altimeters are widely used for various applications including studies on ocean dynamics, marine ecology, and climate change. However, present SSC products estimated on the assumption of an idealized geostrophic balance is biased. To overcome this idealization, this study considers flow curvature in the estimation of sea surface velocities in the global ocean, with the computation scheme validated using numerical model results. It is demonstrated that the inclusion of curvature into SSC estimations significantly changes SSC dynamic features in terms of kinetic energy, enstrophy (the maximum spatial difference is about 15%), and strain rate (the maximum spatial difference is about 10%). Such correction is of importance to the application studies relying on the SSC products from the satellite measured SSHA. Plain Language Summary: Sea surface currents (SSC) are usually derived from satellite altimetry measurements based on the balance between the pressure gradient and Coriolis effect, that is, geostrophic approximation. In this study, it is presented for the first time that the effect of flow curvature is considered in the calculation of the SSC for the global ocean using the satellite remote sensing sea surface height anomalies. Such correction is significant in terms of eddy kinetic energy, enstrophies and strains. The results show that maximum spatial differences in eddy kinetic energy and enstrophy (15%) is significantly greater than the strain rate (10%). In addition, it is of great significance for cyclostrophic‐correcting the surface geostrophic velocities derived from altimeters in meandering, boundary currents and eddy‐rich regions. Key Points: An iterative method is used to cyclostrophic‐correct global sea surface velocities from Archiving, Validation and Interpolation of Satellite Oceanographic Data‐gridded altimetryThe impact of curvature on the ocean currents can be seen in the dynamics of meandering, boundary currents and eddy‐rich regionsMaximum spatial differences in eddy kinetic energy and enstrophy (15%) is significantly greater than the strain rate (10%) [ABSTRACT FROM AUTHOR]

Data from two moorings deployed at 166°W on the northern Chukchi shelf and slope from summer 2002 to fall 2004, as part of the Western Arctic Shelf‐Basin Interactions program, are analyzed to investigate the characteristics and variability of the flow in this region. The depth‐mean velocity at the outer‐shelf mooring is northeastward and bottom‐intensified, while that at the upper‐slope mooring is northwestward and surface‐intensified. This, together with results from a high resolution ocean and sea ice reanalysis, indicates that the outer‐shelf mooring sampled the seaward edge of the Chukchi Shelfbreak Jet, while the upper‐slope mooring sampled the shoreward edge of the Chukchi Slope Current. The coupled variability in velocity at both sites is related to the wind stress curl over the Chukchi Sea shelf, likely via Ekman dynamics and geostrophic set up, analogous to the dynamics of both currents closer to Barrow Canyon near 157°W. Hydrographic signals are analyzed to elucidate the origin of the water masses present at this location. It is argued that the annual appearance of Pacific‐origin warm water at the outer‐shelf (upper‐slope) mooring in late‐fall and winter originates from Herald (Barrow) Canyon some months earlier. Our results constitute the first robust evidence that the westward‐flowing Chukchi Slope Current persists this far west of Barrow Canyon. Plain Language Summary: The boundary currents along the shelfbreak and slope of the Chukchi Sea play a key role in the regional ecosystem via advection and shelf‐basin exchange of water and properties. Two‐years of observational data obtained at two fixed locations on the Chukchi Shelfbreak and slope at 166°W revealed that the mean current in this region was northeastward on the outer shelf and northwestward at the upper slope. This is the signature of the Chukchi Shelfbreak Jet and Chukchi Slope Current, respectively, further verified using output from a high resolution ocean model over the same time period. The velocity at the two locations co‐varies, driven predominantly by the changing rotation of the wind over the Chukchi shelf. Warm water was found annually in the fall/winter at the two locations. Using additional mooring data, it is argued that the warm water in the Chukchi Shelfbreak Jet stems from Herald Canyon in the west, while the warm water in the Chukchi Slope Current stems from Barrow Canyon in the east. Our results demonstrate for the first time that the Chukchi Slope Current persists far west of Barrow Canyon. Key Points: Mooring data reveal the presence of the Chukchi Shelfbreak Jet and Chukchi Slope Current at 166°WThe dominant variability of both currents is driven by the wind stress curl on the Chukchi shelf via geostrophic setupThe warm water present in fall/winter in the Chukchi Shelfbreak Jet and Slope Current originates from Herald Canyon and Barrow Canyon [ABSTRACT FROM AUTHOR]

The Deep Western Boundary Current (DWBC) is the main component of the deep limb of the Atlantic Meridional Overturning Circulation (AMOC). Off northeast Brazil, the DWBC breaks up into southwestward‐propagating anticyclones. In this study, we investigate the breakup mechanism with hydrographic observations, eddy‐resolving numerical model outputs, and theory. Here, we present a quasi‐synoptic map of geostrophic velocities and stream function at the DWBC core level between 2.5°S and 11°S. We observe, in horizontal distributions of velocities, that the DWBC breakup site is linked to a topographic feature of the Brazilian continental margin centered at 8°S: the Pernambuco Plateau (PP). Moreover, both observations and model outputs hint at a possible DWBC separation near the PP preceding anticyclone genesis. We test, with three different theories from the literature, whether or not the DWBC separates at 8°S. The results of the tests converge to indicate that the DWBC undergoes a local and intermittent inertial separation while contouring the PP. Downstream of its separation at the plateau, the DWBC sheds eddies similarly to previously reported laboratory experiments. In addition, a regional analysis of energy transfer shows that barotropic instability significantly contributes to the anticyclones growth between 8°S and 13°S. Analysis of the energy budget and separation of waters related to the AMOC pathways into the basin interior provide a better understanding for later studies about heat fluxes and ventilation in the deep tropical South Atlantic. Plain Language Summary: The Deep Western Boundary Current transports southward the cold and dense waters linked to the deep limb of the Atlantic Meridional Overturning Circulation (AMOC). Among many properties, these waters transport energy from the Northern to the Southern Hemisphere, thousands of meters below the surface. Along the deep ocean off northeast Brazil, this current breaks up into large whirlpools of water that rotate counterclockwise while propagating southwestward. These whirlpools in the ocean are know as eddies. We observe that the eddy formation is linked to a feature of the Brazilian continental margin: the Pernambuco Plateau (PP). Moreover, both shipboard data and numerical model outputs hint at a possible separation of the deep current near the PP prior to the eddy formation. We test the separation with theories and the results converge to indicate that the deep current undergoes a local separation off the continental slope while contouring the PP. In addition, we present that the eddies grow by feeding off the energy from the deep current. The local separation and regional energy exchange along this deep current contribute to further discussions about the pathways of the AMOC and heat fluxes in the South Atlantic. Key Points: Part of the Deep Western Boundary Current (DWBC) separates inertially off the continental slope while crossing the Pernambuco Plateau at 8°SThe DWBC separation plays a crucial role in the formation of the DWBC deep anticyclonic eddiesBarotropic instability significantly contributes to the growth of the deep anticyclonic eddies [ABSTRACT FROM AUTHOR]

The trajectories of 114 000 passive particles regularly distributed on a grid in the Sea of Okhotsk are calculated for each day of the interval from January 31, 1997 to March 10, 2021, based on satellite altimetry data on geostrophic current speeds. All particles coming to the Sea of Okhotsk from the Pacific Ocean are identified and the age of the Pacific-origin waters and the area where these waters enter the sea are determined. Using daily vessel-position data in the walleye pollock fishery from 1997 to 2021, Lagrangian maps of the origin and age of the Pacific waters over the same period, and statistical analysis, it is shown that the Walleye pollock fishing grounds are more often in the Pacific waters which passed through the northern Kuril Straits not more than 100 days before the catch dates than in the waters which entered the sea to the south or have been in the sea for more than 100 days. Analysis of satellite imagery of sea surface temperatures leads to the conclusion that the reasons for the aggregation of walleye pollock catch locations in intrusions and Lagrangian fronts of Pacific waters are their warmer temperatures and higher concentrations of feed. [ABSTRACT FROM AUTHOR]

The distribution of fresh water in the East Siberian Sea (ESS) and Laptev Sea (LS) in the period of the least ice coverage (August–October) has been analyzed using satellite data on sea level and CTD data from ship observations. It is shown that there is a good agreement between the salinity/density distribution in the surface water layer and the sea level in the study area. An increase (decrease) in the runoff of the Lena River during the summer flood leads to an increase (decrease) in the sea level in the eastern part of the LS and in the western and southwestern parts of the ESS in September and October (r = 0.52–0.59 in 1993–2019). The intensification of the western winds is accompanied by a decrease in salinity and a rise in sea level in the coastal areas of the LS and ESS. Eastern winds cause a decrease in sea level in the southern regions of the LS and ESS and the flow of waters with low salinity from the coastal to the open part of the sea. Western/eastern winds are accompanied by eastern/western directed geostrophic currents in the region of the Sannikov Strait and in the southwestern part of the ESS. [ABSTRACT FROM AUTHOR]

The Soya Current is formed from the transformed subtropical waters of the Tsushima Current, which flows from the Sea of Japan through the La Perouse Strait, spreads along the northern coast of Hokkaido, and enters the Pacific Ocean through the Yekaterina and Kunashir straits. An analysis of satellite data (1993–2019) on sea level, geostrophic current velocity, surface water temperature, and chlorophyll concentration in the southern part of the Sea of Okhotsk, in the zone of the La Perouse Strait (Sea of Japan), and in the southern Kuril region of the Pacific Ocean has been carried out. It has been established that an increase in the sea level difference between the Sea of Japan and the Pacific Ocean leads to the intensification of the Soya current. The main contribution to the change in the level difference between the Pacific Ocean and the Sea of Japan in autumn belongs to anticyclonic eddies and meanders of the Subarctic Front in the Pacific Ocean. An increase (decrease) in the penetration of transformed subtropical waters through the La Perouse Strait is accompanied by an increase (decrease) in the temperature of the surface water layer in the southern part of the Sea of Okhotsk (r = 0.67 in 2000–2019) and a decrease (increase) in the chlorophyll concentration in the Kuril region of the Pacific Ocean (r = –0.70 in 1998–2019) in September–October. [ABSTRACT FROM AUTHOR]

We describe an idealized primitive-equation model for studying mesoscale turbulence and leverage a hierarchy of grid resolutions to make eddy-resolving calculations on the finest grids more affordable. The model has intermediate complexity, incorporating basin-scale geometry with idealized Atlantic and Southern oceans and with non-uniform ocean depth to allow for mesoscale eddy interactions with topography. The model is perfectly adiabatic and spans the Equator and thus fills a gap between quasi-geostrophic models, which cannot span two hemispheres, and idealized general circulation models, which generally include diabatic processes and buoyancy forcing. We show that the model solution is approaching convergence in mean kinetic energy for the ocean mesoscale processes of interest and has a rich range of dynamics with circulation features that emerge only due to resolving mesoscale turbulence. [ABSTRACT FROM AUTHOR]

The variation of boundary layer circulation caused by the influence of complex underlying surface is one of the reasons why it is difficult to forecast hourly heavy rainfall (HHR) in the middle Yangtze River Valley (YRV). Based on the statistics of high-resolution observation data, it is found that the low resolution data underestimate the frequency of HHR in the mountain that are between the twain-lake basins in the middle YRV (TLB-YRV). The HHR frequency of mountainous area in the TLB-YRV is much higher than that of Dongting Lake on its left and is equivalent to the HHR frequency of Poyang Lake on its right. The hourly reanalysis data of ERA5 were used to study the variation of boundary layer circulation when HHR occurred. It can be found that the boundary layer circulation corresponding to different underlying surfaces changed under the influence of the weather system. Firstly, the strengthening of the weather system in the early morning resulted in the strengthening of the southwest low-level air flow, which intensified the uplift of the windward slope air flow on the west and south slopes of the mountainous areas in the TLB-YRV. As a result, the sunrise HHR gradually increases from the foot of the mountain. The high-frequency HHR period of sunrise occurs when the supergeostrophic effect is weakened, the low-level vorticity and frontal forcing are strengthened, and the water vapor flux convergence begins to weaken. Secondly, the high-frequency HHR period of the sunset is caused by stronger local uplift and more unstable atmospheric stratification, but the enhanced local uplift is caused by the coupling of the terrain forcing of the underlying surface and the enhanced northern subgeostrophic flow, which causes the HHR to start closer to the mountain top at sunset than at sunrise. [ABSTRACT FROM AUTHOR]

The newly developed fast Earth system model CLIMBER-X is presented. The climate component of CLIMBER-X consists of a 2.5-D semi-empirical statistical–dynamical atmosphere model, a 3-D frictional–geostrophic ocean model, a dynamic–thermodynamic sea ice model and a land surface model. All the model components are discretized on a regular lat–long grid with a horizontal resolution of 5∘×5∘. The model has a throughput of ∼ 10 000 simulation years per day on a single node with 16 CPUs on a high-performance computer and is designed to simulate the evolution of the Earth system on temporal scales ranging from decades to >100000 years. A comprehensive evaluation of the model performance for the present day and the historical period shows that CLIMBER-X is capable of realistically reproducing many observed climate characteristics, with results that generally lie within the range of state-of-the-art general circulation models. The analysis of model performance is complemented by a thorough assessment of climate feedbacks and model sensitivities to changes in external forcings and boundary conditions. Limitations and applicability of the model are critically discussed. CLIMBER-X also includes a detailed representation of the global carbon cycle and is coupled to an ice sheet model, which will be described in separate papers. CLIMBER-X is available as open-source code and is expected to be a useful tool for studying past climate changes and for the investigation of the long-term future evolution of the climate. [ABSTRACT FROM AUTHOR]

To assess the contribution of wind drag and Stokes drift on the near-surface circulation, a methodology to isolate the geostrophic surface current from high-frequency radar data is developed. The methodology performs a joint analysis utilizing wind field and in situ surface currents along with an unsupervised neuronal network. The isolation method seems robust in the light of comparisons with satellite altimeter data, presenting a similar time variability and providing more spatial detail of the currents in the coastal region. Results show that the wind-induced current is around 2.1% the wind speed and deflected from the wind direction in the range [18°, 23°], whereas classical literature suggests higher values. The wave-induced currents can represent more than 13% of the ageostrophic current component as function of the wind speed, suggesting that the Stokes drift needs to be analyzed as an independent term when studying surface sea currents in the coastal zones. The methodology and results presented here could be extended worldwide, as complementary information to improve satellite-derived surface currents in the coastal regions by including the local physical processes recorded by high-frequency radar systems. The assessment of the wave and wind-induced currents have important applications on Lagrangian transport studies. [ABSTRACT FROM AUTHOR]

Tide-induced near-inertial internal waves (NIWs) are generated by tide–topography interaction and are energized by internal tides through triadic resonant interaction of internal waves. They are located above topography and could be in close contact with wind-induced NIWs when the topography is a tall ridge, like in the Luzon Strait of the northern South China Sea (SCS). A natural question arises as to whether there is significant interaction between wind- and tide-induced NIWs. By using moored velocity observations, a satellite-tracked surface drifter dataset, and idealized numerical simulations, we find that in the presence of tide-induced NIWs, the wind can inject slightly more near-inertial energy (NIE), while in the presence of wind-induced NIWs, significantly more tidal energy is transferred to NIWs. Thus, wind- and tide-induced NIWs can mutually enhance each other, producing more NIE than a linear superposition of that generated by wind and tide forcing alone. Increasing wind intensity and tidal excursion lead to saturation of NIE enhancement, while a taller ridge leads to stronger enhancement. The high mixed layer NIE near Luzon Strait is mostly generated by the wind, while the mutual enhancement between wind- and tide-induced NIWs can further enhance this pattern. The interaction between wind- and tide-induced NIWs leads to an enhancement of 25% more NIE. If tide-induced NIWs are neglected, as is usually the case in the estimation of NIE, the total NIE will be underestimated by almost 50%. This might imply that tide-induced NIWs are important for the energetics of NIWs in Luzon Strait. Significance Statement: Near-inertial internal waves (NIWs) usually occupy the most kinetic energy of internal waves and contribute significantly to ocean mixing. Near the surface they are usually generated by wind forcing, but near the bottom they can be generated by geostrophic or tidal flow interacting with topography. Above the tall ridge in Luzon Strait, wind- and tide-induced NIWs are in close contact, leading to potential interactions. It is found that these NIWs can mutually enhance each other, with most of the additional near-inertial energy (NIE) coming from the tides. If tide-induced NIWs are neglected, the total NIE will be underestimated by almost 50%. This suggests that tide-induced NIWs are important for the energetics of NIWs in Luzon Strait. [ABSTRACT FROM AUTHOR]

A remarkably consistent Lagrangian upwelling circulation at monthly and longer time scales is observed in a 17-yr time series of current profiles in 12 m of water on the southern New England inner shelf. The upwelling circulation is strongest in summer, with a current magnitude of ∼1 cm s−1, which flushes the inner shelf in ∼2.5 days. The average winter upwelling circulation is about one-half of the average summer upwelling circulation, but with larger month-to-month variations driven, in part, by cross-shelf wind stresses. The persistent upwelling circulation is not wind-driven; it is driven by a cross-shelf buoyancy force associated with less-dense water near the coast. The cross-shelf density gradient is primarily due to temperature in summer, when strong surface heating warms shallower nearshore water more than deeper offshore water, and to salinity in winter, caused by fresher water near the coast. In the absence of turbulent stresses, the cross-shelf density gradient would be in a geostrophic, thermal-wind balance with the vertical shear in the along-shelf current. However, turbulent stresses over the inner shelf attributable to strong tidal currents and wind stress cause a partial breakdown of the thermal-wind balance that releases the buoyancy force, which drives the observed upwelling circulation. The presence of a cross-shelf density gradient has a profound impact on exchange across this inner shelf. Many inner shelves are characterized by turbulent stresses and cross-shelf density gradients with lighter water near the coast, suggesting turbulent thermal-wind-driven coastal upwelling may be a broadly important cross-shelf exchange mechanism. Significance Statement: A remarkably consistent upwelling circulation at monthly time scales is observed in a 17-yr time series of current profiles in shallow water off southern New England. This is not the traditional wind-driven coastal upwelling; instead, it is forced by cross-shelf buoyancy (density) gradients, released by turbulent stresses in shallow water. The persistent upwelling circulation is strongest in summer, when wind and wave forcing are weak, and flushes the inner portion of the continental shelf in a few days. Consequently, this buoyancy-driven coastal upwelling is important for cooling the inner shelf and provides a reliable mechanism for cross-shelf exchange. Many inner shelves are characterized by cross-shelf density gradients and turbulent stresses, suggesting this may be a broadly important cross-shelf exchange mechanism. [ABSTRACT FROM AUTHOR]

An analytical-numerical method is developed for solving a problem for the potential vorticity equation in the quasi-geostrophic approximation with allowance for vertical diffusion of mass and momentum. The method is used to analyze small perturbations of ocean currents of finite transverse scale with a general parabolic vertical profile of velocity. For the arising spectral non-self-adjoint problem, asymptotic expansions of the eigenfunctions and eigenvalues are constructed for small values of the wave number . It is shown that, for small , there exist two bounded eigenvalues and a countable set of unboundedly growing eigenvalues. For a varying wave number , the trajectories of eigenvalues are calculated for various dimensionless parameters of the problem. As a result, it is shown that the growth rate of unstable perturbations depends significantly on the physical parameters of the model. [ABSTRACT FROM AUTHOR]

The interaction between ocean and ice shelf is a critical physical process in relation to water mass transformations and ice shelf melting/freezing at the ocean-ice interface. However, it remains challenging to thoroughly understand the process due to a lack of observational data with respect to ice shelf cavities. This is the first study to simulate the variability and circulation of water mass both overlying the continental shelf and underneath an ice shelf and an ice tongue in the Terra Nova Bay (TNB), East Antarctica. To explore the properties of water mass and circulation patterns in the TNB and the corresponding effects on sub ice shelf basal melting, we explicitly incorporate the dynamic-thermodynamic processes acting on the ice shelf in the Regional Ocean Modeling System. The simulated water mass formation and circulation in the TNB region agree well with previous studies. The model results show that the TNB circulation is dominated by the geostrophic currents driven by lateral density gradients induced by the releasing of brine or freshwater at the polynya of the TNB. Meanwhile, the circulation dynamics in the cavity under the Nansen Ice shelf (NIS) are different from those in the TNB. The gravity-driven bottom current induced by High Salinity Shelf Water (HSSW) formed at the TNB polynya flows towards the grounding line, and the buoyance-driven flow associated with glacial meltwater generated by the HSSW emerges from the cavity along the ice base. Both current systems compose the thermohaline overturning circulation in the NIS cavity. This study estimates the NIS basal melting rate to be 0.98 m/a, which is comparable to the previously observed melt rate. However, the melting rate shows a significant variation in space and time. [ABSTRACT FROM AUTHOR]

Purpose. The aim of the work is to clarify the spatial structure of the climatic dynamic fronts (geostrophic current jets) and to estimate the relationship between their position and that of the large-scale temperature fronts on the surface of the Scotia Sea and the adjacent water areas in the southwestern part of the Atlantic sector of Antarctica. Methods and Results. The daily averaged data arrays of the CMEMS (1993–2017) and NOAA OI SST (1982–2017) reanalysis at the regular 0.25° grid were used. The CMEMS reanalysis contains the sea surface geostrophic velocity values, the NOAA OI SST reanalysis – the sea surface temperature ones which were reduced to the climatic form through their averaging for each month of the corresponding periods. Position of the current jets and the temperature fronts was determined using the maximums of the geostrophic velocity components and the extremes of the temperature horizontal gradients. The updated scheme of the average long-term position of dynamic fronts was constructed. It shows that in the areas of the most pronounced bottom topography inhomogeneities (the northern boundary of the Falkland Plateau and the Tierra del Fuego shelf, the boundaries of the Falkland Islands shelf and the Birdwood Bank, the Shackleton Ridge and the South Shetland Islands shelf), the fronts do not change their latitudinal position during a year. It is revealed that in most of the water area, the temperature horizontal gradient extremes (temperature fronts) correspond to the geostrophic velocity maximums (dynamic fronts). The Northern and Central Branches of the Antarctic Circumpolar Current are most clearly manifested in the temperature field. In general, in the water area under study, the average annual latitudinal position of the Subantarctic and Antarctic Polar Fronts is displaced to the south relative to the position of the Northern and Central branches jets of the Antarctic Circumpolar Current by 0.25–0.5° and 0.25–1°, respectively. Conclusions. It is shown that, being influenced by the bottom topography, the large-scale jets of geostrophic currents form intense topographic meanders and recirculation branches which are stably manifested on the climatic scale. The Antarctic Circumpolar Current branches being affected by the bottom topography, can merge forming the joint flows, and then diverge forming a system of separate jets again. It is found that the main spatial features of frontal structure in the geostrophic velocities field persist throughout the whole year and are conditioned mainly by the bottom topography. The most of the dynamic fronts are shown to be clearly pronounced in the temperature field on the surface during a year. A high level of linear correlation between the positions of temperature fronts and current jets was revealed; the correlation coefficient values are 0.6–0.97.

Filtering methods are usually used to combine the mean sea surface (MSS) and geoid (computable by global geopotential model (GGM)) into a common subspace, to model mean dynamic topography (MDT), which may lead to signal leakage and distortion problems. The use of the rigorous least squares (LS) method and multivariate objective analysis (MOA) alleviates these problems, and the derived MDTs from these two methods show better performance than MDTs derived from filtering methods. However, the advantages and disadvantages of these two methods have not been evaluated, and no direct comparison has yet been conducted between these two approaches regarding the performances in MDT recovery. In this study, we compare the performances of the MOA method with the LS method, providing information with respect to the usability of different methods in MDT modeling over regions with heterogeneous ocean states and hydrological conditions. We combined a recently published mean sea surface called DTU21MSS, and a satellite-only GGM named GO_CONS_GCF_2_DIR_R6, for MDT computation over four typical study areas. The results showed that the MDTs derived from the LS method outperformed the MOA method, especially over coastal regions and ocean current areas. The root mean square (RMS) of the discrepancies between the LS-derived MDT and the ocean reanalysis data was lower than the RMS of the discrepancies computed from the MOA method, by a magnitude of 1–2 cm. The formal error of the MDT estimated by the LS method was more reasonable than that derived from the MOA method. Moreover, the geostrophic velocities calculated by the LS-derived MDT were more consistent with buoy data than those calculated by the MOA-derived solution, by a magnitude of approximately 1 cm/s. The reason can be attributed to the fact that the LS method forms the design matrix segmentally, based on the error characteristics of the GGM, and suppresses high-frequency noise by applying constraints in different frequency bands, which improves the quality of the computed MDT. Our studies highlight the superiority of the LS-derived method versus the MOA method in MDT modeling. [ABSTRACT FROM AUTHOR]