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This study presents a generalised characterisation of the surface circulation over the northern shelf of the Gulf of Cadiz, based on 4 years of high-resolution satellite altimetry data from Sentinel-3A and wind model data. The altimetry-based surface zonal currents, adjusted for bottom-drag and wind effects, are compared with a generic CMEMS product and validated against in-situ ADCP measurements. The proposed altimetry product demonstrates superior performance than the CMEMS product, accurately reflecting surface circulation direction compared to in-situ measurements (r = 0.77, RMSE = 0.10 m/s, bias = 0.01 m/s). The use of the bottom-drag and wind-corrected/uncorrected altimetry product for spatiotemporal analysis of the shelf circulation revealed the distinct contributions of wind-driven and geostrophic components in different basin sectors. The results show that over the western basin, positive (eastward) surface currents were predominantly driven by westerly winds, while only occasionally, westward flows coincided with easterly winds, suggesting a higher control of the geostrophic component over the westward flows. In contrast, over the eastern basin, both eastward and westward flows were found to be primarily driven by favourable winds. Additionally, the analysis of Absolute Dynamic Topography (ADT) values along the whole basin showed the presence of ADT gradients both along-shore and cross-shore over the shelf, contributing to geostrophic flows. Finally, the seasonal analysis showed that eastward circulation tends to dominate during the spring and summer months, related to the upwelling season in the Gulf of Cadiz and associated westerly winds. Westward flows prevail during the winter months, related to easterly winds and the rebalancing of the along-shore sea level gradient during relaxed upwelling conditions. The findings demonstrate a significant improvement in the use of satellite altimetry data to study complex oceanographic dynamics in coastal areas, where both spatial and temporal variability are high. Moreover, the similarity of our results to those obtained from in-situ systems supports the use of altimetry data and publicly available wind models to support oceanographic studies in remote or resource-limited areas. [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]

The Slope Sea is the dynamic ocean region located between the United States and Canadian northeast continental shelves and the northeastward flowing Gulf Stream (GS) located farther offshore. Here we define it as located between the 200-m isobath and the monthly GS sea surface temperature (SST) front from −75° to −55° E. Monthly mean near-surface eddy kinetic energy (EKE) was computed for the Slope Sea using surface geostrophic current anomalies derived from gridded 1993–2016 Copernicus Marine Environment Monitoring Service (CMEMS) sea height anomalies. Long-term, monthly mean Slope Sea EKE anomalies show a robust seasonal cycle with a winter (February) minimum and summer (June) maximum. This agrees with both seasonally-varying density stratification and dissipation and also the seasonal variation in the formation of GS WCRs within the Slope Sea. The RMS of the Slope Sea EKE seasonal cycle generally increased after 2002 by a factor of up to ∼2 relative to prior years. The seasonal cycle of Slope Sea EKE displayed higher EKE in the vicinity of the New England Seamount Chain (NESC) that extends towards the shelf break front from approximately −67° E to −63° E. Interannual variability of annual mean near-surface EKE from individual digitized GS warm core ring (WCR) observations from a Bedford Institute of Oceanography (BIO) WCR database is highly correlated with Slope Sea EKE. However, interannual variability of annual mean near-surface EKE computed from a census of all newly formed WCRs displayed only a weak correlation. Many of the WCRs from both the BIO and WCR census displayed anomalously low EKE values and were observed within the northern Slope Sea away from the GS. Some were located inshore of the position of the climatological mean shelf break front. WCRs with higher EKE were located throughout the Slope Sea, with higher numbers in the vicinity of the NESC. The many observations of the less energetic features located close to or inshore of the mean shelf break front suggest they are important to cross-shelf fluxes of heat, salt, nutrients, shelf biota. They therefore likely impact the shelf ecosystem, similar to the more energetic and typical WCRs impacting the outer shelf as discussed by earlier workers. • Altimeter-derived currents describe eddy kinetic energy changes in the slope sea. • Slope sea eddy kinetic energy displays strong seasonal and inter-annual variation. • Slope sea eddies display large variation in strength and formation location. • Eddy kinetic energy changes do not support the warm-core ring regime shift of 2000. [ABSTRACT FROM AUTHOR]

In the past three decades, altimeter-based remote sensing has been a widely used system to estimate ocean surface currents. However, it remains a great challenge to effectively resolve scales below ∼100 km at high latitudes and ∼ 300 km at mid-latitudes. In this study, we propose a scheme that utilizes geostrophic equilibrium and surface quasigeostrophy theory (SQG) to improve surface current resolution by incorporating remote sensing sea surface temperature (SST), sea surface height (SSH), and sea surface salinity (SSS) observations. The scheme separately characterizes the larger-scale flows and smaller-scale motions of surface currents. A case study encompassing the Agulhas surface current demonstrates that the smaller-scale motions associated with temperature fronts are well captured by introducing high spatial-temporal resolution SST data. Furthermore, the reconstructed surface current is systemically evaluated by using surface drogued drifters and a Lagrangian synthetic particle tracking tool throughout the South Indian Ocean (SIO) for 2011–2015. Notably, the reconstructed zonal velocity component is closer to the drifter observations than the meridional counterpart and corresponding velocity phase. Regionally, the Antarctic Circumpolar Current (ACC) showcases superior reconstruction performance, with higher skill scores and lower Lagrangian separation distances. However, a relatively large uncertainty is observed around the Agulhas Retroflection (AR) and Greater Agulhas System (GAS), which are linked to complicated regional dynamic regimes. We finally conduct four simulation experiments to explore the effect of different SST products on surface current reconstruction within the subdomain AR. The results indicate the varying potentials of the four evaluated SST products for informing surface current applications. Specifically, the MWIRSST enhances the likelihood of particles reaching the target field, while DMI OI shortens the average deviation distance of the arrived particles. • A new scheme estimating the surface current is proposed based on the SQG theory. • Small-scale motions are resolved by introducing high-resolution SST. • Uncertainty of the estimated currents partially depends on different SST products. [ABSTRACT FROM AUTHOR]

The gravitational field of the Earth reflects Earth's surface mass redistribution and its inner structure and dynamics. Satellite gravimetry techniques have been used to observe the Earth's external gravitational field and its temporal variations on a global scale. The global gravitational models from satellite gravimetry, typically in terms of spherical harmonic coefficients, are crucial in geodetic, geodynamic, geophysical, hydrological, glaciological, oceanographic, and many other geoscience applications. In this paper, we provide a comprehensive overview of theoretical definitions describing relationships between the spherical harmonic coefficients and different satellite gravimetry observables such as orbital perturbations in terms of satellite positions, velocities, and accelerations; satellite-to-satellite range rates; and gravitational gradients. Products and applications of the Earth's static global gravitational models are presented and discussed in the context of determination of the gravimetric geoid and physical heights, gravimetric and isostatic crustal thickness, bathymetric depths, glacier bedrock relief, sediment thickness, geostrophic and eddy currents, Earth's inertia tensor and dipole, precession and nutation parameters of the Earth's rotation, and prediction of the satellite orbital geometry. Furthermore, applications and advances of the Earth's time-variable gravitational models for monitoring large earthquakes, hydrological mass transport, Earth's rotation parameters, and vertical crustal motions (due to the glacial isostatic adjustment and other phenomena) are presented. Finally, future trends and prospects in the satellite gravimetry are discussed. • A comprehensive mathematical foundations of satellite gravimetry is presented. • A complete progress for applications of satellite gravimetry products is shown. • An overview for future satellite gravimetry missions and trend is given. [ABSTRACT FROM AUTHOR]

• This study provided a new approach for constructing the eddy real-time internal structure using the gradient-dependent optimal interpolation (OI) method. • The reliability of reconstruction was evaluated by satellite observation, in-situ data, and numerical output. • From three eddy cases, the real-time structures reflect more small fluctuations with a great change comparing with the background structure. • The results indicated that the gradient-dependent OI was a hopeful technique for representing the real-time internal features during eddy evolution. The internal structures of mesoscale eddies are variable due to different generation mechanism, spatiotemporal scale, and movement characteristics. Based on the principle of data assimilation, this study provided a new approach for constructing the eddy real-time internal structure using the gradient-dependent optimal interpolation (OI) method. Three eddy cases with different types and locations were selected to validate the construction results. The reliability of construction was evaluated by satellite observation, in-situ comparisons, and contrast with the numerical output. The results showed that the real-time geostrophic current characters were basically consistent with the surface and deep current (observed by satellite and acoustic doppler current profiler, ADCP). The density features exhibited by constructed results (eddy centers, mean radii, and variation tendency) were similar to the expendable conductivity-temperature-depth (XCTD) survey data and numerical output. Compared to the composite structure, the real-time structures represented more small or mesoscale fluctuations. During eddy cases evolution, the mean radii at the same depth first increased and then decreased. The horizontal scale, influence depth, and internal stability of an anticyclonic eddy with low-latitude were more powerful than the other two eddy cases owing to the domination of warm water in the Kuroshio extension region. The results indicated that the gradient-dependent OI was a hopeful technique for representing the real-time internal features during eddy evolution. [ABSTRACT FROM AUTHOR]

The North Brazil Current (NBC) flows offshore of the mouth of the Amazon River and seasonally sheds anticyclonic rings (NBC rings) that propagate northwestward and interact with the Amazon River plume (ARP). Mesoscale features have a high temporal variability that is hard to monitor from current weekly and monthly sea surface salinity (SSS) satellite fields. Novel SSS fields with a higher temporal resolution analyzed together with satellite geostrophic currents, chlorophyll-a, and wind speed and in-situ data from the "Microbiomes cruise" on the SV Tara in August–September 2021 revealed a late summer freshwater pathway, which was not well documented in earlier studies. By combining these datasets, we improved the characterization of summer ARP pathways. In 2021, the ARP was a succession of freshwater patches cut off from the main plume by the NBC rings. A patch of about 200.000 km2 with salinity <33.5 pss was observed in September 2021, bringing 0.5 Sv of Amazon water northwestward in a period where the mean ocean currents lead to eastward transport. This patch was shallow, very stratified, and it created a surface steric-height anomaly that was identified as an anticyclonic feature in altimetric sea level products. Once separated from the NBC retroflection, it was mainly driven by Ekman currents. Other similar patches were observed during the 2021 summer, leading to a strong intermittency of the ARP transport. They strongly contributed to make 2021 the year with the largest northwestward freshwater transport in late summer within the 2010–2021 time-period investigated. This freshwater transport pathway is important for all plume-related phenomena, and show the ability of combined SMOS and SMAP data to accurately represent the day-to-day SSS variability. • New high-resolution satellite fields monitor mesoscale SSS day-to-day variability. • Satellite and in-situ data reveal freshwater patches detached from the Amazon plume. • The pathways of freshwater patches are northwestward and eastward. • Eddies, geostrophic and Ekman currents drive the fresh patches pathways. • Sept. 2021 mesoscale freshwater transport to the north is the largest since 2010. [ABSTRACT FROM AUTHOR]

The blue shark (Prionace glauca), a highly migratory and wide-ranging shark, has been classified as Critically Endangered in the Mediterranean Sea by the International Union for Conservation of Nature since 2016 due to overfishing. Surprisingly, basic information on its biology and ecology, essential for its conservation, remains unknown, and its regional stock has not been assessed. A multi-year large-scale telemetry tagging programme was implemented to fill these gaps. A total of 39 blue sharks were equipped with satellite tags in three areas of the Western Mediterranean, and their movements and home ranges were examined by sex and size class. Their distributions and habitat preferences across seasons were identified using satellite-based environmental data. Tagged sharks were capable of long-range movement between distant regions, but they did not move neither towards the Eastern Mediterranean basins nor the North Atlantic. We identified the Gulf of Lions as the mating, parturition and nursery grounds. We demonstrated that sharks can use the general geostrophic current system for large movements. Our results support the current stock boundary in the Mediterranean based on international conventional tagging programmes and genetic studies, but the absence of connectivity between the two Mediterranean basins and the North Atlantic basin suggests that they have distinct stocks. Our findings provide critical input for developing effective conservation and management plans for this cosmopolitan species. For the first time in the Mediterranean, this study provides insights into the movement patterns of blue sharks and their habitat preferences. [Display omitted] • First large-scale telemetry tagging documenting the distribution and movements of blue sharks in the Mediterranean. • The Gulf of Lions (France) is a major mating, parturition and nursery ground in the Western Mediterranean. • Blue sharks use the general geostrophic circulation system to move long distances. • A tentative migration pattern is proposed. • No connectivity between the two Mediterranean basins and the North Atlantic basin was found. [ABSTRACT FROM AUTHOR]

• Overview of NASA's EXport Processes in the Ocean from Remote Sensing 2021 Field Campaign in the North Atlantic (EXPORTS NA). • A multi-ship, multi-asset field program conducted Lagrangian sampling of a mesoscale eddy for carbon pump analyses. • Storms modulate springtime primary productivity through entrainment and wind-driven advection. This manuscript presents an overview of NASA's EXport Processes in the Ocean from Remote Sensing 2021 Field Campaign in the North Atlantic (EXPORTS NA) and provides quantitative and dynamical descriptions of the physical processes modulating water transformations during the study. A major programmatic goal was to conduct the sampling in a Lagrangian mode so that ocean ecological and biogeochemical changes can be observed independent from physical advective processes. To accomplish this goal, EXPORTS NA conducted a multi-ship, multi-asset field sampling program within a retentive, anticyclonic mode water eddy. Beneath depths of ∼ 100 m, Lagrangian sampling assets remained within the eddy core waters (ECWs) throughout the experiment, demonstrating that the ECWs within the mode water eddy were retentive. However, strong westerly winds from four storm events deepened the mixed layer (ML) of the surface core waters (SCWs) above the eddy's mode water core by 25–40 m and exchanged some of the SCWs with surface waters outside of the eddy via Ekman transport. Estimates of flushing times ranged from 5 to 8 days, with surface exchange fractions ranging from 20 to 75 % and were consistent with particle tracking advected by combined geostrophic and Ekman velocities. The relative contributions of horizontal and vertical advection on changes in ECW tracers depended on the horizontal and vertical gradients of that tracer. For example, horizontal advection played a large role in ECW salinity fluxes, while vertical entrainment played a larger role in the fluxes of nutrients into SCW ML. Each storm injected nutrients and low oxygen waters into the ML, after which the surface ocean ecosystem responded by reducing nutrient concentrations and increasing %O 2 saturation levels. Overall, ECW values of chlorophyll and POC were the largest at the onset of the field program and decreased throughout the campaign. The analysis presented provides a physical oceanographic context for the many measurements made during the EXPORTS NA field campaign while illustrating the many challenges of conducting a production-flux experiment, even in a Lagrangian frame, and the inherent uncertainties of interpreting biological carbon pump observations that were collected in a Eulerian frame of reference. [ABSTRACT FROM AUTHOR]

A group of self-developed surface drifters were released near the Pearl River Estuary (PRE) from November to December 2021, and it is found that most of the drifters were finally stranded off the coast around Hainan Island after 10–30 days. This result makes us want to find out whether the surface floating objects around the PRE are converging on Hainan Island and what factors dominate this process. The velocity of the drifters not only displays prominent characteristics of tidal currents but is also highly correlated to winds associated with winter monsoon. A particle tracking model is employed to investigate the fate of the drifters and detailed process. Model simulation results show that the most important factor is winds, which affect the movement of floating objects through the direct way (direct wind drag) and indirect ways (wind-driven currents and the wave-induced Stokes drift). The influence of non-wind-driven flows (e.g., geostrophic currents, density-driven currents) is relatively limited. Tidal currents can significantly influence trajectory details within a tidal cycle. Extended model experiments with a large number of surface floaters released in October and January 2021 show that ∼34% of the surface floaters are stranded near the PRE, ∼30% are stranded to the east of Hainan Island, and ∼ 25% move to the west of Hainan Island. Our results indicate that a high proportion of the surface floaters from the PRE would converge around Hainan Island in the autumn and winter, of which the environmental impact warrants further investigation. • Drifters released near the Pearl River Estuary quickly moved to the Hainan Island. • The simulation reveals the dominant role of wind on the floater movement. • Experiments display a similar proportion (∼55%) of floaters stranded near Hainan. [ABSTRACT FROM AUTHOR]

• The analysis of daily currents in the GoC and SoG using HFR and SAMPA model revealed an EOF Mode one that captured important coastal current patterns such as CCCs and eastward current intensification episodes. • Applying a real EOF decomposition on the SAMPA time series provided the extension to a spatial dominion that comprised the GoC, SoG and westernmost Alboran Sea areas. • The main CCC and eastward current intensification events are connected along the three areas. These intense and extensive current intensification processes seem to be triggered by a synchronized action of local winds over the GoC and additional remote effects which are coming from (i) the wind forcing over the Alboran Sea and the easternmost side of the SoG and (ii) the MSLP forcing over the Ligurian Sea. Several studies about the coastal circulation over the continental shelf of the Gulf of Cadiz have been published in the last twenty years. One of the most studied features in these papers are the so-called Coastal Counter Currents (CCCs) which are located roughly between the 100 m isobath and the coastline and are directed westwards. Despite the considerable number of articles devoted to the analysis of this phenomenon, the actual along-coast extension and origin of these CCCs currently remain unclear. In the present study, different mechanisms are proposed to explain the development of these CCCs. They have been revised and discussed taking into account the latest findings from a recent analysis that utilized surface currents derived from High Frequency Radar (HFR) as well as numerical model simulations. The most interesting finding is that the most intense CCCs events are initiated on the coastal margin of the westernmost side of the Strait of Gibraltar, and they are extended further west of Cape Santa Maria. Furthermore, these intense and extensive CCCs seem to be triggered by the joint action of local winds over the Gulf of Cadiz and additional remote effects which are coming from (i) the wind forcing over the Alboran Sea and the easternmost side of the Strait of Gibraltar, and (ii) the Sea Level Atmospheric Pressure forcing over the Ligurian Sea (western Mediterranean Sea). In addition, the same forcing mechanisms are valid to explain the eastward intensification of the coastal currents. The most relevant feature of the hydrodynamic pattern originated by this combination of forcing agents is its quasi-geostrophic adjustment against the coast in the whole spatial dominion. In addition, these results offer a clear example of how important the effects of forcing in rather remote regions may be in defining the hydrodynamic patterns of certain oceanographic regions. [ABSTRACT FROM AUTHOR]

We investigated the characteristics of seasonal variations in sea surface salinity (SSS) in the subtropical North Pacific and North Atlantic oceans by analyzing observational datasets. As noted in previous studies, the seasonal variation in SSS markedly differs between the western and eastern parts of the basins. These contrasting seasonal variations in SSS have nonnegligible impacts on surface density for each basin and translate into changes in temperature at the outcropping isopycnals of about 1.0 °C. Further analysis of the mixed-layer salinity budget reveals that the zonally contrasting seasonality is attributed to a combination of seasonal variations in the freshwater flux and mixed layer (ML) depth. In the western basins, the wintertime increase in evaporation and reduction in precipitation, as well as the vertical entrainment of salty water below the ML contribute to salinification, whereas the seasonal freshening in summer is mostly attributed to increase in precipitation, reduction in evaporation, and strengthening of geostrophic currents. In contrast, evaporation exceeds precipitation throughout a year in the eastern basins, and seasonal variations in freshwater flux cannot fully explain the seasonal cycle of SSS in the region. Rather, the wintertime freshening there is aided by the deepening of the ML, which acts to mitigate salinification by the evaporation and sustain vertical entrainment of fresher water below the ML. Summertime salinification there can be understood as a mirror image of the wintertime condition, but seasonal strengthening of Ekman currents and associated salinity advection also play a role. (241 words) [ABSTRACT FROM AUTHOR]

In this study, we use seven years of Cryosat-2 data to improve Mean Sea Surface (MSS) and also to estimate Mean Dynamic Topography (MDT) around Australia. Sea Level Anomaly (SLA) map, obtained from Cryosat-2 data, shows substantial spatial striping effects in the areas where annual signal has considerable amplitudes. This signal causes shifts among the SLAs acquired from adjacent tracks since they have collected at different times of the year. In order to mitigate these effects, we used Topex/Poseidon and follow on missions to estimate the seasonal signals in the Cryosat-2 data points. MSS C2 is then estimated by (1) removing these signals from SLAs, (2) averaging in a 0.1° × 0.1° grid cells, and (3) finally adding them to the DTUMSS13. The resultant surface shows good agreement with the MSS estimated by Jason-1 and Jason-2 data in altimetry nominal points. MDT C2 is also estimated in the study area using the MSS C2 and the geoid. It is in good agreement with two widely used global MDT models although showing higher values due to the effect of sea-level rise. When compared to the estimated MDT in tide gauge stations using geodetic data, MDT C2 statistically performs better than the global models. [ABSTRACT FROM AUTHOR]

• Processing of three different kinds of drifters to extract the geostrophic component. • The use of drifters leads to a higher improvement than an additional altimeter. • Improvement is of 1% in SLA and 10% in meridional velocities. • Both components of the velocity vector have similar accuracy. Knowledge of ocean surface geostrophic circulation has been greatly improved due to satellite observations. However, it is essential to use different observation sources (satellites and in situ data) in order to improve accuracy and resolution. This study combines along-track Sea Level Anomalies (SLA) with geostrophic velocity estimated from surface drifters to map SLA and associated geostrophic current anomalies in the Gulf of Mexico. Firstly, substantial pre-processing is needed on the drifter data to extract the geostrophic component of the signal in order to be consistent with the physical content provided by altimetry. This step includes estimating and removing the Ekman current, Stokes drift and wind slippage. Three kinds of drifters are used: – Drifters belonging to Woods Hole Group, a CLS Group company that launches their own drifters in the Gulf of Mexico. – Drifters launched in the framework of the Lagrangian Submesoscale ExpeRiment (LASER) campaign (January-April 2016). – Drifters from Atlantic Oceanographic and Meteorological Laboratory (AOML). Secondly, drifters and along-track SLA from Jason2, HY2, SARAL and Cryosat-2 are combined through multivariate objective analysis to map a daily time series of SLA and associated geostrophic current anomalies from 01/09/2015 to 30/04/2016. Finally, comparisons with independent data reveal the improved agreement of maps combining both altimetry and drifter data, especially for the meridional component of geostrophic current. [ABSTRACT FROM AUTHOR]

The coastal mean sea surface (MSS) has applications within oceanography as well as geodesy. Together with a geoid model, it forms an important component for geodetic mapping of ocean surface currents that are in geostrophic balance. Furthermore, it forms a bridge between open ocean MSS and in situ measurements of mean sea level at or close to land, it contributes to the mapping of the geoid and the marine gravity field, and it is essential for connecting tidal nautical chart datums to physical height systems or global geodetic reference frames. In this study, we determine a coastal MSS with an associated error field for Norway. The MSS is solely based on new-generation altimetry data, i.e., SAR(In) data from Sentinel-3A and CryoSat-2, as well as Ka-band data from SARAL/AltiKa. The data sets partly overlap in time and cover the time period from 2010 to 2017 inclusive. We have chosen these altimeters because they represent evolutions of conventional altimetry, with reduced footprint sizes as a main benefit. This is especially advantageous in the coastal zone, as a smaller footprint reduces the probability of radar pulses being contaminated by energy backscattered from land areas. The satellite missions were harmonized by applying inter-mission biases determined in a regional crossover analysis. Furthermore, in a zone closer to land than 25 km, we have replaced the global ocean tide model with a regional ocean tide model provided by the Norwegian Mapping Authority (NMA). We use an optimal interpolation technique to determine a coastal MSS grid and discuss it in context of the estimated error field. We assess our coastal MSS by comparison to state-of-the-art MSS products along three sections perpendicular to the coast, as well as ellipsoidal mean sea level as observed by an array of permanent tide gauges within the study area. In addition, we assess a higher-resolution version of our MSS in the NMA testbed for vertical datums, by comparison with temporary tide gauges. We find that the coastal MSS outperforms the global MSS models directly at the coast, with standard deviations of differences of ~ 8 cm to the tide gauges, compared to 14–22 cm, obtained with the global MSS models. All MSS models largely agree along three sections perpendicular to the coast, with standard deviations of differences of 2–4 cm. The higher-resolution version of the coastal MSS performs similarly to the coastal MSS in comparison with the temporary tide gauges (standard deviation of differences of ~ 8 cm), but its formal error field also quantifies large uncertainties at the coast and in the fjords, mainly due to the lack of altimetry observations. A trustworthy error field is decisive for the combination of altimetry with other sea-level observations. [ABSTRACT FROM AUTHOR]

Understanding the role of the Pacific Decadal Oscillation (PDO) during the Last Interglacial (LIG, ∼127 ka) may advance our knowledge on decadal climate variability in a warmer world. In this study, we investigate the PDO–precipitation relationship over eastern China during the LIG based on the NorESM2-LM, which shows the best performance in capturing the observed PDO pattern and the PDO–precipitation relationship among 12 models from the Paleoclimate Modelling Intercomparison Project Phase 4. Composite analysis shows that summer precipitation during the LIG is generally increased over eastern China in the positive PDO phases relative to the negative phases, which is different from the "drier-wetter-drier" triple pattern in the present day. The discrepant precipitation patterns between the LIG and preindustrial are caused by different responses of composite atmospheric circulations. Moreover, moisture budget analysis indicates that variations of vertical velocity over eastern China largely explain the PDO-linked precipitation patterns during both LIG and preindustrial, although the main causes for vertical motion changes are different across regions and periods based on the quasi-geostrophic ω equation. Additionally, these discrepancies in composite atmospheric circulations are largely linked to differences in PDO patterns and mean climate states between the LIG and preindustrial. Our results suggest a varying relationship between the PDO and precipitation over eastern China in a warmer-than-present world induced by Earth's orbit, which have important implications for PDO reconstructions during the LIG via the PDO-precipitation relationship reported here and shed light on decadal variability of precipitation as well as the associated decadal predictions in a future warmer world. • Divergent response of eastern China precipitation to PDO during the LIG. • Discrepant composite atmospheric circulations cause varied precipitation patterns. • PDO pattern/mean climate state regulate varied precipitation during the LIG jointly. [ABSTRACT FROM AUTHOR]

The self-organizing map (SOM) is implemented to explore various ocean surface current patterns in the Bay of Bengal (BoB) from 1993 – 2015 based on a satellite sensor-derived current product estimated by combining geostrophic, wind-driven, and temperature-driven current components. Four major patterns/groups corresponding to some of the primary current systems in the BoB are obtained (G1, G2, G3, and G4). The start and end of all the patterns drastically varied year to year and are primarily related to positive and negative Indian Ocean Dipole (IOD) and El Niño Southern Oscillation (ENSO) years. The correlation of ENSO and the following year's transition date from WBC to SMC is 0.70, while -0.76 for IOD and transition date from EICC to WMC. Through temporal analysis and monthly frequency of occurrence measurements, this study reveals that the first significant dominant pattern is observed during the summer monsoon season when the southwest monsoon current (SMC) is visible near the south of Sri Lanka (pattern G1). The following major pattern is the winter monsoon current (WMC) in the south of Sri Lanka (pattern G4), whose formation is contributed by the northeast monsoon and westward propagating Rossby wave. The other two dominating patterns are East India Coastal Current (EICC, G2) and Western Boundary Current (WBC, G3). SOM analysis is then performed on WBC with 9 nodes (3 × 3 , n a m e d P 1 t o P 9) , which captures the meandering nature (P3), and meandering in the presence of SMC (P2), the straight nature (P7) and in the presence of SMC (P4). The spatial and temporal analysis of nine SOM patterns (P1 – P9) suggests that the non-meandering nature of WBC (P4) occurs more frequently than meandering WBC (P2). • Surface Circulation can be efficiently captured by Self-Organizing Maps (SOM). • Southwest monsoon current is a dominant feature with 44.43% occurrence in a year. • Variability of transition of boundary currents are influenced by IOD and ENSO. • The non-meandering WBC is more frequent than the meandering ones. [ABSTRACT FROM AUTHOR]

Using shipborne and satellite ‒ derived data, we demonstrate the water circulation, temperature, salinity, and chemical parameter distributions in the western Chukchi Sea in the summer 2002 and summer 2019. In summer of 2002, the subsurface layer in the western Chukchi Sea was composed of Bering deep basin water with the salinity of about 33.0. In 2019, the deep layer of the western Chukchi Sea was occupied by winter Bering Sea shelf water with the salinity of 32.2–32.5. An inflow of Arctic water (increased salinity/density and low concentration of nutrients) through the Herald Canyon led to cyclonic gyre water circulation in the southwestern Chukchi Sea. The cyclonic gyre in the southwestern Chukchi Sea was characterized by anomalously low pCO 2 in the surface and subsurface layers, low pH and high pCO 2 in the bottom water, and high biomass of benthos. [ABSTRACT FROM AUTHOR]

Mesoscale eddies are major features in ocean dynamics that significantly influence ocean production depending on their polarity, life-stage, location and time of formation. Eddies in the south-west Indian Ocean have been the focus of many studies, especially in the Mozambique Channel where southward-moving cyclonic and anticyclonic eddies feed into the Agulhas Current. However, eddies formed south of Madagascar that move westwards towards the coast of South Africa have been much less studied, despite their significant contribution to the Agulhas Current. This study aims to understand how the plankton communities (i.e. pico-, nano-, phyto-, microzoo- and mesozooplankton) are distributed in a cyclonic eddy generated off southern Madagascar. We also investigate a possible link between the Madagascar shelf and the eddy. Temperature, salinity, fluorescence and plankton abundance were measured at 25 stations spaced at 18.5 km intervals across a young (~ 1 month old) cyclonic eddy, and at three locations on the Madagascar shelf, in July 2013. A patch of enhanced chlorophyll a concentration was found at four stations in the eddy, homogeneously distributed vertically in the upper mixed layer and characterised by a higher proportion of diatoms compared to the other stations. Abundances of picoplankton, nanoplankton, ciliates and dinoflagellates were very similar within and outside the eddy. Zooplankton followed a contrasting pattern with higher biomass at four stations just to the west of this high diatom patch. The mesozooplankton composition at these four stations revealed greater species similarities with the Madagascar shelf than with the other stations, suggesting a link between the shelf and the western side of the eddy. The periphery of the eddy seems to have been primarily influenced by a strong geostrophic current propagating from the southern shelf of Madagascar which then wrapped itself around the eddy. This filament may be the source of the high zooplankton biomass in the eddy periphery, which would account for the similarities in zooplankton community composition between the eddy periphery and the shelf. [ABSTRACT FROM AUTHOR]

We investigated the effect of the assimilation of surface velocities from two HF-Radars (HFRs) and satellite sea surface temperature (SST) into an ocean circulation model, implemented on the North-Western Mediterranean Sea, using the ROMS 4D-Var Data Assimilation (DA) System. A three months period from September to November 2020 is analysed. A model free run is used as reference simulation to quantify the impact of the data assimilation procedure on the surface circulation. The DA framework consists of a sequence of 3-day windows during which the observations are assimilated into the model, followed by forecast simulations, starting from the last time step of the analysis run. The comparison of simulated velocities and SST against the assimilated observations shows that both forecast and analysis runs improve the solution as compared to the freerun, with error reduction up to the 47% and doubling of correlation values. If SST is not assimilated, the simulated sea surface temperature slightly degrades as a result of the DA procedure. Furthermore, we used surface velocities from Lagrangian drifters to test the model against independent observations. The results show that both the forecast and the analysis simulate surface circulation better than the free run within the areas covered by HFR observations, with error reduction ranging from 10% to 50%, and increase in correlation from 5% to more than doubled. Nevertheless, farther afield, outside the area covered by HFRs, improvements and degradations of the solution due to the DA procedure balance each other. The impact of assimilated observation on the alongshore transport of the upper 50 m is analysed along three transects adjacent to the two HFR areas and along a section in between. The transport increment is significant where the surface velocities are assimilated whereas it is negligible elsewhere. The effect of SST is to modify the velocity distribution along the transects, while keeping the transport increment unchanged. Modifications to the initial conditions have the most significant effect on the alongshore transport, whereas the corrections to boundary conditions and atmospheric forcing have different importance according to the analysed transects. The assimilation of surface velocities and SST affects more the ageostrophic component of the surface velocity field, rather than the geostrophic one. • 4D-Var DA of HF-Radar surface velocities and SST improve analysis and forecast. • Validation with independent data shows that benefits are limited to the HF-Radar area. • HF-Radar data change transport magnitude and SST modulates the velocity distribution. [ABSTRACT FROM AUTHOR]

Despite the increasing volume of oceanographic data, the available ocean salinity data remains patently insufficient which limits studying ocean dynamics, climate change and the calculation of salinity-related ocean elements. Considering that traditional salinity reconstruction methods often suffer from various factors such as additional constraints, priori physical assumptions and large of specific regression coefficients, a generative adversarial networks (GANs) based deep learning (DL) framework is proposed to directly construct a near real-time, high-resolution daily three-dimensional ocean subsurface salinity (3D OSS) dataset from a data-driven perspective in this study. Four models with different structural combinations are designed in the China's marginal seas. Experimental results demonstrate that the models can successfully reconstruct the high-precision and high-resolution 3D OSS on a daily scale in 12 depth levels (from 2 m to 200 m). The asymmetric inception-3DGAN model with all enhanced structures has the highest accuracy, the average root-mean-squared error (RMSE) is 0.135psu, the average coefficient of determination (R2) is 0.5641 and the percent bias is 0.436%. Comparing with model without enhanced structures, the average RMSE is decreased by 22.41% when adding all the enhancement structures. Besides, temporal and spatial error analysis are also conducted to evaluate the models' performance from different aspects. Finally, the model results are used to analyze common ocean elements, such as water mass properties, dynamic fields and geostrophic velocity fields, demonstrating that the 3D OSS dataset construction approach proposed in this study not only provides some new insights into ocean observations, but also has high application values. • High precision daily 3D OSS is reconstructed via T–S mapping from a data-driven view. • Four 3D GAN based models are progressively built by powerful enhancement techniques. • Results are analyzed with in situ observations and reanalysis data. • Best reconstruction performance is found for the asymmetric inception-3DGAN case. • Reliable near real-time data can be provided for operational oceanography research. [ABSTRACT FROM AUTHOR]

Numerical ocean circulation forecasts diverge from reality due to non-deterministic processes. Through data assimilation, ocean forecasting systems routinely make corrections to scalar ocean variables of temperature and salinity derived from ocean observations. Velocity observations, however, are vector quantities that require different processing. To assimilate existing ocean velocity observations, which generally are depth limited, vertical error covariances are needed to relate the velocity errors throughout the water column. In this paper we show a velocity assimilation approach that relies on historical observations and geostrophy to relate velocity, temperature, and salinity. Geopotential calculations transform temperature and salinity covariances into corresponding geopotential covariances. This paper describes the methods and examines details of the solution process for ocean velocity data assimilation. The approach employs two separate three-dimensional variational (3DVAR) analyses combined to create one set of initial conditions during each daily forecast cycle. The first 3DVAR analysis utilizes temperature, salinity, and sea surface height anomaly data and reduces the background error variance based on these observations. The second 3DVAR analysis uses the reduced error variance and performs velocity data assimilation using surface drifter data, and these observations further reduce the error variance of temperature and salinity for the next cycle. The second 3DVAR analysis for velocity data assimilation utilizes the new full depth error covariances. We show diagnostic results from a Gulf of Mexico simulation during extensive drifter deployments in the summer of 2020. Results reveal that the two-step data assimilation cycle applies vector velocity innovations to create increments that correct the background field towards a geostrophic balance in temperature and salinity. • Surface drifter positions create Eulerian innovations for assimilation. • Empirically derived vertical covariances distribute velocity increments. • Velocity forecast error is derived from empirical vertical covariances. • Velocity increments constrain forecasted geostrophic velocities. [ABSTRACT FROM AUTHOR]

Abstract The intensity of the Kuroshio Current east of Taiwan shows a clear seasonality, and the Kuroshio is affected by mesoscale eddies east of Taiwan. Sea surface geostrophic velocity and mesoscale eddies derived from satellite altimeter data and gridded Argo-only geostrophic velocity data were used to analyze the mesoscale eddy contribution to the Kuroshio intensity seasonality in the East Taiwan Channel. The low potential vorticity water stacked at the northern side of the subtropical countercurrent (STCC) and eddy generation favored the cyclonic eddy. Between August and January, the STCC near the western boundary area (125°E-135°E) showed two branches and a significant negative zonal velocity meridional gradient between 19°N and 21°N from October to December, which led to a higher occurrence of cyclonic eddies. These eddies reached the area east of Taiwan in 100 days and resulted in the dominance of cyclonic eddies between January and April and a weakened Kuroshio Current. Highlights • Mesoscale eddy arrived east of Taiwan and its seasonality is analyzed. • Cyclonic eddies weaken the intensity of the Kuroshio Current between February and April. • We suggest that the seasonal subtropical countercurrent at lower latitudes controls the seasonality of eddies east of Taiwan. [ABSTRACT FROM AUTHOR]

Abstract Zooplankton was sampled in the Southern Gulf of Mexico, during autumn 2010, summer 2011 and winter 2013. Hydrographic data, dynamic topography and geostrophic velocities showed the presence of alternating cyclonic and anticyclonic gyres in the area, and a quasi-permanent gyre in the Bay of Campeche, with higher surface temperatures and lower salinities during autumn and summer, than in winter. Stronger rainfall was present during summer, which coincided with a strong salps "bloom" that considerably increased biovolumes. A significant difference in biovolumes was found for the three seasons, with highest zooplankton biovolumes during summer, and lowest during autumn. A shoaling of higher fluorescence and higher nutrients (Nitrate + Nitrite) was found in the BOC, in coincidence with higher biovolumes during the three seasons sampled. Multivariate analysis showed that the variables controlling the productivity were mainly fluorescence and nutrients for summer 2011 and winter 2013. Higher biovolumes were present in autumn, summer and winter at the same stations, being higher during summer. Data obtained show that the Bay of Campeche is a high productivity area, all year round, particularly inside the quasi-permanent gyre where high biovolumes were present in autumn, summer and winter, but with considerable seasonal variations in primary and secondary productivity, probably dependent in great measure from river discharge and rainfall. Highlights • Campeche Bay has high productivity during the entire year, around the quasi-permanent gyre. • A shoaling of high fluorescence and high nutrients produces higher biovolumes of zooplankton. • The seasonal variability is always higher in the studied area of Campeche Bay. [ABSTRACT FROM AUTHOR]

Abstract We have assessed the use of satellite derived currents to calculate oceanic surface drift, as an alternative to using currents from ocean models. Predicted trajectories are compared to observed trajectories of two types of drifting buoys, which are subject to different degree of wind and wave forcing, in addition to the ocean surface current. This variable degree of ocean-, wind- and wave forcing is highly relevant for the practical problem of predicting the drift of e.g. oil to aid cleanup operations, and the drift of floating objects to aid search and rescue operations. The calculated trajectories are evaluated in terms of separation distance, and a calculated skill score. For the submerged drifters (CODE/DAVIS), we find that a high resolution non-assimilated ocean model gives the best results, whereas a coarser scale assimilated model performs similarly to satellite derived surface currents from the GlobCurrent project (geostrophic + Ekman components). For the wind-exposed drifters (iSphere), the wind-drift contribution is found to be dominating, and there is only a small impact of using any of the available information about ocean currents. The wind drift contribution to the iSphere drift is estimated to be 3% of the wind if Stokes drift is included in the calculations, and 4% if the Stokes drift is not included. Including Stokes drift in the calculations gives improvement for the submerged drifters, but shows no improvement for the wind exposed drifters. Highlights • Satellite derived currents are found to be useful for ocean trajectory forecasting. • Ocean currents represent the largest uncertainty in ocean trajectory forecasting. • Ocean surface drift may be fairly well predicted from wind only. [ABSTRACT FROM AUTHOR]

Abstract In this article, there is an overview of the potential of different (mostly altimetry-derived) products for manifesting features of basin- and mesoscale circulation of the Western Mediterranean Basin. In terms of detection of mesoscale eddies and larger gyres, the comparison was performed between eddy statistics extracted directly from fields of sea surface temperature (SST), on the one hand, and reconstructed from different scalar and vector altimetry-derived fields, on the other hand. Among the products being analysed, there are fields of (i) sea level anomaly (SLA), (ii) geostrophic currents, and (iii) finite size Lyapunov exponents (FSLE). Comparison of the eddy statistics provided by such different data sources revealed that altimetry-based techniques tended to overestimate the number of big cyclones and thus failed to reproduce cyclonic/anticyclonic eddy asymmetry which was shown by fields of SST. Defining the spatial scale of eddies via altimetry-based techniques seemed particularly problematic. FSLE product analysed in the study showed significant disagreements with vortical structures observed in fields of SST both in terms of the spatial scale and the sign of rotation. In order to scrutinize basin-scale features of surface circulation in the region of interest, fields of (i) geostrophic currents, (ii) total currents reconstructed by the GlobCurrent project, and (iii) surface currents provided by the reanalysis technique were analysed. As a result of the comparison performed, a conclusion was made on the complementarity of the datasets analysed. Highlights • Results of different eddy detection techniques might significantly diverge. • Direct scrutiny of SST images revealed that biggest eddies were mostly anticyclonic. • Indirect eddy detection techniques tend to identify both anticyclones and cyclones. • Features seen in FSLE fields did not correspond to any vortical structures. • Different products on surface currents analysed seem complementary. [ABSTRACT FROM AUTHOR]

Highlights • Developing a simplified model of the double-gyre problem based on layer-averaged Navier–Stokes equations. • Presenting multi-scale spectral solutions to the simplified double-gyre model. • Verification of the resulting spectral solutions with CABARET solutions of a quasi-geostrophic double-gyre model. • Validation of the spectral solutions for integral dynamic quantities and zonally averaged flow profiles. Abstract Several semi-analytical models are considered for a double-gyre problem in a turbulent flow regime for which a reference fully numerical eddy-resolving solution is obtained. The semi-analytical models correspond to solving the depth-averaged Navier–Stokes equations using the spectral Galerkin approach. The robustness of the linear and Smagorinsky eddy-viscosity models for turbulent diffusion approximation is investigated. To capture essential properties of the double-gyre configuration, such as the integral kinetic energy, the integral angular momentum, and the jet mean-flow distribution, an improved semi-analytical model is suggested that is inspired by the idea of scale decomposition between the jet and the surrounding flow. [ABSTRACT FROM AUTHOR]

Abstract The dynamics and seasonal variability of jet currents on the southern Benguela shelf-edge are investigated using a climatologically forced Regional Ocean Modelling System (ROMS) model. The jet is primarily forced by the intense horizontal gradients that exist across the southern Benguela shelf. These gradients are set up by nearshore cooling via the strongly seasonal upwelling regime and variable offshore warming by the advection of Agulhas waters. While the nearshore cooling is prevalent only during the spring and summer upwelling season, the offshore warming exists throughout the year. As a result intensified geostrophically adjusted currents exist throughout the year, particularly off the Cape Peninsula and Cape Columbine. However, the distinct shelf-edge jet features are most intense during upwelling seasons and extend, more or less continuously, from Cape Agulhas, the southern-tip of the continent, to Cape Columbine. The spring and summer jet off the Cape Peninsula reaches speeds of at least 0.7 m.s −1, bifurcates as it moves northward. The branch that continues northward over the shelf goes on to feed the offshore branch of the Cape Columbine jet (over the 500 m isobath) and to a less extent the nearshore branch (over the 200 m isobath) that is locally enhanced by upwelling processes. During winter, the Cape Peninsula jet is more confined to the shelf region and goes on to feed the whole outer-shelf (200–500 m) region off and beyond Cape Columbine. An ageostrophic component associated with offshore non-linearities related to Agulhas influx causes the mean manifestation of the Cape Peninsula jet to broaden slightly (60 km) compared to its 40 km-wide geostrophic core which is situated over the 350 m isobath. The ageostrophic component is related to the generation of eddies that cause the isopycnals to flatten out. Highlights • The intense cross-shelf density gradient drives jet currents in the S. Benguela. • The seasonal variability of the density gradient is controlled by coastal upwelling. • The gradient is modulated by variability linked to the warm offshore Agulhas waters. [ABSTRACT FROM AUTHOR]

Six years of surface velocities have been calculated over the global ocean by combining altimeter derived geostrophic velocities and Sea Surface Temperature (SST) data. The method is based on the inversion for the velocity of the heat conservation equation using the altimeter velocities as background. By accurately prescribing the error both on the background velocities and on the forcing term (heat fluxes), the altimeter background velocities are successfully improved in areas characterized by strong SST gradients and remain unchanged in low SST gradient regions, where by construction no additional information may be brought by the SST field. This allows us to provide for the first time a global field of surface velocities from the merging of altimetry and SST. Both the spatial and temporal resolution of the altimeter derived velocities is enhanced by the SST information. Validation is performed through comparison to in-situ drifting buoy velocities. Major improvements (10–20% globally, up to 35% locally) are obtained on the meridional component of the velocity and in the equatorial band. Due to the large uncertainty on the heat fluxes and the spatio-temporal resolution of the input datasets, the high frequency ageostrophic components of the circulation is hardly resolved by the method and, outside the equatorial band, improvements are mainly on the geostrophic component. It is shown that the level of accuracy obtained by combining altimeter velocities based on a two satellite configuration and microwave SST data is equivalent or higher to the one from a four altimeter constellation in western boundary currents. This opens the perspective for a systematic use of the method to improve the altimeter derived surface velocities over the 1993–2001 period, for which only 2 altimeters have been flying simultaneously. Finally, over the recent years, the use of higher resolution SST products based on both microwave and infrared measurements leads to further enhancement of the accuracy of the altimeter velocities on both the zonal and the meridional component in strong SST gradients areas. [ABSTRACT FROM AUTHOR]

Several fast, narrow boundary currents flow along the eastern margin of New Zealand forming part of the western boundary current system of the South Pacific. In this study, using over 20 years of satellite altimeter observations and in situ data, we investigate the mean and variability of the current transports at seasonal, interannual and decadal time scales. We relate the means and fluctuations of the currents to the Southern Oscillation Index and to potential drivers of the circulation, including local and basin-scale winds. We also investigate the barotropic and baroclinic responses of the currents to basin-scale wind forcing using Rossby wave models. We find the currents are highly variable and there are no discernible trends in the transports since 1993. There is little coherence between transports along the boundary, suggesting a range of mechanisms drive the variability of the currents individually rather than the system as a whole. There is little correspondence between transports in any of the boundary currents and the El Niño/Southern Oscillation. We also find that the transports are not well described by the arrival of linear Rossby waves. At decadal time scales, only transports in the Subantarctic Front show a close correspondence with the variability of the wind stress curl averaged over the South Pacific. Seasonality is significant in two of the currents, likely driven by seasonality in local winds. In contrast with the larger East Australian Current, which forms part of the same South Pacific western boundary current system, we see collectively little evidence of a link between trends and variability in the New Zealand boundary currents and large-scale or local wind forcing. [ABSTRACT FROM AUTHOR]

Since the 1960s, the Instituto del Mar del Perú (IMARPE) collected tens of thousands of in-situ temperature and salinity profiles in the Northern Humboldt Current System (NHCS). In this study, we blend this unique database with the historical in-situ profiles available from the World Ocean Database for the period 1960–2014 and apply a four-dimensional interpolation scheme to construct a seasonal climatology of temperature and salinity of the NHCS. The resulting interpolated temperature and salinity fields are gridded at a high spatial resolution (0.1° × 0.1° in latitude/longitude) between the surface and 1000 m depth, providing a detailed view of the hydrology and geostrophic circulation of this region. In particular, the mean distribution and characteristics of the main water-masses in the upper ocean of the NHCS are described, as well as their seasonal variations between austral summer and winter. The coastal upwelling region is well documented due to the increased data density along 3 highlighted cross-shore vertical sections off Paita (∼5°S), Chimbote (∼9°S) and San Juan (∼16.5°S). The large and long-term database also allowed us, through a composite analysis, to investigate the impact of the eastern Pacific El Niño and La Niña events on the NHCS hydrology. On average, during these periods, large temperature (±3–4 °C) and salinity (±0.1–0.2) anomalies are observed, impacting the water column of the coastal ocean off Peru down to 100–200 m depth. At 100 km from the coast, these anomalies are associated with a maximum deepening (shoaling, respectively) of the thermocline of 60 m (25 m) during composite El Niño (La Niña) events. At interdecadal scale, a similar approach reveals sea-surface temperature variations of ±0.5°C, associated with a deepening (shoaling) of the thermocline of 5–10 m during warm (cold) periods. [ABSTRACT FROM AUTHOR]

Abstract The synoptic monthly gridded (SMG) world ocean database (WOD) were constructed with 1° × 1° horizontal resolution and 28 standard vertical levels from the surface to 3000 m deep using the optimal spectral decomposition technique. Monthly acceleration potential (AP) relative to 1000-dbar is calculated for the isopycnal surface 26.4 kg m−3 from January 1960 to December 2014 to investigate the spatial and temporal variability of the California Current. A composite analysis was conducted to obtain the total-time mean AP field and the climatological monthly mean AP variability, which is two orders of magnitude smaller than the total-time mean. Residual data were used to examine interannual variations of the AP field. An empirical orthogonal function (EOF) analysis was conducted to analyze the AP anomaly data. The EOF-1 (40% variance) represents strengthening/weakening of the North Pacific Gyre and California Current. EOF-2 (20% variance) represents onshore/offshore geostrophic flow. The first principal component was negatively correlated with the PDO; the second principal component was negatively correlated with both the PDO and ENSO, which implied that the AP anomaly contains climate variability signals. [ABSTRACT FROM AUTHOR]

We investigate the nature of the dynamo bifurcation in a configuration applicable to the Earth’s liquid outer core, i.e. in a rotating spherical shell with thermally driven motions with no-slip boundaries. Unlike in previous studies on dynamo bifurcations, the control parameters have been varied significantly in order to deduce general tendencies. Numerical studies on the stability domain of dipolar magnetic fields found a dichotomy between non-reversing dipole-dominated dynamos and the reversing non-dipole-dominated multipolar solutions. We show that, by considering weak initial fields, the above transition disappears and is replaced by a region of bistability for which dipolar and multipolar dynamos coexist. Such a result was also observed in models with free-slip boundaries in which the geostrophic zonal flow can develop and participate to the dynamo mechanism for non-dipolar fields. We show that a similar process develops in no-slip models when viscous effects are reduced sufficiently. The following three regimes are distinguished: (i) Close to the onset of convection ( Ra c ) with only the most critical convective mode (wave number) being present, dynamos set in supercritically in the Ekman number regime explored here and are dipole-dominated. Larger critical magnetic Reynolds numbers indicate that they are particularly inefficient. (ii) in the range 3 < Ra / Ra c < Ra c , the bifurcations are subcritical and only dipole-dominated dynamos exist. (iii) in the turbulent regime ( Ra / Ra c > 10 ) , the relative importance of zonal flows increases with Ra in non-magnetic models. The field topology depends on the magnitude of the initial magnetic field. The dipolar branch has a subcritical behavior whereas the multipolar branch has a supercritical behavior. By approaching more realistic parameters, the extension of this bistable regime increases. A hysteretic behavior questions the common interpretation for geomagnetic reversals. Far above the dynamo threshold (by increasing the magnetic Prandtl number), Lorentz forces contribute to the first order force balance, as predicted for planetary dynamos. When Ra is sufficiently high, dipolar fields affect significantly the flow speed, the flow structure and heat transfer which is reduced by the Lorentz force regardless of the field strength. This physical regime seems to be relevant for studying geomagnetic processes. [ABSTRACT FROM AUTHOR]

In this paper, we consider the two-dimensional dissipative surface quasi-geostrophic equation, and establish a logarithmically improved regularity criterion. Consequently, our result extends the regularity criterion result of Dong and Pavlovié (2009). In addition, a logarithmically improved regularity criterion to the three-dimensional Navier–Stokes equations is also derived by the same arguments. Therefore, this result extends and improves many previous works. [ABSTRACT FROM AUTHOR]

Previous modeling and observational studies have established that it is possible to accurately monitor the Atlantic Meridional Overturning Circulation (AMOC) at 26.5°N using a coast-to-coast array of instrumented moorings supplemented by direct transport measurements in key boundary regions (the RAPID/MOCHA/WBTS Array). The main sources of observational and structural errors have been identified in a variety of individual studies. Here a unified framework for identifying and quantifying structural errors associated with the RAPID array-based AMOC estimates is established using a high-resolution (eddy resolving at low-mid latitudes, eddy permitting elsewhere) ocean general circulation model, which simulates the ocean state between 1978 and 2010. We define a virtual RAPID array in the model in close analogy to the real RAPID array and compare the AMOC estimate from the virtual array with the true model AMOC. The model analysis suggests that the RAPID method underestimates the mean AMOC by ∼1.5 Sv (1 Sv = 10 6  m 3  s −1 ) at ∼900 m depth, however it captures the variability to high accuracy. We examine three major contributions to the streamfunction bias: (i) due to the assumption of a single fixed reference level for calculation of geostrophic transports, (ii) due to regions not sampled by the array and (iii) due to ageostrophic transport. A key element in (i) and (iii) is use of the model sea surface height to establish the true (or absolute) geostrophic transport. In the upper 2000 m, we find that the reference level bias is strongest and most variable in time, whereas the bias due to unsampled regions is largest below 3000 m. The ageostrophic transport is significant in the upper 1000 m but shows very little variability. The results establish, for the first time, the uncertainty of the AMOC estimate due to the combined structural errors in the measurement design and suggest ways in which the error could be reduced. Our work has applications to basin-wide circulation measurement arrays at other latitudes and in other basins as well as quantifying systematic errors in ocean model estimates of the AMOC at 26.5°N. [ABSTRACT FROM AUTHOR]

Three-dimensional abyssal and deep circulation of the region to the east and north of the Emperor Seamount Chain/Hawaiian Ridge is determined from a compilation of CTD and Argo float data, using a new overdetermined inverse technique for the geostrophic reference velocity and diapycnal/lateral mixing coefficients. The Northeast Pacific Basin is primarily sourced from its northern boundary, at a rate of 3.5 Sv across 47°N below 3000 m. Bottom water in the western subarctic gyre recirculates cyclonically between the Emperor Seamount Chain and 155°W. Bottom water east of 155°W takes a more direct path southward along the flank of a broad topographic slope. In the deep water, a ridge of potential vorticity lying along the Mendocino Fracture Zone separates circulation systems north and south of ∼40°N. The region has very weak diapycnal and lateral mixing, and an aspect ratio for the overturning circulation that is correspondingly flat, with bottom water parcels rising less than 1 km during their long transit from the Aleutian Trench to the latitude of Hawaii. [ABSTRACT FROM AUTHOR]

The Lofoten Vortex (LV), a quasi-permanent anticyclonic eddy in the Lofoten Basin of the Norwegian Sea, is investigated with an eddy-permitting primitive equation model nested into the ECCO2 ocean state estimate. The LV, as simulated by the model, extends from the sea surface to the ocean bottom at about 3000 m and has the subsurface core between 50 m and 1100 m depths. Above and below the vortex core the relative vorticity signal decreases in amplitude while the radius increases by as much as 25–30% relative to the values in the core. Analyzing the model run, we show that the vertical structure of the LV can be casted into four standard configurations, each of which forms a distinct cluster in the parameter space of potential vorticity anomalies in and above the LV core. The stability of the LV for each of the configurations is then studied with three-layer and a two-layer (in winter) quasi-geostrophic (QG) models over a flat bottom as well as over a realistic topography. The QG results show a number of common features with those of the primitive equation model. Thus, among the azimuthal modes dominating the LV instability, both the QG model and the primitive equation model show a major role the 2nd and 3rd modes. In the QG model simulations the LV is the subject of a rather strong dynamic instability, penetrating deep into the core. The results predict 50–95% volume loss from the vortex within 4–5 months. Such a drastic effect is not observed in the primitive equation model, where, for the same intensity of perturbations, only 10–30% volume loss during the same period is detected. Taking into account the gently sloping topography of the central part of the Lofoten basin and the mean flow in the QG model, brings the rate of development of instability close to that in the primitive equation model. Some remaining differences in the two models are discussed. Overall, the LV decay rate obtained in the models is slow enough for eddy mergers and convection to restore the thermodynamic properties of the LV, primarily re-building its potential energy anomaly. This justifies the quasi-permanent presence of the LV in the Lofoten Basin. [ABSTRACT FROM AUTHOR]

Hydrographic measurements together with Ship mounted Acoustic Doppler Current Profilers and Lowered Acoustic Doppler Current Profilers (LADCP) obtained in October 2014 are used to describe water masses, geostrophic circulation and mass transport of the Canary Current System, as the Eastern Boundary of the North Atlantic Subtropical Gyre. Geostrophic velocities are adjusted to velocities from LADCP data to estimate an initial velocity at the reference layer. The adjustment results in a northward circulation at the thermocline layers over the African slope from an initial convergent flow. Final reference velocities and consequently absolute circulation are estimated from an inverse box model applied to an ocean divided into 13 neutral density layers. This allows us to evaluate mass fluxes consistent with the thermal wind equation and mass conservation. Ekman transport is estimated from the wind data derived from the Weather Research and Forecasting model. Ekman transport is added to the first layer and adjusted with the inverse model. The Canary Current located west of Lanzarote Island transports to the south a mass of − 1.5 ± 0.7 Sv (1 Sv = 10 6 m 3 s − 1 ≈ 10 9 kg s − 1 ) of North Atlantic Central Water at the surface and thermocline layers (~ 0–700 m). In fall 2014, hydrographic data shows that the Canary Current in the thermocline (below at about 80 m depth to ~ 700 m) recirculates to the north over the African slope and flows through the Lanzarote Passage. At intermediate layers (~ 700–1400 m), the Intermediate Poleward Undercurrent transports northward a relatively fresh Antarctic Intermediate Water in the range of 0.8 ± 0.4 Sv through the Lanzarote Passage and west of Lanzarote Island beneath the recirculation of the Canary Current. [ABSTRACT FROM AUTHOR]

This work updates the methods of Lumpkin and Johnson (2013) to obtain an improved near-surface velocity climatology for the global ocean using observations from undrogued and 15-m drogued Global Drifter Program (GDP) drifters. The proposed procedure includes the correction of the slip bias of undrogued drifters, thus recovering about half of the GDP dataset; and a new approach for decomposing Lagrangian data into mean, seasonal and eddy components, which reduces the smoothing of spatial gradients inherent in data binning methods. The sensitivity of the results to method parameters, the method performance relative to other techniques, and the associated estimation errors, are evaluated using statistics calculated for a test dataset consisting of altimeter-derived geostrophic velocities subsampled at the drifter locations, and for the full altimeter-derived geostrophic velocity fields. It is demonstrated that (1) the correction of drifter slip bias produces statistically similar mean velocities for both drogued and undrogued drifter datasets at most latitudes and reduces differences between their variance estimates, (2) the proposed decomposition method produces pseudo-Eulerian mean fields with magnitudes and horizontal scales closer to time-averaged Eulerian observations than other methods, and (3) standard errors calculated for pseudo-Eulerian quantities underestimate the real errors by a factor of almost two. The improved decomposition method and the inclusion of undrogued drifters in the analysis allows resolving details of the time-mean circulation not well defined in the previous version of the climatology, such as the cross-stream structure of western boundary currents, recirculation cells, and zonally-elongated mid-ocean striations. [ABSTRACT FROM AUTHOR]

Understanding the mechanisms that drive exchanges between the Arctic Ocean and adjacent oceans is critical to building our knowledge of how the Arctic is reacting to a warming climate, and how potential changes in Arctic Ocean freshwater export may impact the AMOC (Atlantic Meridional Overturning Circulation). Here, freshwater pathways from the Arctic Ocean to the North Atlantic are investigated using a 1 degree global model. An EOF analysis of modeled sea surface height (SSH) demonstrates that while the second mode accounts for only 15% of the variability, the associated geostrophic currents are strongly correlated with freshwater exports through CAA (Canadian Arctic Archipelago; r = 0.75), Nares Strait (r = 0.77) and Fram Strait (r = −0.60). Separation of sea level into contributing parts allows us to show that the EOF1 is primarily a barotropic mode reflecting variability in bottom pressure equivalent sea level, while the EOF2 mode reflects changes in steric height in the Arctic Basin. This second mode is linked to momentum wind driven surface current, and dominates the Arctic Ocean freshwater exports. Both the Arctic Oscillation and Arctic Dipole atmospheric indices are shown to be linked to Arctic Ocean freshwater exports, with the forcing associated with the Arctic Dipole reflecting the out-of-phase relationship between transports through the CAA and those through Fram Strait. Finally, observed freshwater transport variation through the CAA is found to be strongly correlated with tide gauge data from the Beaufort Sea coast (r = 0.81), and with the EOF2 mode of GRACE bottom pressure data (r = 0.85) on inter-annual timescales. [ABSTRACT FROM AUTHOR]

The circulation structure in the western tropical Indian Ocean (WTIO) was investigated based on surface drifters and Simple Ocean Data Assimilation (SODA), with particular attention to the characteristics of the circulation structure associated with seasonal monsoon transitions. Surface drifter trajectories manifest three distinct seasonally dependent patterns of western boundary currents in the WTIO. The northward Somali Current (SC) flows across the equator to ∼10 °N in the boreal summer (June to August; JJA). The northward SC turns offshore near the equator during monsoon transitions. The southward SC feeds the South Equatorial Countercurrent together with the northward East African Coast Current in the boreal winter (December to February; DJF). While during the monsoon transition (MAM and SON), drifters prefer to turn eastward and feed the strong equatorial currents. Based on satellite observations, the results of a balanced diagnostic model demonstrate that the geostrophic component U ζ and the wind-driven component U τ predominate the mean state and the seasonal variations of circulation in the WTIO. The vertical motion shows that upwelling primarily occurs along the coasts of Somali and Oman and is induced by offshore Ekman transport. Ekman transport and associated vertical motions constitute meridional overturning circulation. The SODA results confirm the existence of an equatorial roll in the boreal summer and winter. The results elucidate the dynamic relationships between 3-D circulation and monsoon transitions in the WTIO. • Drifter trajectories describe three seasonally dependent circulation patterns in the western tropical Indian Ocean. • The geostrophic component U ζ and wind-driven component U τ predominate the mean state and the soasonal variability of circulation. • The 3-D structures, show seasonal variability associated with monsoon transitions. [ABSTRACT FROM AUTHOR]

The tropical Indian Ocean receives the world's most significant tropical ocean rain and river runoff per unit area. The 3-dimensional spreading of Rain and River Water (RRW) entering the tropical Indian Ocean and associated salinity and circulation anomalies are explored for 60 years using ocean reanalysis data tailored to a tracer transport model. Over 60 years, the cumulative impact of RRW entering the tropical Indian Ocean is to freshen the Indian Ocean basin as large as 2.0–0.1 p.s.u from the surface to 500 m via subduction and transport. The RRW propagates to a vast extent of the Indian Ocean and a limited extent of the Atlantic and Pacific Oceans via general circulation pathways. A quasi-equilibrium model of accumulation of RRW over the tropical Indian Ocean suggests that it induces clockwise geostrophic currents from the Bay of Bengal to the Arabian Sea over 0–500 m depths with a net transport tendency of 0.80 ± 0.14 Sv year−1. The study implies that coupled climate models with precipitation biases may miscalculate such salinity and circulation anomalies due to RRW and aggravating biases in simulated climate dynamics. • The spreading of rain and river water entering the tropical Indian Ocean for 60 yr is identified from ocean reanalysis data. • RRW mixes in the surface, subducts and spreads, offering a dilution in salinity and expansion in dynamic heights. • RRW offers a net transport tendency of water from the Bay of Bengal to the Arabian Sea at a rate of 0.80 ± 0.14 Sv year−1. [ABSTRACT FROM AUTHOR]

To understand long-term mean and seasonal variations of the Oyashio transport off the southeastern coast of Hokkaido, time series of estimated transports were generated by combining satellite altimetry and CTD data from the period 1993–2014 along a monitoring transect referred to as the “A-line”. Linear regressions between CTD-derived geostrophic transport and altimetry-derived sea level anomaly differences were calculated by minimizing a cost function newly proposed in this study. The horizontal structure of 21-year mean transport was characterized by a southwestward flow on the continental slope (SW-1) associated with the First Oyashio Intrusion, a northeastward flow (NE-2) likely trapped against the Kuril-Kamchatka trench, a southwestward flow (SW-3) related to the Second Oyashio Intrusion, and a very stable northeastward flow (NE-4) corresponding to a combination of the Oyashio Return Flow and quasi-stationary Jet. Analysis of long-term mean monthly transports revealed four alternating flows that were present throughout all seasons of the year. The boundaries between adjacent pairs of the four flows on the A-line did not change on a seasonal timescale as much as the southernmost latitude of the First Oyashio Intrusion. At a 0-month (1-month) time lag, the sum of the SW-1, NE-2, and SW-3 (SW-1 and NE-2) transports, which corresponds to net Oyashio transport (partial net Oyashio transport related to the southernmost latitude of the First Oyashio Intrusion), was correlated with Sverdrup transport more robustly than was each of the four alternating flows. This result suggests that as the Oyashio crossed the A-line, it responded barotropically to basin-scale wind stress by adjusting transports composed of both southwestward and northeastward flows. [ABSTRACT FROM AUTHOR]

Variability in along-shelf and cross-shelf circulation in the South Atlantic Bight (SAB) is investigated using altimetry observations. Satellite-derived along-shelf velocity anomalies are in good agreement with independent near-surface current measurements from moored acoustic Doppler current profilers and surface velocities from high frequency radar at adjacent locations. This is especially true if wind-driven Ekman velocities are added to the geostrophic velocities, suggesting that the influence of Ekman dynamics to surface along-shelf flow in the SAB is unusually large. The decade-long time series reveals substantial seasonal variability in surface velocities, with peak poleward anomalies during late spring and summer and strong equatorward flow during autumn. Convergences and divergences in the along-shelf transport between two cross-sections are compared with three-dimensional numerical model results and used to estimate cross-shelf transport across the 50 m isobath in the SAB. The calculation suggests a pattern of weak offshore flow during spring followed by prolonged and relatively stronger offshore flow during summer and early autumn, while cross-shelf velocity anomalies during winter are weak and slightly onshore. Prolonged offshore flow following the peak in river discharge that generally occurs in spring indicates the potential for the establishment of a conduit for offshore export of riverine material. The long-term time series also reveals several large events of interannual variability, including the 2003 cold event observed in the SAB. [ABSTRACT FROM AUTHOR]

This paper considers ergodicity of the stochastic quasigeostrophic equations driven by degenerate Gaussian noise. Uniqueness of invariant measures is shown by proving the asymptotically strong Feller property of the probability transition semigroups. [ABSTRACT FROM AUTHOR]

Our task here is to use a version of the ‘vanishing viscosity technique’ to study the critical 2D Quasi-Geostrophic equation . The present paper extends and specializes the results reported in Dlotko et al. (2015). We treat now in more detail the solutions of the critical problem ( α = 1 2 ); in particular their uniqueness, regularity and long time behavior. [ABSTRACT FROM AUTHOR]

The National Oceanic and Atmospheric Administration׳s (NOAA) Alaska Fisheries Science Center bottom trawl survey samples demersal fish at over 350 sites on the eastern Bering Sea continental shelf each summer on a 37×37 km 2 grid. Rugged conductivity–temperature-depth sensors (CTDs) were added to the net hauls to obtain gridded data sets of temperature and salinity measurements for 2008–2010. Results reveal the three-dimensional thermohaline structure of the shelf including the Cold Pool and areas of fresher water around St. Matthew Island and in Bristol Bay. Horizontal gradients are often strongest roughly along the 50-m and 100-m isobaths that traditionally separate the inner- and outer-shelf from the middle-shelf centered along the 70-m isobath. The summer mixed layer depth is less than 30 m over much of the region. It reaches the bottom along the Alaska Peninsula in water depths greater than 70 m, showing that the boundary of the well-mixed, inner shelf is not always at the 50-m isobath. The greatest upper-to-lower layer density difference is found across the shelf north of 59°N. The salinity difference is the main contributor to this density difference over most of the region in 2008 and 2010, but the temperature difference dominates in 2009 due to decreased ice melt and reduced freshening near St. Matthew Island. The geostrophic velocity relative to the bottom shows northwestward flow seaward of the 100-m isobath and northwestward transports integrated across the shelf of 0.10–0.25×10 6 m 3 /s. In 2008 and 2010 there was clockwise circulation in a region of less-saline water around St. Matthew Island. In 2009 that fresher lens did not exist, and flow was more concentrated along the 100-m isobath bringing saltier water across the shelf. [ABSTRACT FROM AUTHOR]

More than 20 years of data from moorings, satellite-tracked drifters and hydrographic surveys are integrated to provide a comprehensive view of currents and transport on the eastern Bering Sea shelf. The major sources of water onto the eastern Bering Sea shelf are North Pacific water flowing through Unimak Pass and Bering Slope water flowing onto the shelf usually via the canyons that intersect the shelf break. Absolute geostrophic transport through Unimak Pass varies from an average of 0.25×10 6 m 3 s −1 (Sv) in the warm months to 0.43 Sv in the cold months. Flow along the 50-m isobath is weak, with a transport of <0.1 Sv (calculated from current meters) in summer and fall. The transport along the 100-m isobath measured at two locations is more than twice that along 50-m isobaths; in the summer at the Pribilof Islands it was 0.2 Sv and during spring and summer at 60°N the northward geostrophic transport (referenced to the bottom) was 0.31 Sv. Northward transport along the 100-m and 50-m isobaths accounts for approximately half of the transport through Bering Strait. A typical transit time from Unimak Pass to Bering Strait is >13 months and from Amukta Pass to Bering Strait via the Bering Slope Current is >8 months. Consequently, the source of most of the heat transported into the Arctic through Bering Strait is a result of air–sea interactions local to the northern Bering Sea. Analysis of the currents and water properties on the southern shelf indicates that ~50% of the shelf water is exchanged with slope water during October–January each year. This exchange elevates the October mid-shelf average nitrate level from 6 μM to 14–16 μM by the end of January. [ABSTRACT FROM AUTHOR]

We study the production and dissipation of the eddy kinetic energy (EKE) in a submesoscale eddy field forced with downfront winds using the Process Study Ocean Model (PSOM) with a horizontal grid resolution of 0.5 km. We simulate an idealized 100 m deep mixed-layer front initially in geostrophic balance with a jet in a domain that permits eddies within a range of O(1 km–100 km). The vertical eddy viscosities and the dissipation are parameterized using four different subgrid vertical mixing parameterizations: the k − ϵ , the KPP, and two different constant eddy viscosity and diffusivity profiles with a magnitude of O ( 10 − 2 m 2 s − 1 ) in the mixed layer. Our study shows that strong vertical eddy viscosities near the surface reduce the parameterized dissipation, whereas strong vertical eddy diffusivities reduce the lateral buoyancy gradients and consequently the rate of restratification by mixed-layer instabilities (MLI). Our simulations show that near the surface, the spatial variability of the dissipation along the periphery of the eddies depends on the relative alignment of the ageostrophic and geostrophic shear. Analysis of the resolved EKE budgets in the frontal region from the simulations show important similarities between the vertical structure of the EKE budget produced by the k − ϵ and KPP parameterizations, and earlier LES studies. Such an agreement is absent in the simulations using constant eddy-viscosity parameterizations. [ABSTRACT FROM AUTHOR]

The seasonal variability of the hydrology and the poleward subsurface Peru-Chile Undercurrent (PCUC) off the central Chilean coast (29–41°S) were examined using a high-resolution regional model. The model realistically reproduced observed sea level variability, such as intense anticyclonic eddies, the offshore intensification of the poleward flow and the reduced nearshore equatorward flow during autumn, as well as the equatorward intensification of nearshore meandering flow during spring. Values for geostrophic eddy kinetic energy were high along the coast between 30° and 37°S, and lower south of this area. The modelled poleward undercurrent showed latitudinal variability in velocity and transport. The maximum average transport reported was 0.8 Sv near 30°S, consistent with previous modelling studies and estimations derived from in situ observations. The poleward reduction in undercurrent strength was shown to be partly generated by the poleward decrease in wind stress curl and by the formation of a westward jet near 35°S associated with westward-propagating eddies. A Lagrangian analysis of the modelled water parcels transported by the undercurrent shows that only 14–20% of the subsurface floats transported by the undercurrent upwelled into the surface layer within the subsequent six months after their release. The floats remaining within the subsurface layer were likely transported further south by the current, offshore by westward-propagating eddies or equatorward by the deeper part of the surface coastal current. [ABSTRACT FROM AUTHOR]