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Exchange flows past straits and sills play a vital role in controlling water exchange between parts of the oceans with different properties. We modelled such flows in the laboratory using a long tank with a constricted channel in the center, where a barrier was inserted. After barrier removal, the exchange occurred and the corresponding volume transport was measured. Varying channel geometries and shapes were used to investigate the effects of the geometry and shape on the transport. The scaling analysis made for laboratory non-rotating strait exchange flows served as a theoretical basis, giving an upper bound on the transport. The applications to real cases in nature, such as Indonesian Through-Flow (ITF) and world ocean straits provided a new insight into water exchange between ocean basins separated by bottom topography. The results showed that all the transports were less than the normalized value of 0.87 (uppermost), predicted by hydraulic theory for baroclinic strait exchanges with mixing but no friction. A further reduction in the transport was arguably due to frictional effects along the bottom boundary. For the world ocean straits considered in the present study, the normalized volume transports were in the range 0.40–0.86, with the highest was 0.86 recorded in the Gibraltar Strait, implying a relatively small transport reduction due to friction. For the ITF passages, the normalized transports ranged between 0.56 and 0.83, similar to those found for the world ocean straits. These are, in the first order, consistent with the measured transports in the range 0.78–0.86 from the laboratory experiments. [ABSTRACT FROM AUTHOR]

The ocean circulation is typically constrained in operational analysis and forecasting systems through the assimilation of sea level anomaly (SLA) retrievals from satellite altimetry. This approach has limited benefits in the Arctic Ocean and surrounding seas due to data gaps caused by sea ice coverage. Moreover, assimilation of SLA in seasonally ice-free regions may be negatively affected by the quality of the Mean Sea Surface (MSS) used to derive the SLA. Here, we use the Regional Ice Ocean Prediction System (RIOPS) to investigate the impact of assimilating Absolute Dynamic Topography (ADT) fields on the circulation in the Arctic Ocean. This approach avoids the use of a MSS and additionally provides information on sea level in ice covered regions using measurements across leads (openings) in the sea ice. RIOPS uses a coupled ice-ocean model on a 3-4 km grid-resolution pan-Arctic domain together with a multi-variate reduced-order Kalman Filter. The system assimilates satellite altimetry and sea surface temperature together with in situ profile observations. The background error is modified to match the spectral characteristics of the ADT fields, which contain less energy at small scales than traditional SLA due to filtering applied to reduce noise originating in the geoid product used. A series of four-year reanalyses demonstrate significant reductions in innovation statistics with important impacts across the Arctic Ocean. Results suggest that the assimilation of ADT can improve circulation and sea ice drift in the Arctic Ocean, and intensify volume transports through key Arctic gateways and resulting exchanges with the Atlantic Ocean. A reanalysis with a modified Mean Dynamic Topography (MDT) is able to reproduce many of the benefits of the ADT but does not capture the enhanced transports. Assimilation of SLA observations from leads in the sea ice appears to degrade several circulation features; however, these results may be sensitive to errors in MDT. This study highlights the large uncertainties that exist in present operational ocean forecasting systems for the Arctic Ocean due to the relative paucity and reduced quality of observations compared to icefree areas of the Global Ocean. Moreover, this underscores the need for dedicated and focused efforts to address this critical gap in the Global Ocean Observing System. [ABSTRACT FROM AUTHOR]

The ocean circulation is typically constrained in operational analysis and forecasting systems through the assimilation of sea level anomaly (SLA) retrievals from satellite altimetry. This approach has limited benefits in the Arctic Ocean and surrounding seas due to data gaps caused by sea ice coverage. Moreover, assimilation of SLA in seasonally ice-free regions may be negatively affected by the quality of the Mean Sea Surface (MSS) used to derive the SLA. Here, we use the Regional Ice Ocean Prediction System (RIOPS) to investigate the impact of assimilating Absolute Dynamic Topography (ADT) fields on the circulation in the Arctic Ocean. This approach avoids the use of a MSS and additionally provides information on sea level in ice covered regions using measurements across leads (openings) in the sea ice. RIOPS uses a coupled ice-ocean model on a 3-4 km grid-resolution pan-Arctic domain together with a multi-variate reduced-order Kalman Filter. The system assimilates satellite altimetry and sea surface temperature together with in situ profile observations. The background error is modified to match the spectral characteristics of the ADT fields, which contain less energy at small scales than traditional SLA due to filtering applied to reduce noise originating in the geoid product used. A series of four-year reanalyses demonstrate significant reductions in innovation statistics with important impacts across the Arctic Ocean. Results suggest that the assimilation of ADT can improve circulation and sea ice drift in the Arctic Ocean, and intensify volume transports through key Arctic gateways and resulting exchanges with the Atlantic Ocean. A reanalysis with a modified Mean Dynamic Topography (MDT) is able to reproduce many of the benefits of the ADT but does not capture the enhanced transports. Assimilation of SLA observations from leads in the sea ice appears to degrade several circulation features; however, these results may be sensitive to errors in MDT. This study highlights the large uncertainties that exist in present operational ocean forecasting systems for the Arctic Ocean due to the relative paucity and reduced quality of observations compared to ice-free areas of the Global Ocean. Moreover, this underscores the need for dedicated and focused efforts to address this critical gap in the Global Ocean Observing System.

The volume transport of the Tsugaru warm current (TWC), defined as the branch current of the western boundary current of the North Pacific subtropical gyre flowing through the Tsugaru Strait, is a critical factor affecting the acidification in the strait as well as the surrounding coastal regions by promoting the enhancement of the vertical mixing of the deep water rich in dissolved inorganic carbon. The in-phase sea-level variation along the coast of mainland Japan with a gap at the Tsugaru Strait permits the discharge of the sea surface water from the Sea of Japan to the Pacific Ocean, which is significantly correlated to the variation in the sea-level difference across the Tsugaru Strait, being possibly related to that in the TWC volume transport. Furthermore, by averaging the wind-driven Sverdrup streamfunction along the eastern coast of mainland Japan—from the southern end of Kyushu (30 ∘ N) to the separation latitude of the Kuroshio (36 ∘ N)—and scaling by a factor of the ratio between the depths of the Tsushima Strait and the East China Sea, we obtained the volume transport varying similarly with the sea-level difference across the Tsugaru Strait. As obtained, the interannual wind stress variation in this latitude band to the east of mainland Japan on the North Pacific is suggested to altered the strength of the TWC. [ABSTRACT FROM AUTHOR]

The Cape Horn Current (CHC) is one of the components of the Antarctic Circumpolar Current system that enables it to fulfill its crucial role as a conduit between ocean basins. Despite this, to‐date there have been very few measurements of CHC strength and none continuous in time or space. Here, we use a combination of ocean models, one free‐running and one data‐assimilating, and satellite altimetry (1993–2021) to estimate the time‐dependent strength of the CHC at 10 transects along its length. We find the time‐mean CHC transport increases from 0.4 ± 0.5 Sv near 49°S to 5.3 ± 2.2 Sv at Cape Horn, with peak‐to‐peak interannual fluctuations of 0.8–3.4 Sv. Although, theoretically, increased run‐off from a wasting Patagonian Ice‐field would strengthen its flow, the CHC appears quite stable over the last 29 years, with little evidence of a coherent, long‐term increase or decrease in the strength of the current. Plain Language Summary: By connecting the Pacific, Atlantic and Indian Oceans, the Antarctic Circumpolar Current system plays a crucial role in shaping Earth's climate. One component of this current system is the Cape Horn Current (CHC) which flows south about 150 km off the coast of Chile, before entering the South Atlantic through Drake Passage. The CHC is especially important as it carries relatively freshwater, nutrients, marine larvae, and pollutants from the South Pacific and into the South Atlantic, home to globally‐important fisheries and sensitive ecosystems. However, we still do not have a continuous record of the CHC strength and how it may be changing as Earth warms. In this study, we develop a method that uses numerical models of the ocean together with satellite observations of sea level to estimate the strength of the CHC over the last 29 years. We find the CHC increases in strength from 400 thousand m3 s−1 in its early stages to more than 5 million m3 s−1 as it rounds Cape Horn. From 1993 to 2021, the CHC appears quite stable, with little evidence of a long‐term increase or decrease in the strength of the current. Key Points: The Cape Horn Current (CHC) contributes to the overall Antarctic Circumpolar Current system's role as a conduit between ocean basinsObservationally‐based timeseries of CHC strength along its length are provided for the first timeCHC strength ranges from 0.4 Sv at 49°S to 5.3 Sv at Cape Horn and is stable over recent decades [ABSTRACT FROM AUTHOR]

This study provides formulae for monthly estimates of the Soya Warm Current (SYC) transport from the sea-level difference (SLD) across the Soya Strait and along the current, and creates a 50-yr data set of the SYC transport. The formulae are based on the dynamical balance both across the strait and along the current by considering the barotropic and baroclinic components. It is suggested that the barotropic and baroclinic components in transport variability are comparable in summer, whereas the barotropic component is dominant in winter. We compared the 50-year time series of the SYC transport with those of the Tsushima (TSC) and Tsugaru Warm Currents estimated from the SLDs from the viewpoint of the Japan Sea Throughflow (JSTF) system. The variabilities of the SYC and TSC transports are mostly shared for any season, showing relatively high coherence at periods of 2–5 years as well as 1 year. The winter variability of the JSTF transport originates partly from that of the SYC transport caused by the winter monsoon variability in the Sea of Okhotsk. The SYC transport is significantly correlated with the transport through the eastern channel of the TSC, but not that through the western channel. We found difficulty in extracting variability and relationships at periods longer than 5 years among the three transports estimated from the SLDs, probably due to incomplete correction of the ground movements for the sea-level data. [ABSTRACT FROM AUTHOR]

Abstract The Cape Horn Current (CHC) is one of the components of the Antarctic Circumpolar Current system that enables it to fulfill its crucial role as a conduit between ocean basins. Despite this, to‐date there have been very few measurements of CHC strength and none continuous in time or space. Here, we use a combination of ocean models, one free‐running and one data‐assimilating, and satellite altimetry (1993–2021) to estimate the time‐dependent strength of the CHC at 10 transects along its length. We find the time‐mean CHC transport increases from 0.4 ± 0.5 Sv near 49°S to 5.3 ± 2.2 Sv at Cape Horn, with peak‐to‐peak interannual fluctuations of 0.8–3.4 Sv. Although, theoretically, increased run‐off from a wasting Patagonian Ice‐field would strengthen its flow, the CHC appears quite stable over the last 29 years, with little evidence of a coherent, long‐term increase or decrease in the strength of the current.

Introduction: A geodetic estimation of the surface geostrophic currents can be obtained from satellite data by combining sea surface height measurements obtained from altimetry missions with geoid data from gravity missions. These surface geostrophic currents serve as a reference for inferring a comprehensive three-dimensional (3D) geostrophy by propagating them downwards using temperature and salinity profiles.Methods: In this work, we revisit this problem for the Southern Ocean, estimating the 3D geostrophy near full depth in 41 layers, with a 1° spatial resolution and monthly temporal resolution, covering the 12 years from 2004 to 2015. We analyze the obtained 3D geostrophy over the Southern Ocean region, where the Antarctic Circumpolar Current (ACC) and its several fronts are depicted, as well as other major currents such as the Agulhas Current, the Brazil-Malvinas Current, or the East Australian Current. From the 3D geostrophic currents, we also estimate the associated water volume transport (VT) and present the results for the ACC and the Drake Passage in the context of existing literature.Results: Our analysis yields a mean VT estimate of 15.9 ± 0.1 Sv per 1° cell within the ACC region and 149.2 ± 2.2 Sv for the Drake Passage ([60.5°S, 54.5°S] x [303.5°E]). Importantly, our study includes a comprehensive validation of the results. The spatial resolution of our space-data-based approach enables us to provide VT estimates for various paths followed in the different in situ campaigns at the Drake Passage, thereby validating our findings.Discussion: The analysis demonstrates a remarkable agreement across different measurement locations, reconciling the differences in estimates reported from different campaigns. Moreover, we have estimated the barotropic and baroclinic components of the currents and their associated VT.

The zonal flow and volume transport in the Luzon Strait play a crucial role in modulating the circulation, heat and material balance, and biogeochemical processes in the South China Sea (SCS), but the quantitative values remain unclear due to lack of long‐term direct observations. Here, this knowledge gap is bridged by analyzing 4‐year velocity measurements from a mooring array along 119.9°E. Based on the novel data, the mean value of the upper‐layer Luzon Strait transport (i.e., LST_up) is estimated at −4.54 ± 1.69 Sv. Seasonally, the westward LST_up attains its peak and trough in January and June with values of −6.80 ± 1.46 Sv and −2.59 ± 0.76 Sv, respectively. At the interannual time scale, the LST_up was strongest in 2016–2017 but weakest in 2017–2018. Further analysis suggested that local winds and the combination of local winds and upstream Kuroshio transport are likely the dominant modulation mechanisms for its seasonal and interannual variations, respectively. In the middle layer, a quasi‐steady cyclonic flow structure is identified and the volume transport is therefore trivial. We further found that seasonal variation of the middle‐layer transport is dominated by the eastward flow of this cyclonic structure. Corresponding to the gravest‐mode response in the Luzon Strait, seasonal‐to‐interannual variations of this middle‐layer eastward flow are nearly in‐phase with the upper‐layer westward flow. Overall, the above observational results provide a benchmark for the flow and transport in the Luzon Strait which can help understand the dynamics of the SCS circulation and validate and improve regional numerical simulations. Plain Language Summary: The Luzon Strait is the main channel connecting the South China Sea (SCS) with the Pacific Ocean and the volume transport through the strait is crucial in regulating physical and biogeochemical processes in the SCS. However, given the sparsity of long‐term direct observations, the knowledge of zonal flow and transports is still poor. In this study, novel insights into the zonal flow and transport in the Luzon Strait are obtained based on 4‐year velocity measurements from six moorings. The upper‐layer Luzon Strait transport is estimated at −4.54 ± 1.69 Sv (1 Sv = 106 m3 s−1; negative denotes westward). The flow in the middle layer presents a relatively steady cyclonic structure and the transport through the observed section is trivial because the westward and eastward flows cancel each other out. Seasonal‐to‐interannual variations of the transports are also analyzed. Further studies revealed that the middle‐layer eastward flow shows nearly the same phase with the upper‐layer westward flow at seasonal‐to‐interannual time scales. This corresponds well to the gravest‐mode response to external forcings. The present observed results will provide a benchmark for better understanding the dynamics of the SCS circulation and can be used to validate and improve numerical simulations in this region. Key Points: A quasi‐steady cyclonic flow structure is revealed in the middle layer of Luzon Strait based on 4‐year mooring observationsThe upper‐layer Luzon Strait transport is estimated at −4.54 ± 1.69 Sv while the middle‐layer transport is trivialThe upper‐layer westward flow and middle‐layer eastward flow show in‐phase temporal variations corresponding to the gravest‐mode response [ABSTRACT FROM AUTHOR]

The seasonal and spatial variation of the current structure and volume transport across the Jeju Strait (JS) is described based on an analysis of the data from five bottom mounted acoustic Doppler current profilers from February 13, 2020 to February 23, 2021. The current was weak and the mixed layer was well developed in winter, so the vertical current shear was not large. Whereas in summer, the inflow of high-temperature and low-salinity surface water built up the stratification, and the surface current velocity increased, resulting in a large vertical current shear. One salient feature to be found was that a westward flowing counter current in the lower layer appeared from June to December near the bottom trough of the JS. We named this seasonal counter current in the lower layer the Jeju Strait Under Current. The eastward net volume transport passing through the JS was large in summer–autumn and small in winter-spring, but did not follow a simple sinusoidal pattern. The annual mean net volume transport was 0.48 Sv (Sv ≡ 106 m3 s−1), with a minimum (0.27 Sv) in December and a maximum (0.79 Sv) in October 2020. When there were strong northerly winds in winter, a net volume transport to the west was temporarily caused by Ekman transport, but the direction reverted to the east as soon as the northerly winds lessened. When there were strong northwesterly (southeasterly) winds due to typhoons, the volume transport decreased (increased) sharply and then recovered rapidly. [ABSTRACT FROM AUTHOR]

The throughflow in the Canadian Arctic Archipelago (CAA) had a significant impact on the North Atlantic Ocean with the Arctic climate change. The findings of physical mechanisms driving the throughflow in the CAA differed and few studies about the impact of sea level pressure (SLP) on the CAA throughflow were made. A high-resolution ice-ocean coupled Arctic Ocean Finite-Volume Community Ocean Model (AO-FVCOM) was used over the period 1978–2016 to examine the interannual and seasonal variability of the outflows in the CAA and the mechanism of SLP in driving the variation of the CAA throughflow quantitively. The simulated volume transport through Davis Strait, Nares Strait, Lancaster Sound and Jones Sound showed consistent increasing trends over 1978–2016 and the larger flux in winter and spring than in summer and fall. The variation of volume transport through Nares Strait contributed more than Lancaster and Jones Sound to the variation through Davis Strait. Five process-oriented experiments were made to further explore the role of SLP in setting up and controlling the sea surface height (SSH) difference and thus the throughflow transport in the CAA. The SLP was a primary forcing to control the SSH difference and the outflow transport compared with the wind forcing. The memory of the SSH to the SLP was short and an equilibrium state could be reached if the SLP varied with time. The upstream and downstream SLP difference, however, made a slight direct contribution to driving the volume transport of the CAA throughflow. In addition to the external forcing of SLP and wind, the variability of the CAA outflow was also influenced by the variability of the inflow/outflow and SSH on boundaries connected to the Pacific and Atlantic Oceans. The choice of SLP datasets from CORE-v2, ECMWF and NCEP was sensitive to the simulated uncertainty of volume transport.

Gravimetry measurements from the GRACE and GRACE-Follow-On satellites provide observations of ocean bottom pressure (OBP), which can be differenced between basin boundaries to infer mass transport variability at a given level in the deep ocean. However, GRACE data products are limited in spatial resolution, and conflate signals from many depth levels along steep continental slopes. To improve estimates of OBP variability near steep bathymetry, ocean bottom pressure observations from a JPL GRACE mascon product are downscaled using an objective analysis procedure, with OBP covariance information from an ocean model with horizontal grid spacing of ∼18 km. In addition, a depth-based adjustment was applied to enhance correlations at similar depths. Downscaled GRACE OBP shows realistic representations of sharp OBP gradients across bathymetry contours and strong currents, albeit with biases in the shallow ocean. In validations at intraannual (3–12 month) timescales, correlations of downscaled GRACE data (with depth adjustment) and in situ bottom pressure recorder time series were improved in ∼79% of sites, compared to correlations that did not involve downscaled GRACE. Correlations tend to be higher at sites where the amplitude of the OBP signal is larger, while locations where surface eddy kinetic energy is high (e.g., Gulf Stream extension) are more likely to have no improvement from the downscaling procedure. The downscaling procedure also increases the amplitude (standard deviation) of OBP variability compared to the non-downscaled GRACE at most sites, resulting in standard deviations that are closer to in situ values. A comparison of hydrography-based transport from RAPID with estimates based on downscaled GRACE data suggests substantial improvement from the downscaling at intraannual timescales, though this improvement does not extend to longer interannual timescales. Possible efforts to improve the downscaling technique through process studies and analysis of alongtrack GRACE/GRACE-FO observations are discussed. [ABSTRACT FROM AUTHOR]

Thirty‐four months (2017–2020) of mooring data were recently obtained at 80.6°N, 7.26°E in the main branch of Atlantic Water inflow to the Arctic, the Yermak Pass Branch. The Yermak Pass Branch was sampled at that same location during 14 months a decade ago (2007–2008) when sea ice was abundant (mean sea‐ice concentration of 74% vs. 39% during the recent deployment). We focus on time scales larger than 50 hr. The new mooring data set shows an increase in the velocity variations of 40% compared to the 2007–2008 period. Year 2018 was exceptional with ice‐free conditions over the entire year and an intensified mesoscale activity compared to other years. Temperature and salinity time series at 340 m showed significant trends over 3 years (freshening of −0.07 g/kg and cooling of about −0.9°C in 3 years). The performance of 1/12° Mercator‐Ocean operational model at the mooring location was precisely assessed. The modeled Atlantic Water transport was on average larger during 2017–2020 (40% larger) than during 2007–2008. The synoptic transport time series ranged between −1 and 5 Sv over 2007–2020 and showed large seasonal and interannual variations. The transport was larger in winter than summer. However, occasionally negative transport (<−0.7 Sv) through the Yermak Pass Branch occurred during winters ("Blocking events"). These blocking events are associated with recirculations and eddy activity and were more common over the last years from 2016 onward. The model suggested that a Northern Branch crossing the Yermak Plateau further north (81.6°N) intermittently developed. Plain Language Summary: The Atlantic Water flowing through the Fram Strait via the West Spitsbergen Current is the main source of heat and salt for the Arctic Ocean. At the entrance to the Arctic, the West Spitsbergen Current splits into several branches as it meets the Yermak Plateau. Thirty‐four months of observations were recently obtained in the main branch of Atlantic Water inflow to the Arctic, the Yermak Pass Branch. This location used to be ice covered year round. Our results show that the exceptional ice‐free conditions in year 2018 were associated with a highly variable flow. An oceanic model, in good agreement with the observations, showed that the Atlantic Water transport through the Yermak Pass Branch was more irregular from 2016 onward. Changes in the Atlantic Water inflow pathways have important implications for the heat, salt, and nutrients reaching the Arctic. Key Points: New mooring data in the Yermak Pass Branch documented larger velocity variations (40%) in 2017–2020 than in 2007Year 2018 stood out with exceptionally ice‐free conditions and enhanced eddy activity at the mooring locationSynoptic model‐based estimate of Atlantic Water volume transport in the Yermak Pass Branch from 2007 to 2020 varies from −1 to 5 Sv [ABSTRACT FROM AUTHOR]

Among Western Boundary Currents, the East Australian Current (EAC) has a more energetic eddy field relative to its mean flow, however, the relationship between upstream transport and downstream eddy kinetic energy (EKE) is still unclear. We investigate the modulation of downstream EKE in the EAC's typical separation region (Tasman EKE Box) (33.1°S–36.6°S) based on a long‐term (22‐year), high‐resolution (2.5–6 km) model simulation and satellite altimeter observations from 1994 to 2016. Our results show that the poleward EAC transport at 28°S leads the EKE in the Tasman EKE Box by 93–118 days. Barotropic instabilities are the primary source of EKE, and they control EKE variability in the EAC system. Anticyclonic eddies shed from the EAC dominate from 33°S–36°S during high‐EKE periods, but in low‐EKE periods anticyclonic eddies penetrate even further south by ∼2°. Plain Language Summary: The East Australian Current (EAC) mean flow is typically coherent from ∼27°S–32°S (upstream), but eddies form after it separates from the coast typically at ∼32°S, associated with high eddy variability downstream. However, we know little about what drives changes in the downstream eddies and the correlation with transport upstream. Here, we use both satellite observations and model simulations to investigate the interannual variability in the eddy field. We find that the transport upstream of separation is well correlated with the variability of sea surface height within the typical EAC separation region. An anomalously higher EAC transport at 28°S corresponds to an anomalously higher sea surface height within the typical EAC separation region. The reverse is true when the EAC transport is weaker, but the current separates from the coast further to the south. We also show that the energy converted from the mean flow to the eddy fields is mainly through mean kinetic energy to eddy kinetic energy. Key Points: Barotropic instabilities dominate eddy shedding and control variability of eddy kinetic energy (EKE) in the East Australian CurrentThere is a clear (inverse) relationship between poleward transport at 28°S and the latitude that anticyclonic eddies are shedTransport at 28°S and sea level anomalies at ∼27°S–29°S (upstream) are significantly correlated with EKE at ∼33.1°S–36.6°S (downstream) [ABSTRACT FROM AUTHOR]

Linear trends in Kuroshio transport during a recent global warming hiatus (1998–2013) were evaluated using long‐term ferryboat ADCP (acoustic Doppler current profiler) data and tidal gauge data in the Tokara Strait south of Japan. The Kuroshio exhibited a remarkable weakening trend of approximately 0.05 Sv year−1 (1 Sv = 106 m3 s−1). The pycnocline in the weakened Kuroshio was relaxed and displayed shoaling at the offshore edge, which was attributed to vertical thermocline displacement rather than to water mass modification. Importantly, Kuroshio transport trends in the Tokara Strait were affected by sea surface height anomalies, which were driven by the combined effects of the clockwise baroclinic‐mode coastal trapped wave propagation along the southern coast of Japan and downstream Kuroshio advection in the East China Sea. Both features were induced by wind stress curl changes related to the global warming hiatus over the North Pacific. Plain Language Summary: The Kuroshio is the western boundary current in the wind‐driven subtropical circulation of the North Pacific. It exits the East China Sea and flows into the North Pacific through the Tokara Strait between the Yakushima and Amami Oshima Islands to the south of Kyushu, Japan. This study revealed that the Kuroshio transport through the Tokara Strait showed a remarkable weakening trend along with a northward shift during 1998–2013. Changes in the wind stress curl in the high (low)‐latitude band over the North Pacific during the global warming hiatus exhibited negative (positive) tendencies, leading to anomalously high (low) sea surface heights. The high (low) sea surface height anomaly arrived at the onshore (offshore) side of Tokara Strait from the high (low)‐latitude band decreasing Kuroshio transport because of geostrophy. Key Points: Kuroshio transport in the Tokara Strait south of Japan decreased by ∼4% during 1998–2013, indicating a rate of about 0.05 Sv year−1Shoaling of the offshore pycnocline in the Kuroshio was due to vertical thermocline displacement rather than to modified water massDynamic‐topography propagation pathways from the northern and southern interior regions caused weakened Kuroshio transport in the hiatus [ABSTRACT FROM AUTHOR]

The importance of the Malacca Strait to trade is important but its importance in terms of ocean connectivity is often ignored. This paper is a study of the propagation of tide and sea level anomalies (SLA) (i.e., non-tidal component) in the Malacca Strait using a well calibrated numerical model. Four main tidal constituents are examined at different sections of the strait. Tidal amplitude in terms of water level and volumetric flux of these four constituents are derived using harmonic analysis. It is shown that the tidal flux propagation pattern behaves differently from the tidal water level propagation pattern in the strait. Detailed analysis of the tidal amplitudes of both water level and volumetric flux makes the location of the tidal mixing zone in the strait becomes apparent. Analyzing the SLA with regards to flux and net flux volume at each cross section in the strait shows that the tidal mixing zone plays a role in obstructing the propagation of SLA from Andaman Sea and Singapore Strait. [ABSTRACT FROM AUTHOR]

For 30+ years, submarine cable voltage measurements have been a critical measurement system used to produce a highly valuable time series of daily Florida Current volume transport at 27°N in the Florida Straits. However, the high cost associated with replacing the measurement system, should the existing telecommunications cable break, represents a significant vulnerability to the continuation of this important transport time series. Six years of data from tide gauges near the ends of the cable at 27°N have been used to test the potential of a paired tide gauge system to replace the cable in the event of a future problem. Validations against the daily cable observations, and against snapshot transport estimates from ship sections, suggest that the tide gauges do represent a viable replacement, however, the accuracy of the transports determined from the tide gauges is lower than for the cable observations (2.7 Sv vs. 1.7 Sv, respectively). The tide gauges capture roughly 55% of the total variance observed by the cable. The correlation between the cable data and the tide gauge differences is fairly constant (r ≈ 0.75) after low-pass filtering the data at periods from 3 to 365 days, illustrating a lack of coherence sensitivity to those time scales. [ABSTRACT FROM AUTHOR]

Ocean circulation in the Challenger Deep is rarely investigated due to sparse observations. Based on hydrographic data collected by two super‐deep Chinese "Sea‐Wing" gliders during September–October 2018, we analyzed water properties and ocean circulation structures in this trench. Results showed that the horizontal distribution of water properties is approximately a three layer structure, changing from a northeast‐southwest structure to a north‐south structure then to an east‐west structure as the depth increases from 3,000 to 7,000 m. This structure is a joint result of the water uplift, advection, and diffusion in the trench. The westward geostrophic flow, with 1.29 Sv (1 Sv = 106 m3 s−1) volume transport, dominates the deep layer of the Challenger Deep, gradually weakening with depth. The unexpectedly large volume transport, twice that of previous studies, might be due to temporal variations of LCDW (Lower Circumpolar Deep Water) intrusion from the Southern Ocean and different reference levels. Plain Language Summary: The Challenger Deep is the world deepest trench. It acts as a transition zone for deep ocean water transport from the Southern Ocean to the northwestern Pacific Ocean. The AABW (Antarctic bottom water) bring large volume of clod, salty, dense, high dissolved oxygen and low‐nutrient to this region and impact the local environment and biomass in this trench. Hence, it is a meaningful thing to explore the water characteristics, circulation and volume transport over this region. Limited by observation technology in the past, they are rarely studied by previous researchers. Present observations by new‐type super‐deep gliders give us a chance to achieve that goal. Results indicate that complex 3‐D structures of water properties and circulation show up in the Challenger Deep. And large water volume of LCDW (Lower Circumpolar Deep Water) is transported into this area. These results promote the knowledge of water mass properties and circulation in the Challenger Deep. Key Points: Water property in the Challenger Deep has a three‐layer structureWestward geostrophic flow dominated the Challenger Deep and its core is shallower in the westLarge volume transport of Lower Circumpolar Deep Water (LCDW) about 1.29 Sv passed through the trench area above 7,000 m [ABSTRACT FROM AUTHOR]

Velocities in Drake Passage from the 25-year GLORYS12 Mercator Ocean reanalysis were compared with satellite altimetry-derived surface velocities and independent in-situ velocity measurements from the DRAKE (2006–2009) and cDrake (2007–2011) experiments. GLORYS12 assimilates satellite along-track sea level anomalies and, as expected, model velocities are in rather good agreement with gridded altimetry derived velocities (r ~ 0.5, above 99% confidence level). GLORYS12 velocities also compared well with the current-meter data from the DRAKE (in the water column) and cDrake (50 m above seafloor) experiments in terms of means and standard deviations; most correlations between the reanalysis and directly measured velocity time series were significant (r ~ 0.5 for DRAKE and r ~ 0.4 for cDrake, above 99% confidence level). We used the GLORYS12 reanalysis to examine the Antarctic Circumpolar Current (ACC) total volume transport across three sections in Drake Passage. The 25 years of the ACC total volume transport has a mean of 155 ± 3 Sv and a standard deviation of 6.7 Sv. Annual mean values span over a range of 12 Sv. The spectrum of the GLORYS12 total transport time series shows energy peaks (above 95% confidence level) at the intraseasonal, semi-annual and annual periods and no significant low-frequency variations (beyond 2 years) or trend, in agreement with previous studies. This first assessment of GLORYS12 velocities in Drake Passage is very encouraging for monitoring the regional variability of the ACC. [ABSTRACT FROM AUTHOR]

The large increase in upper-ocean sampling during the past decade enables improved estimation of the mean meridional volume transport in the midlatitude South Pacific, and hence of the climatically important Meridional Overturning Circulation. Transport is computed using Argo float profile data for geostrophic shear and trajectory data for reference velocities at 1000 m. For the period 2004-2012, the mean geostrophic transport across 32S is 20.6 ± 6.0 Sv in the top 2000 m of the ocean. From west to east, this includes the southward East Australian Current (23.3 ± 2.9 Sv), its northward recirculation (16.3 ± 3.6 Sv), the broad interior northward flow (18.4 ± 4.1 Sv), and the net northward flow (9.2 ± 2.2 Sv) in opposing currents in the eastern Pacific. The basin-integrated geostrophic transport includes 7.3 ± 0.9 Sv of surface and thermocline waters, 4.9 ± 1.0 Sv of Subantarctic Mode Water, and 4.9 ± 1.4 Sv of Antarctic Intermediate Water. Interannual variability in volume transport across 32S in the South Pacific shows a Southern Annual Mode signature characterized by an increase during the positive phase of the Southern Annular Mode and a decrease during the negative phase. Maximum amplitudes in geostrophic transport anomalies, seen in the East Australian Current and East Australian Current recirculation, are consistent with wind stress curl anomalies near the western boundary. Key Points Improved meridional volume transport in the South Pacific using Argo EAC transport variability tied to the SAM © Published 2014. This article is a U.S. Government work and is in the public domain in the USA.

We use 25 years of a high‐resolution ocean reanalysis (1/12°) to revisit the Malvinas Current (MC) from the South (Drake Passage) to the North (Brazil‐Malvinas Confluence) from the synoptic to interannual time scales. The Malvinas Plateau is home to active eddy mixing, eddy dissipation and deep winter mixed layers occasionally reaching 600 m depth. The MC is organized in two jets which merge around 44°S as the continental slope steepens. The upper 900 m transport mean decreases from 40 Sv at 51°S to 35 Sv at 41°S indicating offshore leakage along the MC path. The MC plays a minor role in the velocity variations observed at the Confluence at seasonal and interannual scales; those are driven by changes in the intensity of the Brazil Current over the slope (34–36°S). Computing MC transport time series at different latitudes requires care because the section eastern limits are embedded in an energetic region. Transport time series were produced at selected latitudes using different criteria and showed common features. They show little seasonality (relative seasonal standard deviation of 2%) and no significant trend. The MC is a steady current: the relative standard deviation is on the order of 10% increasing to 20% near and on the Malvinas Plateau and near the Confluence. In contrast, velocity trends are large in the Brazil Current with the overshoot migrating southward. The associated increase in mesoscale activity south of 44°S in the Argentine Basin might contribute to blocking events occasionally reducing the MC transport. Plain Language Summary: The Malvinas Current (MC) is a branch of the Antarctic Circumpolar Current and flows northward along the Patagonian slope. We revisit the MC upper circulation and water masses using a high‐resolution ocean reanalysis from 1993 to 2017. The MC is a strong and rather steady current with little seasonality and no significant trend. The mean MC volume transport reduces from south to north by about 14%, indicating an offshore leakage along its path. The upper 500 m MC water underwent a freshening of 0.1 psu/decade. Key Points: The Malvinas Current is rather steady: Volume transport time series have little seasonality, no trend, and small relative standard deviationsThe upper 900 m transport mean decreases from 40 Sv at 51°S to 35 Sv at 41°S indicating offshore leakage along the MC pathThe Malvinas Plateau is a key region for water mass modification through eddy mixing and deep winter mixed layers [ABSTRACT FROM AUTHOR]

The Brazil Current, the western limb of the subtropical gyre of the South Atlantic Ocean, is one of the major Western Boundary Currents of the global ocean. Here, we present the first multiyear continuous daily time series of Brazil Current absolute volume transport obtained using 6+ years of observations from a line of four pressure‐recording inverted echo sounders (PIES) deployed at 34.5°S. The array was augmented in December 2012 with two current meter‐equipped PIES and in December 2013 with a moored Acoustic Doppler Current Profiler on the upper continental slope. The Brazil Current is bounded by the sea surface and the neutral density interface separating South Atlantic Central Water and Antarctic Intermediate Water, which is on average at a reference pressure of 628 ± 46 dbar, and it is confined west of 49.5°W. The Brazil Current has a mean strength of −14.0 ± 2.8 Sv (1 Sv ≡ 106 m3 s−1; negative indicates southward flow) with a temporal standard deviation of 8.8 Sv and peak‐to‐peak range from −41.7 to +20 Sv. About 80% of the absolute transport variance is concentrated at periods shorter than 150 days with a prominent peak at 100 days. The baroclinic component accounts for 85% of the absolute transport variance, but the barotropic variance is not negligible. The baroclinic and barotropic transports are uncorrelated, demonstrating the need to measure both transport components independently. Given the energetic high frequency transport variations, statistically significant seasonal to interannual variability and trends have yet to be detected. Plain Language Summary: Western Boundary Currents are the most intense currents of the global ocean and are key to the redistribution of mass, heat, salt, and carbon throughout the globe. In the South Atlantic, the Brazil Current transports warm and salty waters off the South American coast toward the pole and is a major driver of climate variability. This study presents, for the first time, multiyear continuous‐in‐time direct observations of the Brazil Current at 34.5°S. Roughly 6 years of daily measurements from moored sensors, together with high‐resolution snapshots of temperature, salinity, oxygen, and velocity collected during seven oceanographic cruises since 2009, provide the ability to characterize the daily to seasonal to year‐to‐year variability of the Brazil Current with unprecedented detail. These observations reveal strong and rapid changes in the Brazil Current on time scales as short as 14–60 days, allow quantification of the required sampling to resolve longer‐term variability, and improve estimates of the Brazil Current transport. The Brazil Current variability is dominated by the east‐west density variations in the water column although east‐west differences in bottom pressure are not negligible. Understanding of the strength, structure, and time variability of the Brazil Current is needed to improve model representations of this important flow. Key Points: The multiyear continuous absolute Brazil Current transport measured at 34.5°S has significant variability on daily to monthly time scalesThe baroclinic component accounts for the largest part of the absolute transport variance, but the barotropic variance is not negligibleNo meaningful seasonal cycle, interannual variability, or trend is detected during the roughly 6 years of daily transport measurements [ABSTRACT FROM AUTHOR]

Geostrophic deep circulation in the Japan Sea is modeled using the primitive equation multilayer model developed by N.B. Shapiro and E.M. Mikhaylova (Marine Hydrophysical Istitute, RAS). The main simulated patterns are the cyclonic gyres along the sea margins and in the deep basins and the anticyclonic circulation above underwater rises, that generally corresponds with the schemes based on deep floats tracking. The deep circulation is more intense in the subarctic sector than in the subtropical sector and the most intense cyclonic gyre is simulated in the Japan Basin, with the current speed up to 7–8 cm/s at its eastern margin. Speed of the simulated currents compares well with that derived from Argo data. Weakening anticyclonic circulation in the southwestern part of the sea under forcing of cyclonic wind stress curl or decrease of volume transport in the Korea Strait causes increasing of the simulated deep currents in the entire sea and the cyclonic gyres in the whole water column in its subarctic sector. Therefore, the wind and thermal forcing indirectly affect the deep circulation through the water mass redistribution by the surface currents. The deep cyclonic gyre in the Japan Basin has strong seasonal variation, strengthening in winter and weakening in summer, in line with the buoys data. The gyre intensification is accompanied with its shrinkage and the current speed decreasing at its periphery, that explains the seasonal variation of the deep currents at the northern slope of the Japan Basin observed by deep mooring. This winter strengthening of the gyre, both in the deep and intermediate layers, increases the currents barotropy, whereas the gyre strengthening in the surface layer in summer, when it weakens below 100–150 m, increases the baroclinicity. Despite of relatively low spatial resolution, the model captures deep dynamic structures related to local bottom topographic features, such as anticyclonic eddies around underwater rises and seamounts and cyclonic eddies above topographic depressions in the subarctic sector. The reverse currents were simulated above Yamato Rise, Korea Plateau, and in the Ulleung Basin in condition of the weakened deep circulation adjusting in one or two months. The reverse currents are tracked by Argo buoys, too, they follow the changes of surface currents.

The export of dissolved organic carbon (DOC) from the Arctic Ocean is expected to change due to warming and increased river runoff. Here we present a method to quantify DOC transport with the East Greenland Current combining synoptic and year‐round data (2009–2018). An algorithm based on quantitative and qualitative aspects of colored dissolved organic matter (CDOM) was developed to provide DOC estimates. Combined with mooring‐derived monthly Polar Surface Water (PSW) volume transports, we estimate DOC exports for the period from 2003–2017. For much of the period DOC exports have been reasonably constant at 46.8 (±6.2) Tg yr−1, while the reduction in PSW export in recent years has resulted in lower annual DOC exports (below 39 Tg C yr−1). We now have a technique to resolve seasonal and annual fluctuations in Arctic carbon export, offering a significant improvement over earlier bulk estimates and represents a baseline to detect future change. Plain Language Summary: The Arctic Ocean has higher concentrations of dissolved organic carbon (DOC) compared to the other oceans due to the supply from large Siberian rivers that drain expansive carbon‐rich watersheds. Climate change in the Arctic is altering the supply of river water and DOC to the Arctic Ocean. This is eventually expected to also be reflected in the subsequent export of DOC from the Arctic to the North Atlantic via a major gateway, the Fram Strait. Here we present a new approach to quantify the transport of DOC through the Fram Strait based on year‐round mooring data and spectroscopic measurements of colored dissolved organic matter (CDOM). We calculate DOC transports using our measurements over a 10‐yr period, and the results provide a baseline from which to observe future changes in the Arctic Ocean that were not resolved with earlier estimates based on bulk concentrations and bulk volume transports. Key Points: Spectral characteristics of CDOM can predict DOC concentrations in the Fram StraitDOC transports are strongly correlated with Polar Surface Water volume transports from the Arctic Ocean with the East Greenland CurrentAnnual DOC transport estimates for 2009 to 2018 varied between 27 and 60 Tg C yr−1 [ABSTRACT FROM AUTHOR]

The North Atlantic Current (NAC) supplies the subpolar gyre with warm and saline water from the subtropics as part of the upper branch of the Atlantic Meridional Overturning Circulation (AMOC). In the context of climate changes, the North Atlantic Ocean is one of the key regions to investigate the variability of the overturning circulation in which salinity and freshwater variability are playing a central role. Through the gravest empirical mode (GEM) method, we reconstructed salinity and velocity fields in the same spatiotemporal resolution as the sea surface height (SSH) product. The time series of salinity, freshwater, and volume transport are characterized by strong interannual variability related to most of the recently developed subpolar gyre indices. The variability of the freshwater transport in the study area is as high or even higher than the freshwater fluxes from the Arctic Ocean and thus needs to be considered for the impact of freshwater on the subpolar North Atlantic. Our analysis also revealed that the NAC import into the subpolar gyre is not only occurring in the western Atlantic close to the western boundary current. One o the NAC branches that forms at the Mann Eddy contributes about 15% of the volume transport and 28% of the freshwater transport crossing from the western into the eastern Atlantic north of 45°N. This branch has not been subject to the strong lateral mixing with freshwater at the western boundary, and thus, the salinity of this NAC pathway is higher than the one at the western boundary. Plain Language Summary: The northward prolongation of the Gulf Stream is called North Atlantic Current (NAC). The NAC transports warm and salty water from the subtropics to the north, and part of it even reaches the Arctic Ocean. Up to now, the pathways north of 40°N and their strengths and fluctuations are only poorly known. In this study, we used satellite data and measurements in the ocean to calculate the amount of water (volume transport) and salt (here expressed as negative salinity transport and dubbed freshwater transport) between 40°N and 53°N. Our results show that all transports are very variable during the time period from 1993 to 2016. The fluctuation of the NAC freshwater transport is comparable with the amount of freshwater coming from the Arctic Ocean into our study region and thus cannot be neglected. We also saw that the NAC splits into several branches that cross from the western into the eastern Atlantic Ocean. It has been often considered that the southernmost branches turn back to the subtropics. Our results show, however, that some fraction of them instead flows into the eastern subpolar region and influences the amount of salinity there. Key Points: North Atlantic Current branches forming at the Mann Eddy contribute to the salinity distribution in the subpolar eastern North AtlanticTime series of salinity, freshwater, and volume transport show strong variability related to most of the subpolar gyre indicesNorth Atlantic freshwater transport variability is as high or higher than the fluxes from the Arctic ocean [ABSTRACT FROM AUTHOR]

Two-years of data from an array of pressure-sensor equipped inverted echo sounders (PIESs) deployed across the Kuroshio east of Taiwan are analyzed in this study. Cross sections of temperature, absolute, barotropic and baroclinic velocities, as well as time series of volume transports and maximum velocity axis location are analyzed to better understand the response of the Kuroshio to local arrivals of mesoscale eddies and to upstream variations. A measure of Kuroshio position, Xvmax, defined by the velocity in the upper 500 m, does not always coincide with the position of the Kuroshio core defined by the velocity in the upper 150 m. These respond differently to single eddies, dipole eddies and upstream variations. Single mesoscale eddies affect the Kuroshio baroclinicity via a see-saw like vertical displacement of isopycnals from the surface to at least 1000 m depth. Single anticyclonic (cyclonic) eddies deepen (shoal) the isopycnals on the offshore side of the Xvmax, and shoal (deepen) the isopycnals on the inshore side. As a result of single anticyclonic (cyclonic) eddy-Kuroshio interactions, only the Xvmax shifts towards the east (west) while the Kuroshio core location is unchanged. Upstream variations of the Kuroshio in the Luzon Strait can weaken the Kuroshio and favor the eastward migration of both Xvmax and the Kuroshio core east of Taiwan. Coincident with this, a southward current is observed below 200 m depth off the coast of Taiwan. Dipole eddies were also observed to generate migrations of both the Kuroshio core and Xvmax. [ABSTRACT FROM AUTHOR]

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The comprehensive three-dimensional structures of an anti-cyclonic mesoscale eddy (AE) in the subtropical northwestern Pacific Ocean were investigated by combining the Argo floats profiles with enhanced vertical and temporal sampling and satellite altimetry data. The AE originated near the Kuroshio Extension and then propagated westward with mean velocity of 8.9 cm/s. Significant changes and evolutions during the AE's growing stage (T1) and further growing stage (T2) were revealed through composite analysis. In the composite eddy core, maximum temperature (T) and salinity (S) anomalies were of 1.7 (1.9)°C and 0.04 (0.07) psu in T1 (T2) period, respectively. The composite T anomalies showed positive in almost whole depth, but the S anomalies exhibited a sandwich-like pattern. The eddy's intensification and its influence on the intermediate ocean became more significant during its growth. The trapping depth increased from 400×104 Pa to 580×104 Pa while it was growing up, which means more water volume, heat and salt content in deeper layers can be transported. The AE was strongly nonlinear in upper oceans and can yield a typical mean volume transport of 0.17×106 m3/s and a mean heat and salt transport anomaly of 3.6×1011 W and −2.1×103 kg/s during the observation period. The Energy analysis showed that eddy potential and kinetic energy increased notably as it propagated westward and the baroclinic instability is the major energy source of the eddy growth. The variation of the remained Argo float trapped within the eddy indicated significant water advection during the eddy's propagation. [ABSTRACT FROM AUTHOR]

The Bering Strait throughflow has important implications for the Arctic freshwater, heat, and nutrients. By keeping the interannual variabilities of the atmospheric forcing only inside or outside the Arctic Ocean in numerical simulations, we can quantify their relative contributions to the interannual variability of the throughflow. We found that winds play a much more important role for the throughflow interannual variability than buoyancy forcing. Winds over the western Arctic Ocean and North Pacific determine the direction of Ekman transport, thus changing the sea surface height gradient between the two basins, and consequently influencing the volume transport strength. Although winds over the two basins are similarly important for the variance of ocean volume transport, the North Pacific winds cause stronger variability in freshwater and heat transports through modifying the inflow temperature and salinity. After 1994, winds over the western Arctic Ocean explain a larger part of the variability of Bering Strait volume transport than the winds outside the Arctic Ocean. Plain Language Summary: The Pacific Water entering the Arctic Ocean through Bering Strait can affect the Arctic freshwater storage, sea ice cover, ecosystem, and even the large‐scale ocean circulation. Therefore, it is crucial to understand the mechanisms driving the Bering Strait inflow variability. We designed a set of numerical simulations that allow us to quantify the origins of the interannual variability of the inflow. We found that winds both upstream and downstream the Bering Strait together drive the variability, while ocean surface buoyancy forcing plays a relatively small role. However, the variability induced by winds over the North Pacific and western Arctic Ocean are not in phase. Winds over the North Pacific lead to stronger changes in the freshwater and heat transport intensity than winds over the western Arctic Ocean, whereas the latter are responsible for the major part of the volume transport variability after 1994. We expect that the improved understanding can help to identify reasons for the discrepancy between projected Bering Strait inflow in climate models and thus improve model predictions in future work. Key Points: Winds over the Arctic and Pacific Oceans drive the interannual variability of Bering Strait volume transport, contributing similar varianceWinds over the northern Pacific lead to stronger interannual variability for freshwater and heat transports than the Arctic windsAfter 1994 Arctic winds have larger contribution to the variability of Bering Strait volume transport than winds outside the Arctic [ABSTRACT FROM AUTHOR]

Based on hydrographic data from two cruises (June 2010 and May 2012) off Baja California Sur, Mexico, historical in situ measurements from the World Ocean Database 2018, California Cooperative Oceanic Fisheries Investigations, and Investigaciones Mexicanas de la Corriente de California cruise programs, as well as satellite images and data from the Global Reanalysis, this study describes the California Current System off Baja California Sur. The California Current System is characterized by the interaction of four near-surface currents. Far from the coast, the California Current flows within the North Pacific Subtropical Gyre but does not reach the Pacific Tropical-Subtropical Convergence off Mexico. From December to June, an equatorward flow known as the Tropical Branch of California Current emerges along the coast. This flow intensifies from March to June and is closely related to the mass flux induced by coastal upwelling. In July, a new branch of the Tropical Branch of the California Current turns poleward along the Baja California Sur coast. This flow is often referred to as the California Surface Countercurrent because it flows in the opposite direction to the California Current. Additionally, the California Undercurrent is detected, influencing the water column from 100 to 900 m, with a maximum poleward flow between 200 m and 300 m, decreasing toward the surface. The California Undercurrent persists throughout the year over the continental slope, displaying a significant semiannual component around the Gulf of California entrance. The California Current System off Baja California Sur plays a crucial role in the formation of Transitional Waters within the Pacific Tropical-Subtropical Convergence off Mexico. The importance of equatorward flows by the Tropical Branch of California Current in ventilating the Oxygen Minimum Zone waters in the Central Pacific off Mexico is emphasized. [ABSTRACT FROM AUTHOR]

Tides have significant effects on material and energy transports over the continental shelf. A mooring system located in the shelf region of the northeastern South China Sea observed strong and weak mean currents during the spring and neap tides in summer, respectively. The mechanism leading to this difference is investigated by using a three-dimensional circulation model. We find that the impact of tides (TI) on currents exhibited as current components opposing wind-driven circulation play an important role. The TI current components are stronger than the tidal residual currents in summer when vertical stratification enhances the tidal rectification. Tides can affect pressure gradient by modifying sea surface height on the shelf, while in shallow and shelf break regions, the non-linear advection and vertical diffusion become significant in momentum equations attributable to the interactions of tidal currents, bottom topography and water column stratification. Consequently, the along-shelf volume transport is reduced by about 21.6% due to the TI current components and only by about 6.1% due to the tidal residual currents. This study highlights the importance of both tidal mixing and tidal rectification in regulating shelf circulation in the northeastern South China Sea. • Fields and model results show spring-neap variations of shelf currents in the NESCS. • Volume transport is largely reduced under the influence of ties and stratification. • Interactions of tides, topography and stratification produce spring-neap variations. • Tides can affect pressure gradient by modifying sea surface height on the shelf. [ABSTRACT FROM AUTHOR]

The origin of the Ryukyu Current (RC) and the formation of its subsurface velocity core were investigated using a 23-year (1993–2015) global Hybrid Coordinate Ocean Model (HYCOM) dataset. The volume transport of the RC comes from the Kuroshio eastward branch (KEB) east of Taiwan and part of the North Pacific Subtropical Gyre (p-NPSG). From the surface to 2 000 m depth, the KEB (p-NPSG) transport contributes 41.5% (58.5%) to the mean total RC transport. The KEB originally forms the subsurface velocity core of the RC east of Taiwan due to blockage of the subsurface Kuroshio by the Ilan Ridge (sill depth: 700 m). Above 700 m, the Kuroshio can enter the East China Sea (ECS) over the Ilan Ridge, meanwhile, the blocked Kuroshio below 700 m turns to the right and flows along the Ryukyu Islands. With the RC flowing northeastward, the p-NPSG contribution strengthens the subsurface maximum structure of the RC owing to the blockage of the Ryukyu Ridge. In the surface layer, the p-NPSG cannot form a stable northeastward current due to frequent disturbance by mesoscale eddies and water exchange through the gaps (with net volume transport into ECS) between the Ryukyu Islands. [ABSTRACT FROM AUTHOR]

Twenty‐five years of high‐resolution (1/12°) ocean reanalysis are used to examine the Confluence of the Malvinas Current (MC) with the Brazil Current (BC) from synoptic to interannual time scales. The model transports of the MC (38.0 Sv ± 7.4 Sv 57 at 41°S) and the BC (23.0 Sv ± 11 Sv at 36°S) agree with observations. The model shows the branching of the MC near the Confluence with an offshore branch returning south and an inner branch sinking below the BC and managing to continue northward along the continental slope. Northward velocities associated with the subsurface inner branch peak at 40 cm/s at 36°S at 700‐m depth. The model documents the migrations of the Subantarctic (SAF) and Subtropical Fronts (STF) at the Confluence. The SAF and STF positions vary over a large range at synoptic (800 km) and interannual scale (300 and 200 km, respectively) compared to the rather small seasonal migrations of the STF (150 km) and SAF (50 km). While trends in the MC are small over the 25 years of the reanalysis, the BC becomes more intense (12.5 cm/s), saltier (0.37 psu), and warmer (2.5°C) in the upper 1,000 m. These trends are accompanied with a southward displacement of the STF and the SAF of 150 and 50 km. Plain Language Summary: The Brazil‐Malvinas Confluence is a highly dynamic region. Reproducing the circulation at the Confluence is a stringent test for operational models. Mercator Ocean high‐resolution (1/12°) ocean systems show remarkable performance in this region. We took advantage of 25 years of ocean reanalysis to examine variations in the Malvinas Current at 41°S, in the Brazil Current at 36°S, and in their confluence. The Malvinas Current branches at the Confluence: one branch veers offshore while the inner part subducts and flows north along the slope. At the Confluence, the Subantarctic and Subtropical Fronts show large synoptic (800 km) and interannual migrations (300 km) compared to rather small seasonal changes (<150 km). Key Points: Twenty‐five years of Mercator Ocean reanalysis are examined at the Brazil Malvinas Confluence from synoptic to interannual time scalesThe Malvinas Current branches at the Confluence: the outer part veers offshore, the inner part subducts, and flows north along the slopeThe migrations of the Subtropical and Subantarctic Fronts are large (small) at the synoptic and interannual (seasonal) time scales [ABSTRACT FROM AUTHOR]

While the influence of the subpolar gyre (SPG) on thermohaline variability in the eastern North Atlantic is well documented, the extent and timescale of the influence of the SPG on North Sea is not well understood. This is primarily because earlier investigations on the causes of variability in the North Sea water properties mostly focused on the role of atmosphere and deployed regional models. Here using a historical simulation with the Max Planck Institute Earth System Model (MPI‐ESM), we investigate circulation and water mass variability in key regions, namely, the Rockall Trough and the Faroe‐Scotland Channel, which link the North Atlantic to the North Sea. We find that salinity covaries with advective lags in these three regions and that the northern North Sea salinity follows the Rockall Trough with a lag of 1 year. We show that recurring and persistent excursions of salinity anomalies into the northern North Sea are related to the SPG strength and not to the local acceleration of the inflow. Furthermore, we illustrate that the SPG signal is more pronounced in salinity than in temperature and that this simulated SPG signal has a period of 30–40 years. Overall, our study suggests that, at low frequency, water mass variability originating in the North Atlantic dominates changes in the North Sea water properties over those due to local wind‐driven volume transport. Plain Language Summary: Earlier investigations on the causes of variability in the North Sea water properties have mostly focused on the role of the atmosphere. This follows from the general idea that shallowness of the North Sea makes it more responsive to wind speed and direction than the deeper ocean. In the present contribution, we identify variability in North Sea water properties other than that induced by the atmosphere. For our analysis, we use a historical simulation with a global coupled model for two reasons: (a) In the model, the connections between North Atlantic and North Sea are represented, which would not be the case in a regional model, and (b) the model provides a long continuous time series of water properties, which is not available from spatially and temporally scarce observations. We find that the strength of subpolar gyre (SPG) has an impact on the salinity of the North Sea. When the SPG is weak (strong), water from the subtropical (subpolar) North Atlantic dominates the inflow into the North Sea and thus increasing (decreasing) the salinity. This modulation of inflow properties happens at decadal periods and beyond and is largely independent of the amount of Atlantic water entering the North Sea. Key Points: Low frequency open ocean impact on the North Sea is revealed in a global coupled modelThe properties of Atlantic inflow to the North Sea follow changes in subpolar gyre strength, while the total inflow is wind drivenThe subpolar gyre signal in the Faroe‐Shetland Channel and the North Sea is more pronounced in salinity than in temperature [ABSTRACT FROM AUTHOR]

Taiwan Strait (TWS) plays a crucial role in material exchange and nutrient budget between the South China Sea and the East China Sea. In this study, we investigate the variability of volume transport in the TWS and its response to tropical Madden–Julian Oscillation (MJO) convection based on the simulated results from a three-dimensional operational numerical model. Validated by the observational data, the model generally reproduced the physical field well. The volume transport in the TWS has strong seasonal cycles as well as higher-frequency variations. Intra-seasonal fluctuations dominate the along-strait currents variability, while secondary and the last signal are seasonal one and inter-annual variability, respectively. Overall, the along-strait wind stress plays a more important role in controlling the variability of volume transport in the TWS than the pressure gradient induced by north-to-south sea level slope. At intra-seasonal time scales, the volume transport in the TWS varies with the movement of the tropical MJO convection from Indian Ocean eastward to the western Pacific Ocean. These oceanic anomalies are related to atmospheric anomalies, with a distinct physical linkage from the tropical atmosphere to the mid-latitude ocean. The tropical MJO deep convection can modify the upper tropospheric heights and generate a wave train pattern that propagates to mid-latitudes. In addition, these anomalous upper tropospheric heights modulate the surface pressure, resulting in a cyclonic anomaly. The upper tropospheric heights anomaly and its corresponding mean sea level pressure anomaly move eastward in the MJO cycle following the migrating MJO heat source in the tropics from phase 2 to 5. Consequently, surface winds change as the cyclonic anomaly moves from central China to the east of Japan, resulting in a northeast volume transport anomaly during MJO phase 2 and 3 and a southwest volume transport anomaly during MJO phase 4 and 5. • Hydrodynamics in Taiwan Strait were reproduced by an operational numerical model. • Wind dominates the volume transport in TWS than the surrounding pressure gradient. • Roles of tropical MJO on volume transport in the TWS are elaborated. [ABSTRACT FROM AUTHOR]