Search

In this study, we investigated the impacts of typhoons on the transformation of anticyclonic eddies (AEs) into subsurface anticyclonic eddies (SAEs) in the northwestern subtropical Pacific Ocean (NWSP) based on an ocean reanalysis product and multiple satellite observations. Results suggest that while the heavy precipitation and strong positive wind stress curl (WSC) induced by the passage of typhoons may be two main driving factors that transformed shallow mixed layer depth (MLD) AEs (i.e., those shallower than 50 m at the eddy core) into SAEs, the latter played a greater role in such transformation. In addition, shallow MLD AEs with a less depressed isopycnal structure near the eddy center before the passage of typhoons were more likely to be transformed into SAEs under the impacts of typhoons. The likely timing of such transformation may be within 9 days after the passage of typhoons. For deep MLD AEs (i.e., those deeper than 80 m at the eddy core), the impacts of typhoons may be much less prominent below the mixed layer. Based on a diagnostic analysis of the vertical potential vorticity (PV) flux at the surface, we examined the mechanism and dynamic processes involved in the transformation of deep MLD AEs into SAEs under the impacts of typhoons. Results show that while typhoons played a positive role in maintaining low PV within deep MLD AEs, which was favorable for further transformation into SAEs, the diabatic process associated with the net air–sea heat flux was the crucial favorable condition for the transformation of deep MLD AEs into SAEs. [ABSTRACT FROM AUTHOR]

Based on reanalysis data and numerical model, this study documented an unusually weak southward Somali Current (SSC) circulation, an anticlockwise circulation in the western Arabian Sea, during the winter of 1997 to spring 1998, throughout the two-decade period from 1994 to 2017. Theoretical analysis and model simulations suggest that the exceptionally weak SSC circulation was primarily caused by abnormal wind forcing around the equator, associated with extreme climate events. The strong equatorial easterly anomalies are mainly responsible for the weakening of the SSC circulation north of ∼6°N and contribute to about half of the weakening south of ∼6°N, by regulating the layer thickness along the eastern boundary, following the time-dependent Sverdrup relation. The large negative wind stress curl anomalies near the equator contribute to another half of the weakening south of ∼6°N. This research underscores the dynamic connection between the circulation in the western basin and equatorial dynamics during extreme climate modes. Significance Statement: The dynamic linkage between the western basin circulation and the equatorial forcing remains elusive. Based on reanalysis data, theoretical analysis, and numerical simulations, this study has confirmed that the exceptionally weak southward Somali Current circulation during the winter of 1997 to spring 1998 was significantly influenced by abnormal wind patterns around the equator, triggered by extreme climate modes. [ABSTRACT FROM AUTHOR]

The impact of two global climatic oscillating modes, East Atlantic/Western Russia (EA/WR) and Scandinavia (SCA) patterns, which significantly affect the climate in the Atlantic-Eurasian sector, on wind stress curl (WSC) over the Black Sea is studied. The wind data from the MERRA-2 reanalysis and the series of the SCA and EA/WR indices for 1980–2019 are used. The composite method was applied taking into account nonstationarity of the time series of the indices, and the spectral analysis is used for the stationary series. There is a significant increase in cyclonic WSC in different parts of the sea for most of a year during the positive phase of SCA. In September and October and on average in winter, the positive phase of EA/WR is accompanied by a significant increase in anticyclonic WSC. In general, it has been found that SCA influences the formation of the spatiotemporal structure of WSC more strongly than EA/WR. [ABSTRACT FROM AUTHOR]

The East African Coastal Current (EACC) is an important western boundary current of the tropical south Indian Ocean and plays an important role in the ocean circulation and biogeochemical cycles in the Indian Ocean. This study investigates the interannual variability of the EACC and its dynamical mechanisms. The result shows that the EACC has interannual variability associated with El Niño–Southern Oscillation (ENSO) during 1993–2017. The EACC shows a significantly positive correlation with the Niño-3.4 index with a correlation coefficient of 0.65, lagging the Niño-3.4 index by 18 months. During the decaying phases of El Niño (La Niña) events, the negative (positive) sea level anomaly (SLA) propagates westward as upwelling (downwelling) Rossby waves from the southeast Indian Ocean to the southwest Indian Ocean and then strengthens (weakens) the EACC due to zonal SLA gradient off the East African coast under geostrophic equilibrium. The SLA gradually weakens in the southeast Indian Ocean during its westward propagation but strengthens in the southwest Indian Ocean promoted by local wind stress curl anomaly. This study can improve our understanding of the relationship between the western boundary current of the tropical south Indian Ocean and large-scale ENSO air–sea processes and is important for managing marine fisheries and ecosystems on the East African coast. [ABSTRACT FROM AUTHOR]

Abstract Regional sea level rise varies from the global average and is influenced by climate variability. We studied sea level anomalies in southern Brazil from 1993 to 2022, finding increasing trend from 1993 to 2022. We used oceanic and atmospheric dynamics to understand the rapid sea level rise. Positive trends in the Southern Annular Mode and the South Atlantic Ocean subtropical gyre intensified wind stress curl and Ekman transport. If global warming continues and the Southern Annular Mode remains in a positive trend, sea level rise in southern Brazil is likely to persist and increase risks for the population in this low-lying coastal area.

Mesoscale wind stress perturbations induced by sea surface temperature (SST) perturbations have a feedback effect on the ocean by influencing air–sea heat and momentum fluxes. Unlike the thermal feedback mechanism, which is well understood, the momentum feedback still needs to be studied, especially the respective roles of divergent and rotational wind components. In this study, momentum feedback was examined using an ocean model and an empirical equation, which solved wind stress field perturbations from their divergence and curl estimated from time‐evolving downwind and crosswind SST gradients. Through several numerical experiments, it was found that both the divergent and rotational wind can induce positive and negative temperature changes at varying regions and depths. This modulating effect is weak, with a maximum SST change of about 0.4°C on average, but it can reach the deep ocean beyond 1000 m. [ABSTRACT FROM AUTHOR]

In this paper we summarize improvements in climate model simulation of eastern boundary upwelling systems (EBUS) when changing the forcing dataset from the Coordinated Ocean-Ice Reference Experiments (CORE; ∼2° winds) to the higher-resolution Japanese 55-year Atmospheric Reanalysis for driving ocean–sea ice models (JRA55-do, ∼0.5°) and also due to refining ocean grid spacing from 1° to 0.1°. The focus is on sea surface temperature (SST), a key variable for climate studies, and which is typically too warm in climate model representation of EBUS. The change in forcing leads to a better-defined atmospheric low-level coastal jet, leading to more equatorward ocean flow and coastal upwelling, both in turn acting to reduce SST over the upwelling regions off the west coast of North America, Peru, and Chile. The refinement of ocean resolution then leads to narrower and stronger alongshore ocean flow and coastal upwelling, and the emergence of strong across-shore temperature gradients not seen with the coarse ocean model. Off northwest Africa the SST bias mainly improves with ocean resolution but not with forcing, while in the Benguela, JRA55-do with high-resolution ocean leads to lower SST but a substantial bias relative to observations remains. Reasons for the Benguela bias are discussed in the context of companion regional ocean model simulations. Finally, we address to what extent improvements in mean state lead to changes to the monthly to interannual variability. It is found that large-scale SST variability in EBUS on monthly and longer time scales is largely governed by teleconnections from climate modes and less sensitive to model resolution and forcing than the mean state. [ABSTRACT FROM AUTHOR]

Based on the relation between volume transport and sea level difference across the Korea/Tsushima Strait (KTS), in this study, we estimated the volume transport of the Tsushima Warm Current over the past four decades and examined its long-term trend using an ensemble empirical model decomposition (EEMD) method. The estimated geostrophic transports showed very good agreement with those measured by acoustic Doppler current profiler and those estimated from satellite altimetry-based sea level data. This corroborates the reliability of the long-term estimation. EEMD analysis revealed an acceleration in volume transport in the KTS, with a persistent increase in the trend rate occurring after the mid-1980s. Further analyses indicate that the trend shift in the KTS closely coincided with an upper-ocean warming trend in the East/Japan Sea (EJS) after the mid-1980s. Analyzing large-scale wind patterns suggests that long-term trends of wind stress curls over the North Pacific are likely a key driver of the trend shifts in the inflow through the KTS and the upper-ocean warming in the EJS over the past four decades. [ABSTRACT FROM AUTHOR]

The Agulhas Return Current (ARC), as a primary component of the Agulhas system, contributes to water exchange and mass transport between the southern portions of the Indian and Atlantic Ocean basins. In this study, satellite altimeter data and reanalysis datasets, and a new set of criteria for the piecewise definition of the jet axis are used to explore the long-term change of the ARC's axis position in recent three decades. It is found that the ARC axis exhibits a significant slanting trend with its western part (35–48°E) migrating northward and the eastern part (48–70°E) migrating southward. The meridional movement of the ARC path could be attributed to large-scale wind forcing. The anomalous surface wind stress curl, by Ekman pumping mechanism, leads to positive–negative–positive sea surface height anomalies in the western section and negative–positive–negative anomalies in the eastern section, thus the ARC axis tilts accordingly, in a northwest–southeast direction. Further analysis suggests that this ARC slanting trend is more dependent on the southward shift of the downstream axis and less on the topographic steering upstream. The downstream axis is more likely to interact with the ACC fronts and its migration could dominate the local EKE pattern by changing the background circulation and energy cascade direction. For the headstream west of 35°E, the ARC axis is more subject to topography, thus the EKE change is more dominated by eddy activity processes, including shedding, propagation and merging. This study provides some new insights into the long-term change of ARC and its interaction with the local EKE variability. [ABSTRACT FROM AUTHOR]

This study examines the modifications of air-sea coupling processes by dust-radiation-cloud interactions over the North Atlantic Ocean using a high-resolution coupled atmosphere-wave-ocean-dust (AWOD) regional model. The dust-induced mechanisms that are responsible for changes of sea surface temperature (SST) and latent and sensible heat fluxes (LHF/SHF) are also examined. Two 3-month numerical experiments are conducted, and they differ only in the activation and deactivation of dust-radiation-cloud interactions. Model results show that the dust significantly reduces surface downward radiation fluxes (SDRF) over the ocean with the maximum change of 20-30 W m-2. Over the dust plume region, the dust effect creates a low-pressure anomaly and a cyclonic circulation anomaly, which drives a positive wind stress curl anomaly, thereby reducing sea surface height and mixed layer depth. However, the SST change by dust, ranging from -0.5 to 0.5 K, has a great spatial variation which differs from the dust plume shape. Dust cools SST around the West African coast, except under the maximum dust plume ridge, and extends westward asymmetrically along the northern and southern edges of the dust plume. Dust unexpectedly warms SST over a large area of the western tropical North Atlantic and north of the dust plume. These SST changes are controlled by different mechanisms. Unlike the SST change pattern, the LHF and SHF changes are mostly reduced underneath the dust plume region, though they are different in detail due to different dominant factors, and increased south of the dust plume over the tropic.

The relationship between the wind-wave spectrum and surface wind stress τ in tropical cyclones, especially for the misalignment |ϕ| between the wind and τ, was investigated using data from three Electromagnetic Autonomous Profiling Explorer (EM-APEX) floats deployed in Super Typhoon Megi (2010). The floats measured τ by integrating downward momentum flux in the ocean and, in a recent development, directional spectra of surface waves. The wind was captured by aircraft surveys. At wind speeds from 25 to 40 m s−1, the |ϕ| increased with the increasing angle between the wind and dominant waves. The |ϕ| was small near the eyewall, where wave energy concentrated in a narrow frequency band. At the location far away from the eyewall, where most spectra were bimodal in directions with similar frequencies, the stress direction might be similar to the high-frequency waves. The misalignment between the wind and propagating swell might affect the growth and directional spreading of wind waves under tropical cyclones. The resulting wave breaking might then release wave momentum into the ocean as most stress clockwise from the wind direction. C‖, the downwind drag coefficient, increased with increasing inverse wave age of dominant waves. |C⊥|, the magnitude of the crosswind drag coefficient, was significant when low-frequency waves deviated from the wind by more than 90°. The wave directions are used in the inverse wave age for scaling drag coefficients. The new parameterization based on wave dynamics can be useful for improving the prediction of wind stress curl under storms. Significance Statement: Tropical cyclones can impact the maximum sea surface temperature cooling through the curl of surface wind stress τ, which causes divergence under the storm eye. To investigate the effect of surface waves on τ, measurements of downward momentum flux and surface waves from three EM-APEX floats deployed under Typhoon Megi in 2010 are used. The inverse wave age involving wind forcing on the wave directions of dominant waves and swells can significantly influence the momentum transfer efficiency in the downwind and crosswind directions, respectively. That is, the propagating swell under storms should play a crucial role in the downward momentum flux during its interaction with high-frequency waves and wind. Incorporating wave directions to parameterize the magnitude and orientation of τ will improve future models' ability to predict wind stress curl and thereby the heat fluxes to storm intensification. [ABSTRACT FROM AUTHOR]

Using sea surface temperature (SST) data from satellite observations and reanalysis, we revealed the long-term trend of coastal upwelling in the South China Sea (SCS) during 1982–2020. An upwelling index (topographic position index, TPI), defined as the SST difference between the center cell and its neighbors, was used to quantify the upwelling intensity. There are three typical regions (eastern Guangdong, eastern Hainan, and eastern Vietnam) of coastal upwelling in the SCS, in which the upwelling intensity showed an increasing trend during 1982–2020. For all three regions, the upwelling intensity trend reached ~0.2 °C per 10 years in the past 40 years or so. A satellite wind dataset, CCMP, and reanalysis products, ERA5 and ORAS5, were used to discuss the causes of intensified upwelling. Based on the CCMP, the enhanced upwelling in eastern Guangdong and eastern Hainan may be more related to the strengthening of the wind stress curl. Both alongshore wind stress and wind stress curl contribute to the enhanced upwelling in southern Vietnam, and the increasing trend of upwelling in northern Vietnam is not significantly related to the wind field. This study could contribute to the understanding of coastal upwelling response to the long-term climate change in a marginal sea under the background of global warming.

The winter–summer transition in the southern South China Sea (SCS) western boundary current (WBC) is studied. Two categories have been identified. In case 1, the southern SCS WBC transition in the lower layer (below the thermocline) lags that in the upper layer (above the thermocline). In case 2, there is no transition lag at full depth. In both categories, the geostrophic balance dominates the transition. In case 1, the upper layer geostrophic balance is dominated by the sea surface height pressure gradient (SSHPG) and Coriolis forcing during southern SCS WBC transition. Therefore, there is no transition lag with depth in the upper layer. Below the thermocline layer, the competition between the SSHPG and the density pressure gradient (DPG) determines the transition. During the transition, the amplitudes of the SSHPG and DPG are basically equivalent. The SSHPG needs time to develop sufficiently larger than the DPG. Therefore, the transition in the deeper layer significantly lags that in the shallower layer. The reversal of the SSHPG is mainly attributed to the change in the basin-scale wind stress curl over the southern SCS. The change in the DPG is mainly associated with the cooling of the water along the western continental slope, which is induced by upwelling. In case 2, there is no cooling along the western continental slope, and then the amplitude of the DPG is always far smaller than that of the SSHPG. Responding to the change in the SSHPG, the southern SCS WBC transition behaves consistently at full depth. Significance Statement: We have a comprehensive understanding of the South China Sea (SCS) circulation patterns in winter and summer. However, their seasonal transitions remain unclear, and a better understanding of them is potentially helpful for improving ocean circulation modeling and prediction. This paper focuses on the winter–summer transition in the SCS western boundary current (WBC). Above the thermocline (∼100 m), the transition behaves consistently in the vertical direction and is controlled by the conversion of the sea surface height–induced pressure gradient. Below the thermocline, the transition in the deeper layer of the WBC significantly lags that in the shallower layer of the WBC, which is associated with the competition between the SSH-induced pressure gradient and the density-induced pressure gradient at the sea surface. [ABSTRACT FROM AUTHOR]

Coastal upwelling around Hainan Island has a significant impact on the physical environment, ecological environment, and fishery resources of the surrounding waters. Based on the satellite-observed sea surface temperature (SST) from 2003 to 2021, this study analyzed the interannual variations of SST around Hainan Island and the influence of the sea surface forcing on them, with special attention to the upwelling regions in Qiongdong and Qiongxi. The results show that satellite-observed SST can identify three upwelling regions located in the west (WS), east (ES), and northeast (NES) sides of the Hainan Island. The SSTs in the WS, ES, and NES showed warming trends of 0.045 °C/y, 0.010 °C/y, and 0.036 °C/y, respectively. The range of low-temperature water around Hainan Island has gradually shrunk over the past two decades, and this shrinkage trend was more prominent in the WS upwelling region than in the ES and NES upwelling regions. Both the EOF analysis and the SST difference between upwelling regions and non-upwelling regions suggests that the upwelling becomes stronger after 2013. However, it seems not to compete with the overall SST warming in the northwest area of the South China Sea. The interannual SST variations in the ES upwelling region were affected by the local wind stress curl in summer. Except for the wind stress curl, the interannual SST variations in NES upwelling are influenced by the stratification associated with local precipitation and wind direction, especially in 2018. In addition, the interannual SST variation in the WS upwelling region is closely related to the bottom cold water in the central Gulf of Tonkin, which was preserved from the previous winter and is influenced by winter atmospheric conditions. These results are helpful for understanding the variations in SST and ecosystem in the upwelling regions around Hainan Island.

The southwestern tropical Atlantic is a region of complex ocean dynamic where originates the strong western boundary current system composed of North Brazil current and North Brazil undercurrent. The region includes a variety of features including the Atoll das Rocas (AR) and Fernando de Noronha (FN) ridge that may favour mesoscale eddy dynamics. However, origin, occurrence and characteristics of mesoscale eddies were still not described in the region. Using satellite altimetry data from 1993 to 2018 off Northeast Brazil (37-25°W; 13-1°S), we reconstruct eddy trajectories and analyse the main eddy surface characteristics (e.g., size, amplitude, polarity) and their spatiotemporal variations. The study reveals two distinct dynamic regions before quantifying mesoscale eddies characteristics. Approximately 2000 mesoscale eddies crossed the region during the study period, among which 76% were generated inside the region, with amplitudes and radii ranging between 1 and 2 cm and 25 and 205 km, respectively. Eddies are preferentially formed between August and September and propagate westward. In the region around the FN Archipelago (36-26°W; 6-1°S), the formation of cyclonic eddies is likely favoured by barotropic instabilities of surface currents and the wind stress curl. On the other hand, in the south of the region (36-26°W; 12-8°S), eddies formation is likely associated with the barotropic instabilities, wind stress curl and the meandering of surface currents. Based on vertical temperature and salinity profiles from Argo floats’ data, we determined that in average, the core of cyclonic eddies is centred at ~ 130 m (140 m) in the northern (southern) region while the core of anticyclonic eddies is centred at ~90 m (125 m) in the northern (southern) region. Moreover, mesoscale eddies formed in the tropical Atlantic do not connect the eastern tropical Atlantic and northeast Brazil.

The tropical Pacific currents from January 2004 to December 2018 are computed based on the gridded Argo temperatures and salinities using the P-vector method on an f plane and the geostrophic approximation on a β plane. Three branches of the South Equatorial Current (SEC) are identified, i.e., SEC(N) (2°S–5°N), SEC(M) (7°–3°S), and SEC(S) (20°–8°S), with the maximum zonal velocity of −55 cm s−1 and total volume transport of −49.8 Sv (1 Sv ≡ 106 m3 s−1) occurring in the central-east Pacific. The seasonal variability of each branch shows a distinct and different westward propagation of zonal current anomalies, which are well mirrored by the SLA differences between 2°S and 5°N, between 3°S and 6°S, and between 8°S and 15°S, respectively. Most of the seasonal variations are successfully simulated by a simple analytical Rossby wave model, highlighting the significance of the first-mode baroclinic, linear Rossby waves, particularly those driven by the wind stress curl in the central-east Pacific. However, the linear theory fails to explain the SEC(M) variations in certain months in the central-east Pacific, where the first baroclinic mode contributes only around 50% of the explained variance to the equatorial surface currents. A nonlinear model involving higher baroclinic modes is suggested for a further diagnosis. Considering the crucial role played by the tropical Pacific in the natural climate variability via the El Niño–Southern Ocean dynamics and the ocean response to anthropogenic forcing via the ocean heat uptake in the eastern tropical Pacific, advancing the process understanding of the SEC from observations is critical. [ABSTRACT FROM AUTHOR]

The Indonesian throughflow (ITF) transports a significant amount of warm freshwater from the Pacific to the Indian Ocean, making it critical to the global climate system. This study examines decadal ITF variations using ocean reanalysis data as well as climate model simulations from the Coupled Model Inter-comparison Project Phase 5 (CMIP5). While the observed annual cycle of ITF transport is known to be correlated with the annual cycle of sea surface height (SSH) difference between the Pacific and Indian Oceans, ocean reanalysis data (1959–2015) show that the Pacific Ocean SSH variability controls more than 85% of ITF variation on decadal timescales. In contrast, the Indian Ocean SSH variability contributes less than 15%. While those observed contributions are mostly reproduced in the CMIP5 historical simulations, an analysis of future climate projections shows a 25–30% increase in the Indian Ocean SSH variability to decadal ITF variations and a corresponding decrease in the Pacific contribution. These projected changes in the Indian Ocean SSH variability are associated with a 23% increase in the amplitudes of negative zonal wind stress anomalies over the equatorial Indian Ocean, along with a 12º eastward shift in the center of action in these anomalies. This combined effect of the increased amplitude and eastward shift in the zonal wind stress increases the SSHA variance over the Indian Ocean, increasing its contribution to the ITF variation. The decadal ITF changes discussed in this study will be crucial in understanding the future global climate variability, strongly coupled to Indo-Pacific interactions. [ABSTRACT FROM AUTHOR]

Southern Ocean bathymetry constrains the path of the Antarctic Circumpolar Current (ACC), but the bathymetric influence on the coupled ocean–atmosphere system is poorly understood. Here, we investigate this impact by respectively flattening large topographic barriers around the Kerguelen Plateau, Campbell Plateau, Mid-Atlantic Ridge, and Drake Passage in four simulations in a coupled climate model. The barriers impact both the wind and buoyancy forcing of the ACC transport, which increases by between 4% and 14% when barriers are removed individually and by 56% when all barriers are removed simultaneously. The removal of Kerguelen Plateau bathymetry increases convection south of the plateau and the removal of Drake Passage bathymetry reduces convection upstream in the Ross Sea. When the barriers are removed, zonal flattening of the currents leads to sea surface temperature (SST) anomalies that strongly correlate to precipitation anomalies, with correlation coefficients ranging between r = 0.92 and r = 0.97 in the four experiments. The SST anomalies correlate to the surface winds too in some locations. However, they also generate circumpolar waves of sea level pressure (SLP) anomalies, which induce remote wind speed changes that are unconnected to the underlying SST field. The meridional variability in the wind stress curl contours over the Mid-Atlantic Ridge, the Kerguelen Plateau, and the Campbell Plateau disappears when these barriers are removed, confirming the impact of bathymetry on surface winds. However, bathymetry-induced wind changes are too small to affect the overall wave-3 asymmetry in the Southern Hemisphere westerlies. Removal of Southern Hemisphere orography is also inconsequential to the wave-3 pattern. Significance Statement: Several studies in the past have pointed to the controlling effect of bathymetry on currents in the Southern Ocean circulation, but a clear idea of the importance of the major topographic barriers in the Southern Ocean is lacking. By removing these barriers systematically in a coupled climate model, we can evaluate their impact on several important components of the climate system, such as the Antarctic Circumpolar Current (ACC) pathways and strength, Antarctic Bottom Water formation, sea surface temperature, overlying winds, air–sea fluxes, and even precipitation. This helps in our understanding of what controls the pathways of the ACC and how much it matters for climate. [ABSTRACT FROM AUTHOR]

Subantarctic mode water (SAMW) variations and their forcing mechanisms in the three Southern Hemisphere ocean basins are studied using Argo data and reanalysis products during the 2004–2019 period. In the Indian and Pacific Oceans, SAMW is characterized by potential vorticity (PV) minimums lower than 0.5 × 10−10 m−1 s−1. In the Atlantic Ocean, the PV minimum is generally lower than 1 × 10−10 m−1 s−1. The variations in SAMW thickness are significantly correlated with changes in the mixed layer depth (MLD) and show large differences among different ocean basins. The winter MLD in the Indian Ocean SAMW formation region is significantly positively correlated with the local wind stress curl (correlation coefficient = 0.61; 95% confidence level). In the Pacific and Atlantic SAMW formation regions, the variations in the winter MLD have significant negative correlations with the buoyancy flux; the correlation coefficients reach −0.57 and −0.67 (95% confidence level), respectively. These different mechanisms of the MLD variations in SAMW in the three ocean basins are related to the Southern Annular Mode (SAM) variation in winter. During positive (negative) SAM index periods, a strong positive (negative) wind stress curl anomaly occurs in association with a meridionally asymmetric westerly wind anomaly distribution. This phenomenon results in a corresponding MLD anomaly over the Indian Ocean SAMW formation region. In the Pacific and Atlantic SAMW formation regions, a negative (positive) westerly wind anomaly induces buoyancy gain (loss), leading to mixed layer shallowing (deepening) during positive (negative) SAM periods. Plain Language Summary: Subantarctic mode water (SAMW) forms during austral winter in the Subantarctic Zone. Recent studies indicate that the SAMW variations are crucial to the upper ocean heat content change. This change is concentrated in the extratropical latitudes of the Southern Hemisphere. We show that the SAMW in three oceans should be defined with different criteria and SAMW properties present different changing characteristics during the 2004–2019 period. The SAMW thickness variation correlates with mixed layer depth changes, which are driven by the wind stress and buoyancy flux changes in each ocean basin. The SAMW variations are greatly affected by the wind stress curl variations in the Indian Ocean. In the Pacific and Atlantic Oceans, SAMW variations are mostly affected by buoyancy flux variations. These differences between the variation mechanisms of SAMW are caused by the distribution differences of westerly wind anomalies over the three ocean basins. Key Points: Subantarctic mode water (SAMW) variations are different in each ocean basinsWind stress curl controls the Indian SAMW changes, while buoyancy flux is more important in the Pacific and AtlanticThe wind stress curl and buoyancy flux effects SAMW variations are related with westerly wind anomalies distribution [ABSTRACT FROM AUTHOR]

The thermocline depth in the tropical Indian Ocean has experienced dramatic decadal variations in recent decades. Using analysis and reanalysis datasets, we find that the decadal thermocline depth anomalies show large seasonal differences. The seasonal differences are modulated by two major modes. The first mode shows a zonal dipole pattern, with opposite thermocline depth anomalies in the equatorial eastern Indian Ocean and western Indian Ocean, and is prominent in summer and winter. The second mode is characterized by marked thermocline depth anomalies in the southern Indian Ocean and is significant in spring and fall. The amplitude of the seasonal oscillation in these two modes has increased substantially in the twenty-first century. Their phase change is in good agreement with the observed thermocline depth anomalies in each season. The results also show that the seasonality of the decadal thermocline depth anomalies arises directly from surface wind variations within the Indian Ocean. The first mode is mainly caused by equatorial zonal wind anomalies. The second mode is dominated by local wind stress curl anomalies. These wind anomalies are both significantly correlated with the ENSO-like SST anomalies in the Pacific Ocean. The findings improve our understanding of the decadal thermocline anomalies, and will help to better evaluate their impact on seasonal phase-locked oceanic and atmospheric processes. [ABSTRACT FROM AUTHOR]

The Sverdrup relation is the backbone of wind-driven circulation theory; it is a simple relation between the meridional transport of the wind-driven circulation in the upper ocean and the wind stress curl. However, the relation is valid for steady circulation only. In this study, a time-dependent Sverdrup relation is postulated, in which the meridional transport in a time-dependent circulation is the sum of the local wind stress curl term and a time-delayed term representing the effect of the eastern boundary condition. As an example, this time-dependent Sverdrup relation is evaluated through its application to the equatorial circulation in the Indian Ocean, using reanalysis data and a reduced gravity model. Close examination reveals that the southward Somali Current occurring during boreal winter is due to the combination of the local wind stress curl in the Arabian Sea and delayed signals representing the time change of layer thickness at the eastern boundary. Significance Statement: Sverdrup balance dictates the law of meridional transport of steady circulation in the upper ocean, and has been one of the foundations upon which our understanding of ocean circulation is built. However, for circulation forced by time-varying wind stress, with annual, interannual and decadal frequency, the governing law remains elusive. In this study, we introduce a time-dependent Sverdrup relation applicable to time-dependent wind-driven circulation. As an example, this relation is used to diagnose the monsoon-driven circulation in the Indian Ocean. [ABSTRACT FROM AUTHOR]

Fields of coastal wind stress and wind stress curl in the 10–100 km next to the land control the processes of upwelling and downwelling of nutrients and water properties that are vital to highly productive coastal marine ecosystems. Here we ask the question: Do the present surface wind stress products from a satellite-borne scatterometer (QuikSCAT) and an atmospheric reanalysis model (ERA-5) systematically overestimate the magnitude of wind speed and stress in the 10–50 km next to the coast? We compare QuikSCAT wind speed retrievals to the relatively unused wind speed retrievals from satellite altimeters, which are able to approach closer to the coast than scatterometers without land reflections, due to their smaller radar footprints. Altimeter data on tracks approaching and crossing the coast indicate that the increases in coastal QuikSCAT wind speed values and ERA-5 coastal wind stress values are unrealistic. For analyses of wind speed and stress requiring high accuracy, especially those involving wind stress curl, we suggest considering individual Level 2B scatterometer wind retrievals as suspect at distances of 10 km and less from the coast, along with use of the Poor Coastal Processing flag. We found that similar increases in wind stress values next to the coast in gridded ERA-5 fields are not due to errors in the model physics or wind speeds. They are created during the interpolation of wind stress from the original model grid to a regular rectangular grid. We recommend that researchers who are analyzing wind stress and wind stress curl should calculate wind stress themselves from the gridded ERA-5 vector wind speed fields, rather than using the interpolated model wind stress or curl fields. [ABSTRACT FROM AUTHOR]

We decompose the monthly global ocean bottom pressure (OBP) from GRACE(-FO) mass concentration solutions, with trends and seasonal harmonics removed from the signal, to extract 23 significant regional modes of variability. The 23 modes are analyzed and discussed considering sea-level anomalies (SLA), wind stress curl (WSC), and major climate indices. A total of two-thirds of the patterns correspond to extratropical regions and are substantially documented in other global or regional studies. Over the equatorial band, the identified modes are unprecedented, with an amplitude ranging between 0.5 and 1 cm. With smaller amplitude than extratropical patterns, they appear to be less correlated with the local SLA or WSC; yet they present significantly coherent dynamics. The Pacific Ocean modes show significant correlations with the Pacific decadal oscillation (PDO) and El Niño southern oscillation (ENSO). [ABSTRACT FROM AUTHOR]

In the Southern Ocean, the sub-Antarctic Prince Edward Islands (PEIs) play a significant ecological role by hosting large populations of seasonally breeding marine mammals and seabirds, which are particularly sensitive to changes in the surrounding ocean environment. In order to better understand climate variability at the PEIs, this study used satellite and reanalysis data to examine the interannual variability and longer-term trends of Sea Surface Temperature (SST), wind forcing, and surface circulation. Long-term trends were mostly weak and statistically insignificant, possibly due to the restricted length of the data products. While seasonal fluctuations accounted for a substantial portion (50–70%) of SST variability, the strongest variance in wind speed, wind stress curl (WSC), and currents occurred at intra-annual time scales. At a period of about 1 year, SST and geostrophic current variability suggested some influence of the Southern Annular Mode, but correlations were weak and insignificant. Similarly, correlations with El Niño Southern Oscillation variability were also weak and mostly insignificant, probably due to strong local and regional modification of SST, wind, and current anomalies. Significant interannual and decadal-scale variability in SST, WSC, and geostrophic currents, strongest at periods of 3–4 and 7–8 years, corresponded with the variability of the Antarctic Circumpolar Wave. At decadal time scales, there was a strong inverse relationship between SST and geostrophic currents and between SST and wind speed. Warmer-than-usual SST between 1990–2001 and 2009–2020 was related to weaker currents and wind, while cooler-than-usual periods during 1982–1990 and 2001–2009 were associated with relatively stronger winds and currents. Positioned directly in the path of passing atmospheric low-pressure systems and the Antarctic Circumpolar Current, the PEIs experience substantial local and regional atmospheric and oceanic variability at shorter temporal scales, which likely mutes longer-term variations that have been observed elsewhere in the Southern Ocean. [ABSTRACT FROM AUTHOR]

Upper-ocean heat content and heat fluxes of 10–60-day intraseasonal oscillations (ISOs) were examined using high-resolution currents and hydrographic fields measured at five deep-water moorings in the central Bay of Bengal (BoB) and satellite observations as part of an international effort examining the role of the ocean on monsoon intraseasonal oscillations (MISOs) in the BoB. Currents, temperature, and salinity were sampled over the upper 600–1200 m from July 2018 to June 2019. The 10–60-day velocity ISOs of magnitudes 20–30 cm s−1 were observed in the upper 200 m, and temperature ISOs as large as 3°C were observed in the thermocline near 100 m. The wavelet cospectral analysis reveals multiple periods of ISOs carrying heat southward. The meridional heat-flux divergence associated with the 10–60-day band was strongest in the central BoB at depths between 40 and 100 m, where the averaged flux divergence over the observational period is as large as 10−7 °C s−1. The vertically integrated heat-flux divergence in the upper 200 m is about 20–30 W m−2, which is comparable to the annual-average net surface heat flux in the northern BoB. Correlations between the heat content over the 26°C isotherm and the outgoing longwave radiation indicate that the atmospheric forcing typically leads changes of the oceanic heat content, but in some instances, during fall–winter months, oceanic heat content leads the atmospheric convection. Our analyses suggest that ISOs play an important role in the upper-ocean heat balance by transporting heat southward, while aiding the air–sea coupling at ISO time scales. [ABSTRACT FROM AUTHOR]

The influence of two climate modes (North Atlantic Oscillation (NAO) and East Atlantic pattern (EA)) on the wind stress curl (WSC) over the Black Sea, which determines the intensity of water circulation, is investigated using the MERRA2 reanalysis wind data and NAO and EA indices for 1980–2019. The composite method taking into account the nonstationarity of the time series of the NAO and EA indices is used. It is shown that in some months of the summer-autumn period during the positive phase of NAO, there is a significant increase in the cyclonic WSC over the eastern part of the sea. The highest influence of the EA on the WSC over the Black Sea is observed in November to April: during the negative EA phase, the cyclonic WSC increases over most of the Black Sea area. Such increase should lead to an intensification of the Black Sea water circulation. It is confirmed that the EA has a greater effect on the WSC over the Black Sea than the NAO. [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 South Atlantic subtropical underwater (STUW) is a high-salinity water mass formed by subduction within the subtropical gyre. It is a major component of the subtropical cell and affects stratification in the downstream direction due to its high salinity characteristics. Understanding the interannual variability in STUW subduction is essential for quantifying the impact of subtropical variability on the tropical Atlantic. Using the output from the ocean state estimate of the Consortium for Estimating the Circulation and Climate of the Ocean (ECCO), this study investigates the interannual variability in STUW subduction from 1992 to 2016. We find that heat fluxes, wind stress, and wind stress curl cause interannual variability in the subduction rate. Heat fluxes over the subduction area modulate the sea surface buoyancy and regulate the mixed layer depth (MLD) during its deepening and shoaling phases. Additionally, the wind stress curl and zonal wind stress can modulate the size of the subduction area by regulating the probability of particles entrained into the mixed layer within 1 year of tracing. This analysis evaluates the influence of subtropical wind patterns on the South Atlantic subsurface high-salinity water mass, highlighting the impact of heat and wind on the interannual changes in the oceanic component of the hydrological cycle. [ABSTRACT FROM AUTHOR]

This study demonstrates that the generalization that strong anomalous equatorial Pacific westerly (easterly) winds during El Niño (La Niña) events display strong adjusted warm water volume (WWV) discharges (recharges) is often incorrect. Using ocean model simulations, we categorize the oceanic adjusted responses to strong anomalous equatorial winds into two categories: (i) transitioning (consistent with the above generalization) and (ii) neutral adjusted responses (with negligible WWV recharge and discharge). During the 1980–2016 period only 47% of strong anomalous equatorial winds are followed by transitioning adjusted responses, while the remaining are followed by neutral adjusted responses. Moreover, 55% (only 30%) of the strongest winds lead to transitioning adjusted responses during the pre-2000 (post-2000) period in agreement with the previously reported post-2000 decline of WWV lead time to El Niño–Southern Oscillation (ENSO) events. The prominent neutral adjusted WWV response is shown to be largely excited by anomalous wind stress forcing with a weaker curl (on average consistent with a higher ratio of off-equatorial to equatorial wind events) and weaker Rossby wave projection than the transitioning adjusted response. We also identify a prominent ENSO phase asymmetry where strong anomalous equatorial westerly winds (i.e., El Niño events) are roughly 1.6 times more likely to strongly discharge WWV than strong anomalous equatorial easterly winds (i.e., La Niña events) are to strongly recharge WWV. This ENSO phase asymmetry may be added to the list of mechanisms proposed to explain why El Niño events have a stronger tendency to be followed by La Niña events than vice versa. [ABSTRACT FROM AUTHOR]

Located between the sub-Antarctic Front and the Antarctic Polar Front, the Prince Edward Islands (PEIs) provide an essential breeding ground for top predators and are an ideal location to investigate disturbances linked to climate change. This study provides the first seasonal characterisation of surface hydrography at the PEIs, using satellite and reanalysis products from 1993 to 2016. Sea surface temperature (SST) showed consistently higher values to the north, while wind, currents and eddy kinetic energy all showed higher values to the south of the islands. The highest SST (8 °C) occurred in summer and the lowest (2 °C) in winter, with a one-to two-month delay between the maximum incoming solar radiation (in December–January) and the highest SST (in February). The highest wind speed occurred in July (10.8 m s−1) and the minimum in February (7 m s−1). Geostrophic currents were four-times larger than Ekman currents, showing lower speeds between April and June (0.25–0.30 m s−1) and a peak in August (0.45 m s−1). There were no significant differences for SST and Ekman currents between the regions upstream and downstream of the PEIs. In contrast, surface total and geostrophic current velocities were weaker downstream because the islands act as a barrier to the flow. A zone of lower wind speed, coinciding with enhanced positive wind stress curl (WSC), favouring downwelling, occurred directly upstream throughout the year. Although WSC over the PEIs was negative (upwelling-favourable), no corresponding cooling was evident in SST. This seasonal characterisation provides a baseline against which interannual/decadal variability and future changes in these parameters can be assessed. [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.

Between June 2015 and June 2017, two pressure-recording inverted echo sounders (PIESs) and five current and pressure-recording inverted echo sounders (CPIESs) deployed along a section across the Kerama Gap acquired a dataset of ocean bottom pressure records in which there was significant 21-day variability (Pbot21). The Pbot21, which was particularly strong from July-December 2016, was coherent with wind stress curl (WSC) on the continental shelf of the East China Sea (ECS) with a squared coherence of 0.65 for a 3-day time lag. A barotropic ocean model demonstrated the generation, propagation, and dissipation of Pbot21. The modeled results show that the Pbot21 driven by coastal ocean WSC in the ECS propagated toward the Ryukyu Island Chain (RIC), while deep ocean WSC could not induce such variability. On the continental shelf, the Pbot21 was generated nearly synchronously with the WSC from the coastline to the southeast but dissipated within a few days due to the effect of bottom friction. The detection of Pbot21 by the moored array was dependent on the 21-day WSC patterns on the continental shelf. The Pbot21 driven southeast of the Changjiang Estuary by the WSC was detected while the Pbot21 generated northeast of the Changjiang Estuary was not. [ABSTRACT FROM AUTHOR]

In view of the extremely low sea ice concentration (SIC) appeared at high latitudes of the Arctic in the summer of 2010, the changes of SIC in the central Arctic from 2010 to 2017 were investigated in this paper based on the AMSR-E/AMSR-2 SIC products retrieved by the NT2 algorithm. The results show that the extremely low sea ice concentration in the central Arctic not only occurred in 2010 but also occurred again in 2016, and the daily average sea ice concentration (ASIC) reached a minimum of 0.70, which was significantly lower than the value of 0.78 in 2010 and became a new historical low record. A large area of sea ice in the sector 150°E−180° in 2010 disappeared in 2016, which was the most important difference to produce the new minimum. Also, the ice edge in 2016 retreated into the 85°N circle, whereas in 2010 it was far from the central Arctic. In 2010 and 2016, there were high correlations between the wind stress curl and the relative variation rate of ASIC, which indicates that wind stress curl (WSC) drove the divergence of sea ice. It directly leads to the decrease in the SIC and is the main cause of the extremely low SIC events. The results in this paper show that the decline of Arctic sea ice is represented by not only the reduction of sea ice coverage but also the reduction of SICs. The central Arctic has always been covered by large amount of sea ices, so the drastic reduction of SIC will not only change the structure of the ice field, but also lead to critical climatic effects that deserve further attention. [ABSTRACT FROM AUTHOR]

Fourteen years (September 2002 to August 2016) of high-resolution satellite observations of sea surface temperature (SST) data are used to describe the frontal pattern and frontogenesis on the southeastern continental shelf of Brazil. The daily SST fronts are obtained using an edge-detection algorithm, and the monthly frontal probability (FP) is subsequently calculated. High SST FPs are mainly distributed along the coast and decrease with distance from the coastline. The results from empirical orthogonal function (EOF) decompositions reveal strong seasonal variability of the coastal SST FP with maximum (minimum) in the astral summer (winter). Wind plays an important role in driving the frontal activities, and high FPs are accompanied by strong alongshore wind stress and wind stress curl. This is particularly true during the summer, when the total transport induced by the alongshore component of upwelling-favorable winds and the wind stress curl reaches the annual maximum. The fronts are influenced by multiple factors other than wind forcing, such as the orientation of the coastline, the seafloor topography, and the meandering of the Brazil Current. As a result, there is a slight difference between the seasonality of the SST fronts and the wind, and their relationship was varying with spatial locations. The impact of the air-sea interaction is further investigated in the frontal zone, and large coupling coefficients are found between the crosswind (downwind) SST gradients and the wind stress curl (divergence). The analysis of the SST fronts and wind leads to a better understanding of the dynamics and frontogenesis off the southeastern continental shelf of Brazil, and the results can be used to further understand the air-sea coupling process at regional level. [ABSTRACT FROM AUTHOR]

This study investigates the sensitivity of the Sverdrup transport to the NCEP/NCAR, ERA-Interim, CCMP, and QSCAT wind products over the tropical North Pacific Ocean during the period of 2000–2008. Our analyses show that all of the S-E transports in the reanalysis/analysis winds are northward in the band of 7–9° N except the QSCAT wind, which produces more realistic southward transport as shown in the meridional geostrophic transports (MGTs) calculated from WOA13 and Argo data. At 24° N, high-resolution wind products can better estimate the meridional transport in this region. At 8° N, although the CCMP has the same high resolution as the QSCAT, it fails to produce more realistic ocean circulation just as the coarse resolution wind products do there. The unrealistically large wind stress curl and small zonal wind stress in winter in reanalysis/analysis wind products account for this discrepancy with the QSCAT. This discrepancy also suggests that the model physics used in these reanalysis wind products is deficient in depicting the ocean circulation in this region, where the ocean fronts and eddy activity are both active. [ABSTRACT FROM AUTHOR]

This study focuses on the formation,migration and dissipation mechanisms of the Sri Lanka Dome (SLD) and its combination with Bay of Bengal Dome (BBD) using Simple Ocean Data Assimilation reanalysis data and AVISO data.The SLD is a cyclonic eddy in the southwest Bay of Bengal,which occurs during May through September coinciding with the intrusion of southwest monsoon current into the Bay of Bengal.The results show that,Ekman pumping due to positive wind stress curl east off Sri Lanka is the governing mechanism of the SLD formation and migration during developing stage.Besides,a positive impact of cold Rossby waves propagation contribute to the SLD developing as well.Further,weakening positive wind stress curl and the eastern boundary reflected warm Rossby wave effect are important factors for dissipation of SLD.During the decaying stage of SLD,the BBD evolves independently because of the Ekman Pumping driven by local positive wind curl.The regression analysis proves that the local wind stress curl has a strong positive relationship with upwelling within the BBD.After September,weak positive wind stress curl dominates in the southwest BOB,the BBD starts to merge with the SLD.There are two kinds of links between SLD and BBD.The first is a dynamic interaction revealed by eddy kinetic energy analysis and the second is a thermodynamic interaction described by combination of subsurface cold cores of the SLD and the BBD. [ABSTRACT FROM AUTHOR]

The use of information of net primary productivity (NPP) from remote ocean color sensors is increasingly common in marine sciences. The resulting information has been used to explain variations in productivity at different spatio-temporal scales and in the presence of climate phenomena, such as the El Niño Southern Oscillation, and global warming. Satellite remote sensing data were analyzed in Bahía de La Paz (BLP), Mexico, to determine the spatio-temporal variation in NPP. In addition, in situ hydrographic data were obtained to characterize the water properties in the bay. The satellite data agree with in situ measurements, validating the satellite observations over this region. The NPP generally presented seasonal variation with maximum values in winter-spring and minimum values in summer–autumn. The variance explained by NPP from the measured variables was ranked as Chl-a < DEN < SST < PAR < WSC. The highest NPP values generally occurred when subtropical subsurface (SsStW) water was relatively shallow. Due to divergence and mixing processes, this water provided nutrients to the euphotic zone, and consequently an increase in NPP and changes in plankton biomass were observed. The annual trends of the variation in hydrographic data with respect to that of remote sensing data were similar; however, it is necessary to increase the number of data validation studies. The remote sensing and in situ measurements allowed for the main biophysical variables that modulate NPP in different time scales to be identified. The satellite-derived NPP data classifies the BLP as a high productivity zone with 432 g C m−2 year−1. The use of satellite NPP data is satisfactory and should be incorporated into marine primary productivity studies.

The basin-scale wave propagation in the Southern Indian Ocean (SIO) is inferred to affect the low-frequency variability of eddy kinetic energy (EKE) within the Agulhas current system (ACS). Observations and numerical simulations indicate that westward-propagating waves, in the form of sea level variability from the east boundary, may cause "non-linear" oceanic responses (e.g., sea level and EKE) in the ACS. This is likely due to two factors: (i) the movement of wave crests and troughs continuously alters the stratification of the upper ocean, generating a positive baroclinic instability and prompting conversions of EKE from mean flow in the source regions, and (ii) wave-current interactions induce a rapid intension of an anticyclone gyre in the interior SIO and the corresponding EKE in the downstream regions. These basin-scale wave modulations may provide an additional perspective to the traditional wind driving view based on the barotropic Sverdrup relation. The local winds over the SIO play an important role in maintaining basin-scale Rossby waves in the southwest Indian Ocean and further modulating oceanic low-frequency variability in a "quasi-linear" way. • The basin-scale wave propagation in the Southern Indian Ocean (SIO) have an influence on the dynamics of the Agulhas current system. (ACS). • The waves can alter the stratification of the upper ocean, as well as strengthen the anticyclone gyre in the interior SIO and the corresponding EKE in the downstream regions. • These basin-scale wave modulations may provide an additional perspective to the traditional wind driving view based on the barotropic Sverdrup relation. [ABSTRACT FROM AUTHOR]

This study investigates changes in rainfall patterns over Northwest India (NWI) and the warming of the Northwestern Arabian Sea (NWAS) since 2000. These changes are attributed to the Silk Road Pattern (SRP) phase change in the late 1990s. The SRP positive phase triggers geostrophic deformation over the Tibetan Plateau (TP) by inducing an anomalous circulation pattern along the westerly jet, at 200 hPa. This deformation along the westerly Jet leads to Jet streaks and thermally indirect ageostrophic circulation over TP. In addition, the intensified heating over the TP led to a westward shift of the South Asian High (SAH). Associated with this, the interaction of positive temperature advection and northward ageostrophic wind (Jet entrance) over south-central Asia generates a southward component of ageostrophic wind (Jet Exit), causing upper-level convergence at southeast TP, excites the sinking cold air thereby strengthen the surface pressure below, over TP. This amplifies the anomalous anticyclone effect over there resulting in strong easterly wind anomalies along the monsoon trough at 850 hPa, disrupting the monsoon circulation. These anomalies, combined with a poleward shifting moisture-laden Low-Level Jet (LLJ), contribute to the warming of the NWAS and increased rainfall over NWI. The NWAS has shown significant summer warming over the past two decades. The poleward shift of the LLJ and SRP-induced circulation changes also impact the ocean dynamics. Given the occurrence of severe floods in this region of the Indian subcontinent, these findings have broader implications for predicting and projecting monsoon variability and extremes. • The SRP phase change in the late 1990s led to changes in rainfall patterns over NWI and warming of the NW Arabian Sea. • The positive phase of the SRP over the Tibetan Plateau induces jet streaks and increased surface pressure. • The study found that the LLJ shift and SRP-induced circulation changes impacted ocean dynamics. • The study explained the physical mechanisms behind the changes in rainfall patterns over NWI & warming of the NW Arabian Sea. [ABSTRACT FROM AUTHOR]

Extensive field campaigns in the Mauritanian upwelling region between 2005 and 2016 provide the database for analyzing the seasonal variability of the eastern boundary circulation (EBC) and associated water mass distribution at 18°N. The data set includes shipboard upper ocean current, hydrographic, and oxygen measurements from nine research cruises conducted during upwelling (December to April) and relaxation (May to July) seasons. During the upwelling season, the EBC closely resembles a classical eastern boundary current regime, with a poleward undercurrent flowing beneath an equatorward coastal jet. In contrast, elevated poleward flow exceeding 30 cm/s and extending from the surface down to 250‐m depth is observed during the relaxation season. The pronounced seasonal variability of the across‐shore structure of the EBC can be related to local wind forcing and is in general agreement with Sverdrup balance. The EBC transport is correlated to the wind stress curl leading the transport by 7 days. The short lead time suggests a fast response of locally forced waves adjusting the EBC to wind forcing. The seasonal and vertical water mass distribution is presented based on hydrographic observations. The meridional oxygen distribution and corresponding water mass partitioning into South and North Atlantic Central Water masses reveal a possible northerly ventilation pathway in the deeper layers of the central water stratum. Our results suggest that the poleward surface flow and the poleward undercurrent both are a consequence of the cyclonic wind stress curl forcing and thus propose to name it the Mauritania Current. Plain Language Summary: In this study we use a multiyear data set of shipboard current measurements from nine research cruises conducted between 2005 and 2016. We provide a description of the boundary circulation and its seasonality in the Mauritanian upwelling region off Northwest Africa, which is poorly understood owing to the lack of sufficient observations so far. We find poleward flow in the Mauritania Current to be enhanced and extending from the surface to about 250‐m depth during the relaxation season between May and July. During the upwelling season between December and April, weaker poleward flow is confined to the subsurface layers while equatorward flow dominates at the surface. We relate the observed seasonal variability to local wind forcing and find general agreement with theoretical considerations. Water masses in the Mauritanian upwelling region are of both North and South Atlantic origin. Using measurements of temperature and salinity, we present the seasonal and vertical water mass distribution in relation to the boundary circulation. Meridional oxygen and water mass distributions reveal a possible ventilation pathway for oxygenated waters from the north between 250‐ and 500‐m depth. Key Points: The Mauritania Current exhibits pronounced seasonal variability both at the surface and at depthSeasonal strengthening of the Mauritania Current in boreal spring is linked to variability in local wind stress curlBoundary circulation seasonality impacts the local water mass distribution and ventilation of the lower central water layer [ABSTRACT FROM AUTHOR]

The Agulhas Current plays a significant role in both local and global ocean circulation and climate regulation, yet the mechanisms that determine the seasonal cycle of the current remain unclear, with discrepancies between ocean models and observations. Observations from moorings across the current and a 22‐year proxy of Agulhas Current volume transport reveal that the current is over 25% stronger in austral summer than in winter. We hypothesize that winds over the Southern Indian Ocean play a critical role in determining this seasonal phasing through barotropic and first baroclinic mode adjustments and communication to the western boundary via Rossby waves. Our hypothesis is explored using single‐layer and one‐and‐a‐half layer models. We find that the barotropic contribution to seasonal phasing is small, with the majority of the seasonal signal deflected offshore and along the Mozambique Ridge. The summertime maximum and wintertime minimum can, however, be reproduced by a one‐and‐a‐half layer reduced gravity model in which adjustment time to wind forcing via Rossby waves is in line with observations from satellite altimetry. Additionally, near‐field winds (to the west of 35°E) are shown to have a controlling influence on the seasonal phasing, as signals from farther afield dissipate through destructive interference with overlying winds before reaching the western boundary. These results suggest a critical role for a baroclinic adjustment to near‐field winds in setting the summertime maximum in Agulhas Current transport. Plain Language Summary: The Agulhas Current flows along the east coast of South Africa, transporting warm Indian Ocean water southward, acting as a vital limb of the global ocean conveyor belt and influencing local rainfall and climate. This study looks at the seasonal cycle of the Agulhas Current and uses idealized models to explore how winds over the Southern Indian Ocean may influence this seasonality. The current is 25% stronger in summertime, yet we do not have existing knowledge regarding the principal drivers that set this observed variability. In this study, we find that baroclinic processes communicating the wind stress curl variability from near‐field winds have a dominant contribution to the seasonal phasing. Wind signals from further afield are found to die out during their journey west and so have little effect on the seasonal cycle of the Agulhas Current. Furthermore, correctly capturing the adjustment time to wind forcing is found to be very important when endeavoring to simulate the Agulhas Current seasonal phasing using a reduced gravity model. This study exposes a link between the seasonality of Agulhas Current and propagation of first baroclinic mode Rossby waves communicating the near‐field wind stress signal across the western portion of the Southern Indian Ocean. Key Points: A baroclinic adjustment to Indian Ocean winds can explain the Agulhas Current seasonal phasing, and a barotropic adjustment cannotSeasonal phasing is found to be highly sensitive to reduced gravity values, which modify adjustment times to wind forcingNear‐field winds have a dominant influence on the seasonal cycle of the Agulhas Current as remote signals die out while crossing the basin [ABSTRACT FROM AUTHOR]

This paper investigates the response of the thermocline depth (TD) in the South China Sea (SCS) to the El Niño-Southern Oscillation (ENSO) events using 51-year (from 1960 to 2010) monthly seawater temperature and surface wind stress data acquired from the Simple Ocean Data Assimilation (SODA), together with heat flux data from the National Centers for Environmental Prediction (NCEP), precipitation data from the National Oceanic and Atmospheric Administration (NOAA) and evaporation data from the Woods Hole Oceanographic Institution (WHOI). It is indicated that the response of the SCS TD to the El Niño or La Niña events is in opposite phase. On one hand, the spatial-averaged TDs in the SCS (deeper than 200 m) appear as negative and positive anomalies during the mature phase of the El Niño and La Niña events, respectively. On the other hand, from June of the El Niño year to the subsequent April, the spatial patterns of TD in the north and south of 12°N appear as negative and positive anomalies, respectively, but present positive and negative anomalies for the La Niña case. However, positive and negative TD anomalies occur almost in the entire SCS in May of the subsequent year of the El Niño and La Niña events, respectively. It is suggested that the response of the TD in the SCS to the ENSO events is mainly caused by the sea surface buoyancy flux and the wind stress curl. [ABSTRACT FROM AUTHOR]

This study analyzes monthly variability of thermocline and its mechanism in the South China Sea (SCS). The study is based on 51-year (1960–2010) monthly seawater temperature and surface wind stress data from Simple Ocean Data Assimilation (SODA), together with heat flux, precipitation and evaporation data from the National Centers for Environmental Prediction (NCEP), the National Oceanic and Atmospheric Administration (NOAA) and the Woods Hole Oceanographic Institution, respectively. The results reveal that the upper boundary depth (Zup), lower boundary depth (Zlow), thickness (ΔZ) and intensity (Tz) of thermocline in the SCS show remarkable monthly variability. Being averaged for the deep basin of SCS, Zup deepens gradually from May to the following January and then shoals from February to May, while Zlow varies little throughout the whole year. Further diagnostics indicates that the monthly variability of Zup is mainly caused by the buoyancy flux and wind stress curl. Using a linear method, the impacts of the buoyancy flux and wind stress curl on Zup can be quantitatively distinguished. The results suggest that Zup tends to deepen about 4.6 m when the buoyancy flux increases by 1×10 -5 kg/(m∙s 3), while it shoals about 2.5 m when the wind stress curl strengthens by 1×10 -7 N/m³. [ABSTRACT FROM AUTHOR]

The paper demonstrates that the wind stress curl as an external vorticity source plays an important role in shaping large scale shallow lake circulations. The analysis of purpose-oriented simultaneous wind and current measurements data from the Hungarian part of Lake Neusiedl reasonably fits well the internal boundary layer development theory over the lake surface. A 2-D vorticity formulation of wind-induced flows is used to demonstrate mathematically the IBL-related large scale circulation generation mechanism well reflected in the measured data. Further validation of the findings is carried out by means of simple 2-D numerical flow modelling, in which details on the flow pattern besides the measurement sites could be also revealed. Wind-induced lake circulations linked to IBL development shows a novelty to be implemented in up-to-date numerical flow models.

Subsurface temperatures in the Slope Water region of the Northwest Atlantic from Argo profiling floats and on the adjacent continental shelf from ship-based measurements are compared with the latitudinal position of the Shelf-Slope Front (SSF) and the Gulf Stream North Wall (GSNW). The Slope Water and shelf temperature anomalies at 200 m depth are in agreement for the period, 2002-2015. For the period 1978-2015, shelf temperatures are significantly correlated with the SSF position, and to a lesser extent with the GSNW position. Annual SSF position anomalies near the Grand Banks at 50°W-55°W lead anomalies to the west at 65°W-75°W by 1-2 years. Wind stress curl is compared with the annual change in the SSF and GSNW latitudinal positions, rather than with the positions directly. Changes in the mean position of the SSF are related to the wind stress curl pattern in the mid-Atlantic, with an 8 month lag. It is suggested that a wind pattern favoring a southward shift of the SSF is associated with a southward shift of the zero-curl line near 40°W, resulting in an expanded subpolar gyre and enhanced flow of Labrador Current Water westward from the Tail of the Grand Banks. However, changes in the GSNW position are related to an NAO-like wind stress curl pattern in the eastern Atlantic in the winter-spring period, in agreement with other studies. High sea surface temperatures in the Gulf of Maine and on the Scotian Shelf in recent years can be largely attributed to positive local onshore wind anomalies. [ABSTRACT FROM AUTHOR]