Changes In Wind Stress Curl Research Articles

Observations reveal onshore acceleration and offshore deceleration of the Kuroshio Current in the East China Sea over the past three decades

The Kuroshio Current (KC) in the East China Sea is one of the most prominent components of the ocean circulation system in the North Pacific. The onshore intensification of the KC is found to drive nutrient-rich upwelling in the shelf regions, induce anomalous warming that leads to coastal marine heatwaves, and reduce the ability of the oceans to absorb anthropogenic carbon dioxide. Based on altimeter and in situ observations, we find an onshore acceleration and offshore deceleration of the KC over the past three decades. This intensification is characterized by a spatial mean onshore acceleration (offshore deceleration) of 0.39 (−0.63) cm s−1 per decade. This phenomenon can be attributed to changes in wind stress curl (WSC) and oceanic stratification over the subtropical North Pacific. The weakened WSC decreases the vertical extent of the KC by reducing its transport and contributes to the offshore deceleration, whereas the enhanced stratification drives the uplift of the KC and contributes to the onshore acceleration. Our findings underscore the importance of establishing and maintaining a long-term monitoring network for the zonal variations of the KC in the future to obtain a comprehensive understanding of the associated impacts.

Sudden Reduction of Antarctic Sea Ice Despite Cooling After Nuclear War

AbstractA large‐scale nuclear war could inject massive amounts of soot into the stratosphere, triggering rapid global climate change. In climate model simulations of nuclear war, global cooling contributes to an expansion of sea ice in the Northern Hemisphere. However, in the Southern Hemisphere (SH), an initial expansion of sea ice shifts suddenly to a 30% loss of sea ice volume over the course of a single melting season in the largest nuclear war simulation. In smaller nuclear war simulations an expansion in sea ice is instead observed which lasts for approximately 15 years. In contrast, in the largest nuclear war simulation, Antarctic sea ice remains below the long term control mean for 15 years, indicating a threshold that must be crossed to cause the response. Declining sea ice in the SH following a global cooling event has been previously attributed to shifts in the zonal winds around Antarctica, which can reduce the strength of the Weddell Gyre. In climate model simulations of nuclear war, the primary mechanisms responsible for Antarctic sea ice loss are: (a) enhanced atmospheric poleward heat transport through teleconnections with a strong nuclear war‐driven El Niño, (b) increased upwelling of warm subsurface waters in the Weddell Sea due to changes in wind stress curl, and (c) decreased equatorward Ekman transport due to weakened Southern Ocean westerlies. The prospect of sudden Antarctic sea ice loss after an episode of global cooling may have implications for solar geoengineering and further motivates this study of the underlying mechanisms of change.

Toward Ocean Hindcasts in Earth System Models: AMOC Variability in a Partially Coupled Model at Eddying Resolution

AbstractWhile forced ocean hindcast simulations are useful for a wide range of applications, a key limitation is their inability to simulate ocean‐atmosphere feedbacks. As a consequence, they need to rely on artificial choices such as sea surface salinity restoring and other corrections affecting the surface freshwater fluxes. Fully coupled models overcome these limitations, but lack the correct timing of variability due to weaker observational constraints. This leads to a mismatch between forced and coupled models on interannual to decadal timescales. A possibility to combine the advantages of both modeling strategies is to apply a partial coupling (PCPL), that is, replacing the surface winds stress in the ocean component by wind stress derived from reanalysis. To identify the capabilities, limitations and possible use cases of partial coupling, we perform a fully coupled, two partially coupled and an ocean‐only experiment using an all‐Atlantic nested ocean configuration at eddying resolution in a global climate model. We show that the correct timing of Atlantic Meridional Overturning Circulation (AMOC) variability in PCPL experiments is robust on timescales below 5 years. Mid‐latitude wind stress curl changes contribute to decadal AMOC variability, but North Atlantic buoyancy fluxes are not significantly altered by incorporating reanalyzed wind stress anomalies, limiting the success of PCPL on this timescale. Long term trends of the AMOC in PCPL mode are consistent with fully coupled model experiments under historic atmospheric boundary conditions, suggesting that a partially coupled model is still able to simulate the important ocean‐atmosphere feedbacks necessary to maintain a stable AMOC.

Remote Impact of the Equatorial Pacific on Florida Current Transport

AbstractSatellite and in‐situ measurements are used in this study to investigate a possible link between the Florida Current transport (FCT) and temperature changes in the equatorial Pacific. Consistent with the geostrophic nature of the current, the FCT variability shows good correspondence with the changes in sea surface height differences (∆SSH) between the eastern (SSHeast) and western (SSHwest) sides of the Florida Straits. While the variability of SSHwest is mostly associated with seasonal and shorter timescale fluctuations of ∆SSH and FCT, changes in SSHeast are strongly related to the interannual variability of ∆SSH and FCT. A significant correlation is found between the FCT and the Oceanic Niño Index (ONI) on interannual timescales, which explains 21% of the interannual FCT variance. The connection of ONI with FCT is through its impact on SSHeast, associated with the anomalous convergence/divergence in the Caribbean region and the Bahamas forced by ONI‐induced wind stress curl changes.

Mesoscale eddies in the Black Sea: Characteristics and kinematic properties in a high-resolution ocean model

Mesoscale eddies are ubiquitous features of oceanic circulation and important contributors to the transport of water and heat as well as chemical and biological constituents. In the Black Sea, mesoscale eddies, filaments and meanders are recognized as main agents facilitating the transport and distribution of nutrients and biomass between coastal regions and the open, deep sea. In this study, a high-resolution ocean model, Nucleus for a European Model of the Ocean (NEMO), is implemented for the Black Sea and used to create a hindcast simulation of circulation dynamics for the 1985–2014 period. This hindcast simulation is validated and then used to produce a detailed statistical investigation of the occurrence and behavior of mesoscale eddies in the Black Sea. For this purpose, an eddy identification and tracking algorithm (EddyScan) is applied and the kinematic properties of all eddies are detected, including eddy number, size, lifetime, excursion distance and their spatial distribution. The analysis shows that, on average, anticyclonic eddies are larger in diameter, have a longer lifetime and travel farther distances from the generation site than cyclonic eddies. However, the number of anticyclonic eddies generated is less than cyclones. Spatial distribution of eddies shows that cyclonic and anticyclonic eddies occupy much of the open Black Sea but few are found on the northwestern shelf. The density of anticyclonic eddies is especially high in the Sevastapol, Sinop, Kizilirmak and Batumi regions; while cyclonic eddy densities are high in the northern Black Sea along the Rim Current, north of the Batumi region, as well as in the center of the western and eastern Black Sea gyres. The number of eddies generated increases in spring and summer and decreases again in fall and winter due to changes in wind stress curl and its impact on Rim Current strength. The temporal evolution of long-lived eddies (>30 days) shows that after their creation, the diameters of cyclonic eddies increase for a period of 2–3 months, after which they reach a stable diameter. On the other hand, anticyclonic eddy diameters increase, on average, for a 5–6-month period and then reach a stable value. The vertical effects of these long-lived eddies generated in the Black Sea generally reach down to 150-200 m and hence intersect with the nitrate maximum and oxygen minimum zone. A first estimation shows that the simulated anticyclonic eddies in the Black Sea may have transported up to four times more water than cyclonic eddies over one year. The mean vertical flux induced by all cyclones generated in one year upwell ~ 0.63 Sv of water, while all anticyclones generated in one year downwell two times as much water or ~ 1.27 Sv over the top 100 m of the water column. This highlights the importance of eddies for the vertical transport of water and associated biochemical properties. In addition, eddies are able to connect different regions of the Black Sea as well as offshore/onshore regions along their paths and are of great importance for nutrient distribution and availability in the Black Sea.

Comprehensive Observational Features for the Kuroshio Transport Decreasing Trend During a Recent Global Warming Hiatus

AbstractLinear 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.

The Ekman Streamfunction and the Eulerian and Residual Overturning Circulations of the Southern Ocean

AbstractThe Ekman streamfunction is a wind‐derived metric that can be used to infer the Southern Ocean overturning circulations (SOOCs) in both latitude‐depth and latitude‐potential density spaces. The Ekman streamfunction integrates the Ekman pumping zonally and northwards from Antarctica, either to a given latitude or potential density. Here, we evaluate the relationship between the Ekman streamfunction and SOOCs in a global 0. ocean‐sea‐ice model driven by interannual forcing (1958–2018). In certain regions of the Southern Ocean, strong correlations () exist between the Ekman streamfunction and the Eulerian and residual SOOCs on monthly and annual timescales. Regression analysis identifies regions where Ekman streamfunction variability coincides with Sv changes in the overturning; one such location is where the wind stress curl changes sign and the Ekman pumping is highly variable.

Influence of Long-Term Changes in the Large-Scale Sea Level Pressure Field on the Wind Regime and the Wind Stress Curl in the Black Sea

Purpose. The paper is aimed at studying the relationship between the wind regime and the wind stress curl in the Black Sea and the long-term changes in the large-scale sea level pressure field in winter months. Methods and Results. The data on wind speed and sea level pressure in January – February from the NCEP/NСAR reanalysis for 1948–2018 are used. Based on the 6-hour data, the synoptic conditions accompanied by high and low values of the wind stress curl in the sea were determined. The synoptic situations in which a vast anticyclone is located north and northeast of the sea, and the area of low pressure – to the southwest of the sea in the Mediterranean region, are accompanied by the northeast and east winds, and by the cyclonic curl predominance. On the contrary, passing of the cyclones to the north of the sea and increase of pressure to the southwest are followed by the westerly and southwesterly winds, and by the anticyclonic curl predominance. Extremely high monthly mean values of the cyclonic curl were observed in those years, when the area occupied by the Siberian anticyclone increased and expanded westward, so that the Black Sea was on the southwestern periphery of its spur. Extremely low values of the anticyclonic curl were noted when the Azores anticyclone area expanded to the Mediterranean region. The wind stress curl changes on the multidecadal scales have shown its relation to the global changes in the field of the sea level pressure and the sign of the pressure anomalies at the low latitudes. Conclusions. The opposite sign of the surface pressure anomalies to the northeast and southwest of the sea is accompanied by the highest values of the wind stress curl.

Changes in the Subantarctic Mode Water Properties and Spiciness in the Southern Indian Ocean based on Argo Observations

AbstractThe Subantarctic Mode Water (SAMW) plays an essential role in the global heat, freshwater, carbon, and nutrient budgets. In this study, decadal changes in the SAMW properties in the Southern Indian Ocean (SIO) and associated thermodynamic and dynamic processes are investigated during the Argo era. Both temperature and salinity of the SAMW in the SIO show increasing trends during 2004-2018. A two-layer structure of the SAMW trend, with more warm and salty light SAMW but less cool and fresh dense SAMW, is identified. The heaving and spiciness processes are important but have opposite contributions to the temperature and salinity trends of the SAMW. A significant deepening of isopycnals (heaving), peaking atσθ=26.7-26.8 kg m−3in the middle layer of the SAMW, expands the warm and salty light SAMW and compresses the cool and fresh dense SAMW corresponding to the change in subduction rate during 2004-2018. The change in the SAMW subduction rate is dominated by the change in the mixed layer depth, controlled by the changes in wind stress curl and surface buoyancy loss. An increase in the mixed-layer temperature due to weakening northward Ekman transport of cool water leads to a lighter surface density in the SAMW formation region. Consequently, density outcropping lines in the SAMW formation region shift southward and favor the intrusion and entrainment of the cooler and fresher Antarctic surface water from the south, contributing to the cooling/freshening trend of isopycnals (spiciness). Subsequently, the cooler and fresher SAMW spiciness anomalies spread in the SIO via the subtropical gyre.

Vliyaniye dolgovremennykh izmeneniy krupnomasshtabnogo polya prizemnogo davleniya na vetrovoy rezhim i zavikhrennost' napryazheniya treniya vetra v Chernom more

Purpose. The paper is aimed at studying the relationship between the wind regime and the wind stress curl in the Black Sea, on the one hand, and the long-term changes in the sea level pressure fields in winter months, on the other. Methods and Results. The data on wind speed and sea level pressure in January – February from the NCEP/NСAR reanalysis for 1948–2018 are used. Based on the 6-hour data, the synoptic conditions accompanied by high and low values of the wind stress curl in the sea were determined. The synoptic situations in which a vast anticyclone is located north and northeast of the sea, and the area of low pressure – to the southwest of the sea in the Mediterranean region, are accompanied by the northeast and east winds, and by the cyclonic curl predominance. On the contrary, passing of the cyclones to the north of the sea and increase of pressure to the southwest are followed by the westerly and southwesterly winds, and by the anticyclonic curl predominance. Extremely high monthly mean values of the cyclonic curl were observed in those years, when the area occupied by the Siberian anticyclone increased and expanded westward, so that the Black Sea was on the southwestern periphery of its spur. Extremely low values of the anticyclonic curl were noted when the Azores anticyclone area expanded to the Mediterranean region. The wind stress curl changes on the multidecadal scales have shown its relation to the global changes in the field of the sea level pressure and the sign of the pressure anomalies at the low latitudes. Conclusions. The opposite sign of the surface pressure anomalies taking place to the northeast and southwest of the sea is accompanied by the highest values of the wind stress curl.

Regional Versus Remote Atmosphere‐Ocean Drivers of the Rapid Projected Intensification of the East Australian Current

AbstractLike many western boundary currents, the East Australian Current (EAC) extension is projected to get stronger and warmer in the future. The CMIP5 multimodel mean (MMM) projection suggests up to 5°C of warming under an RCP85 scenario by 2100. Previous studies employed Sverdrup balance to associate a trend in basin wide zonally integrated wind stress curl (resulting from the multidecadal poleward intensification in the westerly winds over the Southern Ocean) with enhanced transport in the EAC extension. Possible regional drivers are yet to be considered. Here we introduce the NEMO‐OASIS‐WRF coupled regional climate model as a framework to improve our understanding of CMIP5 projections. We analyze a hierarchy of simulations in which the regional atmosphere and ocean circulations are allowed to freely evolve subject to boundary conditions that represent present‐day and CMIP5 RCP8.5 climate change anomalies. Evaluation of the historical simulation shows an EAC extension that is stronger than similar ocean‐only models and observations. This bias is not explained by a linear response to differences in wind stress. The climate change simulations show that regional atmospheric CMIP5 MMM anomalies drive 73% of the projected 12 Sv increase in EAC extension transport whereas the remote ocean boundary conditions and regional radiative forcing (greenhouse gases within the domain) play a smaller role. The importance of regional changes in wind stress curl in driving the enhanced EAC extension is consistent with linear theory where the NEMO‐OASIS‐WRF response is closer to linear transport estimates compared to the CMIP5 MMM.

Remote Forcing of Tasman Sea Marine Heatwaves

AbstractRecent marine heatwave (MHW) events in the Tasman Sea have had dramatic impacts on the ecosystems, fisheries, and aquaculture off Tasmania’s east coast. However, our understanding of the large-scale drivers (forcing) and potential predictability of MHW events in this region off southeast Australia is still in its infancy. Here, we investigate the role of oceanic Rossby waves forced in the interior South Pacific on observed MHW occurrences off southeast Australia from 1994 to 2016, including the extreme 2015/16 MHW event. First, we used an upper-ocean heat budget analysis to show that 51% of these historical Tasman Sea MHWs were primarily due to increased East Australian Current (EAC) Extension poleward transports through the region. Second, we used lagged correlation analysis to empirically connect the EAC Extension intensification to incoming westward-propagating sea surface height (SSH) anomalies from the interior South Pacific. Third, we dynamically analyzed these SSH anomalies using simple process-based baroclinic and barotropic Rossby wave models forced by wind stress curl changes across the South Pacific. Finally, we show that associated monthly SSH changes around New Zealand may be a useful index of western Tasman Sea MHW predictability, with a lead time of 2–3 years. In conclusion, our findings demonstrate that there is potential predictability of advection-dominated MHW event likelihoods in the EAC Extension region up to several years in advance, due to the deterministic contribution from baroclinic and barotropic Rossby waves in modulating the EAC Extension transports.

Salinity Trends within the Upper Layers of the Subpolar North Atlantic

Examination of a range of salinity products collectively suggests widespread freshening of the North Atlantic from the mid-2000s to the present. Monthly salinity fields reveal negative trends that differ in magnitude and significance between western and eastern regions of the North Atlantic. These differences can be attributed to the large negative interannual excursions in salinity in the western subpolar gyre and the Labrador Sea, which are not apparent in the central or eastern subpolar gyre. This study demonstrates that temporal trends in salinity in the northwest (including the Labrador Sea) are subject to mechanisms that are distinct from those responsible for the salinity trends in the central and eastern North Atlantic. In the western subpolar gyre a negative correlation between near-surface salinity and the circulation strength of the subpolar gyre suggests that negative salinity anomalies are connected to an intensification of the subpolar gyre, which is causing increased flux of freshwater from the East Greenland Current and subsequent transport into the Labrador Sea during the melting season. Analyses of sea surface wind fields suggest that the strength of the subpolar gyre is linked to the North Atlantic Oscillation– and Arctic Oscillation–driven changes in wind stress curl in the eastern subpolar gyre. If this trend of decreasing salinity continues, it has the potential to enhance water column stratification, reduce vertical fluxes of nutrients, and cause a decline in biological production and carbon export in the North Atlantic Ocean.

Sea Level Variability around Japan during the Twentieth Century Simulated by a Regional Ocean Model

Sea level variability around Japan from 1906 to 2010 is examined using a regional ocean model, along with observational data and the CMIP5 historical simulations. The regional model reproduces observed interdecadal sea level variability, for example, high sea level around 1950, low sea level in the 1970s, and sea level rise during the most recent three decades, along the Japanese coast. Sensitivity runs reveal that the high sea level around 1950 was induced by the wind stress curl changes over the North Pacific, characterized by a weakening of the Aleutian low. In contrast, the recent sea level rise is primarily caused by heat and freshwater flux forcings. That the wind-induced sea level rise along the Japanese coast around 1950 is as large as the recent sea level rise highlights the importance of natural variability in understanding regional sea level change on interdecadal time scales.

Synchronicity of Kuroshio Current and climate system variability since the Last Glacial Maximum

The Kuroshio Current (KC) is the northward branch of the North Pacific subtropical gyre (NPG) and exerts influence on the exchange of physical, chemical, and biological properties of downstream regions in the Pacific Ocean. Resolving long-term changes in the flow of the KC water masses is, therefore, crucial for advancing our understanding of the Pacific's role in global ocean and climate variability. Here, we reconstruct changes in KC dynamics over the past 20 ka based on grain-size spectra, clay mineral, and Sr–Nd isotope constraints of sediments from the northern Okinawa Trough. Combined with published sediment records surrounding the NPG, we suggest that the KC remained in the Okinawa Trough throughout the Last Glacial Maximum. Together with Earth-System-Model simulations, our results additionally indicate that KC intensified considerably during the early Holocene (EH). The synchronous establishment of the KC “water barrier” and the modern circulation pattern during the EH highstand shaped the sediment transport patterns. This is ascribed to the precession-induced increase in the occurrence of La Niña-like state and the strength of the East Asian summer monsoon. The synchronicity of the shifts in the intensity of the KC, Kuroshio extension, and El Niño/La Niña-Southern Oscillation (ENSO) variability may further indicate that the western branch of the NPG has been subject to basin-scale changes in wind stress curl over the North Pacific in response to low-latitude insolation. Superimposed on this long-term trend are high-amplitude, large century, and millennial-scale variations during last 5 ka, which are ascribed to the advent of modern ENSO when the equatorial oceans experienced stronger insolation during the boreal winter.

Inter-annual sea level variability in the southern South China Sea

The South China Sea (SCS) is the largest marginal sea in the western Pacific Basin. Sea level anomalies (SLAs) in the southern South China Sea (SSCS) are assumed to be governed by various phenomena associated with the adjacent parts of the Indian Ocean and the Pacific Ocean. We have used monthly sea level anomalies obtained from 12 tide gauge stations of PSMSL and UHSLC and merged and gridded AVISO products of SLAs (sea level anomalies) derived from satellite altimeter. We find that IOD-influenced inter-annual variations are found only in the southwestern and southeastern coastal regions of SSCS. Our analysis reveals that inter-annual regional sea level drops are associated with positive phase of the IOD, and the rises with negative phase of the IOD. SLA variations at decadal scale in the southeastern and northern Gulf of Thailand correlate with Pacific Decadal Oscillations (PDO). Multiple linear regression analysis of inter-annual SLAs and climate indices shows that IOD induced inter-annual variations dominate in the southwestern SCS and it contributes to about ~40% of inter-annual sea level variation. Meanwhile, ENSO contributes to around ~30% variation in sea level in the southwestern and ~40% variation in the southeastern SSCS. The present study also suggests that inter-annual SLA variations in the SSCS can occur by ENSO and IOD induced changes in wind stress curl and volume transport variations.

Weddell Sea Export Pathways from Surface Drifters

AbstractThe complex export pathways that connect the surface waters of the Weddell Sea with the Antarctic Circumpolar Current influence water mass modification, nutrient fluxes, and ecosystem dynamics. To study this exchange, 40 surface drifters, equipped with temperature sensors, were released into the northwestern Weddell Sea’s continental shelf and slope frontal system in late January 2012. Comparison of the drifter trajectories with a similar deployment in early February 2007 provides insight into the interannual variability of the surface circulation in this region. Observed differences in the 2007 and 2012 drifter trajectories are related to a variable surface circulation responding to changes in wind stress curl over the Weddell Gyre. Differences between northwestern Weddell Sea properties in 2007 and 2012 include 1) an enhanced cyclonic wind stress forcing over the Weddell Gyre in 2012; 2) an acceleration of the Antarctic Slope Current (ASC) and an offshore shift of the primary drifter export pathway in 2012; and 3) a strengthening of the Coastal Current (CC) over the continental shelf in 2007. The relationship between wind stress forcing and surface circulation is reproduced over a longer time period in virtual drifter deployments advected by a remotely sensed surface velocity product. The mean offshore position and speed of the drifter trajectories are correlated with the wind stress curl over the Weddell Gyre, although with different temporal lags. The drifter observations are consistent with recent modeling studies suggesting that Weddell Sea boundary current variability can significantly impact the rate and source of exported surface waters to the Scotia Sea, a process that determines regional chlorophyll distributions.

Interannual forcing mechanisms of California Current transports I: Meridional Currents

An ensemble of eddy-resolving ocean model hindcasts integrated from 1950 to 2008 is used to examine and quantify the interannual variability of large-scale (>200km) alongshore equatorward flow in the California Current System (CCS). We also develop a single index of this transport in order to determine what fraction of variance is driven locally, by changes in wind stress curl, and remotely, by the arrival of coastally trapped waves of tropical origin.In agreement with previous studies, coastally trapped waves dominate large-scale interannual CCS sea surface height variability. In contrast, we find that large-scale alongshore currents (v) are driven predominantly by local wind stress curl variability rather than coastally trapped waves. A simple wind-driven diagnostic model of the time-dependent large-scale geostrophic meridional transport captures ~50% (R=0.7) of the total variance. The local wind-stress curl gradient that controls the largest fraction of meridional transport is not independent of the modulations in atmospheric circulation that drive the Pacific Decadal Oscillation (PDO), and a significant fraction of the monthly transport variability in the model ensembles is correlated to the PDO (R=0.4).

Upper ocean warming pattern in the past 50 years

A long-term warming pattern of global subsurface ocean was detected separately from other natural variations. Three dominant modes were extracted: a long-term warming mode, a mode related to the El Nino/Southern Oscillation, and a mode related to the Atlantic Multidecadal Oscillation. The long-term warming mode explained 78 % of the global mean temperature variance from the surface to a depth of 300 m, and the other two modes could explain most of the residual variance. Subsurface warming associated with the long-term warming mode was strong in the subtropics. In contrast, there was a local minimum of warming in the northern hemisphere subarctic ocean, and warming was suppressed in subsurface waters south of the equator. Atmospheric changes associated with the long-term warming mode showed negative (positive) sea level pressure anomalies at high (middle) latitudes in both hemispheres, and an intensification and/or a poleward expansion of mid-latitude westerlies. Wind stress curl changes were negative in the subtropics and positive in the subarctic of the northern hemisphere; changes that were consistent with the strong warming in the subtropics and the local minimum of warming in the subarctic. Warming of Southern Ocean subsurface waters coincided with southward migration and intensification of westerly winds, whereas surface warming to the south of 50°S was suppressed, probably by strengthened northward Ekman transport. Positive wind stress curl off the equator with weakening of the tropical easterly winds in the Pacific and Indian Oceans was consistent with the subsurface negative temperature anomaly there.

Interannual variability in the hydrography of the Norwegian Atlantic Current: Frontal versus advective response to atmospheric forcing

[1] The warm and saline inflow of the North Atlantic Current to the Nordic Seas is highly relevant for the region and the global climate. North of the Greenland-Scotland Ridge, the Norwegian Atlantic Current consists of two 40–60 km wide branches, situated at the slope and 150–200 km offshore, respectively. To interpret changes in these branches in terms of climate variability in the northern North Atlantic, it is important to understand their spatiotemporal response to both atmospheric forcing and advection. We analyzed three decades of synoptic hydrographic observations of the branches' variability, with particular focus on the response to the North Atlantic Oscillation (NAO) and related wind stress curl changes in the Nordic Seas. To do so, we separated the effect of fluctuating position and thickness of the branches from the variability in temperature and salinity in the spatially fluctuating cores. As a rapid response to the NAO we find a deflection of both branches toward the coast which is consistent with an enhanced basin-wide cyclonic circulation. While the immediate correlation between hydrographic properties and the NAO is weak, we find a significant negative correlation when the NAO leads temperature and salinity by 4–6 years. We attribute the overall delayed response to the advection of anomalies generated in the northwestern North Atlantic through NAO induced air-sea interaction and changes in the position of the subpolar front.