Positive Salinity Anomalies Research Articles

Sea Surface Salinity Anomaly in the Bay of Bengal during the 2010 Extremely Negative IOD Event

Based on Soil Moisture and Ocean Salinity (SMOS) data and the Ocean Reanalysis System 5 (ORAS5) dataset, positive salinity anomalies exceeding 2 psu in the northern Bay of Bengal (BoB) and negative salinity anomalies with the peak of the freshening anomalies reaching −2 psu around Sri Lanka were observed in autumn 2010. Here, an analysis of the anomalous salt budget revealed that anomalous horizontal advection contributed most to the variability in salinity in the BoB. With the development of La Niña and negative Indian Ocean dipole (nIOD) in summer and autumn, the strong summer monsoon current and Wyrtki jet combined with the anomalous basin-scale cyclonic circulation led to more high-salinity water entering the northern BoB. In addition, more freshwater was transported southward along the eastern coast of India by east Indian coastal current (EICC) in autumn, resulting in extremely negative salinity anomalies around Sri Lanka and positive salinity anomalies in the northern BoB. Moreover, the freshwater around Sri Lanka was carried farther into the southeastern Arabian Sea by the west Indian coastal current (WICC) in November, which affected the salinity stratification in winter and then influenced the variation of the Arabian Sea Mini Warm Pool (ASMWP) in the following spring. The ASMWP could affect the Indian summer monsoon (ISM) through its influence on the monsoon onset vortex (MOV) over the southeast Arabian Sea (SEAS).

Sensitivity of the Atlantic meridional overturning circulation and climate to tropical Indian Ocean warming

A salient feature of anthropogenic climate change is the enhanced warming of the tropical Indian Ocean (TIO) relative to the tropics. Recent studies show that this warming can remotely modulate the Atlantic meridional overturning circulation (AMOC). Motivated by these results, we systematically study the sensitivity of the AMOC and Atlantic climate to changes in TIO temperature using the latest coupled climate model from the Institut Pierre Simon Laplace (IPSL-CM6A-LR). Ensemble experiments nudging the TIO surface temperatures by − 2 °C, − 1 °C, + 1 °C, and + 2 °C are conducted. Within a few years after the forcing is imposed, different atmospheric teleconnections begin to drive the AMOC “fast” and “slow” responses, yielding after 150 years an AMOC equilibrium sensitivity of about + 9.4 Sv per 1 °C of relative TIO warming. A water mass transformation analysis shows that the fast response to TIO warming (on decadal timescales) is largely driven by surface cooling in the Labrador Sea caused by an induced positive North Atlantic Oscillation (NAO)-like mean pattern. By contrast, the slow response (on multi-decadal to centennial timescales) is driven by the gradual advection of positive salinity anomalies from the tropical Atlantic, which predominantly affect the Nordic Seas. The response is non-linear in that a TIO warming strengthens the AMOC through increase in Labrador Sea deep water formation, while a TIO cooling slows down the AMOC via sea ice expansion over the Nordic Seas deep-water formation region, ultimately leading to the AMOC shut-down in the − 2 °C-TIO experiment. These results help understand the role of interbasin connections and AMOC drivers in a warming climate.

Correlation between subsurface salinity anomalies in the Bay of Bengal and the Indian Ocean Dipole and governing mechanisms

Abstract. Lead–lag correlations between the subsurface temperature and salinity anomalies in the Bay of Bengal (BoB) and the Indian Ocean Dipole (IOD) are revealed in model results, ocean synthesis, and observations. Mechanisms for such correlations are further investigated using the Hamburg Shelf Ocean Model (HAMSOM), mainly relating to the salinity variability. It is found that the subsurface salinity anomaly of the BoB positively correlates to the IOD, with a lag of 3 months on average, while the subsurface temperature anomaly correlates negatively. The model results suggest the remote forcing from the equatorial Indian Ocean dominates the interannual subsurface salinity variability in the BoB. The coastal Kelvin waves carry signals of positive (negative) salinity anomalies from the eastern equatorial Indian Ocean and propagate counterclockwise along the coasts of the BoB during positive (negative) IOD events. Subsequently, westward Rossby waves propagate these signals to the basin at a relatively slow speed, which causes a considerable delay of the subsurface salinity anomalies in the correlation. By analyzing the salinity budget of the BoB, it is found that diffusion dominates the salinity changes near the surface, while advection dominates the subsurface; the vertical advection of salinity contributes positively to this correlation, while the horizontal advection contributes negatively. These results suggest that the IOD plays a crucial role in the interannual subsurface salinity variability in the BoB.

Multicentennial Variability Driven by Salinity Exchanges Between the Atlantic and the Arctic Ocean in a Coupled Climate Model

AbstractThe IPSL‐CM6‐LR atmosphere‐ocean coupled model exhibits a pronounced multicentennial variability of the Atlantic meridional overturning circulation (AMOC), driven by delayed freshwater accumulation and release in the Arctic. The AMOC fluctuations are preceded by salinity‐driven density anomalies in the main deep convection sites in the Labrador and Greenland seas. During a strong AMOC, a combination of reduced sea ice volume and anomalous currents reduces the freshwater export from the Arctic and leads instead to a slow accumulation of freshwater in the central Arctic. Simultaneously, the saltier Atlantic inflow through the Barents Sea results in a positive salinity anomaly in the Eastern Arctic subsurface. When the surface Central Arctic freshwater pool finally reaches the Lincoln Sea, the oceanic currents around Greenland reorganize, leading to the export of the anomalous Arctic freshwater to the North Atlantic, enhancing the stratification in deep convection sites. The AMOC then decreases, positive salinity anomalies appear in the Central Arctic, and the variability switches to the opposite phase. These AMOC and sea ice fluctuations have broader climate impacts, with a strong AMOC leading to a mean warming of about 0.4°C north of 20°N, reaching up to 1°C in the Arctic lower troposphere during winter. In all seasons, a northward displacement of the intertropical convergence zone is also simulated.

Manifestation and consequences of warming and altered heat fluxes over the Bering and Chukchi Sea continental shelves

A temperature and salinity hydrographic profile climatology is assembled, evaluated for data quality, and analyzed to assess changes of the Bering and Chukchi Sea continental shelves over seasonal to century-long time scales. The climatology informs description of the spatial distribution and temporal evolution of water masses over the two shelves, and quantification of changes in the magnitude and throughput of heat and fresh water. For the Chukchi Shelf, linear trend analysis of the integrated shelf heat content over its 1922–2018 period of record finds a significant summer and fall warming of 1.4 °C (0.14 ± 0.07 °C decade−1); over 1990–2018 the warming rate tripled to 0.43 ± 0.35 °C decade−1. In contrast, the Bering Shelf's predominantly decadal-scale variability precludes detection of a water column warming trend over its 1966–2018 period of record, but sea surface temperature data show a significant warming of 0.22 ± 0.10 °C decade−1 over the same time frame. Heat fluxes over 1979–2018 computed by the European Centre for Medium-Range Weather Forecast (ECMWF) ERA5 reanalysis exhibit no record-length trend in the shelf-wide Bering surface heat fluxes, but the Chukchi Shelf cooling season (October–March) has a trend toward greater surface heat losses and its warming season (April–September) has a trend toward greater heat gains. The 2014–2018 half-decade exhibited unprecedented low winter and spring sea-ice cover in the Northern Bering and Chukchi seas, changes that coincided with reduced springtime surface albedo, increased spring absorption of solar radiation, and anomalously elevated water column heat content in summer and fall. Consequently, the warm ocean required additional time to cool to the freezing point in fall. Fall and winter ocean-to-atmosphere heat fluxes were anomalously large and associated with enhanced southerly winds and elevated surface air temperatures, which in turn promoted still lower sea-ice production, extent, and concentration anomalies. Likely reductions in sea-ice melt were associated with positive salinity anomalies on the Southeast Bering Shelf and along the continental slope over 2014–2018. Negative salinity anomalies during 2014–2018 on the central and northern Bering Shelf may be related to a combination of 1) long-term declines in salinity, 2) an increase of ice melt, and 3) a decline of brine production. We hypothesize that freshening on the Bering Shelf and in Bering Strait since 2000 are linked to net glacial ablation in the Gulf of Alaska watershed. We show that the heat engines of both the Bering and Chukchi shelves accelerated over 2014–2018, with increased surface heat flux exchanges and increased oceanic heat advection. During this time, the Chukchi Shelf delivered an additional 5–9 x 1019 J yr−1 (50–90 EJ yr−1) into the Arctic basin and/or sea-ice melt, relative to the climatology. A similar amount of excess heat (60 EJ yr−1) was delivered to the atmosphere, showing that the Chukchi Sea makes an out-sized contribution to Arctic amplification. A conceptual model that summarizes the controlling feedback loop for these Pacific Arctic changes relates heat content, sea ice, freshwater distributions, surface heat fluxes, and advective fluxes.

Evolution of Mediterranean Sea water properties under climate change scenarios in the Med-CORDEX ensemble

Twenty-first century projections for the Mediterranean water properties have been analyzed using the largest ensemble of regional climate models (RCMs) available up to now, the Med-CORDEX ensemble. It is comprised by 25 simulations, 10 historical and 15 scenario projections, from which 11 are ocean–atmosphere coupled runs and 4 are ocean forced simulations. Three different emissions scenarios are considered: RCP8.5, RCP4.5 and RCP2.6. All the simulations agree in projecting a warming across the entire Mediterranean basin by the end of the century as a result of the decrease of heat losses to the atmosphere through the sea surface and an increase in the net heat input through the Strait of Gibraltar. The warming will affect the whole water column with higher anomalies in the upper layer. The temperature change projected by the end of the century ranges between 0.81 and 3.71 °C in the upper layer (0–150 m), between 0.82 and 2.97 °C in the intermediate layer (150–600 m) and between 0.15 and 0.18 °C in the deep layer (600 m—bottom). The intensity of the warming is strongly dependent on the choice of emission scenario and, in second order, on the choice of Global Circulation Model (GCM) used to force the RCM. On the other hand, the local structures reproduced by each simulation are mainly determined by the regional model and not by the scenario or the global model. The salinity also increases in all the simulation due to the increase of the freshwater deficit (i.e. the excess of evaporation over precipitation and river runoff) and the related increase in the net salt transport at the Gibraltar Strait. However, in the upper layer this process can be damped or enhanced depending upon the characteristics of the inflowing waters from the Atlantic. This, in turn, depends on the evolution of salinity in the Northeast Atlantic projected by the GCM. Thus a clear zonal gradient is found in most simulations with large positive salinity anomalies in the eastern basin and a freshening of the upper layer of the western basin in most simulations. The salinity changes projected for the whole basin range between 0 and 0.34 psu in the upper layer, between 0.08 and 0.37 psu in the intermediate layer and between − 0.05 and 0.33 in the deep layer. These changes in the temperature and salinity modify in turn the characteristics of the main water masses as the new waters become saltier, warmer and less dense along the twenty-first century. There is a model consensus that the intensity of the deep water formation in the Gulf of Lions is expected to decrease in the future. The rate of decrease remains however very uncertain depending on the scenario and model chosen. At the contrary, there is no model consensus concerning the change in the intensity of the deep water formation in the Adriatic Sea and in the Aegean Sea, although most models also point to a reduction.

Dipole Structure of Mixed Layer Salinity in Response to El Niño‐La Niña Asymmetry in the Tropical Pacific

AbstractThe asymmetry of mixed layer salinity (MLS) anomalies in response to El Niño and La Niña events in the tropical Pacific is examined for the first time based on Argo observations and Estimating the Circulation and Climate of the Ocean simulation. The difference of MLS anomalies between El Niño and La Niña shows a dipole structure, with El Niño featuring strong negative salinity anomalies east of 160°E while La Niña shows remarkable positive salinity anomalies west of 160°E. A salinity budget analysis suggests that nonlinear zonal advection plays a dominant role in generating the asymmetric MLS structure. This dipole MLS structure acts to generate a dipole structure of barrier layer thickness and thus likely plays an important role in the development of El Niño–Southern Oscillation asymmetries in sea surface temperature.

Salinity Variability Associated with the Positive Indian Ocean Dipole and Its Impact on the Upper Ocean Temperature

Abstract Both surface and subsurface salinity variability associated with positive Indian Ocean dipole (pIOD) events and its impacts on the sea surface temperature (SST) evolution are investigated through analysis of observational/reanalysis data and sensitivity experiments with a one-dimensional mixed layer model. During the pIOD, negative (positive) sea surface salinity (SSS) anomalies appear in the central-eastern equatorial Indian Ocean (southeastern tropical Indian Ocean). In addition to these SSS anomalies, positive (negative) salinity anomalies are found near the pycnocline in the eastern equatorial Indian Ocean (southern tropical Indian Ocean). A salinity balance analysis shows that these subsurface salinity anomalies are mainly generated by zonal and vertical salt advection anomalies induced by anomalous currents associated with the pIOD. These salinity anomalies stabilize (destabilize) the upper ocean stratification in the central-eastern equatorial (southeastern tropical) Indian Ocean. By decomposing observed densities into contribution from temperature and salinity anomalies, it is shown that the contribution from anomalous salinity stratification is comparable to that from anomalous thermal stratification. Furthermore, impacts of these salinity anomalies on the SST evolution are quantified for the first time using a one-dimensional mixed layer model. Since enhanced salinity stratification in the central-eastern equatorial Indian Ocean suppresses vertical mixing, significant warming of about 0.3°–0.5°C occurs. On the other hand, stronger vertical mixing associated with reduced salinity stratification results in significant SST cooling of about 0.2°–0.5°C in the southeastern tropical Indian Ocean. These results suggest that variations in salinity may potentially play a crucial role in the evolution of the pIOD.

Characteristics, vertical structures, and heat/salt transports of mesoscale eddies in the southeastern tropical Indian Ocean

AbstractSatellite altimetry sea surface height measurements reveal high mesoscale eddy activity in the southeastern tropical Indian Ocean (SETIO). In this study, the characteristics of mesoscale eddies in the SETIO are investigated by analyzing 564 cyclonic eddy (CE) tracks and 695 anticyclonic eddy (AE) tracks identified from a new version of satellite altimetry data with a daily temporal resolution. The mean radius, lifespan, propagation speed, and distance of CEs (AEs) are 149 (153) km, 50 (46) days, 15.3 (16.6) cm s−1, and 651 (648) km, respectively. Some significant differences exist in the eddy statistical characteristics between the new‐version daily altimeter data and the former weekly data. Mean vertical structures of anomalous potential temperature, salinity, geostrophic current, as well as heat and salt transports of the composite eddies, are estimated by analyzing Argo profile data matched to altimeter‐detected eddies. The composite analysis shows that eddy‐induced ocean anomalies are mainly confined in the upper 300 dbar. In the eddy core, CE (AE) could induce a cooling (warming) of 2°C between 60 and 180 dbar and maximum positive (negative) salinity anomalies of 0.1 (−0.3) psu in the upper 50 (110) dbar. The meridional heat transport induced by the composite CE (AE) is southward (northward), whereas the salt transport of CE (AE) is northward (southward). Most of the meridional heat and salt transports are carried in the upper 300 dbar.

Salinity anomaly as a trigger for ENSO events.

According to the classical theories of ENSO, subsurface anomalies in ocean thermal structure are precursors for ENSO events and their initial specification is essential for skillful ENSO forecast. Although ocean salinity in the tropical Pacific (particularly in the western Pacific warm pool) can vary in response to El Niño events, its effect on ENSO evolution and forecasts of ENSO has been less explored. Here we present evidence that, in addition to the passive response, salinity variability may also play an active role in ENSO evolution, and thus important in forecasting El Niño events. By comparing two forecast experiments in which the interannually variability of salinity in the ocean initial states is either included or excluded, the salinity variability is shown to be essential to correctly forecast the 2007/08 La Niña starting from April 2007. With realistic salinity initial states, the tendency to decay of the subsurface cold condition during the spring and early summer 2007 was interrupted by positive salinity anomalies in the upper central Pacific, which working together with the Bjerknes positive feedback, contributed to the development of the La Niña event. Our study suggests that ENSO forecasts will benefit from more accurate salinity observations with large-scale spatial coverage.

Combined influences of seasonal East Atlantic Pattern and North Atlantic Oscillation to excite Atlantic multidecadal variability in a climate model

The physical processes underlying the internal component of the Atlantic Multidecadal Variability (AMV) are investigated from a 1,000-yr pre-industrial control simulation of the CNRM-CM5 model. The low-frequency fluctuations of the Atlantic Meridional Overturning Circulation (AMOC) are shown to be the main precursor for the model AMV. The full life cycle of AMOC/AMV events relies on a complex time-evolving relationship with both North Atlantic Oscillation (NAO) and East Atlantic Pattern (EAP) that must be considered from a seasonal perspective in order to isolate their action; the ocean is responsible for setting the multidecadal timescale of the fluctuations. AMOC rise leading to a warm phase of AMV is statistically preceded by wintertime NAO+ and EAP+ from ~Lag −40/−20 yrs. Associated wind stress anomalies induce an acceleration of the subpolar gyre (SPG) and enhanced northward transport of warm and saline subtropical water. Concurrent positive salinity anomalies occur in the Greenland–Iceland–Norwegian Seas in link to local sea-ice decline; those are advected by the Eastern Greenland Current to the Labrador Sea participating to the progressive densification of the SPG and the intensification of ocean deep convection leading to AMOC strengthening. From ~Lag −10 yrs prior an AMOC maximum, opposite relationship is found with the NAO for both summer and winter seasons. Despite negative lags, NAO− at that time is consistent with the atmospheric response through teleconnection to the northward shift/intensification of the Inter Tropical Convergence Zone in link to the ongoing warming of tropical north Atlantic basin due to AMOC rise/AMV build-up. NAO− acts as a positive feedback for the full development of the model AMV through surface fluxes but, at the same time, prepares its termination through negative retroaction on AMOC. Relationship between EAP+ and AMOC is also present in summer from ~Lags −30/+10 yrs while winter EAP− is favored around the AMV peak. Based on additional atmospheric-forced experiments, both are interpreted as the local seasonal-dependent atmospheric response to warmer North Atlantic. Finally, advection of fresher water from the tropical basin created by local atmosphere/ocean anomalous circulation on one hand and from the Arctic on the other hand due to large-scale sea ice melting leads to decrease of density in the SPG and contributes terminating the model AMOC/AMV events. All together, the combined effects of NAO and EAP, their intertwined seasonal forcing/forced role upon/by the ocean and the primary role of salinity anomalies associated with oceanic dynamical changes acting as an integrator are responsible in CNRM-CM5 for an irregular and damped mode of variability for AMOC/AMV that takes about 35–40 (15–20) years to build up (dissipate).

Stable near-surface ocean salinity stratifications due to evaporation observed during STRASSE

Under conditions with a large solar flux and low wind speed, a stably stratified warm layer forms at the ocean surface. Evaporation can then lead to an increase in salinity in the warm layer. A large temperature gradient will decrease density enough to counter the density increase caused by the salinity increase, forming a stable positive salinity anomaly at the surface. If these positive salinity anomalies are large in terms of the change in salinity from surface to the base of the gradient, if their areal coverage is a significant fraction of the satellite footprint, and if they persist long enough to be in the satellite field of view, they could be relevant for calibration and validation of L-band microwave salinity measurements. A towed, surface-following profiler was deployed from the N/O Thalassa during the Subtropical Atlantic Surface Salinity Experiment (STRASSE). The profiler measured temperature and conductivity in the surface ocean at depths of 10, 50, and 100 cm. The measurements show that positive salinity anomalies are common at the ocean surface for wind speeds less than 4 m s−1 when the average daily insolation is >300 W m−2 and the sea-to-air latent heat flux is greater than zero. A semiempirical model predicts the observed dependence of measured anomalies on environmental conditions. However, the model results and the field data suggest that these ocean surface salinity anomalies are not large enough in terms of the salinity difference to significantly affect microwave radiometric measurements of salinity.

Structure and surface properties of eddies in the southeast Pacific Ocean

AbstractA number of studies have posited that coastally generated eddies could cool the southeast Pacific Ocean (SEP) by advecting cool, upwelled waters offshore. We examine this mechanism by characterizing the upper‐ocean properties of mesoscale eddies in the SEP with a variety of observations and by estimating the surface‐layer eddy heat flux divergence with satellite data. Cyclonic and anticyclonic eddies observed during two cruises featured deep positive salinity anomalies along the 26.5 kg m−3isopycnal, indicating that the eddies had likely trapped and transported coastal waters offshore. The cyclonic eddies observed during the cruises were characterized by shoaling isopycnals in the upper 200 m and cool near‐surface temperature anomalies, whereas the upper‐ocean structure of anticyclonic eddies was more variable. Using a variety of large‐scale observations, including Argo float profiles, drifter records, and satellite sea surface temperature fields, we show that, relative to mean conditions, cyclonic eddies are associated with cooler surface temperatures and that anticyclonic eddies are associated with warmer surface temperatures. Within each data set, the mean eddy surface temperature anomalies are small and of approximately equal magnitude but opposite sign. Eddy statistics drawn from satellite altimetry data reveal that cyclonic and anticyclonic eddies occur with similar frequency and have similar average radii in the SEP. A satellite‐based estimate of the surface‐layer eddy heat flux divergence, while large in coastal regions, is small when averaged over the SEP, suggesting that eddies do not substantially contribute to cooling the surface layer of the SEP.

Interannual Variations of Subsurface Spiciness in the Philippine Sea: Observations and Mechanism

Abstract The Philippine Sea (PS) is a key region connecting North Pacific subtropics to the equator via western boundary currents. Using available measurements from Argo profiling floats, satellite altimeters, and research surveys, the authors investigate the characteristics and mechanism of subsurface spiciness variability (represented by salinity changes between 23.5 and 24.5 σθ) in the PS. During the past decade, low-frequency salinity variability was dominated by interannual signals characterized by out-of-phase changes between the southern and northern PS with peak-to-peak amplitudes exceeding 0.1 psu. These salinity anomalies are mainly generated locally by anomalous cross-front geostrophic advections. In 2003, an anomalous cyclonic circulation developed in the PS, which transported greater (less) than normal high-salinity North Pacific Tropical Water to the northern (southern) PS and produced positive (negative) salinity anomalies there. In 2009, an anomalous anticyclone emerged, which produced negative (positive) salinity anomalies in the northern (southern) PS. These year-to-year variations are closely associated with ENSO cycle. During strong El Niño (La Niña) episodes, positive (negative) wind stress curl anomalies between 8° and 18°N evoke westward-propagating upwelling (downwelling) Rossby waves in the central Pacific and positive (negative) anomalous Ekman pumping in the western Pacific, resulting in the observed current and salinity changes in the PS. Further analysis suggests that these locally generated spiciness anomalies disperse quickly while propagating to the equatorial Pacific in the Mindanao Current (MC). In the meantime, anomalies advected from higher latitudes are nearly diminished upon reaching the PS. The western boundary of the North Pacific seems quite efficient in damping extratropical signals.

High‐resolution sea surface reconstructions off Cape Hatteras over the last 10 ka

This study presents high‐resolution foraminiferal‐based sea surface temperature, sea surface salinity and upper water column stratification reconstructions off Cape Hatteras, a region sensitive to atmospheric and thermohaline circulation changes associated with the Gulf Stream. We focus on the last 10,000 years (10 ka) to study the surface hydrology changes under our current climate conditions and discuss the centennial to millennial time scale variability. We observed opposite evolutions between the conditions off Cape Hatteras and those south of Iceland, known today for the North Atlantic Oscillation pattern. We interpret the temperature and salinity changes in both regions as co‐variation of activities of the subtropical and subpolar gyres. Around 8.3 ka and 5.2–3.5 ka, positive salinity anomalies are reconstructed off Cape Hatteras. We demonstrate, for the 5.2–3.5 ka period, that the salinity increase was caused by the cessation of the low salinity surface flow coming from the north. A northward displacement of the Gulf Stream, blocking the southbound low‐salinity flow, concomitant to a reduced Meridional Overturning Circulation is the most likely scenario. Finally, wavelet transform analysis revealed a 1000‐year period pacing the δ18O signal over the early Holocene. This 1000‐year frequency band is significantly coherent with the 1000‐year frequency band of Total Solar Irradiance (TSI) between 9.5 ka and 7 ka and both signals are in phase over the rest of the studied period.

Effect of Decadal Kuroshio Extension Jet and Eddy Variability on the Modification of North Pacific Intermediate Water

Abstract Salinity modifications in the North Pacific Intermediate Water (NPIW) core layer of 26.7–26.8 σθ in the western North Pacific Ocean are investigated using temperature–salinity data from available profiling float and hydrographic measurements in 2002–09. During 2002–05, when the Kuroshio Extension (KE) jet was intense and zonally elongated, coherent positive salinity anomalies appeared along the inflow KE jet southeast of Japan and in the downstream Mixed Water region east of 152°E. Broad-scale negative salinity anomalies were detected south of the KE jet and in the upstream Mixed Water region west of 152°E. The signs of these observed salinity anomalies were reversed in 2006–09, when the KE jet transitioned to a weakened and zonally contracted dynamic state. By adopting an isopycnal advection–diffusion model and conducting model runs with the time-dependent advective field inferred from the eddy-resolving satellite altimeter sea surface height data, it is found that the observed salinity anomalies are oscillatory in nature and are determined not only by the decadally varying KE jet itself but also by mesoscale eddy signals that modulate temporally and longitudinally along the path of the KE jet.

Malvinas-slope water intrusions on the northern Patagonia continental shelf

Abstract. The Patagonia continental shelf located off southeastern South America is bounded offshore by the Malvinas Current, which extends northward from northern Drake Passage (~55° S) to nearly 38° S. The transition between relatively warm-fresh shelf waters and Subantarctic Waters from the western boundary current is characterized by a thermohaline front extending nearly 2500 km. We use satellite derived sea surface temperature, and chlorophyll-a data combined with hydrographic and surface drifter data to document the intrusions of slope waters onto the continental shelf near 41° S. These intrusions create vertically coherent localized negative temperature and positive salinity anomalies extending onshore about 150 km from the shelf break. The region is associated with a center of action of the first mode of non-seasonal sea surface temperature variability and also relatively high chlorophyll-a variability, suggesting that the intrusions are important in promoting the local development of phytoplankton. The generation of slope water penetrations at this location may be triggered by the inshore excursion of the 100 m isobath, which appears to steer the Malvinas Current waters over the outer shelf.

The influence of large-scale atmospheric circulation on the variability of salinity at Helgoland Roads station

The variability of April salinity at Helgoland Roads station (54.12◦N, 7.9◦E, Germany) is analysed in relationship with Elbe river discharge and the observed variability in large-scale atmospheric circulation for the period 1962–2000. It is shown that the main driver of salinity anomalies is the river discharge anomalies from the previous month. These discharge anomalies are strongly related with precipitation anomalies from the Elbe catchment area. Changes in the salinity, discharge and precipitation anomalies are accompanied by a wave train atmospheric circulation pattern that connects the tropical Atlantic Ocean with northern part of Europe, as well as with changes in large-scale water vapour transport over the whole German Bight. Positive sea-surface temperature anomalies, centred in the Caribbean region and the North Sea, are associated with positive salinity anomalies and negative anomalies of discharge and precipitation.

The North Atlantic thermohaline circulation simulated by the GISS climate model during 1970–99

Evidence based on numerical simulations is presented for a strong correlation between the North Atlantic Oscillation (NAO) and the North Atlantic overturning circulation. Using an ensemble of numerical experiments with a coupled ocean‐atmosphere model including both natural and anthropogenic forcings, it is shown that the weakening of the thermohaline circulation (THC) could be delayed in response to a sustained upward trend in the NAO, which was observed over the last three decades of the twentieth century, 1970–99. Overall warming and enhanced horizontal transports of heat from the tropics to the subpolar North Atlantic overwhelm the NAO‐induced cooling of the upper ocean layers due to enhanced fluxes of latent and sensible heat, so that the net effect of warmed surface ocean temperatures acts to increase the vertical stability of the ocean column. However, the strong westerly winds cause increased evaporation from the ocean surface, which leads to a reduced fresh water flux over the western part of the North Atlantic. Horizontal poleward transport of salinity anomalies from the tropical Atlantic is the major contributor to the increasing salinities in the sinking regions of the North Atlantic. The effect of positive salinity anomalies on surface ocean density overrides the opposing effect of enhanced warming of the ocean surface, which causes an increase in surface density in the Labrador Sea and in the ocean area south of Greenland. The increased density of the upper ocean layer leads to deeper convection in the Labrador Sea and in the western North Atlantic. With a lag of four years, the meridional overturning circulation of the North Atlantic shows strengthening as it adjusts to positive density anomalies and enhanced vertical mixing. During the positive NAO trend, the salinity‐driven density instability in the upper ocean, due to both increased northward ocean transports of salinity and decreased atmospheric freshwater fluxes, results in a strengthening overturning circulation in the North Atlantic when the surface atmospheric temperature increases by 0.3°C and the ocean surface temperature warms by 0.5° to 1°C.

On the potential impact of sea surface salinity observations on ENSO predictions

Multiple regression analysis is used here to construct statistical prediction models for the El Niño/Southern Oscillation (ENSO) to explore the potential impact of monitoring Pacific Ocean sea surface salinity (SSS) on prediction of equatorial Pacific sea surface temperature (SST). This study, one of the firsts focusing on the direct role of SSS in ENSO predictions, is motivated by proposed missions for remote sensing of SSS. A forward stepwise method is used to extract significant predictors of the Niño‐3 SST index from observed monthly anomalies of tropical SST, SSS, sea level, freshwater flux, and components of the wind stress. The results indicate that SSS monitoring would have small impact on the statistical nowcast (reconstruction) of ENSO but a potential role in the 6–12 month forecasts. Correlation maps show two regions of high correlation: an equatorial region (between 170°E and 160°W) and an off‐equatorial region (between 170°E and 140°W and 5°S and 20°S). Short lag correlations display the negative relationship between the warm phase of ENSO and the negative equatorial SSS anomalies related with the increase of local rainfall. Such an equatorial negative correlation coexists with an area of positive correlations off the equator. The region with positive correlations moves eastward as the lag increases, reaching the geographical limit of the SSS observations at 6 months lag. The region of negative correlation moves northward and becomes weaker as the lag increases (it is nonsignificant for 9 months lag). For lags longer than 9 months, significant positive correlations are found south of the equator (5°S–10°S). At these lags, positive salinity anomalies have the potential to modify the subsurface stratification of the western Pacific as they are subducted westward. Thus, the availability of continuous remotely sensed SSS data might add considerably to ENSO predictions at longer lead times as a result of SSS‐induced changes in the subsurface density field.