Sea Surface Salinity Anomalies Research Articles

Unraveling the Influence of Equatorial Waves on Post-Monsoon Sea Surface Salinity Anomalies in the Bay of Bengal

In this study, we investigate the connection between planetary equatorial waves, modulated by the Indian Ocean dipole (IOD) and El Niño Southern Oscillation (ENSO), and the interannual variabilities of the salinity distribution in the Bay of Bengal (BoB) in October–December (OND), along with its associated dynamics, using satellite and reanalysis datasets. In OND 2010 and 2016 (1994, 1997, 2006, and 2019), positive (negative) sea surface salinity anomalies (SSSAs) were distributed in the eastern equatorial Indian Ocean (EIO) and Andaman Sea. Moreover, the southward movement of negative (positive) SSSAs along the eastern Indian coast was observed. This phenomenon was caused by large-scale anomalous currents associated with zonal wind over the EIO. During OND 2010 and 2016 (1994, 1997, 2006, and 2019), due to anomalous westerlies (easterlies) over the EIO and anomalous downwelling (upwelling) Kelvin waves, the strengthened (weakened) Wyrtki jet and the basin-scale anomalous cyclonic (anticyclonic) circulation in the BoB gave rise to positive (negative) SSSAs within the eastern EIO and Andaman Sea. In addition, the intensified (weakened) eastern Indian coastal currents led to the southward movement of negative (positive) SSSAs. It is worth noting that downwelling Kelvin waves reached the western coast of India during OND 2010 and 2016, while upwelling Kelvin waves were only confined to the eastern coast of India during OND 1994, 1997, 2006, and 2019. Furthermore, westward salinity signals associated with reflected westward Rossby waves could modulate the spatial pattern of salinity. The distribution of salinity anomalies could potentially influence the formation of the barrier layer, thereby impacting the sea surface temperature variability and local convection.

Atlantic Niño induced sea surface salinity variability as observed from the satellite

The Atlantic Niño exerts great impact on surrounding weather and climate anomalies, leading to anomalous wind, temperature, precipitation, etc. However, the impact of Atlantic Niño on ocean salinity in the tropical Atlantic has not been well disclosed. The present study examines the Atlantic Niño induced sea surface salinity (SSS) distribution in both boreal summer and winter seasons by using the satellite data as well as various reanalysis and objective analysis data sets. It concludes that the summer Atlantic Niño leads to large fresh SSS anomalies in the eastern equatorial basin, while the winter Atlantic Niño leads to a meridional dipole structure of SSS anomalies. The former is mostly controlled by the dynamical processes of ocean, while the latter is largely controlled by the atmospheric processes. Accordingly, two SSS indices are developed to describe the relationships in the two seasons. The present study advances our understanding of the Atlantic Niño and its associated SSS variability and reveals the possible deficiencies of current reanalysis and objective analysis data sets in the tropical Atlantic Ocean.

Connecting subtropical salinity maxima to tropical salinity minima: Synchronization between ocean dynamics and the water cycle

Recent satellite observations reveal that the annual cycles of tropical sea surface salinity (SSS) minimum (Smin) and subtropical SSS maximum (Smax) are phased locked, with the former leading the latter by six months. The evidence suggests an interconnected nature between the salinity extrema, motivating an investigation into the underlying mechanisms. It is found that the poleward Ekman transport, driven by trade winds, stands as a pivotal conduit linking the influence of tropical freshwater to high salinities in the subtropical oceans. Two key aspects are central to the operation of this advective oceanic pathway. One is the annual formation of the near-equatorial Smin zones by Ekman convergence of the freshwater sourced from the double Intertropical Convergence Zones. The other is the advection time taken by Ekman transport traveling at an average speed of approximately 7 cm s−1. This speed allows the near-equatorial salinity anomalies to be carried to the subtropical Smax fringes in six months, aligning closely with the observed semi-annual phase shift between Smin and Smax. During El Niño - Southern Oscillation (ENSO) warm and cold episodes in recent decade, poleward propagation of near-equatorial low salinity anomalies was particularly prominent, leading to substantial surface freshening in the subtropical Pacific. Interestingly, the latitudinal spread of the ENSO-triggered SSS anomalies is confined within the domain of the poleward Ekman transport. This confinement reaffirms the inherent connection between the tropical Smin and subtropical Smax regions, shedding light on the intricate interplay between oceanic processes, the water cycle, and SSS distributions.

Zonal Structure of Tropical Pacific Surface Salinity Anomalies Affects the Eastern and Central Pacific El Niños Differently

AbstractMaximum sea surface salinity (SSS) anomalies are found in the central Pacific during the eastern Pacific El Niño (EPEN) and located further westward during the central Pacific El Niño (CPEN), but whether these differences affect event strengths is unclear. By performing ocean general circulation model experiments via modifying freshwater flux anomalies, we find salinity effects on surface warming during both types are highly sensitive to zonal locations of SSS anomalies, with the strongest warming induced by the SSS anomalies near the international dateline. Further analysis reveals that vertical mixing and entrainment dominate this temperature sensitivity, with the strongest response to SSS anomalies occurring in the central Pacific. The central‐Pacific SSS anomalies increase EPEN warming by 0.15°C while the westward‐located SSS anomalies make little contribution to CPEN warming. Therefore, the distinct zonal structures of SSS anomalies facilitate stronger EPEN than the CPEN, increasing their difference in intensity by about 10%.

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).

Leading-Mode Connections of the Interannual Variability in Upper-Ocean Salinity in the Tropical Indian Ocean

Abstract Leading modes of interannual variability in upper-ocean salinity in the tropical Indian Ocean (TIO) and their connections were studied based on 17 years (2002–18) of oceanic historical and reanalysis data. Empirical orthogonal function (EOF) analysis depicted the dominant roles of the first two leading modes in salinity variability in the TIO over a wide range of interannual time scales. Among the rich oscillations of the leading EOF modes, a coherent near-biennial band was identified with basinwide loading of sea surface salinity anomalies (SSSa) (EOF1) leading/lagging the northeast–southwest dipolar mode of SSSa (EOF2) by around 4 months across the TIO, with southwestward migration of SSSa center. The spatial loadings of the SSSa leading modes in the TIO were strongly shaped by sea surface temperature–related freshwater fluxes and wind-driven regional ocean circulation on a near-biennial time scale. Composite analysis of the mixed layer salinity budget reflected characteristic features of basin-scale ocean–atmosphere coupling, both temporally and regionally during the life cycle of the near-biennial fluctuation in anomalous salinity in the TIO. Consistent with the intrinsic oscillation paradigm in the observed Indian Ocean dipole (IOD) variation, the dynamic and thermodynamic feedbacks associated with switches from the positive to negative IOD modes provided the phase-connection mechanisms for the SSSa leading-mode displacement over the TIO. Significance Statement This study investigates the leading modes of interannual variability in upper-ocean salinity in the tropical Indian Ocean (TIO). The intrinsic oscillation and associated dynamic and thermodynamic feedbacks over the TIO drive the basinwide connections of upper-ocean salinity variability. Our results show that a coherent near-biennial band is identifiable within the leading modes of sea surface salinity anomalies (SSSa), in which the wind-induced horizontal advections and evaporation-minus-precipitation anomalies associated with the switches from positive to negative Indian Ocean dipole modes mainly provide the phase-transition mechanism of SSSa. This research illustrates substantial evidence for the displacement of basin-scale sea surface temperature anomalies modulating the structures of SSSa and inducing the dynamical connections of leading modes of SSSa on the near-biennial time scale.

Opposite‐Sign Sea Surface Salinity Anomalies Over the Northeastern and Southwestern South Atlantic Ocean From 2010 to 2017

AbstractUsing seven sea surface salinity (SSS) data sets, the dominant modes of interannual SSS variability over the South Atlantic Ocean are examined for the period 2010–2017, which features strong anomalies (>0.1 g/kg). The empirical orthogonal function analysis and composite analysis are used, and the results show that the South Atlantic Ocean features opposite‐sign SSS anomalies over the southwestern (45°–20°S/40°–60°W) and northeastern (0°–15°E/0°–15°S) regions. SSS budget analyses based on the Estimating the Circulation and Climate of the Ocean model estimate suggest that horizontal advection promotes the opposite‐sign SSS anomalies over the two regions. The interannual changes in the horizontal advection can be attributed to changes in the advection of mean salinity by the anomalous horizontal flow, in which the Ekman velocities play the dominant role. The anomalous wind stress and sea level pressure patterns suggest that the opposite SSS variations in poles A and B are probably associated with atmospheric forcings under alternating Atlantic Nino and Benguela Nina conditions. This study offers a connection in interannual SSS variations over two remote regions within the South Atlantic Ocean and suggests the possible role of the climate mode in setting up interannual salinity anomalies over the South Atlantic Ocean.

Asymmetric Response of Sea Surface Salinity to Extreme Positive and Negative Indian Ocean Dipole in the Southern Tropical Indian Ocean

AbstractBased on the Argo and multi‐satellite observations, this study investigates the asymmetric response of sea surface salinity (SSS) to two extreme positive and negative Indian Ocean Dipole (IOD) events in the southern tropical Indian Ocean (TIO). During positive IOD events, the easterly wind anomalies trigger zonal freshwater advection along the equator, decreasing SSS, while cold sea surface temperature (SST) depress the convective precipitation in the southeastern TIO (SETIO), increasing the SSS. Contrast SSS anomalies along the equator and in the SETIO show opposite phases during positive and negative IOD events. These SSS anomalies have enhanced in recent years, reaching the strongest in the extreme negative IOD in 2016 and positive IOD in 2019. The case study shows that positive SSS anomalies occurred in the southern TIO during both positive and negative IOD events in 2016 and 2019, suggesting significant asymmetry in SSS. Analysis of the mixed layer salinity budget shows that the local precipitation and the entrainment associated with the upwelling Rossby waves played an essential role in the SSS increase in 2016, while the decreased precipitation and horizontal advection caused by the westward South Equatorial Current contributed to the SSS increase in 2019. Further analyses suggest that SSS response is more sensitive to thermocline shoaling than deepening, and SSS response to precipitation is greater at shallower mixed layer than at deeper ones, which is the main reason for the asymmetric response of the SSS to negative and positive IOD events.

Skillful Long‐Lead Prediction of Summertime Heavy Rainfall in the US Midwest From Sea Surface Salinity

AbstractSummertime heavy rainfall and its resultant floods are among the most harmful natural hazards in the US Midwest, one of the world's primary crop production areas. However, seasonal forecasts of heavy rain, currently based on preseason sea surface temperature anomalies (SSTAs), remain unsatisfactory. Here, we present evidence that sea surface salinity anomalies (SSSAs) over the tropical western Pacific and subtropical North Atlantic are skillful predictors of summer time heavy rainfall one season ahead. A one standard deviation change in tropical western Pacific SSSA is associated with a 1.8 mm day−1 increase in local precipitation, which excites a teleconnection pattern to extratropical North Pacific. Via extratropical air‐sea interaction and long memory of midlatitude SSTA, a wave train favorable for US Midwest heavy rain is induced. Combined with soil moisture feedbacks bridging the springtime North Atlantic salinity, the SSSA‐based statistical prediction model improves Midwest heavy rainfall forecasts by 92%, complementing existing SSTA‐based frameworks.

Ocean–Atmosphere Variability in the Northwest Atlantic Ocean during Active Marine Heatwave Years

The Northwest (NW) Atlantic has experienced extreme ecological impacts from Marine Heatwaves (MHWs) within the past decade. This paper focuses on four MHW active years (2012, 2016, 2017, and 2020) and the relationship between Sea Surface Temperature anomalies (SSTA), Sea Surface Salinity anomalies (SSSA), North Atlantic Oscillation (NAO), Geopotential Height anomalies (ZA), and anomalous Jet Stream positions (JSPA). Multichannel singular spectrum analysis (MSSA) reveals the strongest temporal covariances between SSSA and SSTA, and JSPA and SSTA for all years, particularly for 2020 (SSSA–SSTA: 50%, JSPA–SSTA: 51%) indicating that this active MHW year was more atmospherically driven, followed by 2012, which had the second highest temporal covariances (SSSA–SSTA: 47%, JSPA–SSTA: 50%) between these parameters. Spatial correlations for SSSA and SSTA between NAO during MHW active years disrupt the long–term (2010–2020) positive relationship in the NW Atlantic. SSSA and JSPA, and SSSA and SSTA were strongly correlated across the NW Atlantic; 2012 SSSA–JSPA correlations were strong and positive between 56–62°W, and 2016, 2017, and 2020 SSSA–JSPA correlations were mostly strong and negative, with strong positive correlations present near the coastline (70–66°W) or off the NW Atlantic shelf (52–48°W). SSSA–SSTA showed the opposite correlations of similar spatial distributions of SSSA–JSPA for all MHW active years. This indicates strong relationships between JSPA, SSSA, and SSTA during MHWs. Understanding the temporal and spatial interplay between these parameters will aid in better monitoring and prediction of MHWs.

Impacts of El Niño on the South China Sea surface salinity as seen from satellites

The impacts of El Niño on sea surface salinity (SSS) in the South China Sea (SCS) are investigated using satellite observations, in-situ data, and reanalysis products. Here, we show that positive SSS anomalies cover most of the SCS during the mature phase of El Niño. The physical processes controlling these positive SSS anomalies are different from region to region, and the differences are especially obvious between the northern and southern SCS. In the northern SCS, the positive SSS anomalies are primarily caused by horizontal advection in response to an enhanced Kuroshio intrusion through the Luzon Strait, while changes in surface freshwater fluxes act to reduce SSS. In the southern SCS, the positive SSS anomalies are largely due to reduced surface freshwater fluxes, with ocean dynamics playing a secondary role. An anomalous anticyclone associated with El Niño is mainly responsible for the reduction of surface freshwater fluxes in the southern SCS.

Ocean salinity indices of interannual modes in the tropical Pacific

This study investigates the interannual modes of the tropical Pacific using salinity from observations, ocean reanalysis output and CMIP6 products. Here we propose two indices of sea surface salinity (SSS), a monopole mode and a dipole mode, to identify the El Niño—South Oscillation (ENSO) and its diversity, respectively. The monopole mode is primarily controlled by atmospheric forcing, namely, the enhanced precipitation that induces negative SSS anomalies across nearly the entire tropical Pacific. The dipole mode is mainly forced by oceanic dynamics, with zonal current transporting fresh water from the western fresh pool into the western-central and salty water from the subtropics into the eastern tropical Pacific. Under a global warming condition, an increase in the monopole and dipole mode variance indicates an increase in both the central and eastern Pacific El Niño variability. The increase in central Pacific El Niño variability is largely due to enhanced vertical stratification during global warming in the upper layer, with intensified zonal advection. An eastern Pacific El Niño-like warming pattern contributes to the increase in eastern Pacific El Niño, with enhanced precipitation over the central-eastern tropical Pacific.

Sea surface salinity reemergence in an updated North Atlantic in-situ salinity data set

AbstractMonthly sea surface salinity (SSS) fields are constructed from observations, using objective mapping on a 1°x1° grid in the Atlantic between 30°S and 50°N in the 1970-2016 period in an update of the data set of Reverdin et al. (2007). Data coverage is heterogeneous, with increased density in 2002 when Argo floats become available, high density along Voluntary Observing Ship lines, and low density south of 10°S. Using lag correlation, the seasonal reemergence of SSS anomalies is investigated between 20°N and 50°N in 5°x5° boxes during the 1993-2016 period, both locally and remotely following the displacements of the deep mixed-layer waters estimated from virtual float trajectories derived from the daily AVISO surface geostrophic currents. Although SSS data are noisy, local SSS reemergence is detected in about half of the boxes, notably in the northeast and southeast, while little reemergence is seen in the central and part of the eastern subtropical gyre. In the same period, sea surface temperature (SST) reemergence is found only slightly more frequently, reflecting the short data duration. However, taking geostrophic advection into account degrades the detection of remote SSS and even SST reemergence. When anomalies are averaged over broader areas, robust evidence of a second and third SSS reemergence peak is found in the northeastern and southeastern parts of the domain, indicating long cold-season persistence of large-scale SSS anomalies, while only a first SST reemergence is seen. An oceanic reanalysis is used to confirm that the correlation analysis indeed reflects the reemergence of subsurface salinity anomalies.

SAM-driven variability of the southwestern Atlantic shelf sea circulation

The objective of the present study is to analyze the interannual variability of the Southwestern Atlantic Continental Shelf (SWACS) circulation and its thermohaline structure. To accomplish it we employed the global ocean reanalysis ORAP5.0, which spans the 35-years period 1979–2013. ORAP5.0 satisfactorily reproduces the most distinctive features of the SWACS circulation and its variability on different timescales, including the interannual scale. The first mode of a joint empirical orthogonal function (EOF) analysis of sea surface temperature (SST), sea surface salinity (SSS), and vertically averaged horizontal velocities is significantly correlated with the first mode of interannual variability of the atmosphere in the Southern Hemisphere, the Southern Annular Mode (SAM). Remarkably, the shelf sea response has been found to be statistically significant not only in the Patagonian Shelf (south of 40 °S), as discussed in the literature, but also in the Northern Shelf (north of 40°S). During the active phases of the leading mode, current anomalies develop as a result of the sea surface height adjustment to Ekman transport at the coastline induced by alongshore wind anomalies. Modification of the advection terms in the salinity and heat budgets due to changes in the circulation and heat surface fluxes generate SST and SSS anomalies along the SWACS. It is suggested that these anomalies drive changes in the mixed layer depth and that these changes can modulate the interannual variability of the chlorophyll-a concentration during austral spring in the frontal zones.

Identifying Climate Modes Contributing to Sea Surface Salinity Decadal Variation in the North Pacific Ocean

AbstractThe North Pacific sea surface salinity (SSS) decadal variation (NPSDV) plays key roles in tracing ocean circulation and hydrological cycle, and its contributing climate modes need to be identified. Three data sets, ORAS4, EN4.2.1, and GODAS, were first evaluated. An autoregressive lag 1 (AR‐1) process model, along with spectrum and correlation analysis, was then applied to the most robust SSS data set during 1980–2017. The AR‐1 reconstructed SSS anomalies explain up to 77% and 55% of the first two modes, respectively. As expected, the NPSDV arises from the superposition of SSS variations with different dynamical origins, including the reemergence, Eastern Pacific and Central Pacific El Niño–Southern Oscillation (ENSO) (EP‐ENSO and CP‐ENSO), Aleutian Low (AL), North Pacific Oscillation (NPO), and Kuroshio and Oyashio Extension (KOE) meridional and zonal modes (KOE‐M and KOE‐Z). Its first mode is 50% EP‐ENSO and 32% CP‐ENSO, while the second mode is 58% NPO and 54% CP‐ENSO. Specifically, stronger persistence in salinity explains most of NPSDV variance. The first mode resembles the EP‐ENSO pattern with a positive loading center in the eastern North Pacific, and the second mode resembles the CP‐ENSO (and also NPO) pattern with a positive loading center near the dateline, alike the contrasting EP‐ENSO and CP‐ENSO SSS flavors in the tropics. Besides, a fraction of the first mode notable in the KOE region is controlled by KOE‐Z and KOE‐M as intrinsic oceanic dynamics. The importance of AL to NPSDV, after removing the indirect impact of ENSO, is insignificant. A general NPSDV evolution was lastly revealed.

Bay of Bengal Sea surface salinity variability using a decade of improved SMOS re-processing

Monsoon rain and rivers bring large freshwater input to the Northern Bay of Bengal (BoB), yielding low Sea Surface Salinity (SSS) after the monsoon. The resulting sharp upper-ocean salinity stratification is thought to influence tropical cyclones intensity and biological productivity by inhibiting vertical mixing. Despite recent progresses, the density of in situ data is far from sufficient to monitor the BoB SSS variability, even at the seasonal timescale. The advent of satellite remotely-sensed SSS (SMOS, Aquarius, SMAP) offers a unique opportunity to provide synoptic maps of the BoB SSS every ~8 days. Previous SMOS SSS retrievals did not perform well in the BoB. Here, we show that improved systematic error corrections and quality control procedures yield a much better performance of the new “debiased v4” CATDS level-3 SSS from SMOS (~0.8 correlation, 0.04 bias and 0.64 root-mean-square difference to more than 28,000 collocated in situ data points over 2010–2019). The SMOS product now performs equivalently to Aquarius, and is slightly inferior to SMAP over the BoB. In particular, SMAP and SMOS are able to capture salinity variations close to the east coast of India (r > 0.8 within 75–150 km of the coast). They thus capture the seasonal freshening there, associated with equatorward advection of the Northern BoB low-salinity water by the East Indian Coastal Current (EICC) after the summer monsoon. The 10-year long SMOS record further allows to describe the BoB interannual SSS variability, which is strongest in boreal fall in relation with the Indian Ocean Dipole (IOD). Positive IOD events induce a weakening of the southward export of freshwater by the EICC, and hence negative SSS anomalies in the Northern BoB and positive ones along the East Indian coast. This confirms results from earlier studies based on modelling, sparse in situ data, or shorter satellite records, but this time from a 10-year long SSS record. Overall, our study indicates that the new SMOS retrieval can be confidently used to monitor the BoB SSS and to study its mechanisms. We end by a brief description of the BoB SSS anomalies associated with the extreme 2019 IOD event and highlight the very good performance over the BoB of a new multi-satellite product developed by the European Space Agency merging SMOS, Aquarius and SMAP data.

Characteristics and vertical structure of oceanic mesoscale eddies in the Bay of Bengal

The signatures of mesoscale eddies induced surface and subsurface changes have not been comprehensively quantified for the Bay of Bengal (BoB) region. This study quantifies the statistical properties and three-dimensional (3D) eddy structures in the BoB. To accomplish this, the satellite altimetry data combined with automated eddy detection and tracking algorithm is used. Horizontal distribution of surface characteristics of eddies is analyzed by using 24 years (1993–2016) of AVHRR infrared satellite sea surface temperature (SST) and 7 years (2010–2016) of sea surface salinity (SSS) of SMOS satellite data. Surface eddy centric composite analysis reveals the existence of warm (cold) and diverse SSS anomalies for anticyclonic (cyclonic) eddies. During winter, it is important to note that the eddy induced SST and SSS anomalies show the dipole patterns show opposite phases for the cyclonic and anticyclonic eddies. Observed diploe structures are consistent with the eddy rotation and background large-scale meridional gradient of temperature and salinity fields. The 3D structure of eddies is investigated by using the ARMOR3D and Argo float profiles. The horizontal distribution of temperature and salinity anomalies from ARMOR3D signify the monopole structure of eddies in the subsurface layers. Further, the analysis of composite averages of 241 (200) Argo temperature profiles indicates the core of anticyclonic (cyclonic) eddies centered at about ∼140 m (∼100 m). However, salinity profiles depict the existence of core at ∼65 m (∼50 m). This study have practical relevance to a variety of stakeholders and finds profound importance in the validation of eddy-resolving ocean models for the BoB region.

Anatomy of Salinity Anomalies Associated With the Positive Indian Ocean Dipole

AbstractMechanisms of salinity anomalies associated with the positive Indian Ocean Dipole (pIOD) are investigated through a series of sensitivity experiments and an online budget analysis using a regional ocean model. Special emphasis is placed on the contribution from the rectified effects due to high‐frequency variability, which was not quantitatively discussed in previous studies. The results from sensitivity experiments show that positive sea surface salinity (SSS) anomalies in the southeastern tropical Indian Ocean are primarily caused by reduction in precipitation and partly by enhanced evaporation due to increased wind speed, while negative SSS anomalies in the central‐eastern equatorial Indian Ocean are generated by zonal advection anomalies induced by anomalous wind stress, consistent with previous studies. Completely new results are that the modulation of nonlinear salinity advection associated with mesoscale eddies also plays an important role in determining the spatial pattern of SSS anomalies, especially in the southeastern tropical Indian Ocean. On the other hand, subsurface salinity anomalies are almost entirely caused by wind stress effects mediated by ocean dynamical processes. Further decomposition of advective anomalies suggests that they are mainly explained by the pIOD‐related current anomalies governed by equatorial wave dynamics. However, a vertical shift of nonlinear freshening due to high‐frequency variability also substantially contributes to the generation of positive subsurface salinity anomalies in the eastern equatorial Indian Ocean. Our results show that large‐scale oceanic changes in response to the pIOD‐related atmospheric anomalies are the key drivers of the observed salinity anomalies, while some nonlinear effects also seem to be at work.

Effects of Salinity Variability on Recent El Niño Events

Ocean salinity variability provides a new way to study the evolution of the the El Niño-Southern Oscillation (ENSO). Comparisons between the salinity variation and related processes responsible for sea surface temperature anomaly (SSTA) were extensively examined for the two strong El Niño (EN) events in 1997/1998 and 2015/2016, and a special EN event in 2014/2015. The results show that the development of EN is significantly correlated with a sea surface salinity anomaly (SSSA) in the tropical western-central Pacific. In the spring of 1997 and 2015 with strong EN events, the western-central equatorial Pacific exhibited significant negative SSSA that propagated eastward to the west of the dateline. The negative SSSA induced increased barrier layer thickness (BLT) which enhanced sea surface temperature (SST) warming in the tropical central Pacific. In contrast, although a negative SSSA occurred during April of the 2014/2015 weak EN event in the western-central equatorial Pacific, this SSSA was mainly confined to between 160° E and 180° E without significant eastward movement, resulting in a weakened BLT thickening process and a weak modulation effect on SST. We also confirm that the surface forcing associated with fresh water flux (FWF: evaporation (E) minus precipitation (P)) plays a prominent role in the surface salinity tendency in the tropical Pacific during EN events. Moreover, the negative FWF anomaly leads a strong negative SSSA by two months. Compared with the two strong ENs, the early negative FWF anomaly in the weak 2014/2015 EN did not present distinct development and eastward propagation and weakened rapidly in the summer of 2015. We demonstrate that change in salinity can modulate the ENSO, and the variation of SSSA and associated physical processes in the tropical western-central Pacific and could be used as an indicator for predicting the development of ENSO.

Madden‐Julian Oscillation‐Induced Sea Surface Salinity Variability as Detected in Satellite‐Derived Salinity

AbstractAs a dominant source of tropical variability, the Madden‐Julian oscillation (MJO) influences the ocean in many ways. One approach to observe the atmosphere‐ocean relationship is by examining sea surface salinity (SSS) due to direct freshening by MJO precipitation. The convectively enhanced (suppressed) phase of the MJO is associated with negative (positive) SSS anomalies that propagate eastward along the equatorial Indian and Pacific oceans. In this study, primary MJO events are identified, and their SSS signatures are compared for the first time across multiple satellite salinity products (the European Space Agency's Soil Moisture Ocean Salinity; the National Aeronautics and Space Administration's Aquarius and Soil Moisture Active Passive) from 2010 to 2017. While all satellite missions are capable of detecting MJO signals and primary events on an unprecedented observational scale, we find that the use of the combined active passive algorithm increases signal robustness, with the strongest signal response in Soil Moisture Active Passive and Soil Moisture Ocean Salinity (±0.2 psu) and the lowest in Aquarius (±0.1 psu).