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The Hadley Circulation and associated westerlies strengthened and moved equatorward across the mid‐Pleistocene transition (MPT). However, the evolution of the intertropical convergence zone (ITCZ) is still elusive due to the scarcity of long‐term hydrological records from regions sensitive to the ITCZ change. Here, high‐resolution sea surface salinity estimates derived from surface‐dwelling planktic foraminiferal δ18O and Mg/Ca in Ocean Drilling Program Site 871 reveal a long‐term freshening trend in the central equatorial Pacific across the MPT. We attribute this secular reorganization of the precipitation‐evaporation balance to the gradual southward migration and intensification of ITCZ in the North Pacific. It is inferred that the long‐term evolution of the ITCZ was modulated by the increased meridional sea surface temperature gradients and the enhancements of trade winds across the MPT. Plain Language Summary The Hadley circulation and westerlies have been revealed to strengthen and shift equatorward across the mid‐Pleistocene transition (MPT), yet how the ITCZ, the uplifting branches of the Hadley Circulation, changed remains elusive. As precipitation is a key indicator of the ITCZ, here we use the residual oxygen isotope (δ18O) of calcite, calculated from planktic foraminiferal δ18O and Mg/Ca, to reconstruct the sea surface salinity (SSS) change and track precipitation change in the central equatorial Pacific during the past ∼1,780 kyr. In contrast to the stable SSS condition in the far western part of the Western Pacific, our new SSS record shows a striking secular decreasing trend across the MPT and represents a hydrological transition from evaporation‐control to precipitation‐control state. We suggest that the long‐term hydrological reorganization was the result of the gradual southward migration and intensification of the North Pacific ITCZ across the MPT, which was closely related to the increased meridional sea surface temperature gradients and the enhancements of trade winds. Key Points Sea surface salinity (SSS) in the central equatorial Pacific since ∼1,780 ka was reconstructed by surface‐dwelling foraminiferal δ18O and Mg/Ca The reconstructed SSS shows a long‐term freshening trend across the mid‐Pleistocene transition The freshening trend primarily reflects the southward shift and intensification of the North Pacific intertropical convergence zone

Based on Argo sea surface salinity ( SSS ) and the related precipitation ( P ), evaporation ( E ), and sea surface height data sets, the climatological annual mean and low-frequency variability in SSS in the global ocean and their relationship with ocean circulation and climate change were analyzed. Meanwhile, together with previous studies, a brief retrospect and prospect of seawater salinity were given in this work. Freshwater flux ( E-P ) dominated the mean pattern of SSS , while the dynamics of ocean circulation modulated the spatial structure and low-frequency variability in SSS in most regions. Under global warming, the trend in SSS indicated the intensification of the global hydrological cycle, and featured a decreasing trend at low and high latitudes and an increasing trend in subtropical regions. In the most recent two decades, global warming has slowed down, which is called the “global warming hiatus”. The trend in SSS during this phase, which was different to that under global warming, mainly indicated the response of the ocean surface to the decadal and multi-decadal variability in the climate system, referring to the intensification of the Walker Circulation. The significant contrast of SSS trends between the western Pacific and the southeastern Indian Ocean suggested the importance of oceanic dynamics in the cross-basin interaction in recent decades. Ocean Rossby waves and the Indonesian Throughflow contributed to the freshening trend in SSS in the southeastern Indian Ocean, while the increasing trend in the southeastern Pacific and the decreasing trend in the northern Atlantic implied a long-term linear trend under global warming. In the future, higher resolution SSS data observed by satellites, together with Argo observations, will help to extend our knowledge on the dynamics of mesoscale eddies, regional oceanography, and climate change.

Benguela Niño and Niña events are episodes of extreme warming and cooling off Angola with impacts on fisheries, ecosystems, and rainfall in southwest Africa. They are typically forced remotely or locally by variations in equatorial or alongshore winds, respectively. We use an extensive in‐situ data set to show that sea surface salinity (SSS) changes can also act as a local forcing that amplifies these extreme warm and cold events by altering the water column stratification and consequently the impact of subsurface mixing. The mixed layer turbulent heat loss during an extreme warm episode with unusually low SSS in 1995 is nearly 3× lower than during a cold event with high SSS in 1997. We also demonstrate that interannual turbulent heat flux variability in early boreal spring off Angola is strongly impacted by salt advection fluctuations, and that this turbulent mixing is significant for altering mixed layer temperatures and restoring its salinities.

The decadal variability of sea surface salinity (SSS) is crucial for the global water cycle and the climate system. Previous studies have indicated that the SSS in the western tropical Pacific (WTP) Ocean exhibits significant decadal variability, which plays an important role in shaping tropical climate. Here we find that the amplitude of SSS decadal variability in the WTP has been intensified significantly since the early 1950s. A comprehensive analysis of the salinity budget indicates that the amplifying of SSS decadal variability in the WTP is primarily attributed to changes in both the freshwater flux and ocean dynamics. During the past decades, surface currents in the WTP get stronger and result in enhanced salinity advection. In addition, the amplitude of decadal variability of freshwater flux is increased as well due to anomalous atmospheric circulation associated with the Victoria mode. Plain Language Summary The changes in sea surface salinity in the western tropical Pacific over decades are important for a better understanding of how the ocean affects climate and marine ecosystems. Our research shows that the ups and downs of sea surface salinity over the decades have gotten bigger in the western tropical Pacific. This trend is mainly driven by two things: changes in rainfall and shifts in ocean currents. The North Pacific Victoria mode, which is a special pattern of how the atmosphere changes, might affect the rainfall and ocean currents over decades. The Victoria mode, in turn, plays a big role in shaping the changes we see in sea surface salinity in the western tropical Pacific. Key Points Decadal variability of sea surface salinity (SSS) in the western tropical Pacific (WTP) has been amplified since the early 1950s Changes in freshwater flux and salinity advection account for the amplifying of decadal SSS variability in the WTP The models show that global warming and the intensification of the Victoria Mode are likely to enhance decadal variability in SSS

Previous studies suggested that tropical sea surface salinity (SSS) can influence tropical Pacific sea surface temperature (SST) through mixing and entrainment and thus it may be a signal for El Niño‐Southern Oscillation (ENSO) prediction. This paper explores the influence of SSS on ENSO spring predictability barrier (SPB) using an empirical dynamic model ‐ Linear Inverse Model (LIM). By coupling and decoupling SSS in the LIM, we find that tropical Pacific SSS plays a significant role in weakening both Central‐Pacific and Eastern‐Pacific ENSO SPB. The evolution of optimal initial structure also shows the importance of SSS dynamics in ENSO. We found an SSS mode that plays the dominant role in SSS impacting ENSO prediction. By the analysis of lead‐lag correlation, we find that this mode can induce easterlies during the spring, which finally leads to a La Niña‐like SST pattern in the winter through zonal advective and thermocline feedbacks. Plain Language Summary The spring predictability barrier (SPB) is a phenomenon of forecast skill reduction of the El Niño‐Southern Oscillation (ENSO) when it comes to the boreal spring, regardless of the initial month. Sea surface salinity (SSS) can influence sea surface temperature by altering sea surface density and thus it may be a signal for ENSO prediction. Using a linear dynamical model, we find that SSS plays an important role in improving the forecast skill of ENSO and weakening SPB. We further find that SSS can induce the easterlies during the spring, which finally leads to a La Niña‐like SST pattern. Our study suggests that SSS can be used to predict ENSO about 1 year later. Key Points A linear dynamical model suggests that taking sea surface salinity (SSS) into consideration can strongly weaken Central‐Pacific and Eastern‐Pacific El Niño‐Southern Oscillation (ENSO) spring predictability barrier (SPB) A new SSS mode is found to be important for weakening ENSO SPB The SSS mode can predict ENSO events about 1 year earlier by inducing sea surface temperature and wind anomalies in the early spring

Previous studies suggest that the winter surface freshening in the southeastern Arabian Sea (SEAS) contributes to the development of very high Sea Surface Temperatures (SST) thereby influencing the following summer monsoon onset. Here, we use forced and coupled simulations with a regional ocean general circulation model to explore the SEAS Sea Surface Salinity (SSS) variability mechanisms and impact on the monsoon. Both configurations capture the main SEAS oceanographic features, and confirm that the winter SSS decrease results from horizontal advection of Bay of Bengal freshwater by the cyclonic circulation around India during fall. A coupled model sensitivity experiment where salinity has no effect on mixing indicates that the salinity stratification reduces the SEAS mixed layer cooling by vertical processes by 3 °C seasonally. Salinity however enhances mixed layer cooling by a similar amount through concentrating negative winter surface heat fluxes into a thinner mixed layer, resulting in no climatological impact on SST and summer monsoon rainfall. The Indian Ocean Dipole (IOD) is the main driver of the winter SEAS SSS interannual variability (r ~ 0.8). Salty anomalies generated in the western Bay of Bengal during fall by positive IOD events are indeed transported by the cyclonic climatological coastal circulation, reaching the SEAS in winter. By this time, warm IOD-induced SST anomalies in the SEAS are already decaying, and the SEAS SSS anomalies hence do not contribute to their development. Overall, our model results suggest a weak climatological and interannual impact of the SEAS winter freshening on local SST and following monsoon onset.

The leading hypothesis attributes the Younger Dryas (YD) event to a disruption in the Atlantic Meridional Overturning Circulation, driven by meltwater input from North America. Determining the origin, timing, and magnitude of YD freshening are crucial for understanding abrupt climate change. This study examines the δ2H values of specific lipids in response to freshwater discharge and provides a quantitative estimate of YD freshening, using a marine sediment core from the Canadian Beaufort Sea, a region with documented evidence of a YD flood event. A pronounced reduction in δ2H values of leaf wax lipids and microalgal dinosterol indicates marked freshening at the onset of the YD, with the YD flood and the melting Laurentide Ice Sheet likely reducing surface water salinity by ∼15–24. In contrast, salinity levels remained high and stable for the last 8 kyr, likely implying a drier climate in the Mackenzie River basin. Plain Language Summary The Younger Dryas (YD) cold event is thought to have been triggered by a weakening of the Atlantic Meridional Overturning Circulation, driven by an influx of freshwater into the North Atlantic Deep Water formation region. Determining the source, timing, and magnitude of this freshwater input is critical for understanding the associated climate change. In this study, we analyzed the hydrogen isotopic composition of various lipids in a sediment core from the Canadian Beaufort Sea to investigate these hydrological changes. Our results show that terrestrial leaf wax lipids and microalgae lipids recorded distinct freshwater signals during the YD, allowing for a better constraint on the source and magnitude of this freshening. Using an established empirical relationship between lipid hydrogen isotopes and sea surface salinity, we estimated that surface waters in the Canadian Beaufort Sea experienced a substantial salinity reduction of approximately 15–24 during the YD. This significant decrease was likely caused by a combination of a freshwater outburst and the Laurentide Ice Sheet (LIS) melting water discharge. Following the LIS retreat, the δ2Hlipid indicates that the region likely experienced a shift toward a drier climate during the mid‐to‐late Holocene (∼8–0 cal. kyr BP). Key Points A pronounced decrease in δ2H values of leaf wax lipids and dinosterol reveals significant freshening at the onset of the Younger Dryas The salinity depression in surface waters is estimated to be approximately 15–24, driven by the flood event and Laurentide Ice Sheet decay

The Kuroshio intrusion (KI) is a northwestward‐flowing branch of the Kuroshio Current, which enters the South China Sea (SCS) and regulates its temperature, salinity, and water mass exchanges. However, limited direct observations hinder our understanding of KI's mechanisms and responses to climate change. Here, we present a 60‐year bi‐monthly resolved coral oxygen isotope (δ18Oc) record from Dongsha Atoll, the northern SCS. The dry‐season (December–March) δ18Oc record reveals interannual to decadal variabilities of the KI. The impact of the East Asian winter monsoon (EAWM) on Dongsha δ18Oc was more pronounced during the 1970s and 1980s and after the early 2000s, while the influence of the El Niño‐Southern Oscillation (ENSO) on Dongsha δ18Oc was higher between the 1980s and 1990s. The Pacific Decadal Oscillation (PDO) may have a relatively minor effect on KI strength or may indirectly modulate KI strength through its influence on ENSO activities. Our Dongsha δ18Oc record highlight the importance of the EAWM, ENSO, and PDO in predicting future KI changes. Plain Language Summary The Kuroshio intrusion is a branch of the Kuroshio Current. It flows into the South China Sea and affects its temperature, salinity, and water movement. We however know very little how the Kuroshio intrusion responds to climate changes. Here we present a study on the chemical composition of a coral core from Dongsha Atoll in the northern South China Sea. Our study shows that the intrusion varies markedly over the past 60 years and is connected to changes in sea surface salinity. Stronger intrusions align with stronger East Asian winter monsoon intensity and El Niño events. Understanding how the Kuroshio intrusion reacts to climate change is important for effectively managing the potential impacts of global warming on the marine ecosystem. Key Points A 60‐year‐long coral oxygen isotope record from Dongsha Atoll, South China Sea reveals interannual to decadal variations in the Kuroshio intrusion Strong Kuroshio intrusion, corresponding to high sea surface salinity, is identifiable over the past 60 years The Kuroshio intrusion variations are primarily driven by East Asian winter monsoon changes and also influenced by El Niño‐Southern Oscillation

We construct freshwater balance in the Bay of Bengal (BoB) within a control volume (CV) bounded by 1,018 kg/m3 isopycnal surface using observations and ocean reanalysis during 2011–2015. Freshwater in CV is maximum in October–November due to monsoonal rain and river inflows, and minimum in April–May. Water lighter than 1,018 kg/m3 is not transported out of BoB, implying that freshwater lost from CV is mixed away entirely within the basin. From freshwater budget, we infer moderate diapycnal mixing rates (∼0.8 × 10−5 m2/s) in boreal spring and summer; in winter (December–January), the rate of freshwater loss to subsurface ocean is 0.015 m/day, corresponding to a median turbulent diffusivity of 4.2 × 10−5 m2/s, with standard error of 25%. We show that enhanced winter mixing across the shallow pycnocline is due to reduced shortwave radiation and subseasonal episodes of surface buoyancy loss when cool, dry northeast monsoon winds blow over BoB. Plain Language Summary The freshwater from monsoonal rivers and rain discharged into the Bay of Bengal (BoB) forms a shallow (<10 m deep) salinity‐stratified layer which has implications for the regional air‐sea interaction. There is very limited understanding of the mixing of the freshwater owing to the absence of near‐surface turbulence measurements in BoB. It is important to understand the disappearance of the shallow fresh layer in the Bay due to it's linkages to the regional hydrological cycle and the near‐surface stratification. In this study, we infer the seasonality of basin‐scale mixing beneath the surface layer of BoB from a freshwater volume balance using a combination of observations and ocean analysis. We find that the highest rates of freshwater mixing (mean and median values of about 5.5 × 10−5 m2/s and 4.2 × 10−5 m2/s) occur during winter (December–January), mainly driven by enhanced surface buoyancy loss thereby reducing the freshwater content at a mean rate of 0.015 m/day. This study has implications for improvement of ocean and climate models, which generally have too‐high mixing rates and poor representation of the salinity‐stratified near‐surface layer in BoB. Key Points Low‐salinity water from monsoon rain and rivers covers nearly 80% of the northern Bay of Bengal's surface in October–November, and nearly vanishes by April–May The freshest (salinity <30 pss) and lightest (density <1,018 kg/m3) water is not transported across the southern boundary, indicating modification or storage within the bay Freshwater balance shows that most of the rain and river water is mixed across the 1,018 kg/m3 isopycnal surface in winter during episodes of enhanced surface buoyancy loss

On the basis of the synergistic seasonal evolution of sea surface salinity over the Maritime Continent (MC SSS) and South Asian rainfall, the singular value decomposition analysis is used to identify a robust coupled mode between springtime MC SSS and South Asian summer monsoon rainfall (SASMR). High MC SSS is accompanied by a SASMR dipole (suppressed rainfall in northern India and enhanced rainfall in the eastern Tibetan Plateau), which is largely caused by the negative and positive moisture flux convergence associated with ocean-to-land moisture transport. In addition, the negative rainfall anomalies in northern India decrease evapotranspiration and further suppress Indian summer monsoon rainfall (ISMR) because of land-to-atmosphere feedback. The seasonal forecast of ISMR can be improved by incorporating SSS into a physics-based empirical model, which was selected according to adjusted R 2 values, Mallows’ C p values, and cross-validation. In this study, we found that a new predictor (ocean salinity) over the MC region can improve ISMR prediction, which highlights the importance of enhanced monitoring of salinity in the MC region.