Sea surface salinity (SSS), an essential climate variable that is sensitive to changes in the global water cycle, varies seasonally in many places due to annual variations in rainfall and evaporation, as well as vertical mixing and advection. The seasonal variability of global mean SSS, with maximum/minimum SSS in March/September has been increasing in amplitude since the start of the satellite observation era in 2010. This variability, with a range of 0.03, implies approximately 2 cm of water leaving from and returning to the surface of the ocean over the course of the year, with its 0.01 increase in range since 2011, roughly equating to an additional 0.7 cm of water cycling out and returning to the ocean. This trend is consistent with predictions of an accelerating global water cycle on a warming planet.

Satellite observations of sea surface salinity (SSS) have been validated in a number of instances using different forms of in situ data, including Argo floats, moorings and gridded in situ products. Since one of the most energetic timescales of variability of SSS is the seasonal, it is important to know if satellites and in situ gridded products are observing the seasonal variability correctly. In this study we validate the seasonal SSS from satellite and in situ products using observations from moorings in the global tropical moored buoy array. We utilize 6 different satellite products, and two different in situ gridded products. For each product we have computed seasonal harmonics, including amplitude, phase and fraction of variance (R2). These quantities are mapped for each product and for the moorings. We also do comparisons of amplitude, phase and R2 between moorings and all the satellite and in situ products. Taking the mooring observations as ground truth, we find general good agreement between them and the satellite and in situ products, with near zero bias in phase and amplitude and small root mean square differences. Tables are presented with these quantities for each product quantifying the degree of agreement.

This study examines salinity barrier layers (BLs) in the eastern tropical North Pacific in the region of the Salinity Processes in the Upper ocean Regional Studies-2 field campaign. We utilize a high- resolution numerical model to study BLs and their relationship to frontal features and small-scale ocean variability, focusing on two specific events. One is associated with a large outbreak of BL presence near 7°N along 125°W. The other is a relatively isolated but persistent BL that forms near 13°N, again along 125°W. In both cases we find that the BL is proximate to a salinity frontal feature in which isohalines tilt toward the fresh side of the front at its base. The BLs studied are associated with divergent flow at the surface on the fresh side of the front and convergent flow on the salty side. Tilting of the front is invoked to explain this, with an additional mechanism involving a vertical circulation which causes the base of the front to tilt preferentially.

Sea Surface Salinity (SSS) can change as a result of surface freshwater forcing (FWF), or internal ocean processes such as upwelling or advection. At the seasonal scale, SSS should follow FWF by ¼ cycle, or 3 months, if FWF is the primary process controlling it at the seasonal scale. In this paper we compare the phase relationship between SSS and FWF (i.e. evaporation minus precipitation over mixed layer depth) over the global (non-Arctic) ocean using in situ SSS and satellite evaporation and precipitation. We find that instead of the expected 3 month delay between SSS and FWF, the delay is mostly closer to 1-2 months, with SSS peaking too soon relative to FWF. We then compute monthly vertical entrainment and horizontal advection terms of the upper ocean salinity balance equation and add their contributions to the phase of the FWF. The addition of these processes to the seasonal upper ocean salinity balance brings the phase difference between SSS and the forcing processes closer to the expected value. We do a similar computation with the amplitude of the seasonal SSS and the forcing terms, with less definitive results. The results of this study highlight the important role that ocean processes play in the global freshwater cycle at the seasonal scale.

The region between a shallow mixed layer and a deep isothermal layer resulting from salinity stratification is called a barrier layer (BL). Since BLs hinder the surface heat and momentum exchange with the ocean subsurface, they play an important role in air-sea interaction. Synoptic features and formation of BLs and associated temperature inversions (TIs) in the Eastern Pacific Fresh Pool (EPFP) were investigated using shipboard observations. BLs and TIs were found in the high precipitation EPFP, a frontal transition zone (FTZ) and in the sea surface salinity (SSS) front during a 2016 boreal summer cruise. During a 2017 boreal autumn cruise, BLs and TIs were found from the southern part of the EPFP to the SSS front. The BLs in the SSS front and the FTZ were meridionally displaced from the precipitation maximum during both the 2016 and 2017 cruises. Freshening and cooling by tilting of both the isohalines and isotherms of the SSS front occurred via Ekman and geostrophic flow during both cruises, and also within the EPFP during 2016. The SSS front was a dominant contributor to the density gradient in the horizontal plane. A high dissolved oxygen content observed within BLs confirmed that the surface flow plays an important role in the BL and TI formation through the tilting process. Patchy distribution of freshening and cooling by tilting implies an intermittent BL and TI formation with short time scales.

Seasonal cycle is the largest source of variability for sea surface salinity (SSS) and has a significant influence on the upper-ocean stratification and water-mass formation. The advent of the Argo profiling floats and L-band passive microwave remote sensing in the past one and half decade has significantly improved the sampling of seasonal variations of SSS over the global ocean. Assessing the seasonality of SSS using these recent measurements is important for understanding its relationships with freshwater forcing and ocean dynamics as well as for identifying potential limitations of the SSS observing system. Here we utilize a suite of SSS products from recent satellite and in-situ platforms to revisit seasonal variations of SSS under different freshwater forcing conditions. The result shows that, although the annual harmonic is the most characteristic feature of the seasonal cycle, the semiannual harmonic is not negligible, especially in regions influenced by monsoon and major rivers. The annual and semiannual harmonics account for 70–80 % and 10–16 % of the total observed variance respectively, which together drive the SSS seasonality. The range of seasonal SSS is approximately ±0.05 practical salinity scale (pss) in the subtropical SSS maximum regions, but greater than ±0.25 pss in the tropical SSS minimum regions. However, the seasonal variations of satellite SSS in the 20-40°N latitude range showed erroneous annual and semiannual phases compared with in situ products, the cause of which needs further examination.

The sea surface salinity (SSS) maximum of the South Indian Ocean (the SISSS-max) is a high-salinity feature centered at 30°S, 90°E, near the center of the South Indian subtropical gyre. It is located poleward of a region of strong evaporation and weak precipitation. Using several different satellite and in situ datasets, we track changes in this feature since the early 2000’s. The centroid of the SISSS-max moves seasonally north and south, furthest north in late winter and farthest south in late summer. Interannually, the SISSS-max has moved on a northeast-southwest path about 1500 km in length. The size and maximum SSS of the feature vary in tandem with this motion. It gets larger (smaller) and saltier (fresher) as it moves to the northeast (southwest) closer to (further from) the area of strongest surface freshwater flux. The area of the SISSS-max almost doubles from its smallest to largest extent. It was maximum in area in 2006, decreased steadily until it reached a minimum in 2013, and then increased again. The seasonal variability of the SISSS-max is controlled by the changes that occur on its poleward, or southern, side, whereas intereannual variability is controlled by changes on its equatorward side. The variations in the SISSS-max are a complex dance between changes in evaporation, precipitation, wind forcing, gyre-scale ocean circulation and downward Ekman pumping. Its motion correlated with SSS changes throughout the South Indian Ocean and is a sensitive indicator of changes in the basin’s subtropical circulation.