Study Discloses the Spatial-temporal Structure of Ocean Salinity Seasonal Variation


Salinity is always regarded as a natural "water gauge". It plays a vital role in regulating ocean density, stratification and circulation and is therefore an indicator of the coupling between the ocean, atmosphere and land through the water cycle. However, a holistic analysis of subsurface salinity down to 2000m is lacking, and there has been no quantification of the uncertainty in the salinity seasonal variation. Therefore, a rigorous analysis on ocean subsurface salinity with uncertainty estimate is needed and performed here.
Using newly provided observational salinity and evaporation-precipitation (E-P) products, a new study, by a group of researchers from China and U.S.A., published in International Journal of Climatology, provides a spatially complete look of the seasonal variation of the upper 2000m ocean salinity from regional to global scales, and assesses the robustness of the signals.
The new data shows a robust seasonal variation in global ocean salinity from the surface down to 350 m depth that exceeds the observational uncertainty, and in some regions the variation can be detected at 2000m. Regions with pronounced seasonal variation of sea surface salinity (SSS) include the Northwest Pacific, Northwest Atlantic, tropical oceans and the northeast Indian Ocean. From 5°N to 30°N (20°N to 5°N), the sea surface is fresher (saltier) in the first half of the year and gets saltier (fresher) in the second half of the year, because of surface precipitation and evaporation changes associated with monsoons and the seasonal changes in atmospheric circulation. In the middle and high latitudes, the evaporation-minus-precipitation is mismatched with local salinity changes, suggesting that river runoff, sea ice change and ocean dynamics have a controlling role. 
The zonal mean salinity variation (Fig. 1) shows that SSS anomalies extend down to 40-80m and are generally consistent with E-P, indicating that the role of salinity as "rain gauge" extends to a depth of 40-80m. Below 100m, ocean dynamics plays a vital role in salinity changes, which should be associated with the seasonal variation of winds driven circulation variations. Spatially, the concept of the "salinity as rain gauge" does not work well in the coastal regions and polar regions. It is likely that other processes are dominant in these regions.
Seasonal variations of the zonal mean salinity changes from 0 to 1800 m in January, March, May, July, September and November. The zonally averaged surface E?P is shown in the upper part of each panel. (Image by CHENG Lijing)
Finally, the new study discloses a notable difference in global SSS variation between Argo-only products and datasets that merge all observations, revealing the insufficiency of Argo network in shallow oceans and polar regions. "This study highlights the needs to integrate multiple instrumental observation systems to obtain a better ocean monitoring system", said CHENG Lijing with the Institute of Atmospheric Physics at the Chinese Academy of Sciences, also the corresponding author of this study.
This study is supported by National Key R&D Program of China (2017YFA0603202), the National Natural Science Foundation of China (Grant 42076202, 42122046), and Key Deployment Project of Centre for Ocean Mega- Research of Science, CAS (COMS2019Q01).
Liu, Y., L. Cheng*, Y. Pan, J. P. Abraham, B. Zhang, J. Zhu, J. Song, Climatological seasonal variation of the upper ocean salinity. International Journal of Climatology, 1– 22.
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