Seasonal salinity stratifications in the near-surface layer from Aquarius, Argo, and an ocean model: Focusing on the tropical Atlantic/Indian Oceans

A newly available sea surface salinity (SSS) measurement from Aquarius/SAC-D satellite reveals strong seasonal variability in the tropical Atlantic and Indian Oceans. The seasonal SSS variability at skin layer differs/agrees regionally in their amplitude from/with Argo-measured salinity at 5 m depth and model salinity at the top layer, indicating various characteristics of near-surface salinity stratifications. By comparing the three different salinity products, we have examined the near-surface salinity stratifications with emphasis on the dynamical processes that differ from one region to another. Our analysis shows that for the western part of tropical Atlantic and southern tropical Indian, a large amount of river runoff and/or surface freshwater significantly stratifies the surface layer above 5 m depth, resulting in the differences among the Aquarius, Argo, and model. Differently for the southern Arabian Sea, the surface water can be mixed down to the depth of 5 m due to seasonally reversing currents driven by monsoons, resulting in an agreement among the data sets. The comparison suggests that dynamical differences can lead to different vertical salinity stratifications locally, which explain the differences between the Aquarius observations in the first cm of the sea surface, the Argo measurements at the 5 m depth, and model's representation of the surface-layer averaged salinity.