Mechanisms for Mixed-layer Salinity Variability in the Indian Ocean
[12-Feb-2018] Köhler, J., Serra, N., Bryan, F., Johnson, B.K., and Stammer, D.
Presented at the 2018 Ocean Sciences Meeting
The physical processes that control the seasonal mixed layer salinity (MLS) budget are examined through a combined analysis of an ensemble-mean sea surface salinity (SSS) product based on SMOS and Aquarius data and a high-resolution ocean model simulation using the National Center for Atmospheric Research Community Earth System Model driven by COREv2 forcing. The analyses reveal that SSS variations can be used as a proxy for MLS variations, taking therefore advantage of the high spatial and temporal resolution of the satellite SSS data.
It was found that the surface freshwater flux controls the annual MLS tendency in the western Indian Ocean, where the Inter-Tropical Convergence Zone has a strong annual cycle. There, surface fluxes account approximately for 70% of the annual MLS tendency variance. In contrast, advection is the dominant contribution in the northern and equatorial Indian Ocean, accounting for approximately 80% of the annual MLS tendency. The vertical diffusion, neglected in previous observational MLS budget studies, is an important contribution to the MLS budget, weakening the overall freshening in precipitation and river runoff dominated regions. The influence of entrainment on the MLS tendency is mainly due to mixed layer depth temporal variations but remains small.
It is further shown that the main differences between the model and observational budget terms in equatorial regions arise from differences in horizontal velocity fields, which lead to different advection contributions. Also high-frequency, small-scale eddy and shear processes, not resolved with observational data, present important contributions to the MLS budget. Horizontal shear advection was found to be important in regions of strong horizontal currents, where the strong vertical shear in the lateral flow contributes to reduce the vertical salinity gradient. This term adds up to 5% of MLS variance in the equatorial ocean region, where the strong equatorial jets are present. Finally, the MLS annual cycle in the Bay of Bengal is dominated by the surface freshwater flux explaining around 60% of MLS tendency variance. Here, the contribution of vertical diffusion due to the strong vertical salinity gradients and the effect of eddy advection compensate each other.