Seasonality of the Somali Current/Undercurrent system

The connected western boundary current system in the Arabian Sea plays an important role in the exchange of heat, mass, and freshwater and their distribution. Nonetheless, the dynamics and time scales of the processes and characteristics of this boundary current are not well understood because most prior measurements were only for a limited period or only resolved the surface circulation. Seasonality of the Somali Current (SC)/Undercurrent (SUC) system is investigated using a multi-decadal time series of ocean temperature and salinity profiles. The SC is driven principally by the seasonal monsoon winds, flowing southwestward in the winter monsoon (January-March), then reverses to northeastward in the summer monsoon (June-September). The subsurface southwestward SUC is found during the transition seasons in April-June and again in September-November. A vertical normal mode decomposition shows that the surface SC is mainly controlled by the seasonally varying first baroclinic mode and the barotropic component (a combined 86.5% of the total variance), while the SUC variations can be explained by the superimposition of the first and second baroclinic modes (a combined 75.9% of the total variance). Here we show that the occurrence of the countercurrent is not only due to the baroclinic adjustment caused by the wind forcing, but also a response to the complex salinity structure caused by the intrusion of high salinity water masses. Specifically, the presence of the salty water masses from the northern Arabian Sea during the boreal summer monsoon inhibits the southwestward flowing SUC and results in a baroclinic structure that supports a subsurface northeastward flow, aligned with the surface current. This suggests that salinity dynamics play a fundamental role in the circulation of the boundary current system of the Arabian Sea.

Automated summary

The connected western boundary current system in the Arabian Sea plays a crucial role in the exchange of heat, mass, and freshwater. The seasonality of the Somali Current/Undercurrent system is driven by seasonal monsoon winds, with the presence of high salinity water masses affecting the flow dynamics. Salinity dynamics play a fundamental role in the circulation of the Arabian Sea boundary current system, influencing the behavior of both the Somali Current and Undercurrent.