Evolution of Thermal Fronts in the Somali Current System

Satellite observations have shown that there is significant covariability between sea surface temperature (SST) and wind stress in the presence of fronts. The marine atmospheric boundary layer is more stable over the cool side and more unstable over the warm side. The surface wind is reduced over the cold water where the surface is decoupled from stronger upper layer winds. In contrast, the decreased stability over the warm side draws momentum from upper layer winds and increases the wind speed.

The western Arabian Sea off the coast of Africa is characterized by an annually reversing winds and currents. The annual Rossby wave reaches the coast in spring and sets up the northward flowing Somali Current (SC) prior to the arrival of the summer southwest monsoon winds. Part of the East African Coastal Current flows northward along the African coast to feed the SC. Some of this flow may retroflect from the coast at about 4°N to form the Southern Wedge/Gyre (SWG) while the remaining flow continues north past the Gulf of Aden or detaches at about 10°N to form the Great Whirl (GW). The sizes and locations of these two gyres vary interannually and intraseasonally as the frontal positions evolve and translate poleward. The geometry of this region is unique with respect to the steady summer winds that flow parallel to the SC and perpendicular to the GW and SWG thermal fronts.

In this study, we use winds from the ASCAT scatterometer with a 12.5 km resolution and SST data from AMSR2 with a 25 km resolution to study the evolution of the covariability between the SST and wind stress during the 2012 – 2019 Indian summer monsoons. Wind stress divergence computed from the scatterometer data clearly show the feedback of the thermal fronts on the wind crossing the SWG and the GW. Between the SWG and GW, the wind stress curl from the wind along the SC shows some influence from the current, however inconsistent data quality limits the analysis near the coast. The data show that the “classic” view of the annual appearance and dissipation of the GW and SWG vary interannually and intraseasonally. In the northern hemisphere, the GW may appear before, after or simultaneously with the SWG. The two gyres frequently appear quasi-stationary, but also migrate north where the SWG either dissipates, or merges with the GW.