Merging Process of the Great Whirl and the Socotra Gyre in 2019

The Great Whirl (GW) and the Socotra Gyre (SoG), two prominent anticyclonic eddies in the western Arabian Sea, exhibit strong dynamic interactions. This study reports a case of the merging of the GW and the SoG recorded by Argo floats in September 2019. Combined with satellite observations and a state-of-the-art ocean reanalysis, we show that the merging process was first detected at the subsurface layer (similar to 150 m depth) rather than the surface. As the original water inside the GW is cooler than the SoG, the merging created a baroclinic structure between the eddies. The density gradients associated with the baroclinic structure drive strong subsurface geostrophic currents following the thermal wind relationship, leading to the fast merging at 100-200 m depth. Energy analysis shows that the predominant energy source for the merged eddy was the barotropic and baroclinic instability. The dissipative processes caused the rapid decay of the merged eddy. The merging process induced submesoscale activities and promoted ocean vertical exchanges south of Socotra Island. The Great Whirl (GW) and the Socotra Gyre (SoG) are two large clockwise eddies in the western Arabian Sea. Ocean observations caught the merging process of these two eddies in September 2019. The Argo floats detected the process first at the subsurface layer. Ocean reanalysis reveals that the water inside the GW was cooler than the SoG, which led to the density difference that enhanced the subsurface flow. The subsurface flow transported the warm and salty water from the SoG faster than the surface in the interior of the eddy. During the merging process, the smaller-scale motions led to increased surface chlorophyll concentrations in the region, with replenished nutrients and dissolved oxygen. The merging process of the Great Whirl (GW) and the Socotra Gyre (SoG) was faster at the subsurface, resulting from density gradientsThe barotropic and baroclinic instability provided the predominant energy for the mergingThe merging process stimulated submesoscale activities and promoted ocean vertical exchanges

Automated summary

This study reports the merging process of the Great Whirl (GW) and the Socotra Gyre (SoG) in the Arabian Sea in September 2019. The merging was first detected at the subsurface layer rather than the surface, and it occurred rapidly at 100-200 m depth. The predominating energy sources for the merged eddy were barotropic and baroclinic instability. The merging process stimulated submesoscale activities and promoted ocean vertical exchanges.