A numerical study of the Somali coastal undercurrents

Abstract

Abstract Subsurface circulation in the western Arabian Sea is studied with an open boundary version of the Miami Isopycnic Coordinate Ocean Model (MICOM). The model solution demonstrates a strong annual cycle and significant alongshore variability of subsurface circulation. Based on the dynamics and water properties, three regions are identified along the coast. A cross-equatorial current, which exists throughout the year, carries low-salinity water northwards. Comparison of the model results with observations in the equatorial region demonstrates that the model reproduces the annual cycle and transport of the currents remarkably well. Although it underestimates the speed of the undercurrent core by about a factor of two, increasing the horizontal resolution from 0.35° to 0.225° improves agreement with the measurements. A spring southward undercurrent between 5°N and the equator owes its existence to the wind forcing in the Arabian Sea. Water with higher salinity values, found in the coastal region north of 5°N, is advected by a southward undercurrent that is present between October and March. The existence of the undercurrent is caused by flows from the east and northeast. The latter originates in the Persian Gulf and provides about 75% of water for the coastal undercurrent. The annual Rossby wave generated in the interior of the domain contributes to the formation of the current in the fall. The third region is an area near 4°N, where the southward undercurrent separates, as velocity and salinity fields suggest. Subsurface circulation north of 5°N is disconnected from flows near the equator during most of the year. The model circulation is not sensitive to the details of coastal bottom topography. In contrast, the presence of the Socotra Island, which is absent in the model, leads to a more realistic solution in that the southward undercurrent north of 5°N is present throughout the spring. Interannual variability of the model subsurface fields increases significantly when observed, rather than climatological, wind forcing is used. The most dramatic changes occur in the coastal and equatorial regions.