Abstract

The dynamics of the Somali Current system during the Southwest Monsoon are investigated using a 2½-layer numerical model that includes entrainment of cool water into the upper layer. Entrainment cools the upper layer, provides interfacial drag, and prevents the interface from surfacing in regions of strong coastal upwelling. Solutions are forced by a variety of wind stress fields in ocean basins with western boundaries oriented either meridionally or at a 45° angle. Solutions forced by southern hemisphere easterlies develop a strong coastal current south of the equator. When the western boundary is slanted, this current bends offshore at the equator and meanders back into the ocean interior. No cold wedge forms on the Somali Coast. These solutions suggest that the southern hemisphere trades are not an important forcing mechanism of the Somali Current circulation. Solutions forced by northward alongshore winds differ considerably depending on the orientation of the western boundary and the horizontal structure of the wind. When the boundary is meridional and the wind is uniform (a curl-free wind field), solutions continuously shed eddies which propagate northward along the coast and weaken. When the boundary is meridional and the wind weakens offshore, they reach a completely steady, eddy-free state with no coastal upwelling. If the boundary is slanted and the wind does not vary alongshore, solutions reach a steady state that now contains stationary gyres and cold wedges. If the boundary is slanted and the forcing is a strong wind patch confined north of the equator, the flow field slowly vacillates between single-gyre and double-gyre states. Solutions are also forced by an idealized representation of the observed alongshore wind field, consisting of two components: a moderate background field (∼1 dyn/cm2) turned on in May, and a Findlater jet (∼4 dyn/cm2) turned on gradually in June. A single gyre, the Southern Gyre, initially develops south of 4N due to the background wind, and a second gyre, the Great Whirl, develops later between 4N–9N in response to the Findlater jet. Cold wedges form on the northern flanks of both gyres. In some of the solutions, the Southern Gyre moves northward and coalesces with the Great Whirl in early September, before the monsoon begins to weaken. Thus the collapse of the two-gyre system is part of the adjustment of the model to the peak phase of the Southwest Monsoon, and is not due to a relaxation of the wind.

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