Western boundary circulation driven by an alongshore wind: With application to the Somali Current system
Abstract:The linear, continuously stratified, eastern-boundary model of McCreary (1981) is extended to apply to a western ocean boundary and to wind fields with curl. The model has vertical and horizontal mixing, and both types of mixing are important in its dynamics. Solutions are represented as expansions in vertical modes, and the dynamics of low-order and high-order modes are very different. Low-order modes tend to be in Sverdrup balance in the interior ocean, and to form a Munk layer at the coast. High-order modes are in Ekman balance in the interior ocean, and the coastal balance is two dimensional.
The model is forced by steady northward winds, both with and without curl. Solutions at the western boundary differ fundamentally from comparable ones at the eastern boundary. For winds without curl, the surface jet is stronger, and there is essentially no coastal undercurrent. This difference is due to the fact that at an eastern boundary the currents associated with low-order modes leak offshore. For winds with curl, a sizeable undercurrent develops, but only south of the region of the wind. A strong onshore current, located near the southern edge of the wind, is generated by offshore wind curl. Part of this current turns southward at the coast, thereby generating the undercurrent in the south.
The existence of this undercurrent is in accord with observations off Somalia, where a southward undercurrent has been observed at 5N during the Southwest Monsoon. The wind at this time is oriented alongshore, reaches maximum strength well to the north of 5N, and is associated with a region of large, negative wind curl off the coast.
Document Type: Research Article
Publication date: 1985-08-01
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