Interannual variability in the Somali Current 1954–1976

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Somali current Arabian Sea Indian Ocean

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Beginning with the pioneering efforts of Cox 6, 7, 8, numerous modelling efforts have sought to explain the observed flows in the tropical Indian Ocean, with particular attention given to the semi-annual reversals in the Somali Current along the east coast of Africa (e.g. 15, 24, 25, 26, 27, 28, 29, 30, 31, see 20, 25for a review). While the Somali Current is similar to mid-latitude western boundary currents in some respects, it is unique in many others. The most striking feature of this current is its reversal with the changing monsoon winds (see 9, 20, 33). During boreal summer, the boundary current flows towards the north and northeast from the coast of Mozambique (11°S) to the island of Socotra at 12°N, driven by the southwest monsoon winds. A “two gyre” system is often observed in this current [36], with a southern gyre that straddles the equator, flowing offshore at 3–4°N, and a northern gyre, called the great whirl, between 5°N and 9°N. Wedge-shaped areas of cold, upwelled water are found along the coast to the north of these gyres. Late in the summer, the southern gyre and its cold wedge migrate rapidly northward and coalesce with the great whirl 3, 11, 37. This appears to be the case in most years; however, there is evidence of some years when the two gyre system does not form [36]. The summer Somali Current to the north of 2–3°S gradually breaks up during the fall transition period, and is replaced by the southwestward winter Somali Current with the onset of the northeast monsoon in December. The boundary current to the south of 3°S flows to the north throughout the year and is called the East African Coastal Current (EACC). During the summer, the EACC feeds the northward Somali Current; during winter, it meets the southwestward Somali Current and both flow offshore into the South Equatorial Counter Current (SECC). Leetmaa 21, 22concludes that the early reversal of the Somali Current south of the equator with the onset of the southwest monsoon is due to both local wind forcing and an inertial overshoot (or switching action) in the EACC. Woodberry et al. [40]show that this region of the EACC is a tropical analog to a mid-latitude western boundary current recirculation region that is strongly modified by the presence of the equatorial wave guide and by the seasonal reversals in the wind. It closes the circulation in a tropical Sverdrup-like gyre in the southern hemisphere, consisting of the eastward SECC that meanders between the equator and 8°S and the westward South Equatorial Current (SEC) between 10 and 20°S. We use “Sverdrup-like” to describe this gyre because it is far from steady state; indeed, it has large seasonal variability 34, 38, 40.

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Nonlinear Analysis: Theory, Methods & Applications, v. 35, issue 1, p. 59-83