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A Numerical Investigation of the Somali Current During the Southwest Monsoon

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A Numerical Investigation of the Somali Current During the Southwest Monsoon Journal of Marine Research, 46. 25-58, 1988 A numerical investigation of the Somali Current during the Southwest Monsoon by Julian P. McCreary, Jr.' and Pijush K. Kundu1 ABSTRACT The dynamics of the Somali Current system during the Southwest Monsoon are investigated using a 21/2-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 (-I 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. 1. Introduction In contrast to the other major western boundary currents, the Somali Current changes direction annually. It seems to respond to the seasonal changes in the wind I. Nova University Oceanographic Center, 8000 North Ocean Drive, Dania, Florida, 33004, U.s.A. 25 26 Journal of Marine Research [46, 1 field, being poleward during the summertime Southwest Monsoon and equatorward during the wintertime Northeast Monsoon. The development of the current through- out the Southwest Monsoon has been studied extensively, and involves the establish- ment and decay of intense gyres and wedges of cold sea-surface temperature (SST). The articles by Schott (1983), Knox and Anderson (1985), and Luther (1987) provide recent overviews of both observations and models. In late April or early May, at the beginning of the northern-hemisphere summer monsoon, the southern hemisphere tradewinds shift equatorward, and northward winds appear along the African coast near and south of the equator; a few weeks later, 2 moderate alongshore winds (-1 dyn/cm ) appear along the entire Somali Coast (Fig. la). In late April, a northward coastal current already exists south ofthe equator, and it bends offshore to form a meandering eastward flow located just south of the equator (Fig. 2a). By mid-May, the coastal current strengthens, crosses the equator, and turns offshore near 2.5N. Part of this current loops back across the equator (Fig. 2b; Leetmaa et al.. 1982; Swallow et al., 1983); we refer to this loop, and its later development, as the Southern Gyre. A wedge of cold SST, extending several hundred kilometers offshore and from 3N to 5N, forms on the northern flank of this gyre (Fig. 14 of Swallow et al., 1983). Farther to the north there is an alongshore current directed poleward, and coastal SST is cold but not wedge-like, as in a typical mid-latitude upwelling region (Schott, 1983). (a) 16 May (b) 16 July ,~.~ ,,~~..;. .;/~::"·f , , t , , t - ...•...• ...•.- ;' ~ "'" " }::'r-c-,-~--~__'\ '\ '\ , " - - '\ , , t f . /~-\"-\;..\__~ '-'Qs\ ' " •......•.." " , \ t f, I<:-"'_~ "-\ '\ '\ "--'-o.:-"'-' __ ~ __'\ , ~ t \\.~..~""~...~ ~.s..'''\:.'''~ " :-::_:':-.......•........'::-,_., t'., .:>", ". , '\ , ,~ .......•....-~..:::-~ . Figure 1. Observed wind stress fields from the FGGE Experiment of 1979, using a drag coefficient of 0.1875 x 10-3• The left panel (a) shows them on 16 May, and the right panel (b) 2 shows them on 16 July. The contour interval is 0.5 dyn/cm • In May the southern hemisphere trades migrate northward, and moderate alongshore winds appear everywhere along the African coast. In July the Findlater jet is well developed, resulting in a region of very large 2 wind stress values (~6 dyn/cm ) over the northern Arabian Sea. [After Luther et al., 1985.] 1988] ¥cCreary & Kundu: Model of the Somali Current 27 Cal 29 April- 5 May 1979 Cbl 20 May - 2 June (ell July - 4 Aug. Figure 2. Surface currents (arrows) and depths (m) of the 20°C isotherm in 1979 during (a) April 29-May 5, (b) May 2o-June 2, and (c) July I-Aug 4. There are indications of a weak eddy and a meandering eastward current just south of the equator in panel (a), a well-developed Southern Gyre in panel (b), and both the Southern Gyre and the Great Whirl in panel (c). [After Swallow et al., 1983.] In early June, a strong low-level atmospheric jet (the Findlater jet) appears off nothern Somalia. It strengthens throughout June, attaining speeds of the order of 2 15 mls (T-5 dyn/cm ) in July (Fig. Ib). By the end of June, a second oceanic gyre, called the Great Whirl, develops north of 5N (Fig. 2c) and another wedge of cold SST forms on its nothern flank near 9-10N. There is usually a third gyre even farther north near the island ofSocotra (Fig. 3), and we refer to this gyre as the Socotra Eddy. The winds off Somalia weaken in September, and the Southwest Monsoon is essentially over by October. In either August or September, the Southern Gyre and its associated cold wedge have been observed to migrate northward and to interact with the Great Whirl. In 1979, for example, satellite images of SST indicate that the two gyres coalesce (Fig. 4). Coalescence, however, does not always occur; in 1978 the Great Whirl appeared to be pushed out of the way to the north by the movement of the Southern Gyre (Fig. 9 of Schott, 1983). What aspects of the wind field over the Indian Ocean force this remarkable series of events? An obvious possibility is the local alongshore wind. Such a wind forces offshore Ekman drift, coastal upwelling and a poleward surface current. A number of studies have already examined the response of the Somali Current region to a forcing of this kind (Hurlburt and Thompson, 1976; Lin and Hurlburt, 1981; Cox, 1979, 1981; Philander and Delec1use, 1983; Delec1use and Philander, 1983; Luther and O'Brien, 1985; Luther et al.• 1985; McCreary and Kundu, 1985). A second possibility is remote forcing by features of the offshore wind field that excite baroclinic Rossby waves. These waves subsequently propagate to the coast of Africa, adjust the interior ocean toward Sverdrup balance, and thereby affect the Somali Current. One type of remote 28 Journal of Marine Research [46, 1 6-11 JUly, 1976 o 20'W 100 200 10' 300 400 500 m 2'$ 0 2 4' 6' 8' 10' 12' 14' 16' 18' 20' 22'N 50'E 60' Figure 3. Temperature section obtained during July 6-11, 1978, along the tanker lane shown in the right panel. The island of Socotra is also shown in the right panel. Note the Great Whirl between 5N-ION, and the smaller Socotra Eddy between lON-14N. The depth of the 20°C isotherm varies markedly from about 75 m at the edge of the Great Whirl to more than 200 m at its center. [After Bruce, 1979.] forcing was proposed by Lighthill (1969), who hypothesized that the onset of the Somali Current is forced entirely by remote wind curl, without any coastal wind; however, Leetmaa (1972, 1973) pointed out that this interesting hypothesis cannot be correct, noting that the Somali Current turns northward considerably before the remote winds strengthen. Another type of remote forcing, and the one that is addressed in this paper, involves wind curl associated with an alongshore wind that weakens offshore. A third possibility is forcing by local wind curl, which generates geostrophic July 1979 (mean) 17 Aug. 25 Aug. 3 Sep. Figure 4. Time sequence of satellite-observed temperature fronts observed during 1979. The northward propagation of the southern cold wedge and its eventual coalescence with the northern wedge are evident. [After Brown et al .• 1980.] 1988] McCreary & Kundu: Model of the Somali Current 29 currents via Ekman pumping. There is a region of very strong negative wind curl on the eastern side of the Findlater Jet just off the Somali Coast (Fig. 1b). This region drives an anticyclonic circulation, and several workers have suggested that the Great Whirl may be a directly forced response to this curl (Oiling, priv.
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