The Somali Current is a cold ocean boundary current that runs along the coast of Somalia and Oman in the Western Indian Ocean and is analogous to the Gulf Stream in the Atlantic Ocean. This current is heavily influenced by the monsoons and is the only major upwelling system that occurs on a western boundary of an ocean. The water that is upwelled by the current merges with another upwelling system, creating one of the most productive ecosystems in the ocean. The Somali current is characterized by seasonal changes influenced by the Southwest monsoon and the Northeast Monsoon. During the months of June to September, the warm Southwest monsoon moves the coastal waters northeastward, creating coastal upwelling. The upwelled water is carried offshore by Ekman transport and merges with water that was brought to the surface by open-ocean upwelling. The Somali Jet, a.k.a. the Findlater Jet, a narrow low-level, atmospheric jet, also develops during the Southwest monsoon, and blows diagonally across the Indian Ocean, parallel to the coasts of Somalia and Oman. As a result, an Ekman transport is created to the right of the wind. At the center of the jet, the transport is maximum and decreases to the right and left with increasing distance. To the left of the jet center, there is less water movement toward the center than is leaving, creating a divergence in the upper layer and resulting in an upwelling event (Ekman suction). In contrast, to the right of the center of the jet, more water is coming from the center than is leaving, creating a downwelling event (Ekman pumping). This open-ocean upwelling in combination with the coastal upwelling causes a massive upwelling.
History The history of physical oceanographic approaches to the Somali current has begun from the mid-1960s with serious interests. From the mid-1960s until the late 1970s several magnificent theoretical studies had been proposed and gave physical answers of the current behaviors and formations. After the late 1970s, the physics of the Somali current enhanced by ocean data analyses with outstanding field measurements of the current properties. The research footprints during the early 1960s to the late 1970s are presented as below. (early research histories before 1981) 1966 Warren et al. : Oceanographers and Meteorologists agreed the existence of the Somali current and its behaviors, but its exact processes and involved nature sources had not been clearly understood. 1969 Lighthill : the source of the Somali current is mass flux deposited by baroclinic and barotropic waves in the western boundary region. 1970 Düing : presented the presence of alternative cyclonic and anti-cyclonic gyre, the Indian Ocean Gyres. Found eddy size of the Indian Ocean Gyres are much larger than the gyres of the other mid-latitude western boundary currents (the Indian Ocean Gyres ~ 300-500 nmi > the Gulf Stream / Kuroshio ~ 50 – 100 nmi) 1971 Düing & Szekelda : the baroclinic mode is the dominant mode of the Somali current 1972,1973 Leetma : local winds are crucial to the onset of the Somali Current. 1975 Colborn : climatological analysis of the entire Indian Ocean from bathythermograph and hydrocast observations 1976 Hurlburt & Thompson : Characterize the Somali current as a time-dependent, baroclinic, inertial boundary current. 1976 Bruce : time-series analyses, XBT cross-section measurement by the EXXON tankers 1979 USNS WILKES : the Great Whirl (Prime Eddy) and the Socotra Eddy together with the strong shear zone along the eastern edge of the Great Whirl were observed during late August and early September 1979.
Properties The Somali Current is rapid response, shallow and shifts its direction seasonally. Especially from 5°N and southward, the Somali current is extremely shallow (under 150m depth southward flow all year through). Further north, the jet deepens somewhat, reaching the permanent thermocline. The structure of the current around the equator is extremely complex and similarly layered to the equatorial flows, though the Somali Current is oriented north-southward instead of east-westward. The typical water volume transported by the Somali Current amounts to 37 ± 5 Sv (0.037 ± 5 km3/s) during mid-September, making the current circulation generally weaker than other mid-latitude western boundary currents (such as the Gulf Stream, the Kuroshio current). However, volume transport maxima as high as 60-70 Sv (comparable to the Gulf Stream) have been measured around the south of Socotra Island.
The Somali Current reaches speeds of up to 7 knots during the summer months weakening and eventually reversing direction during the winter.
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