Somali Current
[3] (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.
[4] 1969 Lighthill : the source of the Somali current is mass flux deposited by baroclinic and barotropic waves in the western boundary region.
[3] The Somali Current reaches speeds of up to 7 knots during the summer months weakening and eventually reversing direction during the winter.
Before the onset of the monsoon (March–May): During this season, shallow northward coastal current flows with 50–100 km width near the Somali coast, overlying a southward undercurrent.
The upper layer Somali Current flows northwards along the East African coast, and finally enters the Gulf of Aden between the Socotra Archipelago and the Horn of Africa.
Finally, in early December, beginning south of 5°N and expanding rapidly to 10°N by January, the current reverses its direction, now flowing southwardly (velocities of 0.7 – 1.0 m/s).
[14] During the winter monsoon, after crossing the equator, the southward Somali Current comes into confluence with the northward EACC and thereafter flows to the East.
The Great Whirl can be observed between 5-10°N and 52-57°E off the Somali coast in the summer season, a location typically around 200 km southwest of the Socotra Gyre (between June and September).
[17] How the Great Whirl forms exactly isn't yet fully understood, but the analytical approach of applying Rossby wave theory can explain its basic formation mechanism.
By observation and analysis of [Schott and Quadfasel (1982)], the summer monsoon develops suddenly during June–July, and drives westward water flow in the location.
Schott and Quadfasel applied first-mode Rossby waves to the water flows, and concluded that "the formation of the Great Whirl is a response to the very strong anti-cyclonic wind-stress curl".
As the Somali Current developed through the summer, it was only in September that the Great Whirl finally maximized and began to disperse, entering the winter season (the wane).
[18][19] Conversely, the Great Whirl may also continue to exist until mid-October, still exhibiting large size and its curl structure may remain evident even longer, underneath the winter Somali Current.
Not only does the biological productivity of the region depend on these upwelling currents, they (and thus, the Great Whirl itself) also play a large part in regulating the heat flux budget of the North Indian Ocean.
Especially during the southwest Indian monsoon, strong upwelling pumps cold (17-22 °C) and highly nutrient rich (about 5 to 20 μm of nitrate [21]) subsurface water to the coastal region.
[22][23] However, the fast-moving Somali Current reduces the residence time of the upwelled nutrient-rich water, thus limiting biological productivity in this region somewhat.
