OC4331-Mesoscale Oceanography
Final Project Summary

Topic Area

Somali Current Eddies

Project Team Member(s)

LT Sean Robinson, USN

Major Findings

The Somali Current (SC) is a unique western boundary current as the surface current reverses direction during the year.  The northeast monsoon (weaker monsoon) is responsible for the Laccadive High (LH), however, the southwest monsoon (stronger monsoon) is responsible for three separate anticyclonic eddy events located off the Somali Coast (western Arabian Sea):

·        Southern Eddy

·        Prime Eddy

·        Socotra Eddy

The Southern Eddy (SE) forms south of 50 N from a retroflection of the northward flowing Zanzibar current.  This eddy consists of fresher water and is rather large at 400-600 km in diameter.  The SE is not always present, but historical evidence was found 37 out of 63 years (1906-1968).  Located between the SE and its northern cousin, the Prime Eddy, is a large wedge of cold water from the intense coastal upwelling associated with the southwestern SC.

The Prime Eddy (PE) forms at ~80 N and is developed from horizontal shear instability (Bruce) or from barotropic instability (Jensen).  PE is also rather large (400-600 km in diameter) and like the SE remains stationary until mid-August.  Then the SE will migrate north to join the PE and form the “Great Whirl”.  The Great Whirl is a large anticyclonic eddy (600-800 km in diameter) with a large mass transport (58 Sv) that causes the constant southward SC undercurrent to temporally disappear.  The Great Whirl is responsible for the recirculation of the increased transport associated with the southwest SC and can survive for 3 months after the onset of the northeast monsoon.

The Socotra Eddy does not develop until late summer and is the last eddy to form in the SC system.  Located east of Socotra Island, this third eddy is north of the Great Whirl and can have a diameter of 400 km with a cross-section mass transport of 23 Sv.

The Laccadive High is a large (500-800 km diameter) seasonal anticyclonic eddy formation found in the upper 300-400 m of the eastern Arabian Sea during the northeast monsoon.  In some ways it is a mirror like counterpart to the Great Whirl, which develops during the southwest monsoon off the Somali coast.  The LH occurs at the same latitude but on the opposite side of the basin (located east of the Laccadive Islands) during the reversed monsoon.  It is different from the Great Whirl, however, in its formation process, its intensity, and its decay.


References

Bruce, J.G., Eddies off the Somali coast during the southwest monsoon, J. Geophys. Res., 84, 7742-7748, 1979.

Bruce, J.G., D. R. Johnson, J. C. Kindle, Evidence for eddy formation in the eastern Arabian Sea during the northeast monsoon, J. Geophys. Res., 99(C4), 7651-7664, 1994.

Fischer, J., F. Schott, and L. Stramma, Currents and transports of the Great Whirl – Socotra Gyre system during the summer monsoon, J. Geophys. Res., 101(C2), 3573-3587, 1996.

Jensen, T.G., Modeling the seasonal undercurrents in the Somali current system., J. Geophys. Res., 96(C12), 22151-22167, 1991.

Swallow, J.C., and M. Fieux, Historical evidence for two gyres in the Somali Current, J. Mar. Res., 40 (suppl.), 747-755, 1982.

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