OC4331-Mesoscale
Oceanography
Final Project Summary
Topic Area
Cdr Manoj Kumar Singh, India
Major Findings
North Western Indian Ocean is a unique area bounded by continent from three sides having marginal seas and gulfs. The area experiences intense annually reversing monsoon winds. The Somali Current (SC) is a unique western boundary current with reversing surface current direction during the year. The northeast monsoon (during winter) is responsible for the Laccadive High (LH), however, the southwest monsoon is responsible for three separate anticyclonic eddies located off the Somali Coast (Western Arabian Sea):-
The Southern Gyre (SG) forms around 50 N from a retroflection of the northward flowing Zanzibar current in June. This eddy consists of fresher water and is rather large at 500-600 km in diameter and remains stationery until Mid-August. The SG is not always present, but historical evidence was found 21 out of 23 (1954-1976). Between SG and Great Whirl, a large wedge of cold water flows from the intense coastal upwelling associated with the southwestern SC.
The Great Whirl (GW) forms between 9-100 N during July and is developed from horizontal shear instability (Bruce) or from barotropic instability (Jensen). GW also remains stationary until mid-August like the SG. Then the SG migrates north to join the GW. 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 Socotra Eddy (SE) does not develop until late SW monsoon 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 200 km with a cross-section mass transport of 23 Sv.
During NE monsoon season, the Laccadive High (LH) eddy forms in the eastern Arabian Sea. LH is a large (~500 km diameter) seasonal anticyclonic eddy found in the upper 300-400 m.
In addition, number of eddies are seen in the Red Sea, Gulf of Aden and Gulf of Oman varying in location, size, number and dynamics.
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