Sumatra Lee Vortices and Tropical Cyclogenesis in the Indian Ocean

The role of the island of Sumatra in tropical cyclone (TC) genesis over 
the Indian Ocean is investigated using comprehensive datasets from two recent tropical campaigns, the 2008-10 Year of Tropical Convection 
(YOTC) and the 2011 Dynamics of the Madden-Julian Oscillation or MJO 
(DYNAMO). Sumatra is a long, mountainous island straddling the equator, extending in a northwest-to-southeast direction from approximately 6°N to 6°S. Under typical environmental conditions, flow is partially blocked by the island. When 
the low-level flow is easterly, counter-rotating lee vortices are often produced 
over the Indian Ocean. Because the island spans the equator, both wake vortices are cyclonic. The YOTC and DYNAMO data sets reveal these cyclonic 
circulations downstream of Sumatra over the Indian Ocean on numerous occasions, most commonly during October through December. Once shed, the 
vortices move westward both north and south of the equator. During the 2.25 years covering the two campaigns, twelve wake vortices (~20% of the shed circulations identified) were tracked and observed to subsequently develop into TCs 
over the northern and southern Indian Ocean. In several cases, TC genesis 
was assisted by developing MJOs over the central Indian Ocean.