Effects of the topography of Sumatra on tropical cyclone formation over the Indian Ocean

One-quarter of the world’s tropical cyclones (TCs) occur in the Indian Ocean (IO) basin. The mechanisms for TC initiation in the IO are varied, but one recently discovered process involves the flow around the steep topography of Sumatra. When the low-level flow impinges on Sumatra, it is partially blocked under typical environmental stratification. Easterly low-level flow, which is commonplace over northern Sumatra during boreal winter and southern Sumatra during boreal summer, frequently generates wake vortices at the northern and southern ends of the island that shed and move downstream over the IO. The wake vortices originating from the island tips are counter-rotating, but since Sumatra straddles the equator, the circulations are cyclonic in both hemispheres and thus have the potential for TC development. Using data from 2.5 years of observations from DYNAMO and YOTC, over three hundred wake vortices originating from Sumatra were tracked, one-third of which shed and moved away from the island over the IO. Thirteen of these vortices became TCs, constituting 25% of the TCs that occurred over IO basin during the 2.5-year period. A more recent analysis for a longer period (2008-17) has shown that vortex counts at the north and south ends of Sumatra are highest during the initiation phase of the Madden-Julian Oscillation (MJO) when low-level easterly flow maximizes at these locations. A secondary peak in vortex formation occurs during the MJO active phase when low-level westerlies exist near the equator west of Sumatra. The latter finding suggests that MJO-related, low-level westerly surges on the equator impinging on Sumatra contribute to an increase in wake vortex development. Numerical simulations have shown that circulations farther to the east, such as western Pacific remnant TCs and the Borneo vortex, can influence the development of Sumatra wake vortices and their growth into TCs over the IO.