Spanning across the equator with a northwest-southeast orientation, the island of Sumatra can exert significant influences on low-level flow. Under northeasterly flow, in particular, lee vortices can form and some of them may subsequently develop into tropical cyclones (TCs) in the Indian Ocean (IO). Building upon the recent work of Fine et al., this study investigates the roles of the Sumatra topography and other common features on the formation of selected cases for analysis and numerical experiments. Four cases in northern IO were selected for analysis and two of them [Nisha (2008) and Ward 2009)] for simulation at a grid size of 4 km. Sensitivity tests without the Sumatra topography were also performed. Our results indicate that during the lee stage, most pre-TC vortices tend to be stronger with a clearer circulation when the topography is present. However, the island's terrain is a helpful but not a deciding factor in TC formation. Specifically, the vortices in the no-terrain tests also reach TC status, but just at a later time. Some common ingredients contributing to a favorable environment for TC genesis are identified. They include northeasterly winds near northern Sumatra, westerly wind bursts along the equator, and migratory disturbances (TC remnants or Borneo vortices) to provide additional vorticity/moisture from the South China Sea. These factors also appear in most of the 22 vortices in northern IO during October-December in 2008 and 2009. For the sole case (Cleo) examined in southern IO, the deflection of equatorial westerlies into northwesterlies by Sumatra (on the windward side) is also helpful to TC formation.
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