Throughout much of the experiment, the low-level flow over the Indian Ocean was westerly, consistent with the seasonal-mean flow during this period. However, during the development of the MJO active phase over the Indian Ocean, the flow over Sumatra and the surrounding area was easterly such that the eastern portion of the equatorial Indian Ocean was downstream of the island. Sumatra is a long, narrow island, oriented northwest to southeast, extending from approximately 5 N to 6 S, and has mountainous terrain along its western edge rising locally to above 3 km.
During the period of easterly flow, wake vortices occasionally developed at the northern and southern ends of Sumatra as a result of flow splitting by the island barrier. The vortices at both ends are counter-rotating, but because the island straddles the equator, a rather unique situation exists that the circulations are cyclonic in both hemispheres. On a several occasions, these vortices moved westward over the Indian Ocean and intensified as deep convection was incorporated into their circulations. Once such instance in late November led to tropical cyclogenesis as the system passed across the northern sounding array. Strong southwesterly flow occurred at the R/V Revelle on 24 November during its passage followed by the eventual development of Tropical Storm 05A over the Arabian Sea. Another case in early December, this time associated with a wake vortex to the lee of the southern tip of Sumatra, led to the formation of Tropical Storm Alenga over the southern Indian Ocean. Analyses of these events will be presented.
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