Wake Vortex Formation and Tropical Cyclogenesis West of Sumatra During DYNAMO

The anomalously strong easterly winds that precede the active phase of the Madden-Julian Oscillation (MJO) may lead to counter-rotating wake vortices and tropical cyclones (TCs) forming downstream of Sumatra. Any tropical cyclone, including those that may be linked to strong, MJO-related easterly winds, constitutes a threat to shipping, property, economic wellbeing, and lives. Meanwhile, the Madden-Julian Oscillation is the leading source of intraseasonal variability on a global scale and has been connected to many high-impact weather phenomena around the world. This presentation explores the contribution of wake vortices to TC formation in the Indian Ocean, and the multi-scale interaction of those wake vortices or their ensuing tropical cyclones and the MJO itself.

When easterly winds impinge upon the mountainous island of Sumatra and conditions are favorable, flow splitting occurs. This movement around the tips of Sumatra generates counter-rotating lee vortices downstream that feature low central pressures and geopotential heights. Because Sumatra straddles the equator, one wake vortex forms in the northern hemisphere, and one in the southern hemisphere. Though the lee vortices rotate in opposite directions, they are both cyclonic.

During the Dynamics of the Madden-Julian Oscillation (DYNAMO) field campaign in 2011, cyclonic wake vortices were observed downstream of Sumatra on either side of the equator during periods of easterly flow that occurred prior to the passage of the MJO's convective component over Sumatra. During DYNAMO, two cases of tropical cyclogenesis were also observed west of Sumatra following easterly flow, producing Severe Tropical Storm Alenga in the Southern Hemisphere and Tropical Storm 05A in the Northern Hemisphere. Tropical Storm 05A propagated westwards, towards the eastward-propagating active phase of the MJO, and damaged thousands of homes and caused 19 deaths in Sri Lanka. Analyses of the conditions leading to tropical cyclogenesis in these cases, including the contribution of cyclonic lee vortices formed off either end of Sumatra during easterly flow, and of the relationship between these tropical cyclones once formed and the active phase of the MJO, will be discussed.

Understanding how easterly winds associated with the MJO contribute to tropical cyclogenesis in the Indian Ocean may enhance disaster preparedness and improve TC prediction in the region. Furthermore, investigating the influence of nearby TCs upon the mature phase of the MJO or vice versa may give insight into the behavior of the Madden-Julian Oscillation.