Twin tropical cyclones in the Indian Ocean: the role of equatorial waves

Abstract

This study explores the background atmospheric conditions that led to the development of recent two sets of twin tropical cyclones in the Indian Ocean. One set of twins formed in late December 2011 (“pair A”) and other in May 2013 (“pair B”). An extensive area of active convection prevailing over wide-equatorial region and low-level westerly wind over near-equatorial region is major precursors to the two cases of twins. Convection and accompanying westerlies slowly strengthen, and the two sets of twins tend to emerge within it. The atmospheric equatorial convectively coupled Kelvin wave (CCKW) and convectively coupled equatorial Rossby (CCER) wave played key role in the formation of the two twins’ events. Wave-enhanced equatorial convection and low-level westerly anomalies produced by the combination of CCKW and CCER wave are crucial for the development of “pair A” twins. CCKW prior to “pair B” twins produced convection and westerly anomalies over equatorial region. The equatorial westerlies are further modulated by CCER wave favoring twins of “pair B.” Moreover, convection reinforced by CCER wave in OLR aided occurrence of Northern Hemispheric cyclone of “pair A” and Southern Hemispheric cyclone of “pair B.” While the CCKW and CCER wave appear to be associated with the formation of the two twin cyclone cases, more thorough understanding of the mechanisms involved needs considerable attention in forthcoming research.