Formation of Tropical Cyclones in the Northern Indian Ocean Associated with Two Types of Tropical Intraseasonal Oscillation Modes

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Over the northern Indian Ocean (NIO), a substantial number (∼60%) of tropical cyclones (TCs) form in association with significant intraseasonal oscillation (ISO) events (i.e., Nargis [2008]). In this paper the relationship between TC genesis and ISO in the NIO was studied using 30-year (1997-2008) observations. Because NIO TCs mainly occur in transitional seasons when climatological environmental forcing favors TC genesis, two types of ISO modes, boreal summer intraseasonal oscillation (BSISO) and Madden-Julian oscillation (MJO), which represents boreal winter ISO, were objectively and quantitatively defined, and their connection with TC genesis was examined. It was found that over 70% of ISO-related genesis is associated with the northward propagating BSISO mode and up to 30% with the eastward propagating MJO mode. The BSISO mode primarily affects TC formation in May-June and September-November, while the MJO mode affects TC formation primarily from November-December. Because of their distinct structures and lifecycles, the BSISO and MJO modes affect TC formation differently. For the BSISO mode, TC formation is enhanced during its wet phases overlaying the NIO. For the MJO mode, TC formation is enhanced after the convection passes over the Malay Peninsula and when the Indian Ocean is in a dry phase. The BSISO mode enhances TC genesis by creating favorable environmental forcing for TC genesis, while the MJO mode does not. The most salient feature is that both the ISO modes favor TC genesis by providing a synoptic-scale seeding disturbance at least six days prior to TC formation. The seeding disturbance provided by the BSISO is a cyclonic vorticity anomaly to the north of the equatorial convection/westerly wind burst, whereas the seeding provided by the MJO is a convectively coupled Rossby wave that breaks away from the major body of the MJO convection. The seasonality of the NIO TC genesis, intensity, and prevailing tracks are also explained in terms of the effect of environmental forcing on TC genesis potential, steering flow, and maximum potential intensity. The results imply that monitoring the evolution of the two types of ISO modes, especially the BSISO, may provide a useful medium-range forecast for NIO cyclogenesis.