Formation of tropical cyclones in the northern Indian Ocean associated with two types of tropical intraseasonal oscillation modes

Over the northern Indian Ocean (NIO), a substantial part (~60%) of tropical cyclone (TC) forms in association with significant intraseasonal oscillation (ISO) events; Nargis (2008) is a typical example. Here the relationship between TC genesis and ISO in the NIO is studied by using 30-year (1997-2008) observations. Since NIO TCs mainly occur in transitional seasons when the climatological environmental forcing is preferable to TC genesis, two types of ISO modes, boreal summer intraseasonal oscillation (BSISO) and Madden-Julian oscillation (MJO) that represents boreal winter ISO, were objectively and quantitatively defined, and their connection with TC genesis was examined. It is found that Over 70% of the ISO-related genesis is associated with northward propagating BSISO mode and up to 30% with eastward propagating MJO mode. BSISO mode primarily affects TC formation in May-Jun and September to November, while MJO mode affects TC formation primarily from November to December. Due to their distinct structure and lifecycle, the BSISO and MJO modes affect TC formation differently. For BSISO mode, TC formation is enhanced during its wet phases overlaying the NIO. For MJO mode, TC formation is enhanced after the convection passes over the Malay Peninsula and the Indian Ocean is in a dry phase. The BSISO mode enhanced TC genesis through creating favorable environmental forcing for TC genesis, while the MJO mode does not. The most salient feature is that both ISO modes favor TC genesis through providing a synoptic-scale, seeding disturbance at least six days prior to the TC formation. The seeding disturbance provided by the BSISO is a cyclonic vorticity anomaly to the north of equatorial convection/westerly wind burst, whereas the seeding provided by the MJO is a convectively coupled Rossby wave escaped 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 environmental forcing determining TC genesis potential, steering flow and maximum potential intensity. The result here implies that monitoring evolution of the two types of ISO modes, especially BSISO, may provide a useful medium-range forecast for NIO cyclogenesis.