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

Abstract

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 a¤ects TC formation in May-June and September-November, while the MJO mode a¤ects TC formation primarily from November-December. Because of their distinct structures and lifecycles, the BSISO and MJO modes a¤ect TC formation di¤erently. 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 e¤ect 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.

Real-time multivariate indices for the boreal summer intraseasonal oscillation over the Asian summer monsoon region

“…As will be shown, BSISO1 represents the canonical northward propagating BSISO over the ASM region addressed in many previous studies with a 30-60 day quasi-oscillating period (e.g., Annamalai and Sperber 2005;Wang et al 2005;Kikuchi et al 2012). BSISO2, on the other hand, mainly captures the northward and northwestward propagating BSISO with periods of both around 30 days and 10-20 days (e.g., Kikuchi and Wang 2010).…”

“…It is also a possible source of seasonal climate predictability for precipitation (Wang et al 2009a;Lee et al 2010) and extratropical atmospheric circulation (Ding and Wang 2005;Lee et al 2011;Wang et al 2012). Two different periodicities of the BSISO have been identified: periods of 30-60 days (e.g., Wang et al 2005) and 10-20 days (e.g., Kikuchi and Wang 2010). The wet and dry spells of the BSISO strongly influence extreme hydro-meteorological events, major driving forces of natural disasters, and thus the socio-economic activities in the World's most populous monsoon region (Lau and Waliser 2005).…”

Seasonal Environmental Characteristics for the Tropical Cyclone Genesis in the Indian Ocean during the CINDY2011/DYNAMO Field Experiment

“…The temporal and horizontal resolutions of this dataset, which is available from 1979, are 6 h and 1.25 degree, respectively. The JRA-25 dataset has been widely used in investigating the environmental characteristics for the generation of TCs [8,[23][24][25][26]. There is also a study [27] which compared the representations of the environmental conditions for TCs in various reanalysis datasets and found that there are no pronounced differences among the datasets.…”

Strong modulations on the Bay of Bengal monsoon onset vortex by the first northward-propagating intra-seasonal oscillation

“…It is understood that the FNISO plays the critical role in triggering the southwest monsoon over the BoB and has unique structure different from boreal winter Madden-Julian Oscillation (MJO) and boreal summer intra-seasonal oscillation (BSISO) (Li et al 2013a). Even though there are some previous studies on the ISO's modulation on the BoB cyclones (Camargo et al 2009;Kikuchi and Wang 2010;Yanase et al 2012;Krishnamohan et al 2012), (2)…”

“…This peculiar feature is attributed to the strong modulation by seasonal monsoon circulation (Gray 1968;Yanase et al 2012;Li et al 2013b). It is also identified that BoB TCs are strongly influenced by the intra-seasonal oscillation (ISO) (Camargo et al 2009;Kikuchi and Wang 2010;Yanase et al 2012;Krishnamohan et al 2012). Monsoon onset represents the critical time when the seasonal transition is realized by the intrusion of the first northward-propagating intra-seasonal oscillations (FNISOs) into the central BoB (Li et al 2013a).…”