Seasonal and Intraseasonal Modulation of Tropical Cyclogenesis Environment over the Bay of Bengal during the Extended Summer Monsoon

Abstract

The environmental field of tropical cyclogenesis over the Bay of Bengal is analyzed for the extended summer monsoon season (approximately May-November) using best-track and reanalysis data. Genesis potential index (GPI) is used to assess four possible environmental factors responsible for tropical cyclogenesis: lower-tropospheric absolute vorticity, vertical shear, potential intensity, and midtropospheric relative humidity. The climatological cyclogenesis is active within high GPI in the premonsoon (;May) and postmonsoon seasons (approximately October-November), which is attributed to weak vertical shear. The genesis of intense tropical cyclone is suppressed within the low GPI in the mature monsoon (approximately June-September), which is due to the strong vertical shear. In addition to the climatological seasonal transition, the authors' composite analysis based on tropical cyclogenesis identified a high GPI signal moving northward with a periodicity of approximately 30-40 days, which is associated with boreal summer intraseasonal oscillation (BSISO). In a composite analysis based on the BSISO phase, the active cyclogenesis occurs in the high GPI phase of BSISO. It is revealed that the high GPI of BSISO is attributed to high relative humidity and large absolute vorticity. Furthermore, in the mature monsoon season, when the vertical shear is climatologically strong, tropical cyclogenesis particularly favors the phase of BSISO that reduces vertical shear effectively. Thus, the combination of seasonal and intraseasonal effects is important for the tropical cyclogenesis, rather than the independent effects.

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

“…2d, where the vorticity, vertical wind shear, and mid-level humidity are three leading terms in defining the intra-seasonal GPI ′ . The vorticity and mid-level humidity terms are shown to vary nearly in phase with the cyclogenesis, while the vertical wind shear and PI terms slightly lead, which is similar with Yanase et al (2012).…”

“…At seasonal scale, BoB TC activity exhibits a notable bimodal character with peaks in pre-and post-monsoon seasons, in contrast to the single peak distribution in the Pacific and Atlantic basins. 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).…”

“…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).…”

Development of tropical cyclone wind field for simulation of storm surge/sea surface height using numerical ocean model

“…The favorable hydrographic conditions such as formation of stably stratified thick barrier layer in & Yashvant Das yashvantdas@rediffmail.com 1 the upper ocean that acts as barrier to vertical mixing and entrainment cooling induced by TC's leading to an increase in heat fluxes from the ocean into the atmosphere play a dominant role for the enhanced TC activity over BoB during post monsoon seasons (Sprintall and Tomczak 1992;Sengupta et al 2008;Balaguru et al 2012). TC activity in BoB is strongly modulated by the tropical intraseasonal oscillation (ISO) (Yanase et al 2010(Yanase et al , 2012. The influence of coupled ocean-atmospheric phenomena such as El-Nino, El-Nino Modoki, MJO and Indian Ocean Dipole (IOD) etc.…”