Abstract
Monsoon onset vortex (OV) in the form of tropical cyclone is often observed in the pre-monsoon period and contributes to the subsequent abrupt establishment of summer monsoon over the Bay of Bengal (BoB). It is identified here that all historical OVs occurred during the convection-enhanced phase of the first northward-propagating intra-seasonal oscillation (FNISO). The individual contributions from the four large scale environmental fields associated with the intra-seasonal variations to the cyclone genesis are diagnosed with the aid of the genesis potential index. The significant moistening of mid-level atmosphere, which is embedded in the FNISO convection-enhanced phase, is shown to be the primary factor leading to the cyclone genesis. The water vapor budget analysis is further done to understand the governing process for the mid-level humidity increase. It is clearly seen that the vertical advection process, dominated by the anomalous vertical advection of the mean vertical water vapor gradient, plays the critical role. Hence the OVs are shown to be strongly modulated by FNISOs, both of which are important elements of the complex story of the BoB monsoon onset.
Highlights
The abrupt onset is one of the salient features of the Asian monsoon system (Yeh et al 1959), which first occurs very explosively in the southern part of Bay of Bengal (BoB) (Lau and Yang 1997; Wu and Zhang 1998; Wang and LinHo 2002)
According to the onset vortex (OV) definition as well as the onset date of BoB monsoon in Wu et al (2012), 11 OVs are identified from Joint Typhoon Warning Center (JTWC) archive during 1991–2010 (Table 2), which means OV is a frequent phenomenon during the onset of BoB monsoon
It is understood that the first northward-propagating intra-seasonal oscillation (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)
Summary
The abrupt onset is one of the salient features of the Asian monsoon system (Yeh et al 1959), which first occurs very explosively in the southern part of Bay of Bengal (BoB) (Lau and Yang 1997; Wu and Zhang 1998; Wang and LinHo 2002). The development of an onset vortex (OV) is often observed in the lower troposphere (Krishnamurti et al 1981; Rao and Sivakumar 1999). It is found recently that some OVs could evolve into tropical cyclones (TCs) and eventually lead to the monsoon onset by their northward movement (Wu et al 2011, 2012). Some OVs even got quickly intensified into super cyclones along with the monsoon onset and loaded severe destruction to the local inhabitants. Documenting and understanding of the OV development is of vital importance for the Southern Asia and beyond
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