Active/break cycles: diagnosis of the intraseasonal variability of the Asian Summer Monsoon

In this study, various diagnostics have been applied to daily observed outg oing longwave radiation (OLR) and ECMWF ReAnalysis (ERA) products to provid e a comprehensive description of the active/break cycles associated with th e Asian Summer Monsoon and to address the differing behaviour of the two do minant time scales of intraseasonal variability, 10-20 days and 30-60 days. Composite analysis of OLR based on filtered daily All-India rainfall (AIR) for the 40 day (30-60 days) intraseasonal mode indicates that during activ e phases, convection is significantly enhanced over the Indian continent, e xtending over the Bay of Bengal, Maritime continent and equatorial west Pac ific, while convection is suppressed over the equatorial Indian Ocean and n orthwest tropical Pacific, resulting in a 'quadrapole' structure over the A sian monsoon domain. In response to this heating pattern, the large-scale H adley (lateral) and the two cast-west (transverse) tropical circulations ar e enhanced. There is also a significant impact on the extra-tropical circul ation through excitation and propagation of Rossby waves. In contrast, the 15-day mode is more regional to the monsoon domain and has a prominent cast -west orientation in convection. Only the local Hadley circulation over the monsoon region is modulated by this mode. The evolution of these two modes as revealed by POP (principal oscillation pattern) analysis, shows that th e 40-day mode originates over the equatorial Indian Ocean. Once formed it h as poleward propagation on either side of the equator, and eastward propaga tion into the equatorial west Pacific. From the equatorial west Pacific, no rthward propagation over the west Pacific and westward propagation into the Indian longitudes are prominent. The propagative features are complex and interactive and are responsible for the 'quadrapole' structure in convectio n seen from the composites. The interannual variability, assessed from the POP coefficient time series, indicates that the 40-day mode is strong durin g the onset phase of the monsoon in all the years but systematic propagatio n over the entire season depends crucially on the activity of the oceanic T CZ (tropical convergence zone). The POP analysis of the 15-day mode indicat es that this event originates over the equatorial west Pacific, associated with westward propagating Rossby waves, amplifies over the northwest tropic al Pacific and modulates both the continental and oceanic TCZs over Indian longitudes simultaneously. This mode is pronounced during the established p hase of the monsoon. Due to the complexity in the propagational features of both the intraseasonal modes, it is concluded that understanding the subse asonal variability of one regional component of the Asian Summer Monsoon (A SM), requires understanding the entire ASM system.