Behaviour of onset phase and its probable impact on Overall nature of Indian southwest monsoon - an useful approach

Indian southwest monsoon has large inter-annual and intra-seasonal variabilities and the variabilities arise in every aspect of the monsoon i.e. onset date/onset phase, total seasonal rainfall, active/break spells and number of synoptic scale disturbances etc. Once the onset takes place at Kerala coast, its northward propagation and progressive activity depends on favorable large circulation i.e. enhanced westerlies and cyclonic circulation at least upto mid troposphere. If well-organized cyclonic circulation is missing then the monsoon progress is abnormal and weak. Keeping this fact in mind, various dynamical parameters (ζ, D and ω) are computed from surface to 200 hPa, at least for 2 to 3 weeks period near the onset date during onset phase of southwest monsoon for different years over India and neighbouring seas (EQ-25°N and 50 °E-90°E), for 20 years from 1987 to 2006 using NCEP/NCAR reanalysis daily horizontal wind fields. It is found that whenever the horizontal extent of cyclonic vorticity are of synoptic scale, the performance of monsoon season is normal or above normal and the mesoscale extent of ζ and ω during onset phase resulted weak or drought case of the monsoon season. The results are presented for three normal / above normal years (1997, 2000 and 2005) and three drought years (1987, 2002 and 2004