On the relationship between Arabian Sea warm pool and formation of onset vortex over east-central Arabian Sea

The interannual variability in the formation of mini warm pool (MWP, SST >= 30.5A degrees C) and its impact on the formation of onset vortex (OV) over the east-central Arabian Sea (ECAS) are addressed by analyzing the NCEP OIV 2-weekly SST data and NCEP-NCAR reanalysis 850 hPa wind fields from May to June (prior to the onset of monsoon) over the north Indian Ocean for a period of 12 years from 1992 to 2003. Strong interannual variability in the formation and intensification of MWP was observed. Further, the 850 hPa wind fields showed that OV developed into an intense system only during 1994, 1998 and 2001. It formed in the region north of the MWP and on the northern flank of the low-level jet axis, which approached the southern tip of India just prior to the onset of monsoon, similar to the vortex of MONEX-79. The area-averaged zonal kinetic energy (ZKE) over the ECAS (8-15A degrees N, 65-75A degrees E) as well as over the western Arabian Sea (WAS, 5A degrees S-20A degrees N, 50-70A degrees E) showed a minimum value of 5-15 m(2) s(-2) prior to monsoon onset over Kerala (MOK), whereas a maximum value of 280 m(2) s(-2) (40-70 m(2) s(-2)) was observed over the ECAS (WAS) during and after MOK. The study further examined the plausible reasons for the occurrence of MWP and OV.