An investigation into the conditions leading to monsoon onset over Kerala

Monsoon onset over the Kerala (MOK) coast has been studied using the recently released high resolution Hamburg Ocean Atmosphere Parameters and Fluxes from Satellite data (HOAPS 3). Columnar water vapour content, sea surface temperature and evaporation have been utilised to examine the conditions leading to MOK from 7 pentads. The role of monsoon hadley cell (MHC) has also been assessed using the NCEP=NCAR Reanalysis dataset. In addition the role of the cross-equatorial flow and low level jet at 850 hPa has also been explored, as has the importance of the Madden Julian Oscillation in initiating the MOK, the latter using outgoing longwave radiation data. An analysis of sea surface temperature over the Arabian Sea (AS) and Bay of Bengal for contrasting MOKs, showed that the AS warm pool plays a crucial role in MOK. Contrary to the popular notion that moisture builds up only 2–3 weeks in advance of MOK, our study has shown that the integrated columnar water peaks around 35–40 days (almost around the same time as that of the pre-monsoon rainfall peak) prior to MOK and can be a potential predictor of MOK. The 850 hPa winds strengthen over the extreme southern peninsula and Sri Lanka about 3 pentads prior to MOK, and the evaporation rates over the southern Arabian Sea show a dramatic increase with onset of MOK. The MHC can be a useful predictor for extreme monsoon onsets (early or delayed).