Distinct atmosphere-ocean coupling processes on the onset phase of Indian summer monsoon during 2017 and 2018 as revealed through SCATSAT-1 and its comparison with CFSv2

Abstract

ABSTRACT Distinct atmosphere-ocean coupled processes determine the inter-annual variability in the onset processes of Indian Summer Monsoon. In this work, we have analysed the distinct characteristics of onset process during 2017 (MOK17) and 2018 (MOK18) using SCATSAT-1 observations. Two years of dedicated SCATSAT-1 analysed winds and other flux datasets are used to understand distinct-coupled processes and further compared with coupled model forecasts from National Centres for Environmental Prediction (NCEP) Climate Forecast System version 2 (CFSv2). Air-sea coupled mechanism during 2017 and 2018 suggests that there is a delayed response (about one pentad) of atmosphere-ocean coupling over North Indian Ocean on the monsoon onset and its propagation during 2018. It is found that the influence of cyclone Mekunu formed over Western Arabian Sea disturbed the onset vortex formed over Eastern Arabian Sea during MOK18. The presence of cyclone Mekunu results in a sudden spike of East-west Sea Surface Temperature gradient, Latent Heat Flux and wind stress over Arabian Sea during one pentad before monsoon onset over Kerala. This created an onset like situation in the next pentad, and later the conditions are not favoured for the further progress of the monsoon. It leads to a weak onset followed by a sluggish and halted northward progression of monsoon during 2018. In the absence of a cyclonic system during the onset phase of MOK17, conventional onset and its further progress took place during 2017. Further comparison of SCATSAT-1 and other analysed data sets with the CFSv2 model show that the CFSv2 model is able to capture the atmosphere-ocean coupled processes during the onset phase up to four pentads in advance during MOK17. However, due to the presence of Mekunu cyclone before the monsoon onset limited the forecast skill of CFSv2 only up to two pentads in advance during MOK18. We found that the presence of strong convective instability due to strong synoptic-scale system can influence the skill of coupled model forecast in the extended range. The new SCATSAT-1 observations provide high-quality observations that can be used to quantify the errors and identify common biases in the coupled models.