Dynamical influence of West Pacific Typhoons on the 2018 historic flood of Kerala as revealed by the weather research and forecasting (WRF) model

The remote influence of west Pacific typhoons on the historic Kerala flood in the 2018 Indian summer monsoon (ISM) season is investigated using the weather research and forecasting (WRF-ARW) model. The flood occurred as a result of vigorous monsoon intra-seasonal activity with some districts receiving excess rainfall exceeding 405%. Observational data reveals that a deflection of the monsoon low level jet (LLJ) occurred from south westerly to north westerly direction together with a slow-down of cloud movement over the central Kerala region due to a split in the LLJ core nearby the Arabian sea coast. The split and deflection of LLJ core produced cyclonic vorticity along 10 & DEG; N latitude which favored severe convection. Nevertheless, a detailed analysis of the flow pattern reveals that the simultaneous formation of a low-pressure system (near Orissa coast in the Bay of Bengal) and several typhoons in the west-Pacific Ocean had a crucial role in the deflection and hence the production of cyclonic vorticity. By employing the tropical cyclone bogussing scheme in WRF-ARW, the formation of a west-Pacific typhoon was suppressed and subsequently a comparison was made against the control run in order to quantify the typhoon's effect on the Kerala rainfall. It is found that the presence of low-latitude typhoon was primarily responsible for the deflection of the monsoon LLJ and hence the production of cyclonic vorticity through remote forcing. The study thus provides an insight into the teleconnection of west-Pacific typhoons on the intraseasonal variation of ISM and associated extreme rainfall events.