Contrasting features of northeast monsoon during 2019 and 2020 in response to the delayed withdrawal of southwest monsoon

Kerala conside red the land of monsoons, located on the southwest coast of India, received contrasting rainfall during the northeast monsoon (NEM) seasons of 2019 (excess) and 2020 (deficit). The reasons for the contrasting NEM rainfall during these two consecutive years were studied by analysing the atmospheric circulation pattern and its interaction with warm ocean waters by using wind data from the stratosphere-troposphere (ST) radar and reanalysis products. The study reveals that weak and variable westerly winds over the southern Indian Peninsula coupled with the location of the Inter-tropical Convergence Zone (ITCZ) north of the peninsula prevented the timely onset of NEM over its core regions during 2019. However, 2019 was an anomalous year with an extreme positive Indian Ocean Dipole (IOD) with unusual cyclonic activity over the Arabian Sea (AS) during the NEM season. These anomalous conditions favoured large amounts of rainfall, especially over the northern parts of Kerala during the latter half of October and compensated for the rainfall deficiency during the early half of October. In 2020, the subdued cyclogenesis over the AS combined with near-normal sea surface temperatures (SST) resulted in reduced convective activity over Kerala during the initial part of the NEM season. After the withdrawal of the southwest monsoon (SWM), the formation of two cyclones in the Bay of Bengal (BoB) and the frequent passage of easterly troughs from the BoB across the southern peninsula brought in anomalous rainfall over the east coast and some parts of Kerala, but the central and northern parts of Kerala largely remained rainfall deficient.