Spatial Variations of Northeast Monsoon Rainfall over South Peninsular India

Abstract

This study investigates the spatial variability of Northeast Monsoon (NEM) rainfall over South Peninsular (SP) India, focusing on five meteorological subdivisions: Coastal Andhra Pradesh, Rayalaseema, Tamil Nadu, South Interior Karnataka, and Kerala. Using rainfall data from the Indian Institute of Tropical Meteorology (IITM), seasonal rainfall patterns are assessed for the NEM period (October-December), identifying active and poor monsoon years based on standardized departures. Composite rainfall values and percentage deviations provide insights into NEM performance across subdivisions. The analysis also explores key meteorological parameters, including lower tropospheric winds, relative vorticity and divergence at 1000 hPa, 850 hPa, and 200 hPa, using NCEP/NCAR Reanalysis datasets from 1948 to 2016. Outgoing Longwave Radiation (OLR) data from NOAA (1974–2016) further elucidate atmospheric circulation patterns associated with active and poor monsoon years. Findings reveal that active NEM seasons display negative OLR anomalies over SP India and surrounding seas, indicating enhanced convection, particularly in Coastal Andhra Pradesh, Rayalaseema and Tamil Nadu. Conversely, poor monsoons show positive OLR anomalies, signalling reduced convective activity. Wind and divergence anomalies highlight that active monsoons strengthen easterly flows and promote moisture transport to the Intertropical Convergence Zone (ITCZ), amplifying rainfall. During poor monsoons, weakened easterlies and reduced atmospheric convergence correspond with diminished rainfall. Notably, El Niño events generally enhance NEM rainfall, while La Niña years typically result in deficits, with some exceptions, such as in 2016.