Sub-India summer monsoon rainfall variability and its implications for all-India summer monsoon rainfall prediction

Abstract:

The Indian summer monsoon is an iconic feature of Earth's physical climate, and its rainfall sustains the socioeconomic activities of over one billion citizens of India and beyond. Improving the skill of predictions of interannual variations in seasonal-mean (June through September) summer monsoon rainfall averaged over all of India --- so-called All-India Rainfall or AIR --- would therefore be of great scientific and societal value. We use observational data and high-resolution seasonal ensemble hindcast simulations performed by the Indian Institute for Tropical Meteorology (IITM) to investigate AIR seasonal predictability. Past studies have disagreed on sub-India heterogeneity of summer monsoon rainfall variability; some argue that monsoon rainfall anomalies are coherent throughout the country and season, while others have argued that AIR primarily reflects two regions, the Western Ghats and the Central Monsoon Zone, which are uncorrelated with one another. Using high-resolution gridded rainfall data from the Indian Meteorological Department (IMD), we reconcile these viewpoints by showing that prior definitions of the Western Ghats region based on coarser, 2x2 degree data combines rainfall from points within the region that are actually anti-correlated with one another. When the Western Ghats region is confined to the narrow coastal strip over which JJAS rainfall anomalies tend to be coherent, we find that they are highly correlated with rainfall over the Central Monsoon Zone, and that a linear combination of these two region averages is nearly perfectly correlated with AIR. The Western Ghats region exhibits more decadal-scale variability, and the Central Monsoon Zone more interannual variability, the physical mechanisms behind which we discuss including links to ENSO and the more recently discovered Equatorial Indian Ocean Oscillation (EQUINOO). We further discuss the relationships between rainfall in these sub-regions on monthly timescales within the monsoon season, and finally we discuss the predictability from the sub-India, monthly timescales through to AIR in the IITM seasonal forecast data.