Exploring Spatial Coherence in Inter-annual Changes and Annual Extremes of Rainfall over India

Abstract: Forecasts of monsoon rainfall for India are made at national scale. But there is spatial coherence and heterogeneity that is relevant to forecasting. This paper considers year-to-year rainfall change and annual extremes at sub-national scales. We use Data Mining techniques to gridded rain-gauge data for 1901-2011 to characterize coherence and heterogeneity and identify spatially homogeneous clusters. We study the direction of change in rainfall between years (Phase), and extreme annual rainfall at both grid level and national level. Grid-level Phase is found to be spatially coherent, and significantly correlated with all-India mean rainfall (AIMR) phase. Grid-level extreme-rainfall years are not strongly associated with corresponding extremes in AIMR, although in extreme AIMR years local extremes of the same type occur with higher spatial coherence. Years of extremes in AIMR entail widespread phase of the corresponding sign. Furthermore, local extremes and phase are found to frequently co-occur in spatially contiguous clusters.