  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.
