Macro‐regional definition and characteristics of Indian summer monsoon rainfall, 1871–1985

A case is made for regional‐scale Indian summer monsoon rainfall data and analysis, intermediate between the widely used all‐India and meteorological subdivision data sets. Macro‐regional units are constructed from the data provided by Parthasarathy et al. , 10 being defined using principal components analysis and a classification algorithm. The temporal changes of summer monsoon rainfall over the period 1871–1985 are analysed and described for each of these regions, and the marked diversity of fluctuations between the regions is emphasized by a variety of methods. The degree of relationship is considered for each region between drier and wetter conditions and (i) El Niño and non‐El Niño years, and (ii) SST anomalies in the eastern tropical Pacific. Clear regional differences are apparent, but even statistically highly significant relationships are not large in any absolute sense. The need for explanatory analyses at the regional scale, in addition to those at the more common all‐India scale, is stressed.