On the spatial coherence of sub-seasonal to seasonal Indian rainfall anomalies

The spatial coherence of interannual variations of sub-seasonal to seasonal anomalies in Indian summer monsoon rainfall is investigated at 0.25° spatial resolution using various metrics, including estimates of the number of degrees of freedom, the spatial scale of daily wet “patches”, as well as relationships between local and regional-scale rainfall anomalies and the monsoon circulation. Spatial coherence of interannual rainfall variations is generally found to peak near monsoon onset, in late May–June over Monsoonal India, and again during the withdrawal stage in September–October. However, the spatial coherence and correlations between local rainfall and the regional-scale monsoon circulation decrease during the core phase of the monsoon between early July and late August, when the interannual variability of local-scale amounts tend to be more tightly related to mean daily intensity rather than to the frequency of wet days. The drop in spatial coherence during the core phase is related to increases in the number and intensity of wet “patches” of daily rainfall while their mean spatial extent remains similar throughout the monsoon season. The core phase, with large values of precipitable water, is characterized by very variable daily rainfall amounts across gridpoints, as a consequence of the near exponential distribution of daily rainfall. The interannual variations of sub-seasonal amounts are then dominated by very wet days, which tend to be rather randomly distributed. This contrasts with the onset and withdrawal phases where the mean of the exponential is smaller, and where interannual variations in the timing of regional-scale onset and end of the monsoon predominate. The early and late phases may thus be more potentially predictable than the core of the monsoon season.