Persistent Wet and Dry Spells of Indian Summer Monsoon Rainfall: A Reexamination of Definitions of “Active” and “Break” Events

Abstract

Persisting wet events (PWEs) and persisting dry events (PDEs) over central India (CI), defined by rain spells lasting for 5 days or more above and below climatology, respectively, represent an important component of the Indian summer monsoon's intraseasonal variability. However, half of such PREs and PDEs that do not overlap with conventionally defined “active” and “break” spells over CI while contributing about 20% to the seasonal mean remained poorly studied. Here we find that, in contrast to more abundant longer (.5 days) wet and dry spells over the CI, the intraseasonal rainfall variability over northeast India (NEI) is characterized by higher abundance of intense shorter spells (,5 days). Physically, the difference is linked to the fact that monsoon intraseasonal oscillations with a 30-60-day time scale dominate subseasonal variability over the CI, whereas the 10-20-day quasi-biweekly mode dominates subseasonal variability over NEI. While non-overlapping PDEs are associated with large-scale but lower-intensity breaks, non-overlapping PWEs are associated with synoptic events with relatively smaller spatial scales rather than large-scale active events. Here, a percentile-based definition of active and break spells as daily rainfall in excess of the 90th percentile and below the 30th percentile, respectively, persisting for more than 3 days is proposed; this encompasses almost all non-overlapping PWEs and PDEs and is expected to be more useful to the users. Contributions of the subseasonal fluctuations to the seasonal mean and their association with predictable drivers indicate that the seasonal mean rainfall over the NEI is significantly less predictable than that over the CI.