Predicting Extreme Phases of the Indian Summer Monsoon*

Extreme active and break phases of the Indian summer monsoon (ISM) often bring about devastating floods and severe draughts. Here it is shown that these extreme phases exhibit distinctive precursory circulation conditions in both the tropics and extratropics over a range of antecedent periods. The extremely active monsoon over northern India is preceded by a strengthening of the upper-tropospheric central Asian high and enhancement of the tropical convection over the equatorial Indian Ocean and the South China Sea. The concurrent buildup of the anomalous high over central Asia and the arrival of tropical convection over northern India increase the likelihood of occurrence of a heavy rainy period there. Similarly, the concurrent anomalous low over central Asia and the arrival of suppressed convection originating from the equatorial Indian Ocean and the South China Sea precede extremely strong monsoon breaks over northern India. Two predictors can be used to predict the extreme active/break phases of the northern ISM: normalized 200-hPa geopotential height over central Asia and outgoing longwave radiation over southern India. Once the mean of the two predictors exceeds a threshold unit (1.0), an extreme phase is anticipated to occur over northern India after 4–5 days and reach peak intensity after an additional 2 days. In general, an event forecast by this simple scenario has a 40% probability of developing into an extreme phase, which is normally a small probability event (a less than 4% occurrence).