Leading role of internal dynamics in the 2009 Indian summer monsoon drought

Abstract

Understanding the underlying dynamics of the Indian summer monsoon (ISM) extremes such as severe droughts is key to improving seasonal prediction of the ISM rainfall. A large number of ISM droughts over the past century occurred unrelated to external forcing like the El Nino-Southern Oscillation (ENSO). In this study, we challenge the perception that the 2009 ISM drought was driven by ENSO and show that it was caused by internally driven processes. The 2009 drought of ISM was the result of two very long breaks, one in the month of June and the other in July-August (JA). While some studies provide strong evidence that the June break was caused by dry air intrusion associated with extratropical waves, a mechanism for the equally important JA break has not been elucidated so far. In this study, we unravel a new process in which westward propagating convectively coupled planetary-scale equatorial Rossby (PSER) waves emanating from the eastern Pacific as a remnant of Madden-Julian Oscillation (MJO), interact with the monsoon intraseasonal oscillation (MISO), modulate the active/break spells, and thereby influence the seasonal mean. It was found that during JA 2009 the arrival of the divergent phase of this PSER mode over the ISM domain reinforced and extended the break condition initiated by the northward propagating MISO, thereby creating a long break. Nonlinear kinetic energy exchanges between the PSER mode and the northward propagating MISO were found to be at the heart of such interactions. Evidence of such interactions can be seen during different active/break events in other monsoon seasons as well. As both long breaks were primarily driven by internal dynamical processes of the atmosphere, the study underscores the major role played by internal dynamics in causing the 2009 ISM drought. Our discovery that interactions between PSER waves and MISO can lead to significant modulations of the active/break spells opens up a new unexplored mechanism for understanding monsoon variability.