Impact of Northward-Propagating Intraseasonal Variability on the Onset of Indian Summer Monsoon

Abstract

Abstract The onset of the Indian summer monsoon (ISM) has a pronounced interannual variability, part of which originates from the large-scale circulation and its thermodynamic properties. While the northward-propagating intraseasonal variabilities (ISVs) are a prominent characteristic of the ISM, they tend to initiate an early onset by transferring moisture and momentum from the deep tropics to the Indian subcontinent. However, not all early onsets of ISM are attributable to strong ISVs and not all strong ISVs can lead to early ISM onsets. With a daily Indian monsoon index and a simple regression model, the onsets of ISM from 1982 to 2011 are separated into two groups. The years in which the early onsets of ISM are closely related to the northward-propagating ISVs are categorized as the ISVO years, and the other years in which the ISM onsets are not closely related to ISVs are categorized as non-ISVO years. The former category is the focus of this study. Before the onset of ISM in the ISVO years, the convective features are prominent, such as a cyclone over the Bay of Bengal (BoB) and the associated strong convection. The ocean–atmosphere interaction is found to be important for the northward-propagating ISVs before the ISM onset in the ISVO years. Evidence shows that warm SST anomalies drive the atmosphere and lead to atmospheric instability and convection. This reinforces the more recent view that the ocean does not just play a passive role in the northward-propagating ISVs. This process understanding helps shape the path to enhancing predictive understanding and monsoon prediction skills with obvious implications for the prediction of El Niño–Southern Oscillation.