Northward Propagation of the Intertropical Convergence Zone and Strengthening of Indian Summer Monsoon Rainfall

Abstract

Since 2002, there has been a clear increase in Indian summer monsoon rainfall (ISMR). We demonstrate that this increase is associated with a change in the dynamics of the Intertropical Convergence Zone (ITCZ). Using a recently released reanalysis product from 1980–2016, we show that the ITCZ has strengthened and propagated northward since 2002. Analysis of the total energy budget reveals an increase in energy divergence and atmospheric diabatic heating, which is consistent with the changes in the ITCZ. Although global aerosol optical depth shows a significant positive trend during 1980–2016, it has declined over many parts of India since 2002. We put forward the hypothesis that this is the driver of the changing characteristics of the ITCZ. Our results suggest that changes in the dynamics of the ITCZ, together with changes in the energy/moisture budget, are responsible for the strengthening of ISMR since 2002, consistent with the emergence of a greenhouse gas-induced signal.