Boreal Spring El Nino Convective State and Its Impact on Monsoon Onset

Abstract

A lesser known relationship between boreal spring El Ni̱o state and the Indian summer monsoon (ISM) is explored, considering the differences in the evolution of sea surface temperature and convection over the Pacific. Rossby wave tracing analysis reveals that the boreal spring El Ni̱o convective state permits two distinct eastward waveguides from the central Pacific to the ISM domain. The first waveguide reaches the northern Indian region roughly along the African‰Û Asian jet, and the second waveguide crosses over to the Southern Hemisphere over the eastern Pacific and extends up to the south Indian Ocean region. The net result is an altered free tropospheric temperature distribution over the ISM domain in the month of May leading to a delayed monsoon onset. The existence of the two waveguides and the modulation of monsoon onset makes the boreal spring El Ni̱o state an important component to be accounted for in ISM prediction.