Multidecadal variations in ENSO-Indian summer monsoon relationship at sub-seasonal timescales

Abstract

Customarily, the strength of the impact of El Niño-Southern Oscillation (ENSO) on Indian summer monsoon is evaluated with respect to the seasonal mean rainfall. As the rainfall occurring over the Indian subcontinent experiences strong variations intraseasonally, the impact of ENSO on these intraseasonal fluctuations is often more important for the agribusiness and economy when contrasted with the seasonal mean. The annual cycle of ISM rainfall peaks during July and August months. In this study, we investigate the impact of ENSO on ISM precipitation during July and August. We show that the relationship between ENSO and ISM rainfall during July and August has experienced significant changes at multidecadal timescales. While the impact of ENSO was significantly strong in August and significantly weak in July during 1948–1980, post-1980s August shows a weak ENSO-Monsoon relationship than that in July. We investigate these multidecadal changes in the ENSO-monsoon relationship using 68 years of observational records. ENSO produces strong (weak) upper tropospheric divergence over equatorial eastern Pacific Ocean in August (July) before 1980. A stronger divergence helps Rossby wave within the Pacific-Asian jet cools the upper tropospheric mid-latitude region by modulating the vorticity there. A colder mid-latitude temperature weakens the Indian summer monsoon. Thus, the impact of ENSO on the Indian summer monsoon was weaker (stronger) in July (August) before 1980 and vice versa after the 1980s. Further, we deploy a linear barotropic vorticity model to assess the association between upper tropospheric divergence over the Pacific Ocean region and vorticity (temperature) over the mid-latitude Asian region. A stronger forcing of divergence in the upper tropospheric Pacific region reinforces stronger positive vorticity in mid-latitude Asia resembling the findings above. Our study suggests that these multidecadal fluctuations in the sub-seasonal ENSO-monsoon relationship is linked to the physical changes in the state of the climate and could not be attributed to the stochastic processes. Such insight into the variability in ENSO-monsoon relationship at sub-seasonal timescales would help predictability and decision-making.