Influence of El Niño Southern Oscillation on Indian Summer Monsoon Intraseasonal Circulation and Cluster Dynamics

Abstract

The Indian Summer Monsoon (ISM) is a large-scale seasonal event responsible for providing the majority of
annual rainfall to India. However, when the El Niño Southern Oscillation (ENSO), categorized by anomalous sea surface
temperatures (SST) in the central and eastern Pacific Ocean, is in a warm year (higher SST), the seasonal mean ISM
rainfall generally decreases, although the robustness of this relationship has been questioned. Our study focuses on the
extent to which the preferred intraseasonal active-break cycle of the ISM defined by five circulation regimes (obtained by
cluster analysis in Straus 2021), is independent of the seasonal mean SST of the Pacific Ocean, a current debate among
researchers. Using the Niño 3.4 Index (average SST over the central-eastern Pacific), we define warm years as those for
which the summer average anomaly of Nino3.4 exceeds 0.7 degrees K. Cold years have the seasonal mean index
anomaly less than -0.7 degrees K. (All other years are considered neutral.) Following Straus (2021), we composite
precipitable water (PW) and diabatic heating (Q) anomalies during each of the five circulation regimes to show their
evolution during the active-break cycle. Going beyond this, we composite PW and Q during each regime for warm, cold
and neutral ENSO years separately. Differences between the warm and cold ENSO regime composites for each phase of
the active-break cycle are nearly independent of the phase of the cycle, suggesting (but not proving) that the seasonal
mean ENSO effect is not related to the cycle. We pursue this line of enquiry with further research to determine how the
principal components of u and v-winds at 850 mb (the variables used to define the regimes) are affected by ENSO.