Explaining how SST patterns influence monsoon interannual variability using a moist static energy framework

Sea surface temperature (SST) patterns strongly influence tropical convection, large-scale circulation, and the global energy balance. Yet, the physical mechanisms linking SST patterns to monsoon variability remain insufficiently understood, particularly from an energetic perspective. This study aims to understand how SST patterns, particularly those related to the El Niño Southern Oscillation (ENSO), have influenced Northern Hemisphere monsoons using a subcloud moist static energy (MSE) framework. Utilising 6-hourly ERA5 reanalysis and GPCP precipitation data, we find that Northern Hemisphere monsoon systems exhibit significant negative regressions with boreal summer SST anomalies in the eastern equatorial Pacific, consistent with ENSO-driven variability. Removing the ENSO signal strengthens relationships with other SST patterns, including those over the Mediterranean and tropical North Atlantic for the West African monsoon. Findings also reveal that the theoretical monsoon extent, defined by the latitude of peak subcloud MSE, remains relatively stable interannually, independent of ENSO conditions. ENSO phases instead modulate the distribution and local gradient of subcloud MSE, producing a dipole structure in MSE anomalies. In El Niño years, reduced subcloud MSE poleward of the climatological MSE maximum corresponds to suppressed precipitation, consistent with the upped-ante mechanism in which enhanced tropospheric warming increases the energetic threshold for deep convection at the northern edge of the monsoon where moisture is limited. These results highlight that ENSO-driven SST patterns primarily alter the energetics of monsoon systems remotely through a top-down mechanism that modulates atmospheric stability and local MSE gradients. They also underscore the importance of region-specific processes in mediating SST–monsoon interactions.