Balancing Role of the Arabian Sea Mini Warm Pool in the ENSO-Monsoon Onset Relationship

Abstract The onset of the Indian Summer Monsoon in Kerala (MoK) has large socio-economic implications for the densely populated Indian subcontinent, yet understanding the associated air-sea interactions remains challenging for the scientific community. In this study, we first show that the preceding El Niño-Southern Oscillation (ENSO) accounts for approximately two-thirds of the interannual variability of the Arabian Sea Mini Warm Pool (MWP), whereas the preceding Indian Ocean Dipole (IOD) has minimal influence. Following an El Niño event, a strong anticyclone develops over the northern Arabian Sea from April to June due to the anomalous easterlies that weaken the mean southwesterly winds over the southeastern and central Arabian Sea. It thus simultaneously favors the MWP expansion in May and delays the monsoon onset in Kerala (MoK), establishing a positive correlation between the two (a strong MWP being statistically associated with a delayed onset). In contrast, after a La Niña event, the southwesterly monsoon winds remain strong, resulting in a weak or absent MWP. To examine the direct influence of the MWP on the MoK, we have employed a Regional Coupled Atmosphere-Ocean Numerical Model (RCM). The ‘noMWP’ sensitivity experiments suggest that following an El Niño year, as the MWP intensifies, it draws winds toward the Kerala coast, enhancing convective activity and causing an early MoK. Consequently, the MWP helps advance the MoK, mitigating the delay that would otherwise be more pronounced after an El Niño. These findings underscore the importance of improving the observation and simulation of MWP dynamics to enhance monsoon forecasting.