Impacts of precipitation anomaly on the ENSO development

During El Niño, positive sea surface temperature (SST) anomalies in the central-eastern Pacific are accompanied by positive precipitation anomalies. While it has been known that precipitation can impact the SST through changing near-surface stratification, observations are spatiotemporally sparse to quantify its role in ENSO. On the other hand, modeling studies are mostly based on ocean-only models or a hybrid coupled model that ignores cloud variations, thus shortwave radiation anomalies, despite precipitation varying interannually.

In this study, we investigate the roles of freshwater flux (particularly precipitation) anomaly in SST anomalies associated with ENSO in a coupled system using a state-of-the-art climate model, MIROC6. In addition to the control experiment (CTRL), where the atmosphere and ocean are fully coupled everywhere, two sensitivity experiments are conducted where freshwater flux over the tropical Pacific Ocean is partially replaced with the 3-hourly climatology from CTRL. In the Pclim/Eclim experiment, the precipitation/evaporation flux into the ocean is replaced over the tropical Pacific (the atmosphere can "feel" the ocean variations everywhere). It is found that ENSO variability significantly reduces (up to about 17% for Niño3.4), particularly in the developing season without precipitation anomaly, while evaporation anomaly does not significantly change the ENSO amplitude. Mixed layer heat budget analysis for the CTRL and Pclim reveals that precipitation anomaly enhances the ENSO variability through modifying oceanic "vertical processes," such as the entrainment.