Diabatic Contribution of Warm Water Volume Variability Over ENSO Events

The Pacific Warm Water Volume (WWV), defined as the volume of water warmer than 20°C near the Equator, is a key ENSO predictor and yet much about the individual processes that influence it remains unknown. In this study we simulate idealised ENSO events in a global ocean model and use the water mass transformation framework to examine the contributions of both adiabatic and diabatic processes to changes in WWV. The WWV discharge and recharge periods are initiated by diabatic fluxes of volume across the 20°C isotherm associated with surface heat and vertical mixing fluxes. Adiabatic horizontal volume exchanges above 20C between the Equator and higher latitudes dominate at a later stage. The anomalous diabatic flux across the 20°C isotherm associated with vertical mixing is responsible for 43% of the total WWV loss during El Niño’s discharge phase, with changes in surface forcing responsible for 6% and adiabatic transport responsible for the remainder. In contrast, during La Niña’s recharge, surface heat fluxes drive a large increase in WWV that exceeds the total change in WWV (accounting for 128% of the total recharge) compensated for by a decrease in WWV driven by vertical mixing (–65% of the total recharge). The increased importance of diabatic processes during La Niña, linked to the shoaling of the 20°C isotherm in the eastern equatorial Pacific, is a major source of asymmetry between the two ENSO phases, while adiabatic volume fluxes are much more symmetric. These results highlight the key role of diabatic processes in driving ENSO-related WWV variability.