The equatorial Pacific warm water volume (WWV), defined as the volume of water warmer than 20°C near the equator, is a key predictor for El Niño–Southern Oscillation (ENSO), and yet much about the individual processes that influence it remains unknown. In this study, we conduct idealized ENSO simulations forced with symmetric El Niño– and La Niña–associated atmospheric anomalies as well as a historical 1979–2016 hindcast simulation. We use the water mass transformation framework to examine the individual contributions of diabatic and adiabatic processes to changes in WWV. We find that in both sets of simulations, El Niño’s discharge and La Niña’s recharge periods are initiated by diabatic fluxes of volume across the 20°C isotherm associated with changes in surface forcing and vertical mixing. Changes in adiabatic horizontal volume transport above 20°C between the equator and subtropical latitudes dominate at a later stage. While surface forcing and vertical mixing deplete WWV during El Niño, surface forcing during La Niña drives a large increase partially compensated for by a decrease driven by vertical mixing. On average, the ratio of diabatic to adiabatic contributions to changes in WWV during El Niño is about 40% to 60%; during La Niña this ratio changes to 75% to 25%. The increased importance of the diabatic processes during La Niña, especially the surface heat fluxes, is linked to the shoaling of the 20°C isotherm in the eastern equatorial Pacific and is a major source of asymmetry between the two ENSO phases, even in the idealized simulations where the wind forcing and adiabatic fluxes are symmetric.