The equatorial Pacific Warm Water Volume (WWV), defined as the volume
of water warmer than 20C near the Equator, is a key ENSO predictor and yet
much about the individual processes that drive its changes 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 20C 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 diabatic volume flux across this
isotherm associated with vertical mixing is responsible for 43% of the total
WWVloss during El Niño’s discharge phase, with 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 inWWVthat
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
Ni~na, linked to the shoaling of the 20C 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 have implications
for understanding the cycling of heat over ENSO and highlight the
key role of diabatic processes for driving ENSO-related WWV variability.