Timing of subsurface heat magnitude for the growth of El Niño events

The subsurface heat buildup in the western tropical Pacific and the recharge phase in equatorial heat content are intrinsic elements of El Nino-Southern Oscillation, leading to changes in zonal wind stress, sea surface temperature, and thermocline tilt that characterize the growing and mature phases of El Nino (EN) events. Here we use numerical simulations to study the impact on subsequent EN episodes of a sudden increase or decrease in ocean heat content during the recharge phase and compare results with previous studies in which this perturbation is prescribed earlier during the tilting mode. We found that while not substantially affected by the phase at which a sudden rise in heat content is prescribed, the timing and magnitude of the events are very sensitive to the phase at which a major decrease is imposed. The different response to the phase of increases and decreases substantiates the importance of nonlinear subsurface ocean dynamics to the onset and growth of EN episodes and provides insight into the irreversibility of the events at different stages of the oscillation.