The role of nonlinearities associated with air-sea coupling processes in El Nino’s peak-phase locking

Abstract

We use conditional nonlinear optimal perturbation (CNOP) to investigate the optimal precursory disturbances in the Zebiak-Cane El Nino-Southern Oscillation (ENSO) model. The conditions of the CNOP-type precursors are highly likely to evolve into El Nino events in the Zebiak-Cane model. By exploring the dynamic behaviors of these nonlinear El Nino events caused by the CNOP-type precursors, we find that they, as expected, tend to phase-lock to the annual cycles in the Zebiak-Cane model, with the SSTA peak at the end of a calendar year. However, El Nino events with CNOPs as initial anomalies in the linearized Zebiak-Cane model are inclined to phase-lock earlier than nonlinear El Nino events despite the existence of annual cycles in the model. It is clear that nonlinearities play an important role in El Nino's phase-locking. In particular, nonlinear temperature advection increases anomalous zonal SST differences and anomalous westerlies, which weakens anomalous upwelling and acts on the increasing anomalous vertical temperature difference and, as a result, enhances El Nino and then delays the peak SSTA. Finally, we demonstrate that nonlinear temperature advection, together with the effect of the annual cycle, causes El Nino events to peak at the end of the calendar year.