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Observed El Nino-Southern Oscillation (ENSO) events exhibit distinct amplitude and evolution asymmetries due to nonlinear interactions between sea surface temperature (SST), equatorial zonal wind stress, and thermocline depth anomalies. Establishment of atmospheric convection leads to a nonlinear response of zonal winds to SST, but its relative importance to nonlinear oceanic feedbacks remains unclear. Using a recharge oscillator model modified to incorporate various nonlinearities estimated from observations, we show that the nonlinear wind response to SST alone can induce a nonlinear response of the thermocline to winds and that of SST to the thermocline, as winds affect the growth of SST and damping of thermocline anomalies through the recharge/discharge process. Oceanic nonlinear feedbacks enhance the simulated ENSO asymmetries only when the nonlinear atmospheric feedback is present. Our result suggests that the observed strengthening of ENSO variability in the past century is attributable to an increase in the nonlinear atmospheric feedback.

Plain Language Summary The El Nino-Southern Oscillation (ENSO) has long been known to affect extreme weather events, ecosystems, and agriculture across the globe. Its warm phase (El Nino) and cold phase (La Nina) are like two sides of the same coin. However, they are not simply a mirror image; instead, they exhibit significant asymmetries in terms of event amplitude, duration, and phase transitions. In the past two decades, nonlinear feedbacks inherent to either atmosphere (i.e., formation of atmospheric deep convection) or ocean (i.e., thermocline response to zonal wind and surface temperature response to thermocline fluctuation) have been proposed to explain some aspects of ENSO asymmetry, but the relative importance of these different nonlinear feedbacks remains unclear. By incorporating observed ENSO-related nonlinearities into a classic conceptual model, here we find that the nonlinear atmospheric feedback alone can lead to nonlinear oceanic feedbacks. The nonlinear oceanic feedbacks enhance the simulated ENSO asymmetry only in the presence of the nonlinear atmospheric feedback, indicating that the latter plays a dominant role. The magnitude of nonlinear atmospheric feedback is found to enhance variability and asymmetry of simulated sea surface temperature anomalies, which could explain the strengthening of ENSO variability and asymmetry observed in the past century.