A theoretical model of strong and moderate El Niño regimes
Ken Takahashi, Christina Karamperidou, Boris Dewitte - Submitted to Climate Dynamics
Several nonlinear processes have been proposed to play a role in El Niño-Southern Oscillation (ENSO). The existence of two regimes for El Niño (EN) events, moderate and strong, has been specifically proposed to originate from the nonlinearity in the Bjerknes feedback associated with the existence of a threshold in sea surface temperature for deep atmospheric convection to occur in the eastern Pacific. The 2015-16 EN event provides further evidence for the two regimes. Here, this nonlinear mechanism is implemented in the recharge-discharge ENSO model. The model is stable with weak nonlinearity, requiring stochastic forcing to maintain the variability against the damping, and is able to simulate the two EN regimes. The sustained low-frequency component of the stochastic forcing plays a key role during the onset of strong EN events, i.e. those in which the sea surface temperature anomaly (T) exceeds the threshold Tc, as it produces increase in both T and in the heat content anomaly (h). High-frequency forcing helps some EN events to exceed the threshold and become strong events, enhancing the total number of strong EN events by 31%. Using the Fokker-Planck equation we show how the bimodal probability distribution of EN events arises from the nonlinear Bjerknes feedback and show that the damping strength determines both the adjustment time-scale and equilibrium value of the ensemble spread associated with the stochastic forcing.
Figure Caption: Composite mean evolution in T-h phase space of strong (red) and moderate (blue) El Niño events in observations (Obs, dotted) and the model (CTL, solid). The deterministic tendencies are shown with black arrows with sample deterministic trajectories in green, while blue arrows illustrate the effect of the stochastic forcing.