Nonlinear boosting during extreme El Niño through local air-sea interaction

The delayed negative feedback is a key process for the turnabout between El Niño and La Niña. Since the intensity of this dynamical negative feedback is determined by itself, the stronger event is supposed be strongly damped during the decaying phase. However, the extreme El Niño actually lived longer than the normal El Niño. Here, we propose that the far-eastward extension of the warm pool promotes the positive SST tendency during the decaying phase of El Niño so disrupting a strong decay. The warm pool expansion accompanies by the expansion of the convective threshold region toward the eastern Pacific. During and after the mature phase of the extreme El Niño, therefore the rainfall band and the enhanced westerly anomalies over the eastern Pacific move to the east, which enhances the upwelling. This eastward migration of surface winds also plays a role of out-of-phase relationship between SST anomaly and subsurface temperature anomaly. All these processes results in the positive SST tendency through the positive nonlinear dynamical heating. This positive SST tendency maintains the warm eastern Pacific until the following summer.