Asymmetric Changes of ENSO Diversity Modulated by the Cold Tongue Mode Under Recent Global Warming

Instead of the La Nina events with no significant change in their surface expressions, the recent increasing frequency of central Pacific El Nino events is suggested to be related to global warming. There is yet no consensus on the impacts of the recent global warming on such asymmetric changes of El Nino-Southern Oscillation (ENSO) events. Here we show the frequency of the extreme cold/moderate warm events both increases in the central equatorial Pacific over the past decades. We attribute the change of ENSO diversity to the positive cold tongue mode under recent global warming, which gives rise to an intensification, contraction, and westward shift of Walker circulation accompanied by an uplift of the thermocline. Our results propose a unified explanation for the changes of ENSO diversity under the recent global warming, which carries important implications for the relationship between global warming and ENSO. Plain Language Summary El Nino-Southern Oscillation (ENSO) events, characterized by sea surface temperature anomaly patterns in the tropical Pacific, have great global climate impacts by alternating between warm El Nino and cold La Nina phases. At the expense of the conventional El Nino peaking in the eastern Pacific, the recent increasing frequency of El Nino events with warming sea surface temperature anomalies concentrated in the central equatorial Pacific is suggested to be related to global warming, whereas for the La Nina events, there seems to be no significant change in their surface expressions. There is yet no consensus on the impacts of the recent global warming on such asymmetric changes of ENSO events. Our results show the increasing frequency of both the extreme cold and moderate warm events (ENSO diversity) can be ascribed to the positive cold tongue mode under the recent global warming. The positive cold tongue mode gives rise to an intensification, contraction, and westward shift of Walker circulation accompanied by an uplift of the thermocline, which enlarges the sea surface temperature morphology changes of El Nino but covers up the surface expression of La Nina.