Impacts of the Tropical Pacific Cold Tongue Mode on ENSO Diversity Under Global Warming

The causes of ENSO diversity, although being of great interest in recent research, do not have a consistent explanation. This study provides a possible mechanism focused on the background change of the tropical Pacific as a response to global warming. The second empirical orthogonal function mode of the sea surface temperature anomalies (SSTA) in the tropical Pacific, namely the cold tongue mode (CTM), represents the background change of the tropical Pacific under global warming. Using composite analysis with surface observations and subsurface ocean assimilation data sets, we find ENSO spatial structure diversity is closely associated with the CTM. A positive CTM tends to cool the SST in the eastern equatorial Pacific and warm the SST outside, as well as widen (narrow) zonal and meridional scales for El Nino (La Nina), and vice versa. Particularly in the positive CTM phase, the air-sea action center of El Nino moves west, resembling the spatial pattern of CP-El Nino. This westward shift of center is related to the weakened Bjerknes feedback (BF) intensity by the CTM. By suppressing the SSTA growth of El Nino in the eastern equatorial Pacific, the CTM contributes to more frequent occurrence of CP-El Nino under global warming.