Sensitivity Difference in the Extratropical Atmosphere to Two Types of El Niño Events

Abstract

A comparison of sensitivity in extratropical circulation in the Northern Hemisphere (NH) and Southern Hemisphere (SH) is conducted through observational analyses and diagnostic linear model experiments for two types of El Nino events, the traditional El Nino with the strongest warmth in the eastern tropical Pacific (EP El Nino) and the El Nino Modoki with the strongest warmth in the central tropical Pacific (CP El Nino). It is shown that CP El Nino favors the occurrence of a negative-phase Northern Annular Mode (NAM), while EP El Nino favors that of the Pacific-North American (PNA) pattern. In SH, both EP and CP El Nino induce a negative phase Southern Annular Mode (SAM). However, the former has a greater amplitude, which is consistent with the stronger sea surface temperature (SST) warmth. The difference in the two types of El Nino events in NH may originate from the dependence of heating-induced extratropical response on the location of initial heating, which may be associated with activity of the stationary wave. In SH, the lack of sensitivity to the location of heating can be associated with weaker activity of the stationary wave therein.

Keywords:

• El Niño Modoki
• El Niño
• Northern Annular Mode
• Pacific-North American
• Southern Annular Mode