Altered atmospheric responses to eastern Pacific and central Pacific El Niños over the North Atlantic region due to stratospheric interference

Abstract

The two types of El Nino that have been identified, namely the eastern Pacific (EP) and central Pacific (CP) El Ninos, are known to exert different climatic impacts on the North Atlantic region during winter. Here, we investigate the characteristics of the teleconnection of the two El Nino types with a focus on the stratosphere-troposphere coupling. During the EP El Nino, polar stratospheric warming and polar vortex weakening frequently occur with a strong tendency for downward propagation near the tropopause. Consequently, the atmospheric pattern within the troposphere over the North Atlantic sector during midwinter closely resembles the negative North Atlantic Oscillation pattern. In contrast, during CP El Nino events stratospheric warming events exhibit a much weaker downward propagation tendency. This difference in the stratospheric circulation response arises from the different seasonal evolution of the tropospheric wave response to the two El Nino types. For the EP El Nino, the Aleutian Low begins growing during December and is sustained throughout the entire winter (December to February), which provides favorable conditions for the continuous downward propagation of the stratospheric warming. We also discuss the origin of the difference in the teleconnections from the two types of El Nino associated with the distinct longitudinal position of the warm SST anomaly that determines troposphere-stratosphere coupling.