Consideration of the origin of the different midlatitude atmospheric responses among El Nino events

The Pacific/North American (PNA) and Western Pacific (WP) teleconnection patterns during the winter are considered to be associated with El Nino events. Although each PNA or WP index is only weakly correlated with the El Nino/Southern Oscillation (ENSO) cycle, when they are combined together a high correlation is obtained. This result suggests that both atmospheric circulation patterns, PNA and WP, are not directly connected to the ENSO cycle, but their overall occurrence frequencies are controlled by El Nino events. In order to examine the condition that determines the type of atmospheric circulation that appears concurrently during an El Nino winter, a composite analysis is conducted by classifying the El Nino winters according to the relative amplitudes of PNA and WP indices. The result suggests that different types of responses are not strongly dependent on the differences in sea surface temperature (SST) distribution among El Nino events, but rather they depend on the circulation condition over the Eurasian continent during the early winter.