The Characteristic Variability of Boreal Wintertime Atmospheric Circulation in El Niño Events

Abstract

The influence of the Pacific sea surface temperature (SST) on the characteristic variability of boreal wintertime atmospheric circulation among El Niño events is investigated by carrying out singular value decomposition (SVD) analysis and general circulation model (GCM) experiments. From the SVD analysis of SST and 300-hPa streamfunction in the El Niño winter, the first mode associated with the strength of the El Niño mode itself has a positive trend in its time series and the second mode associated with the North Pacific SST variation includes substantial event-to-event variability. The second mode is thus important for explaining the inter?El Niño variability in atmospheric circulation. Four El Niños among 15 El Niño events from 1950 to 1998 have a mainly positively contribution from the first SVD mode, and the other four events are influenced by the second mode as well as the first mode. The atmospheric responses related to the latter four events show the weakening of the Aleutian low and an eastward-shifted pattern of circulation in contrast with the former ones. Results of ensemble GCM experiments imply that the North Pacific SST leads to differences in the atmospheric circulation among El Niño years. When the SST anomaly exists only in the North Pacific, the atmospheric responses to SST forcing are very weak. However, in the presence of a negative SST anomaly in the North Pacific with a positive SST anomaly in the equatorial eastern Pacific, the atmospheric responses are much stronger than the case with the absence of SST forcing in the North Pacific. When a positive SST anomaly exists in the North Pacific, the responses are weaker. Therefore, the intensity of the atmospheric responses depends partly on the sign of the SST anomaly in the North Pacific. In addition, the potential predictability using ensemble experiments is calculated to investigate the relative importance of SST forcing with respect to the internal dynamics in atmospheric circulation. The predictability of atmospheric responses is poor for the SST forcing in the North Pacific only, but is very good for the negative SST forcing in the North Pacific and the positive SST forcing in the equatorial eastern Pacific.