Disparate Midlatitude Responses to the Eastern Pacific El Niño

Abstract

To investigate the disparate influences of the eastern Pacific (EP) El Niño on the winter climate in the Far East, we conducted composite analyses using long-term reanalysis datasets. Our analysis shows that the western Pacific (WP) pattern dominates in the warm winter (typical) composite and the Pacific–North American (PNA) pattern dominates in the non-warm winter (atypical) composite. In the warm winter case, the amplitudes of the negative sea surface temperature (SST) anomalies in the western Pacific Ocean are large whereas in the non-warm winter case, these amplitudes are small. In addition, the Indian Ocean basin warming occurs following the Indian Ocean dipole mode, as seen in the warm winter composite. We investigated the dynamical mechanisms responsible for the disparate midlatitude responses to the EP El Niño by focusing on Rossby wave sources and propagation. These SST anomalies modulate the Walker and Hadley circulations and the convective activity in the western Pacific Ocean. Upper-tropospheric divergences at the midlatitudes due to the anomalous Hadley circulation result in different teleconnection patterns. In the warm winter composite, the anticyclonic anomaly in the southern part of the WP pattern is created by the upstream negative Rossby wave source, while the other cyclonic anomaly is reinforced by the northward Rossby wave propagation. The cyclonic second and fourth centers of action of the PNA pattern are created by the positive Rossby wave sources. Furthermore, the equatorial SST gradient near the date line is found be a good precursor of the winter climate in the Far East.