Combined Effects of El Niño and the Pacific Decadal Oscillation on Summertime Circulation over East Asia

Abstract

The El Nino-Southern Oscillation (ENSO) and Pacific decadal oscillation (PDO) are the two major modes of sea surface temperature (SST) variability over equatorial and North Pacific regions, respectively. In this study, the combined effects of the ENSO and PDO on summertime circulation and precipitation fields over East Asia are investigated. Results show that SST forcing associated with a positive ENSO phase intensifies the anticyclonic circulation anomaly over the western North Pacific (WNP) region through a meridionally propagating Rossby wave train or via downward motion due to an overturning circulation stemming from equatorial central Pacific warming. A strong negative SST anomaly over the North Pacific during positive PDO phases increases local meridional temperature gradient and thus induces anomalous westerly winds along 35°N. This wind anomaly forms the northern margin of the anticyclonic circulation anomaly over the WNP. The combined effect of positive ENSO and positive PDO phases strengthens the anticyclonic circulation anomaly more than when these forcings are considered separately. Therefore, to the north of the anomaly, precipitation is enhanced due to the increased moisture flux transport and convergence along the rim of the WNP subtropical high.