Decadal changes in the relationship between the tropical Pacific and the North Pacific

Abstract

It has been recognized that the tropical Pacific sea surface temperature (SST) anomaly associated with the El Nino/Southern Oscillation (ENSO) is closely tied to large-scale atmospheric anomalies in the extra-tropical region. Based on observational data, this connection appears to have changed significantly since 1999. This study examines the characteristics of this connection during two sub-periods (1980-1998 and 1999-2010) using cyclostationary empirical orthogonal function (CSEOF) analysis. We found that the atmospheric anomalies induced by the ENSO alter the SST over the North Pacific through surface heat flux change during the earlier period of 1980-1998. During the later period, in contrast, ENSO-related atmospheric anomalies feature the North Pacific oscillation (NPO) pattern during winter, which is characterized by a southward shift of the atmospheric center of action from its climatological position. The NPO-related atmospheric anomalies extend to the subtropical Pacific; this extension potentially links midlatitude and tropical Pacific variability through air-sea interactions. The physical change appears to alter the El Nino characteristics into that of the Central Pacific El Nino through the wind-SST coupling mechanism. This process may be related to the recent change in which the Central Pacific El Nino occurs more frequently than the conventional Eastern Pacific El Nino. Detailed descriptions of the physical changes between the tropical Pacific and the North Pacific are presented in this study along with the possible implications of this change.