Dominant modes of CMIP3/5 models simulating northwest Pacific circulation anomalies during post-ENSO summer and their SST dependence

Abstract

Based on an intermodel empirical orthogonal function (EOF) analysis, this study has investigated the dominant modes of northwest Pacific (NWP) circulation anomalies during post-ENSO summer and their SST dependence involved in 47 Coupled Model Intercomparison Project phase 3 and phase 5 models. The first EOF mode, explaining 33.3% of total intermodel variance, features an anomalous cyclone over the tropical NWP and is controlled by the positive SST anomalies over the equatorial western Pacific (WP). The equatorial WP warming enhances local convection with lower- (upper-) level convergence (divergence), and the anomalous cyclone is a direct Rossby wave response to positive rainfall anomalies there. The second EOF mode, explaining 24.6% of total intermodel variance, is characterized by an anomalous NWP anticyclone (NWPAC). The related SST anomalies show warming in the tropical Indian Ocean (TIO) and equatorial central and eastern Pacific (CEP) and cooling in the NWP. The TIO (CEP) warming induces local wet anomalies, which trigger eastward (westward) Kelvin (Rossby) wave, resulting the adjustment of large-scale circulation. The resultant lower- (upper-) level divergence (convergence) suppresses convection over the NWP, inducing the anomalous NWPAC as a Rossby wave response. The NWP cooling influences NWPAC via positive thermodynamic feedback between local SST and circulation anomalies. Model results further confirm the role of leading mode-related SST anomalies affecting the simulation of NWP circulation.