Low-frequency variability and possible changes in the north pacific simulated by CMIP5 models

Abstract

The Pacific Decadal Oscillation (PDO) and North Pacific Gyre Oscillation (NPGO) are the two dominant lowfrequency modes in the North Pacific. This study focused on the simulation capability of the two leading lowfrequency modes in current-coupled models, based on 24 coupled model outputs from the Coupled Model Intercomparison Project Phase 5 (CMIP5). Results showed that most of these models captured the two low-frequency modes, but the air-sea coupling relationship (covariability of the ocean low-frequency modes with the atmospheric forcing modes) captured by CMIP5 models had drastic differences. Four models (CCSM4, CESM-WACCM, MIROC5, and NorESM1-M) not only captured the spatial and temporal characteristics of the PDO and NPGO modes but also simulated their air-sea coupling relationships. Therefore, we selected these four models to examine changes in the PDO and NPGO modes under different global warming scenarios using RCP4.5 and RCP8.5 forcing (RCP: Representative Concentration Pathway). In future RCP scenarios, the spatial patterns of PDO and NPGO showed no obvious changes. However, the dominant periods of the PDO and NPGO modes were shorter, which is consistent with faster oceanic Rossby waves induced by enhanced upper oceanic stratification in the warming scenarios.