CMIP5 model simulations of the Pacific meridional mode and its connection to the two types of ENSO

摘要

This study examines the Pacific meridional mode (PMM) simulated in the pre-industrial simulations of the Coupled Model Intercomparison Project Phase 5 (CMIP5). The spatial pattern and intensity of the PMM were found to be reasonably simulated by CMIP5 models, as was the subtropical atmosphere-ocean coupling associated with the PMM. However, the persistence of the coupling, which sustains the PMM's duration and extends its pattern equatorward, was found to be underestimated in most of the CMIP5 models. Many of the simulated PMMs do not have a pattern that extends far enough into the tropical Pacific to influence El Nino-Southern Oscillation (ENSO). The CMIP5 models that do produce longer persistence for the PMM coupling show a higher correlation of the PMM with the Central-Pacific (CP) type of ENSO than with the Eastern-Pacific (EP) type of the ENSO. This study concludes that (1) the PMM and its associated subtropical Pacific ocean-atmosphere coupling are important to the generation of the CP type of ENSO; (2) the so-called seasonal footprinting mechanism that sustains an equatorward extension of the PMM is not well simulated in a majority of the CMIP5 models; and (3) the persistence of the subtropical Pacific coupling is more important than the other properties in gauging a model's skill in the PMM simulation.