El Niño–East Asian monsoon teleconnection and its diversity in CMIP5 models

Abstract

In this study, we assess the performance of state-of-the-art coupled models in reproducing El Nino diversity and its impacts on the East Asian Summer Monsoon (EASM), based on simulations taken from the Coupled Model Intercomparison Project phase 5 (CMIP5). All of the 17 models used in this study are identified to give reasonable eastern-Pacific (EP) El Nino-related sea surface temperature anomalies (SSTA) and anomalous Walker circulation compared with observations, whereas one-third of the models fail to reproduce the Walker circulation anomalies for central-Pacific (CP) El Nino. In these models, negative SSTA over the eastern-to-central Pacific are found to be too strong, while positive SSTA shift to the western Pacific during CP El Nino. There is thus sinking (rising) motion over the central (western) part of the Pacific basin, implying westward displaced anomalous Walker cells, relative to the observed. This results in simulated low-level cyclonic flow and positive rainfall anomalies over the far western north Pacific/South China Sea during and after the peak phase of CP El Nino, which are opposite to the observed anomalous climate state. The other two-thirds of models, with realistic CP El Nino-related anomalous Walker cell, can better reproduce the corresponding wind circulation and rainfall anomalies over East Asian (EA) region. The contrasting CP El Nino-related teleconnection among models mainly arises from the different simulated CP El Nino SSTA, which in turn determine the circulation anomalies through a Gill-type response. Further analyses reveal that, in the mean state of models with erroneous teleconnection, there exists a large cold SST bias over the eastern Pacific. The stronger Walker circulation, and possibly the stronger Bjerknes feedback, might be responsible for the overly negative SSTA associated with CP El Nino in the vicinity. Our results highlight the sensitivity of El Nino–EA climate linkage to the oceanic state in models, especially for CP El Nino events.