Relationships between ENSO and the East Asian–western North Pacific monsoon: observations versus 18 CMIP5 models

Abstract

The western North Pacific anomalous anticyclone (WNPAC) plays an essential role in linking El Nino and the East Asian–western North Pacific monsoon. The WNPAC and the corresponding WNP anomalous cyclone (WNPC) associated with La Nina are studied by examining the pre-industrial control runs of 18 Coupled Model Intercomparison Project Phase 5 models. The WNPAC is asymmetric with the WNPC during ENSO mature winters, which can be reproduced by seven out of the eighteen models. Based on the differences between the seven models and the other models, it is verified that the asymmetry results from the combined effects of the asymmetric remote forcing from the equatorial central-eastern Pacific and the asymmetric local sea surface temperature (SST) anomalies. It is further demonstrated that the asymmetric circulation anomalies over the tropical WNP contribute to the asymmetric decaying rates between El Nino and La Nina. Those models that can (cannot) simulate the asymmetry between the WNPAC and WNPC tend to reproduce ENSO with asymmetric (symmetric) decaying rates. In El Nino decaying summers, the WNPAC is still maintained due to the sequential effects of the local forcing of the underlying cold SST anomalies remaining from the previous winter and spring, and the remote forcing from the tropical Indian Ocean. Multi-model results show that the coupling of the local cold SST anomalies and the WNPAC forms a damping mode, which is only conducive to maintaining the WNPAC in the early summer, while the response of the WNPAC to the remote forcing from the tropical Indian Ocean intensifies from June to July, concurring with the establishment of the climatological WNP summer monsoon trough.