ENSO-monsoon relationship in the MRI coupled GCM

Climatological features and interannual variability of the Asian summer monsoon and its relationship with equatorial Pacific sea surface temperature (SST) anomalies simulated by a global coupled atmosphere-ocean GCM (CGCM) are investigated. The coupled model results are compared with the observation as well as the simulations by an atmospheric general circulation model (AGCM). Overall features of the model climatology and variability of the Asian summer monsoon in the CGCM are as close to the observed one as in the AGCM. The simulated SST and its variability in the CGCM shows some bias compared to the observation. The monsoon region in the models, as defined by the seasonal change of wind direction and convection proxy, agrees with the observed. The models are less successful in simulating the summertime wind system in the western Pacific region. The CGCM reasonably well reproduces the observed ENSO-related interannual variability of the tropical circulation system. Its main deficit is associated with a westward displacement of simulated SST variability. There is an underestimation of precipitation around the Philippines. Differences are found between the CGCM and the AGCM in the variability over the Indian monsoon region. The AGCM responds well to the prescribed SST anomaly in the Pacific. It behaves erroneously over the Indian Ocean. This may be related to the fact that the AGCM is only responding to the prescribed SST fields, while the CGCM includes two-way atmosphere-ocean interactions. The CGCM results show that the simulated Indian summer monsoon rainfall anomalies are negatively correlated with the equatorial Pacific SST anomalies. It is consistent with the observed one, i.e., good monsoon is associated with La Nina. As a precursory signal, ground temperature is significantly warmer in spring in central Asia preceding a good monsoon. It is noted that the snow cover anomalies are negative in the above region, but its significance is marginal.