Simulations of the Asian monsoon using a regionally coupled-global model

Abstract

This study examines the simulation of June–September mean of the Asian monsoon in three different numerical experiments with a global climate model in which the atmosphere and ocean are coupled (coupled general circulation model, CGCM), regionally coupled (“Pacemaker”), and with specified sea surface temperature as in the Atmospheric Model Inter-comparison Project (AMIP) applied to a global atmospheric general circulation model (AGCM). The main features of the climatology for wind at 850 hPa, rainfall and zonal wind shear (U200–U850 hPa) over the South China Sea, subtropical western Pacific and Arabian Sea regions are remarkably well simulated in the Pacemaker experiment, compared with observations, whereas the CGCM and AGCM experiments either underestimate or overestimate magnitude of anomalies. The observed relationships between NINO3.4 (an index of El Nino and the Southern Oscillation, ENSO) and the Indian summer monsoon indices are remarkably better captured in the Pacemaker experiment than in the CGCM or AGCM/AMIP experiments. The pattern correlations between the first empirical orthogonal function (EOF) of the model simulations and that of the observed precipitation is higher in the Pacemaker experiment than in the CGCM or AGCM experiments over a large region (40°E–80°W, 20°S–30°N). The temporal correlation between the first principal component of the model simulation and the observations is also higher for the Pacemaker experiment than for the AGCM experiment. The northward/eastward propagation features and the spectral peaks (30–60 days) of rainfall are significantly more realistically captured by Pacemaker in comparison to CGCM and AGCM/AMIP. Based on the correlation coefficient between seasonal and pentad EOF1 and the related composite analysis, we have found that the Pacemaker is able to reproduce the relationship between the intraseasonal and interannual variability of the South Asian monsoon. The observed improvement in simulation in the Pacemaker experiment, as compared to the CGCM experiment, illustrates that a simple system having only local air–sea feedback is capable of greater realism than sophisticated coupled models.