How Well Do Atmospheric General Circulation Models Capture the Leading Modes of the Interannual Variability of the Asian-Australian Monsoon?

The authors evaluate the performances of 11 AGCMs that participated in the Atmospheric Model Intercomparison Project II (AMIP II) and that were run in an AGCM-alone way forced by historical sea surface temperature covering the period 1979-99 and their multimodel ensemble (MME) simulation of the interannual variability of the Asian-Australian monsoon (AAM). The authors explore to what extent these models can reproduce two observed major modes of AAM rainfall for the period 1979-99, which account for about 38% of the total interannual variances. It is shown that the MME SST-forced simulation of the seasonal rainfall anomalies reproduces the first two leading modes of variability with a skill that is comparable to the NCEP/Department of Energy Global Reanalysis 2 (NCEP-2) in terms of the spatial patterns and the corresponding temporal variations as well as their relationships with ENSO evolution. Both the biennial tendency and low-frequency components of the two leading modes are captured reasonably in MME. The skill of AMIP simulation is seasonally dependent. December-February (DJF) [ July-August (JJA)] has the highest (lowest) skill. Over the extratropical western North Pacific and South China Sea, where ocean-atmosphere coupling may be critical for modeling the monsoon rainfall, the MME fails to demonstrate any skill in JJA, while the reanalysis has higher skills. The MME has deficiencies in simulating the seasonal phase of two anticyclones associated with the first mode, which are not in phase with ENSO forcing in observations but strictly match that of Nino-3.4 SST in MME. While the success of MME in capturing essential features of the first mode suggests the dominance of remote El Nino forcing in producing the predictable portion of AAM rainfall variability, the deficiency in capturing the seasonal phase implies the importance of local air-sea coupling effects. The first mode generally concurs with the turnabout of El Nino; meanwhile, the second mode is driven by La Nina at decaying stage. Multimodel intercomparison shows that there are good relationships between the simulated climatology and anomaly in terms of the degree of accuracy.