Ensemble Simulations of Asian–Australian Monsoon Variability by 11 AGCMs*

Ensemble simulations of Asian–Australian monsoon (A–AM) anomalies were evaluated in 11 atmospheric general circulation models for the unprecedented El Niño period of September 1996–August 1998. The models' simulations of anomalous Asian summer rainfall patterns in the A–AM region (30°S–30°N, 40°–160°E) are considerably poorer than in the El Niño region. This is mainly due to a lack of skill over Southeast Asia and the western North Pacific (5°–30°N, 80°–150°E), which is a striking characteristic of all the models. The models' deficiencies result from failing to simulate correctly the relationship between the local summer rainfall and the SST anomalies over the Philippine Sea, the South China Sea, and the Bay of Bengal: the observed rainfall anomalies are negatively correlated with SST anomalies, whereas in nearly all models, the rainfall anomalies are positively correlated with SST anomalies. While the models' physical parameterizations have large uncertainties, this problem is primarily attributed to the experimental design in which the atmosphere is forced to respond passively to the specified SSTs, while in nature the SSTs result in part from the atmospheric forcing. Regional monsoon dynamic indices are calculated for the Indian, the western North Pacific, and the Australian monsoons, respectively. Most models can realistically reproduce the western North Pacific and Australian monsoon, yet fail with the Indian monsoon. To see whether this is generally the case, a suite of five Seoul National University model runs with the same observed lower boundary forcing (and differing only in their initial conditions) was examined for the period 1950–98. The skill in the 49-yr ensemble simulations of the Indian monsoon is significantly higher than the skill for the period 1996–98. In other words for the unprecedented 1997/98 El Niño period, the models under study experience unusual difficulties in simulating the Indian monsoon circulation anomalies. Moreover, the observed Webster–Yang index shows a decreasing trend over the last 50 yr, a trend missed by the models' ensemble simulations.