Thermodynamic Bias in the Multimodel Mean Boreal Summer Monsoon*

Here it is shown that almost all models participating in the Coupled Model Intercomparison Project (CMIP) exhibit a common bias in the thermodynamic structure of boreal summer monsoons. The strongest bias lies over South Asia, where the upper-tropospheric temperature maximum is too weak, is shifted southeast of its observed location, and does not extend as far west over Africa as it does in observations. Simulated Asian maxima of surface air moist static energy are also too weak and are located over coastal oceans rather than in their observed continental position. The spatial structure of this bias suggests that it is caused by an overly smoothed representation of topography west of the Tibetan Plateau, which allows dry air from the deserts of western Asia to penetrate the monsoon thermal maximum, suppressing moist convection and cooling the upper troposphere. In a climate model with a decent representation of the thermodynamic state of the Asian monsoon, the qualitative characteristics of this bias can be recreated by truncating topography just west of the Tibetan Plateau. This relatively minor topographic modification also produces a negative anomaly of Indian precipitation of similar sign and amplitude to the CMIP continental Indian monsoon precipitation bias. Furthermore, in simulations of next-century climate warming, this topographic modification reduces the amplitude of the increase in Indian monsoon precipitation. These results confirm the importance of topography west of the Tibetan Plateau for South Asian climate and illustrate the need for careful assessments of the thermodynamic state of model monsoons.