Delayed Seasonal Transition of Tropical Wave Activity in the CMIP3 Global Climate Models

This study evaluates the seasonal cycle of the activity of convectively coupled equatorial waves (CCEWs), including mixed Rossby-gravity (MRG) and tropical depression-type (TD-type) waves, based on the twentieth century experiments of 18 global climate models (GCMs) from the Coupled Model Intercomparison Project phase 3 (CMIP3). The ensemble result of the 18 GCMs shows that the observed seasonal cycle of MRG and TD-type wave activity cannot be well reproduced. The seasonal transition of wave activity from the southern hemisphere to the northern hemisphere is delayed from April in the observations to May in the simulations, indicating that the simulated active season of tropical waves in the northern hemisphere is delayed and shortened. This delayed seasonal transition of tropical wave activity is associated with a delayed seasonal transition of simulated mean precipitation. The mean precipitation in April and May shows a double-ITCZ problem, and the horizontal resolution is important to the delayed seasonal transition of wave activity. Because of the coincident seasonal cycle of MRG and TD-type wave activity and tropical cyclone (TC) geneses, the delayed seasonal transition of wave activity may imply a similar problem of TC genesis in the GCMs, namely, a delayed and shortened TC season in the northern hemisphere.