Impact of the Atmospheric Mean State on Tropical Instability Wave Activity

The features of simulated tropical instability waves (TIWs) in the Pacific Ocean are compared between atmospheric models of two different resolutions coupled with a uniform oceanic model. Results show that TIWs are more active in the high-resolution model, even though it includes atmospheric negative feedback. Such negative feedback is not identified in the low-resolution atmospheric model because of the absence of atmospheric responses. Comparison of the energetics between the two models shows that the large TIW activity in the higher-resolution model is due to the difference in barotropic energy sources near the surface. A high-resolution atmosphere results in a tighter intertropical convergence zone and associated stronger wind curl and shear. This causes a stronger surface current shear between the South Equatorial Current (SEC) and North Equatorial Counter Current (NECC), which is one of the main sources of TIW kinetic energy. These results indicate the important role of the atmospheric mean field on TIW activity and the advantage of using high-resolution models to represent coupling among multiscale phenomena.