The role of surface friction in tropical intraseasonal oscillation

The role of surface friction in the tropical intraseasonal oscillation, or the Madden-Julian oscillation (MJO), is investigated by comparing two 4-yr integrations with the Goddard atmospheric general circulation model: one wi th the original model design with one added feature to enhance the intensit y of the MJO and the other is identical but with surface friction in the Tr opics replaced by its zonal mean value. This comparison indicates that in t he second experiment the MJO not only still exists but also exists with sim ilar intensity. The oft-cited frictional wave-CISK (FWC) interpretation for the origin of the MJO, which emphasizes the role of frictionally induced c onvergence in the surface layer, is reassessed in light of these experiment s. The possibility of the MJO in the second experiment being forced by the middle latitudes is excluded by a third experiment. These experiments do no t support one of the central ideas in FWC that surface friction plays an in stability-enhancing role.