Equatorial Variability and Transition to Superrotation in Warm Climate Simulations

Previous work shows that a transition to superrotation, a state with westerly winds at the equator, occurs in a general circulation model when tropical temperatures are sufficiently high. The transition is caused by enhanced transient eddies on the equator, similar in structure to the modern day MJO. The MJO-like wave activity converges zonal momentum onto the equator, driving strong superrotation at tropopause levels. Two questions remain open: what drives the increase of the MJO-like waves with temperature and how does superrotation affect the angular momentum budget, in particular at the surface. Here these questions are studied using a set of aquaplanet simulations employing the NCAR CAM3 model. To understand the driving mechanism behind the increase in MJO-like activity the moist static energy budget is analyzed. Results show, in agreement with previous studies employing the superparameterized version of the model, that the waves are initially driven by a gross moist stability feedback. However, this is not the case for a strong superrotating state. Instead the change in driving seems to depend on a more complex relation involving the horizontal advection of moist static energy and to a lesser degree the latent heat. Regarding the angular momentum budget, the eddy momentum convergence at the tropopause is mostly balanced by cross-equatorial zonal-mean flow and downward momentum transport by the eddies themselves. Below tropopause levels, zonal-mean equatorial winds are strongly sheared and the zonal mean become negative at the surface. However, for the warmest cases the surface wind can become seasonally westerly, with important implications for the fate of ENSO variability in such warm climates.