Equatorial Superrotation and the Factors Controlling the Zonal-Mean Zonal Winds in the Tropical Upper Troposphere

The response of the zonal-mean zonal winds in the tropical upper troposphere to thermal forcing in the Tropics is studied using an idealized general circulation model with 18 vertical levels and simplified atmospheric physics. The model produces a conventional general circulation, with deep easterly flow over the equator, when integrated using zonally invariant and hemispherically symmetric boundary conditions, but persistent equatorial superrotation (westerly zonal-mean flow over the equator) is obtained when steady longitudinal variations in diabatic heating are imposed at low latitudes. The superrotation is driven by horizontal eddy momentum fluxes associated with the stationary planetary wave response to the applied tropical heating. The strength of the equatorial westerlies is ultimately limited by vertical steady eddy momentum fluxes, which are downward in the tropical upper troposphere, and by the zonally averaged circulation in the meridional plane, which erodes the mean westerly shear via vertical advection. The transition to superrotation can be prevented by specifying offsetting zonally invariant heating and cooling anomalies on either side of the equator to create a “solstitial” basic state with a single dominant Hadley cell straddling the equator. Superrotation is restricted in the solstitial climate because the strength of the mean meridional overturning is enhanced, which increases the efficiency of vertical advection, and because the cross-equatorial flow in the upper troposphere provides an easterly zonal acceleration that offsets some of the momentum flux convergence associated with tropical eddy heating. The cross-equatorial flow aloft also reduces the stationary planetary wave response in the summer hemisphere. These results suggest that hemispheric asymmetry in the mean meridional circulation is responsible for maintaining the observed mean easterly flow in the tropical upper troposphere against the westerly torques associated with tropical wave sources.