The response of the zonal-mean zonal winds to steady thermal forcing in the tropics was studied using an idealized general circulation model with 18 vertical levels and simplified atmospheric physics. The model produces a conventional general circulation, with weak easterly flow over the equator, when integrated using zonally-invariant and hemispherically-symmetric boundary conditions, but an exotic general circulation characterized by 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 components of the zonal-mean zonal wind balance in the superrotating state, along with experiments performed using equivalent zonally-invariant torques, indicate that the equatorial westerlies are driven by the net equatorward flux of westerly zonal momentum associated with the horizontal stationary wave response to the applied tropical heating. The zonal overturning associated with the vertical component of the steady eddy response along the equator acts to redistribute the zonal momentum anomaly downwards into the mid-troposphere, but the strength of the superrotation is ultimately limited by the mean meridional circulation, which erodes the mean westerly vertical shear at the equator via vertical advection.

Additional experiments were performed to determine which aspects of the idealized model promote the exotic response to eddy heating. The transition to superrotation can be prevented (or reversed) by imposing offsetting zonally-invariant heating and cooling anomalies on either side of the equator to create a "solstitial" basic state with a single Hadley cell straddling the equator. Westerly mean flow over the equator is restricted in a solstitial climate because the mean meridional circulation is enhanced and the cross-equatorial flow in the upper troposphere both restricts the propagation of planetary waves into the summer hemisphere and provides an easterly acceleration that offsets the momentum flux convergence associated with low-latitude thermal forcing. These results are consistent with previous axisymmetric model experiments and observational analyses suggesting that the seasonal cycle in the mean meridional circulation, especially the tendency for the maximum rising motion to occur off the equator throughout much of the year, is responsible for maintaining the observed mean easterly flow in the tropical upper troposphere against the westerly torques associated with tropical wave sources.