Superrotation in an axisymmetric shallow water model of the upper troposphere

Abrupt transitions to strongly superrotating states have been found in some idealized models of the upper troposphere. These transitions are thought to be caused by feedbacks between the eddy momentum flux convergence in low latitudes and the strength of the equatorial flow. The behavior of an axisymmetric shallow water model with an applied tropical torque is studied here to determine if an abrupt transition to a superrotating state can be realized without eddy feedbacks. The layer is relaxed to a radiative equilibrium thickness, exchanging mass and thus momentum with the non-moving lower layer. For low values of the applied torque, the circulation is earth-like; however, for larger values, an abrupt transition to a strongly superrotating state can occur, in conjunction with multiple equilibria and hysteresis. A simple analytical model is used to better understand the system. The bifurcation is caused by a feedback between the applied torque and the strength of the Hadley cell. As the torque increases, the cell weakens, reducing the strength of the damping caused by momentum transfer from the lower layer.