Karen M. Shell     

 

Superrotation in Earth-like atmospheres


Relevant Paper:

Shell, Karen M., and Isaac M. Held, 2004: Abrupt transition to strong superrrotation in an axisymmetric model of the upper troposphere. J. Atmos. Sci., 61, 2928-2935. (*)



Figure from Shell and Held (2004). Nondimensional zonal wind at the Equator for a range of Equatorial forcing values. Circles correspond to full model results, asterisks to stable solutions of the analytic model, and X's to unstable solutions. Analytic solutions with U>1 are not valid and are shown for illustrative purposes only.

As part of the 2000 Geophysical Fluid Dynamics Summer Program at Woods Hole, I wrote a zonally-average shallow water model to study a feedback between a superrotating atmosphere and the Hadley circulation. The term superrotating refers to an atmosphere in which the zonal average absolute angular momentum at some latitude or height exceeds any surface values. On a counterclockwise rotating planet, superrotation corresponds to westerly winds at the equator. The earth's troposphere is not superrotating, since the mean equatorial tropospheric winds are slightly easterly. However, superrotation occurs during the westerly phase of the Quasi-Biennial Oscillation (QBO) in the stratosphere, as well as on other planets, such as Jupiter and Saturn. These cases raise the question of whether the Earth's troposphere could be superrotating under somewhat different conditions.

Abrupt transitions to strong superrotation have been found in some idealized models of the troposphere. The transitions correspond to drastic changes in atmospheric circulation patterns and are thought to be caused by feedbacks between zonally asymmetric processes (i.e., eddy momentum flux convergence) in low latitudes and the strength of the equatorial flow.

In collaboration with Isaac Held of the Geophysical Fluid Dynamics Laboratory, I studied the behavior of an axisymmetric shallow water model with an applied tropical torque to determine if an abrupt transition can be realized without eddy feedbacks. The upper tropospheric layer is relaxed to a radiative equilibrium thickness, exchanging mass and thus momentum with the non-moving lower layer through the upward and downward branches of the Hadley cell. For low values of the applied torque, the circulation is earth-like; however, for larger values, an abrupt transition to strong superrotation can occur. In some cases, the system remains strongly superrotating as the torque is subsequently decreased. The bifurcation is caused by a positive feedback between the equatorial zonal wind and the strength of the Hadley cell. As the wind increases in response to the applied torque, the strength of the cell decreases, reducing the vertical flux of momentum and further accelerating the wind.

In a zonally asymmetric atmosphere, eddy feedbacks may play the dominant role in an abrupt transition to superrotation. However, the feedback described here is capable of producing an abrupt transition in the axisymmetric case and may amplify transitions in a zonally asymmetric atmosphere. More work is necessary to determine if an abrupt transition to strong superrotation is possible in the terrestrial setting.


Last modified: Thu Nov 1 11:52:38 PDT 2012