The annual variation of global atmospheric angular momentum (AAM) is dominated by its first and and second harmonic components. The first harmonic is associated with a maximum of global AAM in the boreal winter and a minimum in summer, but the second harmonic is important enough to produce a distinct secondary mid-winter minimum. Locally, the second harmonic has largest amplitude in the tropics and subtropics of the upper troposphere. At present little is known concerning the fundamental cause of this semiannual variation. The problem is investigated here by focusing on the upper tropospheric winds, whose angular momentum is an excellent proxy of global AAM. The annual variation of the rotational part of these winds (the part that contributes to the global AAM) is diagnosed in a nonlinear one-level vorticity-equation model with specified upper tropospheric horizontal wind divergence and transient-eddy forcing. The divergence forcing is the more important of the two, especially in the tropics and subtropics, where it is associated with tropical heating and cooling. Given the harmonics of the forcing, the model predicts the harmonics of the response, i.e. the vorticity, from which the harmonics of angular momentum are then calculated. The surprising but clear conclusion from this diagnosis is that the second harmonic of AAM arises more as a nonlinear response to the first harmonic of the divergence forcing than as a linear response to the second harmonic of the divergence forcing. This result has implications for general circulation model (GCM) simulations of semiannual variations, not only of global AAM but also of other quantities.
Close window or click on previous window to return to the Conference Program.
12th Conference on Atmospheric and Oceanic Fluid Dynamics