Another Look at the Annual and Semiannual Cycles of AtmosphericAngular Momentum

Abstract

The annual variation of global atmospheric angular momentum (AAM) is dominated by its first and second harmonic components. The first harmonic is associated with maximum global AAM in winter (December? January?February) and minimum in summer, but the second harmonic is important enough to produce a distinct secondary midwinter 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 upper-tropospheric vorticity-equation model with specified 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, that is, the vorticity, from which the harmonics of angular momentum can then be 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 simulations of semiannual variations, not only of global AAM but also of other quantities.