The Dynamics Associated with Equatorial Atmospheric Angular Momentum in an Aquaplanet GCM

Abstract

The dynamical processes that drive intraseasonal equatorial atmospheric angular momentum (EAAM) fluctuations in a 4000-day aquaplanet GCM run are examined. The all-ocean lower boundary has a sea surface temperature field that is both independent of longitude and symmetric across the equator. Because of the absence of topography, the model includes an equatorial bulge and friction torque, but not a mountain torque. The methodology adopted is to regress variables such as surface pressure, streamfunction, precipitation, and the two torques against individual components and the amplitude of the EAAM vector. The results indicate that the phase of the EAAM vector is associated with the westward propagation of a zonal wavenumber-1 midlatitude Rossby wave. This wave has characteristics that closely match those of a normal mode of the GCM and also those of the first antisymmetric rotational mode of the shallow water model on the sphere. Fluctuations in the amplitude of the EAAM vector are found to be related to the presence of a zonal wavenumber-1 mixed Rossby–gravity wave in the Tropics. The structure of the precipitation anomalies suggests that the latent heat release associated with the mixed Rossby–gravity wave excites poleward Rossby wave propagation, which alters the EAAM amplitude. The above dynamical processes are also found to determine the phase and amplitude of the equatorial bulge torque. It is this torque that dominates the driving of the EAAM. Lastly, the properties of the friction torque are discussed.