Variations of atmospheric angular momentum associated with intraseasonal oscillations forced by zonally moving prescribed heating

Abstract

Intraseasonal variations of atmospheric angular momentum (AAM) are numerically studied. With prescribed 40‐day‐period external heating which moves eastward from 60°E to the dateline in the tropics, intraseasonal oscillations are forced in a numerical model. In accordance with the oscillations, the variations of AAM also show 40‐day periods. The phase relationship between the model intraseasonal oscillations and AAM variations is similar to that observed. Further, AAM exhibits the largest variation in the equatorial region, and a phase propagation from equatorial regions to the subtropics, and from high latitudes to the subtropics, as found in the real atmosphere. After careful inspection of these features, it was clarified that the source region for intraseasonal variations of AAM is not the equatorial region, but the subtropics. Also, the equatorial region is an apparent or superficial source region, since AAM is transported from the subtropics, in accordance with the propagation of Rossby wave trains to the subtropics. The phase propagation of AAM from high latitudes to the subtropics can also be explained from the behavior of Rossby wave trains in the midlatitudes radiating from the equatorial region. Thus, Rossby wave trains associated with intraseasonal oscillations play an important role in intraseasonal variations of AAM.