Torques and the Related Meridional and Vertical Fluxes of Axial Angular Momentum

Abstract

The budget equation of the zonally averaged angular momentum is analyzed by introducing belts of 1000-km width to cover the meridional plane from pole to pole up to an altitude of 28 km. Using ECMWF Re-Analysis (ERA) data the fluxes of angular momentum are evaluated as well as the mountain and friction torques per belt. Generalized streamfunctions and velocity potentials are introduced to better depict the fluxes related to the angular momentum transferred at the ground during an event of mountain or friction torque. The variance of the total flux divergence per belt is one order of magnitude larger than those of the torques. All variances peak at midlatitudes. As a rule, the structure of the generalized streamfunctions changes little during an event; that is, the structure of the nondivergent part of the fluxes is stable. That of the divergent part, as represented by the velocity potential, undergoes a rapid change near the peak of a torque event. Positive friction torque events in midlatitude belts are preceded by a divergence of angular momentum fluxes in that belt, which is linked to the anticyclonic mass circulation needed to induce the positive torque. The divergence in the belt breaks down shortly before the torque is strongest. Angular momentum is transported upward from the ground after that. Much of the angular momentum generated in a midlatitude belt by positive mountain torques is transported out of the domain, but there is also a short burst of upward transports. Angular momentum anomalies linked to torque events near the equator tend to be symmetric with respect to the equator. Related fluxes affect the midlatitudes of both hemispheres.