THE DYNAMICS OF INTRASEASONAL ATMOSPHERIC ANGULAR-MOMENTUM OSCILLATIONS

The global and zonal atmospheric angular momentum (AAM) budget is comp uted from seven years of National Centers for Environmental Prediction data and a composite budget of intraseasonal (30-70 day) variations d uring northern winter is constructed. Regressions on the global AAM te ndency are used to produce maps of outgoing longwave radiation, 200-hP a wind, surface stress, and sea level pressure during the composite AA M cycle. The primary synoptic features and surface torques that contri bute to the AAM changes are described. In the global budget, the frict ion and mountain torques contribute about equally to the AAM tendency. The friction torque peaks in phase with subtropical surface easterly wind anomalies in both hemispheres. The mountain torque peaks when ano malies in the midlatitude Northern Hemisphere and subtropical Southern Hemisphere are weak but of the same sign. The picture is different fo r the zonal mean budget, in which the meridional convergence of the no rthward relative angular momentum transport and the friction torque ar e the dominant terms. During the global AAM cycle, zonal AAM anomalies move poleward from the equator to the subtropics primarily in respons e to momentum transports. These transports are associated with the spa tial covariance of the filtered (30-70 day) perturbations with the cli matological upper-tropospheric flow. The zonally asymmetric portion of these perturbations develop when convection begins over the Indian Oc ean and maximize when convection weakens over the western Pacific Ocea n, The 30-70-day zonal mean friction torque results from 1) the surfac e winds induced by the upper-tropospheric momentum sources and sinks a nd 2) the direct surface wind response to warm pool convection anomali es. The signal in relative AAM is complemented by one in ''Earth'' AAM associated with meridional redistributions of atmospheric mass. This meridional redistribution occurs preferentially over the Asian land ma ss and is linked with the 30-70-day eastward moving convective signal. It is preceded by a surface Kelvin-like wave in the equatorial Pacifi c atmosphere that propagates eastward from the western Pacific region to the South American topography and then moves poleward as an edge wa ve along the Andes. This produces a mountain torque on the Andes, whic h also causes the regional and global AAM to change.