The Observed Structure of Tropospheric Stationary Waves and the Local Balances of Vorticity and Heat

Abstract

The physical structure and the associated transport properties of stationary waves in the troposphere are described using circulation statistics compiled from twice-daily hemispheric analyses covering 11 winters. The distributions of standing eddy meridional transports in middle latitudes are characterized by momentum flux convergence and equatorward geopotential energy transports in the upper troposphere, and by poleward heat fluxes at the lower levels. The contributions of steady and transient motions to the local, time-averaged budget of vorticity at 300 mb are evaluated. The dominant terms in the time-averaged vorticity equation are the local advection of relative vorticity by the stationary flow and the divergence term. The advection of planetary vorticity by the mean flow (the ?-effect) and the convergence of vorticity fluxes by transient eddies appear to be of secondary importance. The hemispheric distributions of the principal terms in the vorticity balance are closely related to topographical features at the lower boundary. The hemispheric field of stationary flow divergence at various levels is determined as a residual in the time-averaged vorticity balance. This diagnosed divergence field is used to 1) demonstrate the feasibility of retrieving essential stationary flow features in the upper troposphere through solution of the linearized vorticity equation with a prescribed divergence forcing; 2) deduce the velocity potential field; and 3) compute the mean vertical motion field through vertical integration of the continuity equation. Standing eddy statistics involving vertical motions are described. Mean vertical motions in middle latitudes are found to be positively correlated with mean meridional motions and with mean temperature. The distributions of meridional and vertical transports of geopotential energy and westerly momentum in the meridional plane are presented in a vectorial format. The pattern depicting geopotential energy fluxes suggests that the enhanced standing wave kinetic energy over the subtropics is maintained by geopotential energy transports which originate from higher latitudes. A diagnosis of the local, time-averaged balance of heat at 1000 and 700 mb is performed. The heat transports by the transient eddies in the lower troposphere exhibit a strong tendency to destroy the zonally asymmetric component of the stationary temperature field. This dissipative mechanism acts on a time scale of several days. The hemispheric distributions of the diabatic heating deduced from the heat budget are indicative of the central role of geographically fixed influences such as ocean currents and sea-land contrast.