Variability of the Baroclinic and Barotropic Transient Eddy Forcing Associated with Monthly Changes in the Midlatitude Storm Tracks

Abstract

The heat and vorticity transports by synoptic-scale eddies at various levels between 1000 and 100 mb have been compiled for each winter month of the 1966?84 period using time-filtered daily analyses produced by the U.S. National Meteorological Center. These circulation statistics were used to compute the three-dimensional distributions of the quasigeostrophic geopotential tendency and vertical motion induced by baroclinic and barotropic eddy processes in individual months. The latter fields serve as the basis for describing the synoptic-scale eddy forcing associated with the leading modes of month-to-month variability of the storm tracks over the North Pacific and North Atlantic. These modes are associated with the meridional displacements of the storm-track axes from their climatological positions. The placement of a storm track at a certain latitude ? in a certain month is accompanied by enhanced convergence of eddy heat fluxes poleward of ?. In the tropospheric column poleward of the storm track, this baroclinic eddy forcing leads to positive geopotential tendency near the tropopause and negative geopotential tendency near sea level, as well as strong positive temperature tendency and rising motion. In the same month, the convergence of eddy vorticity transport is also enhanced poleward of ?. This barotropic forcing results in negative geopotential tendency throughout the troposphere, as well as rising motion and weak negative temperature tendency poleward of ?. All of these features appear with reversed polarity in latitudes equatorward of ?. In the upper troposphere, the geopotential tendency induced by vorticity fluxes is stronger than the opposing effects due to heat fluxes, so that the net eddy forcing retains most of the characteristics of the forcing associated with barotropic processes alone, but with considerably reduced amplitudes. Near sea level, the geopotential tendencies induced by heat and vorticity fluxes reinforce each other and are comparable in amplitude. Throughout the troposphere, the patterns of net geopotential tendency exhibit a positive spatial correlation with those of the concurrent monthly averaged height anomaly. The characteristic time scale associated with this constructive eddy forcing in the storm-track region ranges from several days at 1000 mb, to 1?2 months near the tropopause. On the other hand, the eddy-induced temperature tendency is negatively correlated with the local monthly mean temperature anomaly. The dissipative time scale for this thermal forcing in the storm-track region is ?10 days at 850 mb. The barotropic geopotential tendency and the baroclinic temperature tendency are essentially determined by the convergences of vorticity and heat fluxes, respectively. The eddy-induced secondary circulation plays a minor role in these tendencies.