Statistical Local Effect of Synoptic-Scale Transient Eddies on the Time-Mean Flow in the Northern Extratropics in Winter

Abstract

Time-filtered grid-point data for the northern extratropics in winter have been used to study the local effect on the time-mean flow, arising from the ?synoptic-scale? transient eddies (TE), which have periods between 2.5 and 6 days. The local TE effect is defined as the net work done on the time-mean flow by the TE induced force. In the interior of an air column, this force is determined by the irrotational TE flux of quasi-geostrophic potential vorticity; at the upper and lower boundaries, it is determined by the irrotational TE heat flux. The net work in the entire air column consists of the sum of a ?baroclinic? part, which affects only the vertical shear of the time-mean flow, and of a ?barotropic? part, which affects mainly the vertically-averaged component of the time-mean flow. The results show that the baroclinic part of the net TE work dominates and, in the storm track regions, gives an energy conversion from the time-mean flow to the synoptic-scale eddies. The barotropic part of the TE work shows in these same regions a partially compensating conversion of the TE energy back to the mean flow. These eddies thus tend to dissipate the baroclinic component and to strengthen the barotropic component of the time-mean flow with the concomitant tendency of strengthening the surface westerlies in the storm track regions and of shifting the time-mean jet stream polewards. The ?energy method? introduced in the present paper for the description of the interplay between the time-mean flow and the transient eddies is compared with other methods suggested in the literature for this purpose. The effect of synoptic-scale eddies on the time-mean flow is further compared briefly with that exerted by low-frequency fluctuations with periods between 10 and 90 days.