The Effect of Large-Scale Transient Eddies on the Time-Mean Flow in the Atmosphere

The effect of horizontal transports of momentum and heat by transient eddies (TE) on the time-mean flow is studied by examining the relevant terms in a local budget of quasi-geostrophic potential vorticity. Two long-term observational data sets are used, and results for the Northern Hemisphere winter are presented. The results indicate that eddy heat fluxes in the free atmosphere exert a dissipative influence on both the zonally averaged flow and the stationary waves. Conversely, eddy momentum transports tend to force cyclonic circulations over the semipermanent Icelandic and Aleutian surface lows and anticyclonic circulations over the oceanic high-pressure cells in the subtropics. The forcing of the time-mean flow arising from horizontal TE heat transports is generally stronger than the forcing associated with eddy momentum transports. The net effect of eddy transports of heat and momentum is to dissipate the potential enstrophy of the stationary waves. The characteristic time scale associated with this dissipative effect is of the order of 4-5 days.The relative contribution to the eddy forcing by low-frequency fluctuations (with periods between 10 days and a season) and by synoptic-scale fluctuations (with periods of 2.5-6 days) is examined. The forcing associated with low-frequency eddies generally dominates. The forcing associated with synoptic-scale eddies is concentrated in the cyclone tracks near the east coasts of Asia and North America, where a certain degree of counterbalancing between the heat flux forcing and the momentum flux forcing occurs.