Abstract
The observed wintertime distribution of the poleward flux of westerly momentum by transient eddies at the jet stream (250 mb) level is characterized by 1) a strong convergence of momentum flux into “storm tracks” over the western and central oceans near 40°N and 2) strong poleward fluxes over the western parts of the continents. The former fluxes are strongly countergradient. This observed distribution is discussed in light of the flux maintenance equation, which is analogous to the equations for the local time rate of change of eddy kinetic energy. The terms of major interest in this equation are a so-called “mixing term,” which is always acting to produce a down-gradient flux, and a pair of terms which involve temporal correlations between the eddy velocity components and their respective ageostrophic departures. It is shown that the observed countergradient fluxes over the oceans must be maintained by these ageostrophic correlations terms.
The geographical distributions of the terms described above are estimated on the basis of ten winters' hemispheric synoptic charts. The ageostrophic correlation terms tend to produce eddy fluxes of westerly momentum into the storm tracks, as observed. It is proposed that the convergence of westerly momentum into the storm tracks is a consequence of the fact that there is a strong tendency for air with high westerly momentum to be accelerated in the direction of the center of the storm track by the imbalance between pressure gradient and Coriolis forces.
