Eddy–Zonal Flow Feedback in the Northern Hemisphere Winter

Abstract

The variability of the zonal-mean zonal wind in the Northern Hemisphere winter (December-March) is studied using EOF analysis and momentum budget diagnostics of NCEP-NCAR reanalysis data (1976-2001). The leading EOF of the zonal-mean zonal wind is well separated from the remaining EOFs and represents the north-south movement of the midlatitude westerlies. Analysis of the momentum budget shows that a positive feedback between the zonal-mean wind anomalies and the eddy momentum fluxes selects the leading EOF of midlatitude variability. Like the Southern Hemisphere, the baroclinic eddies reinforce the zonal wind anomalies while external Rossby waves damp the wind anomalies. In the Northern Hemisphere, the quasi-stationary eddies also reinforce the zonal wind anomalies, but the baroclinic eddies are most important for the positive eddy-zonal flow feedback. The observations support the following feedback mechanisms. 1) Above-normal baroclinic wave activity is generated in the region of enhanced westerlies. This leads to wave propagation out of the westerlies that is associated with reinforcing eddy momentum fluxes. 2) The westerly jet is a waveguide for external Rossby waves that tend to propagate into the jet and remove momentum from it. 3) The quasi-stationary waves respond to a refractive index anomaly in the high latitudes below the tropopause. During the high (low) index this anomaly is negative (positive) leading to an acceleration (deceleration) of the zonal wind in the high latitudes.

Time Scale and Feedback of Zonal-Mean-Flow Variability

“…This meets the necessary requirements for a positive feedback between the eddy momentum fluxes and the zonal-mean zonal flow. Similar results were found by Lorenz andHartmann (2001, 2003) for the annular modes in both hemispheres, but the implied feedback in their study was much weaker. For the CS2 run, the positive feedback, as measured by statistically significant positive correlations at positive time lags, persists for 61 days.…”

“…This reversal in the lag-lead relationship implies that there should be a local minimum correlation near the lag 0 day, which is indeed observed in Fig. 5 of Lorenz and Hartmann (2001) and Fig. 3a of this study.…”

“…Other studies of zonal-meanflow variability have also proposed mechanisms for a positive feedback. For example, Lorenz and Hartmann (2003) indicated that modification of the refractive index can aid the feedback. Gerber and Vallis (2007) discussed the possible role played by the mechanism proposed by Robinson (2006) who showed that the mean meridional circulation driven by the upper-level PV flux can maintain the midlatitude baroclinicity.…”

“…Following Lorenz and Hartmann (2001), the latitudinal profile of total forcing is projected onto the EOF1 pattern. The resulting time series, to be referred to as F1, is then correlated with PC1.…”

“…This time scale is longer than that for synoptic-scale eddies. Several studies have suggested that this increase in the time scale relative to that for synoptic-scale eddies, for both the zonal index and also for the northern and southern annular modes (Thompson and Wallace 2000), arises from a positive eddy-zonal-mean-flow feedback process (Robinson 1991;Yu and Hartmann 1993;Feldstein and Lee 1998;Lorenz andHartmann 2001, 2003). As a result, depending on the pervasiveness of the positive eddy feedback process, the time scale of the zonal index or the annular modes can potentially be much longer.…”