Title: Eddy – zonal flow interactions in the Annular Modes:
the roles of synoptic and low-frequency eddies

Abstract: The annular modes are the leading modes of extratropical variability of the Northern and Southern Hemisphere tropospheric winds, and are often characterized by latitudinal shifts of the midlatitude jets with time scales more persistent than baroclinic eddies.
Using a multi-layer QG channel model, we studied the different response of high and low frequency (phase speed) eddies to the zonal wind anomalies associated with the annular modes and suggest a baroclinic mechanism through which the two eddies work symbiotically creating a positive feedback loop that extends the persistence of the jet shift. In this mechanism, the synoptic eddies drive the zonal wind anomalies directly, while the low-frequency eddies act to restore the lower-level baroclinicity at the jet center. Analysis also indicates that the different roles played by these two eddies in the annular modes are related to the differences in their critical line distributions. The importance of the lower-level baroclinicity restoration by the low-frequency eddies in the annular modes and how the proposed baroclinic mechanism works in the real atmosphere will also be discussed in the talk.

Host: Daniela Domeisen