Stratosphere-troposphere coupling during tropospheric anticyclonic wave breaking

The possible coupling between upper tropospheric anticyclonic wave breaking and stratospheric sudden warmings (SSWs) is examined in re-analysis data over the Northern Hemisphere from January 1987 and January 2009. During each of these months, the wave-breaking event, associated with tropospheric blocking anticyclogenesis, was followed within a few days by the formation of a high-latitude stratospheric anticyclone and a reversal of the zonal-mean zonal flow from westerly to easterly at 60N and 10mb (by definition, a SSW). The stratospheric anticyclone was co-located with the block/wave-break event in the January 2009 case but not during January 1987, so the connection between the two phenomena, at least in the latter case, is unclear. The possible coupling between these phenomena is being investigated with inversion of Ertel's potential vorticity (EPV) and with quasigeostrophic transient wave activity flux diagnostics. Preliminary results with the EPV inversion reveal a potentially important role by the upper troposphere-lower stratosphere (300 – 100 mb) layer in the coupling. The transient wave activity flux diagnostics are being modified to account for the apparently short time scale of the coupling. Usually, transient wave activity is defined by time averaging over one-month periods, but the stratospheric response to tropospheric wave breaking in each case appeared to occur within a week. Therefore, the wave activity flux components will be averaged over continuously running, one-week periods. This diagnosis should indicate if there is a rapid propagation of wave activity from the tropospheric wave breaking region to the stratospheric anticyclone in each case, and also if there was a downward propagation of wave activity from the stratosphere to the troposphere prior to each wave-breaking event.