Tropospheric eddy feedback to different stratospheric conditions in idealised baroclinic life cycles

Tropospheric eddy feedback to different stratospheric conditions in idealised baroclinic life cycles

A pronounced signature of stratosphere-troposphere coupling is a robust negative anomaly in the surface northern annular mode (NAM) following sudden stratospheric warming (SSW) events, consistent with an equatorward shift of the tropospheric jet. It has previously been pointed out that tropospheric synoptic-scale eddy feedbacks, mainly induced by anomalies in the lowermost extratropical stratosphere, play an important role in creating this surface NAM-signal.
P. Rupp and T. Birner used the basic setup of idealised baroclinic life cycles to investigate the influence of stratospheric conditions on the behaviour of tropospheric synoptic-scale eddies. Particular focus is given on the enhancement of the tropospheric eddy response by surface friction, as well as the sensitivity to wind anomalies in the lower stratosphere.
They find systems that include a tropospheric jet only (modelling post-SSW conditions) to be characterised by an equatorward shift of the tropospheric jet in the final state of the life cycle, relative to systems that include a representation of the polar vortex (mimicking more undisturbed stratospheric winter-time conditions), consistent with the observed NAM-response after SSWs. The corresponding relative surface NAM-signal is increased if the system includes surface friction, presumably due to a direct coupling of the eddy field at tropopause level to the surface winds. The authors further show that the jet shift signal observed in their experiments is mainly caused by changes in the zonal wind structure of the lowermost stratosphere, while changes in the wind structure of the middle and upper stratosphere have almost no influence.

Rupp, P., and T. Birner (2021), Tropospheric eddy feedback to different stratospheric conditions in idealised baroclinic life cycles. Weather Clim. Dynam., 2, 111–128, doi: 10.5194/wcd-2-111-2021, https://wcd.copernicus.org/articles/2/111/2021/