Dynamics of the coupled system stratosphere/troposphere at strong polar vortex

The dynamical coupling between stratosphere, troposphere and lower mesosphere is investigated during strong stratospheric Northern Annular Mode (NAM) using the middle atmospheric version of the ECHAM5 general circulation model (MAECHAM5) coupled to a mixed layer ocean model. A lag composite analysis based on the 50hPa NAM-Index (NAMI) shows that strongly positive stratospheric NAM-phases are associated with strong anomalous westerlies in high latitudes and a temperature quadripole between the troposphere and lower mesosphere. The lower part of this quadripole shows a dipole structure with an anomalous cold (warm) polar (mid-latitude) troposphere and stratosphere and is associated with a positive shear of the anomalous westerlies there. This dipole strengthens (weakens) together with the anomalous westerlies in the NAM-increase (decrease) phase. The upper part of the temperature quadripole shows a coherent dipole with an anomalous warm (cold) polar (mid-latitude) upper stratosphere and lowermost mesosphere and is associated with a negative shear of the anomalous zonal flow there. The warm pole in the mesosphere and upper stratosphere strengthens and propagates downward together with anomalous easterlies, especially in the NAM-decrease phase.

The dynamics during strong vortex is analyzed by computing the wave forcing, residual Coriolis forcing and non resolved (residual) forcing in the Transformed Eulerian Mean formulation (TEM). The NAM temperature, zonal flow and geopotential height patterns and their downward propagation can be explained by the response of the stretching vorticity to residual Coriolis forcing changes, which in turn are caused by the resolved and non resolved wave forcing. The stretching vorticity approach can be seen as another form of the downward control and potential vorticity inversion mechanisms.

The persistence, increase, and downward propagation of the NAM-patterns is enhanced by two positive feedbacks that accelerate the westerlies within the coupled troposphere/stratosphere system: The stationary wave-vortex feedback, caused by vertical wind shear and curvature, and the residual forcing-vortex feedback, which can be explained by the enhanced baroclinic instability due to the westerly vertical wind shear.

The decrease and modulation of the strong NAM-anomalies is enhanced by two negative feedbacks that decelerate the westerlies in the upper and lower boundaries of the coupled troposphere/stratosphere system. The first feedback is a vortex-residual forcing feedback acting in the upper stratosphere and lower mesosphere and can be explained by the filtering of gravity waves by the stratospheric jet. The second feedback acts near the bottom and is driven by both residual and stationary wave forcing.