Flow regimes in the winter stratosphere of the northern hemisphere

A stratosphere-mesosphere model is used to investigate the response of the northern-hemisphere stratospheric flow regime to changes in tropospheric wave forcing and equatorial wind direction. An ensemble approach is employed to assess changes in variability. The flow regime of the modelled atmosphere is broadly determined by the strength of the imposed troposheric forcing. However, there exists a region of parameter space, where the tropospheric forcing is intermediate, in which the tropospheric control of the stratospheric response is substantially weakened. Under these circumstances the ambient background flow of the stratospheric into which the tropospheric planetary waves propagate is important. We define three flow regimes based upon the likelihood of stratospheric warming events, which may have a correspondence in the real atmosphere. With low tropospheric forcing major warming events, never occur. With strong tropospheric forcing major warmings always occur. With intermediate forcing, which we suggest may be the case for most of the time in the northern hemisphere, there is reduced tropospheric control, greater uncertainty in the stratospheric response and an increased window of opportunity for influence from other factors such as early-winter initial conditions, the Quasi-Biennial Oscillation (QBO) and possibly the solar cycle. We emphasize the importance of using Eliassen-Palm (EP) flux diagnostics not only to examine the extent of 'wave activity' but also to examine where the interaction of the waves with the mean flow is strongest. The latter suggests that flow modification in the upper stratosphere as the Aleutian high is developing is a key aspect of the early stages of the stratospheric warming. The model shows strong evidence that the flow in the mid and upper stratosphere modifies the EP fluxes at the lower boundary and that the wave fluxes through the lower boundary should be considered as part of the response to forcing rather than as the forcing per se. We also highlight the role of travelling anticyclones in the build up of the Aleutian high, the subsequent sudden stratospheric warming and, hence, the modelled variability. These anticyclones are features of the subtropical upper stratosphere with maximum amplitude at the stratopause. Warm disturbed winters are associated with travelling anticyclones that have larger amplitude than average and are more effective in penetrating polewards.