Q. J. R. Meteorol. Soc. (2003), 129, pp. 925–945 doi: 10.1256/qj.02.82 Flow regimes in the winter stratosphere of the northern hemisphere By L. J. GRAY 1¤ , S. SPARROW 2 , M. JUCKES 1;2 , A. O’NEILL 3 and D. G. ANDREWS 2 1 Rutherford Appleton Laboratory, UK 2 Oxford University, UK 3 Reading University, UK (Received 11 March 2002; revised 13 September 2002) SUMMARY A stratosphere–mesosphere model is used to investigate the response of the northern-hemisphere strato- spheric ow regime to changes in tropospheric wave forcing and equatorial wind direction. An ensemble approach is employed to assess changes in variability. The ow regime of the modelled atmosphere is broadly determined by the strength of the imposed tropospheric 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 ow of the stratosphere into which the tropospheric planetary waves propagate is important. We dene three ow 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 inuence 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) ux diagnostics not only to examine the extent of ‘wave activity’ but also to examine where the interaction of the waves with the mean ow is strongest. The latter suggests that ow modication 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 ow in the mid and upper stratosphere modies the EP uxes at the lower boundary and that the wave uxes 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 strato- spheric 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. KEYWORDS: Interannual variability Northern-hemisphere winter Quasi-biennial oscillation Stratospheric warming 1. I NTRODUCTION In their quasi-geostrophic model of wave-mean-ow interaction in the stratosphere, Holton and Mass (1976) identied two different ow regimes depending on the ampli- tude of the planetary waves used to represent the tropospheric forcing. At low amplitude the model remained in a steady state with strong westerly ow. At a critical amplitude a bifurcation occurred (see also Yoden 1987a,b)beyond which the ow became disturbed, with vacillating polar temperatures and high-latitude winds that switched between mean westerlies and mean easterlies, as seen in observations of major stratospheric warm- ings. Scaife and James (2000)—hereafter referred to as SJ2000—extended this study of the internal variability of the stratosphere by using a full primitive-equation model of the stratosphere and mesosphere. They reproduced the steady-ow regime at low forcing (geopotential-height amplitude at the lower boundary Z o D 100 m) and the vac- illating regime at very high forcing (Z o D 300 m); the latter resembled observations of warmings with intermittent easterlies developing on a time-scale of months. SJ2000 also noted the existence of a third regime at intermediate forcing, in which the ow remains westerly but is unsteady, with an oscillation whose period is around 4–7 days. The max- imum amplitude of this oscillation was at the level of the jet maximum and manifests ¤ Corresponding author: Rutherford Appleton Laboratory, Chilton, Didcot, Oxon. OX11 0QX, UK. e-mail: l.j.gray@rl.ac.uk c ° Royal Meteorological Society, 2003. 925