Stratosphere–Troposphere Coupling in a Relatively Simple AGCM: Impact of the Seasonal Cycle
Journal of Climate
The seasonal time dependence of the tropospheric circulation response to polar stratospheric cooling in a simple atmospheric general circulation model is investigated. When the model is run without a seasonal cycle, polar stratospheric cooling induces a positive annular-mode response in the troposphere that takes a remarkably long time, several hundred days, to fully equilibrate. One is thus lead to ask whether the tropospheric response would survive in the presence of a seasonal cycle. When a
... onal cycle. When a seasonal cycle is introduced into the model stratosphere, the tropospheric response appears with a distinct time lag with respect to the stratospheric cooling but, in the long-term mean, the pattern of the wind response is very similar to the one that results from stratospheric forcing in the absence of a seasonal cycle. 1 Recent observational studies (e.g. Thompson and Solomon 2002, hereafter TS02) and comprehensive climate-modeling studies (e.g. Sexton 2001; Gillett et al. 2002; Gillett and Thompson 2003, hereafter GT03) have suggested that polar stratospheric cooling associated with photochemical ozone loss might influence the extratropical tropospheric circulation. Studies using relatively simple atmospheric GCMs (e.g. Polvani and Kushner 2002; Taguchi and Yoden 2002; Kushner and Polvani 2004; Song and Robinson 2004) have provided dynamical insights into this kind of downward influence. In Polvani and Kushner (2002, hereafter PK02) and Kushner and Polvani (2004, hereafter KP04), we have used a simplified AGCM to analyze the impact of externally imposed stratospheric cooling on the tropospheric circulation. In these studies, we have found that, as the polar-winter stratosphere is cooled, the tropospheric jet shifts poleward in a manner that projects almost entirely and positively (by convention) onto the model's annular mode. This result provides a simple qualitative representation of observed tropospheric circulation trends in the Southern Hemisphere that appear to be, at least in part, the result of ozone depletion (TS02, GT03). We find that the response is robust to variations in the strength of the reference polar vortex, in horizontal and vertical resolution, and in the strength of the sponge-layer damping (PK02, KP04). However, one as yet unexplored simplifying assumption of our ealier studies might call into question the robustness of the tropospheric response. In PK02, to focus first on the simplest case and to improve statistical sampling, we used perpetual solstice (i.e. time-independent) forcing. To examine the transient adjustment to equilibrium of the response, in KP04 we performed additional simulations in which the polar-stratospheric cooling is switched on and thereafter held steady in time. We found that, whereas the stratosphere fully adjusts to the stratospheric cooling within a few radiative timescales, the troposphere fully adjusts to the stratospheric cooling over the course of several hundred days (see Figs. 5 and 6 of 2 KP04 and Fig. 2 in this study) . This an example of the well-known low-frequency variability and persistence that is present in this class of simple AGCM (e.g. James and James 1989, KP04, Reichler et al. 2005) and that may very well be unrealistic. The crux of the problem, therefore, is that these long time adjustment scales might prevent the troposphere from adjusting to the stratospheric cooling if a realistic seasonal cycle were added to the model. This might be the case, if, for example, the troposphere is sensitive to the peak strength of the winter polar vortex, but the influence of the polar vortex takes longer than the winter season to be felt in the troposphere.