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Stratosphere–Troposphere Coupling

The stratosphere and troposphere are coupled by radiative, dynamical and chemical processes. Changes in the chemical composition of radiatively active gases, such as ozone (O3), carbon dioxide (CO2) or water vapour (H2O) in the stratosphere lead to significant changes in stratospheric temperature (e.g., Forster and Shine, 1999; Ramaswamy et al., 2001, Langematz et al., 2003; Shine et al., 2003) as well as a set of radiative forcings (RF) on the troposphere-surface system. In a transient climate model simulation Schwarzkopf and Ramaswamy (2008) find a sustained and significant global, annual-mean cooling since ~1920 in the lower-to-middle stratosphere (~20-30 km), a global temperature change signal developing earlier than in any lower atmospheric region, that largely results from carbon dioxide increases. After 1979, stratospheric O3 decreases reinforce the cooling. Forster et al. (2007) use a radiative fixed dynamical heating model to show that the effects of tropical ozone decreases at 70 hPa and lower pressures can lead to significant cooling not only at stratospheric levels, but also in the ‘‘sub-stratosphere/upper tropospheric’’ region around 150-70 hPa. The radiative forcing of the troposphere-system surface strongly depends on the height of the stratospheric composition changes (e.g., Forster and Shine, 1999). The lower stratospheric O3 decreases have been shown to change the radiative forcing of climate comparable in magnitude to other trace gases (e.g., Chapter 5 in WMO, 2007). It is therefore important to assess the radiative forcing in a future climate induced by changes in atmospheric composition. 

In addition, a two-way dynamical coupling exists between the troposphere and the stratosphere. The stratosphere in winter is largely influenced by upward propagating tropospheric dynamical disturbances (waves) that may dissipate and decelerate the polar stratospheric polar night jet. On the other hand, stratospheric variability modes, like the Northern Annular Mode (NAM), appear to propagate downward to the troposphere where they affect weather and climate (Baldwin and Dunkerton, 2001; Thompson et al., 2002; Norton, 2003; Limpasuvan et al., 2004). Baldwin et al. (2003; 2007) and Christiansen (2005) found an additional predictive skill for extended-range weather forecasting from the stratospheric memory effect, however statistical forecast models incorporating the stratosphere underestimate the coupling between the stratosphere and troposphere (Charlton et al., 2003). 

On climate time scales, stratospheric polar O3 losses and cooling were shown to lead to more positive phases of the Arctic and Antarctic Oscillations (AO, AAO) (Kindem and Christiansen, 2001; Schnadt and Dameris, 2003; Gillett and Thompson, 2003). GCM simulations of the influence of increasing greenhouse gas (GHG) concentrations revealed a positive trend in the AO in the troposphere (Fyfe et al., 1999; Shindell et al., 1999; Gillett et al., 2002). However, these studies came to contradictory conclusions on the relevance of the stratospheric contribution. Volcanic aerosols caused positive AO phases (e.g., Kirchner et al., 1999; Stenchikov et al., 2004). Changes in radiative forcing associated with the 11-year solar cycle were found to influence the near-surface AO/NAO (Kodera, 2002; Matthes et al., 2006). Stratospheric O3 is believed to be recovering in the first half of the 21st century, leading to a weakening of the polar vortex and the Southern Annular Mode (SAM), while rising CO2 levels might counteract this process (Arblaster and Meehl, 2006).

The mechanisms involved in the interaction between the troposphere and the stratosphere are still under debate. Explanations include: Planetary wave propagation (Chen and Robinson, 1992; Limpasuvan and Hartmann, 1999; Perlwitz and Harnik, 2003), planetary scale wave-mean flow interaction (Kodera, 1994; Christiansen, 1999), responses to the rearrangement of potential vorticity (Hartley et al., 1998; Black, 2002), or the effect of secondary circulations according to the downward control principle (Haynes et al., 1991). Only simulations with simplified models exist so far (e.g., Polvani and Kushner, 2002; Rind et al., 2005). Song and Robinson (2004) indicate that a tropospheric enhancement of the initial stratospheric signal might be necessary. Stratospheric conditions seem to influence baroclinic instability in the troposphere (Wittman et al., 2004).

The relevance of the stratosphere for tropospheric climate and weather is under debate. While Thompson and Solomon (2002) for example imply an active role of the stratosphere in the development of extreme tropospheric weather events, Polvani and Waugh (2004) argue that the stratosphere acts as a mediator transferring initial tropospheric anomalies back to the troposphere. While Fyfe et al. (1999) and Gillett et al. (2002) argued that the effects of increasing GHGs can be simulated in GCMs without stratospheric dynamics, Shindell et al. (1999; 2001) emphasised the important role of the stratosphere for simulating realistic tropospheric AO trends in their GCM. Scaife et al. (2005) showed that the IPCC climate projections of the 20th century indeed revealed a positive trend in the NAO - even in models without stratospheric resolution - however the magnitude of the observed NAO trend was underestimated by the models. Whether the under-representation of the vertical coupling in the models is related to the missing stratospheric resolution remains to be solved. 

An innovative aspect to be studied in the future is the link between stratospheric variability, ocean dynamics and the feedback to tropospheric weather. Hall and Visbeck (2002) and Gupta and England(2006) found an impact of SAM anomalies on the southern circumpolar ocean circulation with feedback on atmospheric climate variables.