The Brewer-Dobson Circulation
The BDC, how it may be affected by climate change and the discussion of possible consequences for the future distribution of trace gases (e.g. stratospheric ozone and water vapour) has intensively discussed in the last WMO ozone assessment report (Chapter 4 in WMO, 2011) and the “SPARC Report on the Evaluation of Chemistry-Climate Models” (Chapter 4 in SPARC CCMVal, 2010), providing excellent overviews of recent research activities in this field. Moreover, Butchart et al. (2010) compared simulations of 11 CCMs regarding the response of stratospheric climate and circulation to increasing greenhouse gas (GHG) concentrations and ozone recovery in the 21st century. Among others they found that the subtropical jets accelerate (see also Garny et al., 2011b) in response to climate change and that there is a strengthening of the BDC throughout the depth of the stratosphere, which reduces the mean age of stratospheric air nearly everywhere at a rate of about 0.05 yr/decade. Garcia et al. (2011) used the CCM WACCM to derive estimates of the age of stratospheric air from SF6 and CO2 over the period 1965-2006. The calculated age yielded trends that are smaller than the trend derived from a synthetic, linearly growing tracer. Garcia et al. (2011) also discussed the effect of the sparse sampling in available observations and suggested that the resulting trends could have very large uncertainties. These results suggested that trends in the age of air are difficult to estimate unambiguously. Shepherd and McLandress (2011) analysed simulations from the CCM CMAM to demonstrate that changes in resolved wave drag are largely explainable in terms of a simple and robust dynamical mechanism, namely changes in the location of critical layers within the subtropical lower stratosphere. They showed that the strengthening of the upper flanks of the subtropical jets due to tropospheric warming caused by enhanced GHG concentrations pushes the critical layers upward, allowing more wave dissipation in the subtropical lower stratosphere. Transient planetary-scale waves and synoptic-scale waves are both found to play a crucial role in this process which is consistent with findings carried out in SHARP-BDC (Garny et al., 2011b). In connection with the BDC and how it may change in future climate, discussions have arisen again about the specific role of gravity waves (GWs). Alexander et al. (2010) demonstrated that the tuning of GW-parameters in numerical models is not only crucial for a realistic reproduction of the climatological mean dynamical properties of the middle atmosphere, but that the tuning of the non-orographic GW parameterisation, especially in CCMs, is also essential for a reasonable modelling of the seasonal cycle. Okamoto et al. (2011) showed that GW breaking effects have large impact on the BDC, especially in the low and middle latitudes of the lower stratosphere. Geller et al. (2011) demonstrated that it is very important to specify effects from orographic GWs and non-orographic GWs (e.g. stimulated by convection) differently in the Northern and Southern Hemisphere. A very important discussion occurred during the Chapman Conference on ‘Atmospheric Gravity waves’ (March 2011), directly relating to a central topic of our research in SHARP-BDC: There it was discussed that GW drag could interact with resolved (i.e. large-scale) waves and partially cancel the effect of strengthening of the BDC. There are indications that the acceleration of the BDC can lead to seemingly paradoxical increases in stratospheric age of air depending on the latitude-altitude pathways of air in the stratosphere.