The Brewer-Dobson Circulation
Recent analyses of radiosonde data by Thompson and Solomon (2005) indicate for the first time that the annual mean tropical lower stratosphere has cooled over the past few decades. This contradicts previous assessments of the tropical lower stratosphere (e.g., Ramaswamy et al., 2001; WMO, 2003). Although a large number of the tropical radiosondes exhibit systematic cooling biases relative to satellite data (Randel and Wu, 2006), these analyses indicate significant temperature changes in the tropical upper troposphere and lower stratosphere (UTLS). In addition, there are obvious indications in recent simulations with state-of-the-art GCMs and CCMs that stratospheric temperatures may further decrease in future as a result of increasing GHG concentrations. The models demonstrate that stratospheric dynamics could change significantly, in particular that the Brewer-Dobson circulation (BDC; Holton et al., 1995) might accelerate (e.g., Sigmond et al., 2004; Giorgetta and Doege, 2005; Fomichev et al., 2007; Butchart et al., 2006; Eyring et al., 2007), which would have a direct impact on the exchange of air masses between the troposphere and the stratosphere. A common explanation for this behaviour is that climate change leads to an intensification of extra-tropical planetary wave (PW) forcing, propagation and dissipation in the stratosphere (e.g., Rind et al., 2001; Butchart and Scaife, 2001; Salby and Callaghan, 2004; Butchart et al., 2006). The expected changes of PW generation and propagation, particularly in winter, are a major source of uncertainty for predicting future levels of stratospheric composition (Chapter 5 in WMO, 2007).
Tropical upwelling can also be modified by changes of GHG concentrations via variation of net heating rates. Also clouds (i.e. formation, coverage, type, etc.) are directly influenced. Basic investigations of these processes and their feedbacks are rare, comprehensive studies based on CCM simulations are not available so far. Finally, the quasi-biennial oscillation (QBO) is sensitive to changes in the BDC, which affect the balance between wave driving and advective tendencies of the QBO (Giorgetta and Doege, 2005), with possible implications for variations of stratospheric tracers.