WACCM: Whole Atmosphere Community Climate Model
NCAR’s Whole Atmosphere Community Climate Model (WACCM) has been used to simulate climate change in the 21st century. The model includes interactive chemistry and a fully resolved middle atmosphere. WACCM simulations, like those of other models, indicate that the Brewer–Dobson circulation will accelerate under rising concentrations of greenhouse gases. The Brewer-Dobson circulation is a large-scale circulation that brings air from the tropics to the poles in the stratosphere.
The circulation strengthens as a result of increased wave driving in the subtropical lower stratosphere, which in turn occurs because of enhanced propagation and dissipation of waves in this region. Enhanced wave propagation is due to changes in tropospheric and lower-stratospheric zonal-mean winds, which become more westerly. Ultimately, these trends follow from changes in the zonal-mean temperature distribution caused by the greenhouse effect. The circulation in the middle and upper stratosphere also accelerates as a result of filtering of parameterized gravity waves by stronger subtropical westerly winds.
Figure 1: The trend in the vertical mass flux in the tropics as simulated by the Whole Atmosphere Community Climate Model for two time periods (REF1: 1950-2003) and REF2 (1990-2050). Most of the trend in mass flux occurs in the lowermost stratosphere (below 20 km) and is due to changes in the forcing of the circulation by tropical Rossby waves. In the middle/upper stratosphere, the trend is due mainly to changes in parameterized gravity wave forcing although in REF1, some also comes form extratropical Rossby waves.
Reference: Garcia and Randel, Acceleration of the Brewer–Dobson Circulation due to Increases in Greenhouse Gases, J. Atmos. Sci., 65, 2731-2739, 2008.