Simulated Anthropogenic Changes in the Brewer–Dobson Circulation, Including Its Extension to High Latitudes

Abstract

Recent studies using comprehensive middle atmosphere models predict a strengthening of the Brewer–Dobson circulation in response to climate change. To gain confidence in the realism of this result it is important to quantify and understand the contributions from the different components of stratospheric wave drag that cause this increase. Such an analysis is performed here using three 150-yr transient simulations from the Canadian Middle Atmosphere Model (CMAM), a Chemistry–Climate Model that simulates climate change and ozone depletion and recovery. Resolved wave drag and parameterized orographic gravity wave drag account for 60% and 40%, respectively, of the long-term trend in annual mean net upward mass flux at 70 hPa, with planetary waves accounting for 60% of the resolved wave drag trend. Synoptic wave drag has the strongest impact in northern winter, where it accounts for nearly as much of the upward mass flux trend as planetary wave drag. Owing to differences in the latitudinal structure of the wave drag changes, the relative contribution of resolved and parameterized wave drag to the tropical upward mass flux trend over any particular latitude range is highly sensitive to the range of latitudes considered. An examination of the spatial structure of the climate change response reveals no straightforward connection between the low-latitude and high-latitude changes: while the model results show an increase in Arctic downwelling in winter, they also show a decrease in Antarctic downwelling in spring. Both changes are attributed to changes in the flux of stationary planetary wave activity into the stratosphere.

Changes in Northern Hemisphere stratospheric variability under increased CO2 concentrations

“…Lack of a robust temperature response, however, is perhaps not surprising given that (1) any temperature trend in the wintertime ALS will likely be small in amplitude, due the close balance between enhanced diabatic CO 2 cooling and the predicted adiabatic warming accompanying a strengthened mean-meridional circulation McLandress and Shepherd, 2009b) and (2) trends may also be contaminated by the large component of interannual variability common to the region, in particular that due to the occurrence of stratospheric sudden warming (SSW) events.…”

“…The predicted changes in stratospheric wave drag that influence the mean-meridional circulation Garcia and randel, 2008;McLandress and Shepherd, 2009b) are also likely to have an impact on the variability of the NH polar vortex, although relatively few studies to date have explicitly investigated such a possibility. In a 2×CO 2 simulation, Rind et al (1998) reported a decrease in the frequency of SSW events compared to their control simulation.…”

The Climatology of Brewer-Dobson Circulation and the Contribution of Gravity Waves

“…This fact 5 ensures the validity of the steady state assumption for solstitial seasons, which are frequently made for the diagnostics using the downward control principle (e.g., McLandress and Shepherd, 2009). It is interesting that the magnitude of Ψ / , in the summer low-latitude region is comparable to that of Ψ , but confined in the lower stratosphere.…”

“…This principle indicates that the Coriolis torque for the residual mean meridional flow is balanced with the wave forcing in a steady state. The contribution of each wave to the residual mean flow can be evaluated using this 10 principle (McLandress and Shepherd, 2009). Okamoto et al (2011) applied this method to the ERA-Interim data (Dee et al, 2011) and to the outputs of a chemistry climate model (CCM) and showed that the GW forcing contributes to the formation of the summer hemispheric part of the deep branch of the winter circulation where RWs hardly propagate in the mean easterly wind of the summer stratosphere (Charney and Drazin, 1961), and to the formation of the shallow branches where orographic GWs break in the weak wind layer in the lower stratosphere (Lilly and Kennedy, 1973;Sato, 1990;Tanaka, 1986).…”

“…(7) and (9) were performed faithfully along the angular momentum ( ) contour in the vertical because the contribution of GW forcing is expected relatively small and hence the uncertainty should be reduced as much as possible, although a few previous studies performed an approximated integration at a constant in the vertical (McLandress and Shepherd, 2009;Okamoto et al, 2011). Note that as is quite small near the equator, the stream functions 10 of Eq.…”