Changes in the Brewer-Dobson circulation due to the increased CO2 - Radiation and SST induced effects

1. Introduction

We investigate mechanisms of change in the Brewer-Dobson (BD) circulation due to the increased CO₂. The increased CO₂ directly affects the stratospheric temperature through the radiation and may change the BD circulation. Another possible mechanism of the BD circulation change is indirect effects through the stratosphere-troposphere coupling. The increased CO₂ slowly warms up the SST and the change in the tropospheric temperature affects the BD circulation through the modification of wave activities. Considering this, we attempt to separate Radiation-induced (direct CO₂) and SST-induced (indirect CO₂) effects.

2. Model and Experimental Design

This study employs the atmospheric GCM developed at Meteorological Research Institute (MRI). The model has a horizontal truncation wavenumber T42 (2.8°× 2.8°) and 45 vertical levels from the surface to 0.01 hPa. The current and future SSTs prescribed for the GCM are prepared by using a coupled GCM. The future SST is obtained from 2070 to 2079 in the IPCC A2 scenario (approximately doubled CO₂ climate).

Four experiments are constructed, i.e., 348 ppmv CO₂ with the current SST (cntl run), 696 ppmv CO₂ with the future SST (dco2sst run), 696 ppmv CO₂ with the current SST (dco2 run), and 348 ppmv CO₂ with the future SST (dsst run). In this study, Radiation-induced or "direct" effect is defined as the difference between dco2 and cntl run. Similarly, SST-induced or "indirect" effect is defined as the difference between dsst and cntl run.

3. Results and Discussions

The increased CO₂ enhances the Brewer-Dobson (BD) circulations in the stratosphere, This result is consistent with some previous studies, e.g., Rind et al.(2002).

In the Northern Hemispheric (NH) winter, the BD circulation are strongly enhanced in the entire region of NH stratosphere. On the other hand, in the Southern Hemispheric (SH) winter, the BD circulation weakly increases only in the region outside the polar vortex. The large difference between the two hemispheres may result from the wave activities of stationary waves. The changes in both winter hemispheres are caused primarily by the direct effect.

In both summer hemispheres, the increased CO₂ enhances BD circulations. The changes in both summer hemispheres are caused primarily by the indirect effect. A Eliassen-Palm flux analysis suggests that the indirect effects enhance upward wave propagation from the troposphere to the stratosphere in both summer hemispheres. The indirect effect also weakens westerlies in the stratospheric summer hemispheres. So we think that the weakening of easterlies may be favorable for upward wave propagations and then enhances BD circulations in both summer hemispheres.