Decomposing the Brewer–Dobson Circulation Response to an Abrupt 4 x CO2 Perturbation

Climate models consistently simulate a strengthening of the stratospheric Brewer Dobson circulation (BDC) in response to increasing greenhouse gas (GHG) concentrations, but there remains debate around the processes contributing to this change.One mechanism to explain the increased BDC under raised GHGs invokes changes to the distribution of wave forcing in the lower stratosphere associated with increases in the strength of zonal winds in the subtropical upper troposphere/lower stratosphere^[1]. The sea-surface temperature (SST) response to increasing GHG modifies deep convection affecting latent heat release, ultimately altering tropospheric wave generation^[2]. Thus, the proposed mechanisms for the projected acceleration of the BDC are all strongly tied to the details of surface and tropospheric warming. In this study, we perform experiments with the HadGEM3 model to decompose the response of the BDC to an abrupt 4xCO₂perturbation into three components: 1) a global uniform SST warming; 2) an SST pattern effect; and 3) a direct atmospheric CO₂component (rapid adjustment). We analyze the Transformed Eulerian Mean (TEM) diagnostics and associated wave forcing from resolved and parameterized waves, as well as changes to the zonal atmospheric structure, to understand the contributions of the three distinct components and assess the extent to which they can be combined linearly to explain the overall BDC response.

References

1. Shepherd, T. G. & McLandress, C. A Robust Mechanism for Strengthening of the Brewer–Dobson Circulation in Response to Climate Change: Critical-Layer Control of Subtropical Wave Breaking. J. Atmos. Sci.68, 784–797 (2011).
2. Garny, H., Dameris, M., Randel, W., Bodeker, G. E. & Deckert, R. Dynamically Forced Increase of Tropical Upwelling in the Lower Stratosphere. J. Atmos. Sci.68, 1214–1233 (2011).