Observational evidence of strengthening of the Brewer-Dobson circulation since 1980

The change of the Brewer-Dobson circulation (BDC) over the period of 1980-2009 is examined through a combined analysis of satellite Microwave Sounding Unit (MSU/AMSU) lower stratospheric temperatures (T-LS), ERA-Interim reanalysis data, and observed estimates of changes in ozone, water vapor, well-mixed greenhouse gases, and stratospheric aerosols. The MSU/AMSU-observed tropical T-LS trend is first empirically separated into a dynamic component associated with the BDC changes and a radiative component due to the atmospheric composition changes. The derived change in the dynamic component suggests that the annual mean BDC has accelerated in the last 30 years (at the 90% confidence interval), with most of the change coming from the Southern Hemisphere. The annual mean Northern Hemisphere contribution to the acceleration is not statistically significant. The radiative component of tropical T-LS trends is independently checked using observed changes in stratospheric composition. It is shown that the changes in ozone, stratospheric aerosols, well-mixed greenhouse gases, and water vapor make important contributions to the radiative component of tropical T-LS trends. Despite large uncertainties in lower stratospheric cooling associated with uncertainties in observed ozone and water vapor changes, this derived radiative component agrees with the empirically inferred radiative component, both in terms of its average value and small seasonal dependence. By establishing a relationship between tropical residual vertical velocity at 70 hPa and T-LS, we show that the relative strengthening of the annual mean BDC is about 2.1% per decade for 1980-2009, supporting the results from state-of-the-art chemistry-climate model simulations.