Overestimated acceleration of the advective Brewer-Dobson circulation due to stratospheric cooling

Abstract

Tropospheric warming and stratospheric cooling influence the vertical structure of the atmosphere. Numerous studies have analysed the thermal expansion of the troposphere, however, stratospheric cooling reverses the sign of this shift in the middle stratosphere, causing a downward shift in the upper stratosphere and mesosphere. This is a robust feature in transient climate model simulations, but its impact is commonly unappreciated. Here, we quantify the trend difference of the residual mean vertical velocity (w*), a proxy for diagnosing the advective Brewer-Dobson circulation (BDC) strength, which arises from implicit neglect of the shrinking distance between stratospheric pressure levels in the CCMI-1 (Chemistry-Climate Model Initiative part 1) data request. There, a log-pressure formula with constant scale height is recommended to compute w*. However, stratospheric cooling in transient climate simulations causes a reduction of the geometrical distance between pressure levels and thereby also the scale height significantly decreases over time. Using the general scale height definition for the transformation, the w* trends are therefore smaller. In both cases, the units are m·s-1, but in the latter case it is the constant measure of length geopotential metres and not log-pressure metres. We quantify that, due to the temperature dependence of log-pressure metres, past studies that based w* trend analyses on log-pressure w* overestimated the advective BDC acceleration by ~20%. This result is consistent among the CCMI-1 projections over the 1960-2100 period. We highlight that other diagnostics can also be affected by the neglect of the declining stratospheric pressure level distance. A detailed description of the diagnostics is necessary for consistent assessments of trends. Data processing tools should generally not include the constant scale height assumption if the data are used for trend analyses.