The latitudinal structure of recent changes in the boreal Brewer–Dobson circulation

Abstract. Upwelling branch of the Brewer–Dobson circulation (BDC) controls the tropical lower stratospheric water vapor (WV) through dynamic cooling near the tropopause. Downwelling branch of BDC dominates the extratropical middle-lower stratospheric Hydrogen Chloride (HCl) by dynamic transport. Climatologically, a symmetric weakening BDC indicates increasing tropical lower stratospheric WV and decreasing extratropical middle-lower stratospheric HCl. However, the global ozone chemistry and related trace gas data records for the stratosphere data (GOZCARDS) show that the tropical lowermost stratospheric WV increased by 18 % decade⁻¹ during 2001–2011 and the boreal mid-latitude lower stratospheric HCl rose 25 % decade⁻¹ after 2006. We interpret this as resulting from a slowdown of the tropical upwelling and a speedup of the mid-latitude downwelling. This interpretation is supported by composite analysis of Eliasen–Palm Flux (EPF), zonal wind and regression of temperature on the EPF from the ERA-Interim data. Results present that the enhancing polar vortex and weakening planetary wave activity leads to a downwelling branch narrowing equatorward and a local speedup of 24 % at 20 hPa in the mid-latitudes. Moreover, there are regressive temperature increase of 1.5 K near the tropical tropopause and that of 0.5 K in the mid-latitude middle stratosphere, which also indicates the tropical upwelling slowdown and the mid-latitude downwelling speedup during 2001–2011.