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Variability of the Brewer-Dobson circulation's meridional and vertical branch using Aura/MLS water vapor

This discussion paper is/has been under review for the journal Atmospheric Chemistry and Physics (ACP). Please refer to the corresponding final paper in ACP if available. Abstract We use Aura/MLS stratospheric water vapor measurements to infer interannual variations in the speed of the Brewer-Dobson circulation (BDC) from 2004 to 2011. Strato-spheric water vapor (H 2 O) is utilized as a tracer for dynamics and we follow its path along the vertical and meridional branch of the BDC from the tropics to mid-latitudes. 5 We correlate one year time series of H 2 O in the lower stratosphere at two subsequent altitude levels (68 hPa, ∼18.8 km and 56 hPa, ∼19.9 km at the Equator) and determine the time lag for best correlation. The same calculation is made on the horizontal on the 100 hPa (∼16.6 km) level by correlating the H 2 O time series at the Equator with the ones at 40 • N and 40 • S. From these lag coefficients we derive the vertical and hori-10 zontal speeds of the BDC in the tropics and extra-tropics respectively. We observe a clear interannual variability of the vertical and horizontal branch. The variability reflects signatures of the Quasi Biennial Oscillation (QBO). Our measurements confirm the QBO meridional circulation anomalies and show that the speed variations in the two branches of the BDC are out of phase and fairly well anti-correlated. Maximum ascent 15 rates are found during the QBO easterly phase. We also find that the transport towards the Northern Hemisphere (NH) is on the average two times faster than to the Southern Hemisphere (SH) with a mean speed of 1.15 m s −1 at 100 hPa. Furthermore, the speed towards the NH shows much more variability with an amplitude of about 21 % whilst the speed towards the SH varies by only 10 %. An amplitude of 21 % is also observed in 20 the variability of the ascent rate at the Equator which is on the average 0.2 mm s −1 and hence about 5000 times slower than the meridional branch.