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The ground‐based microwave radiometers GROMOS and MIAWARA at Bern (Switzerland) continuously measure ozone and water vapor profiles from 20 to 70 km altitude. A major sudden stratospheric warming occurred around 19 February 2008 with minimal temperatures of 189 K at 40 hPa and maximal temperatures of 300 K at 4 hPa. During the stratospheric warming the Swiss ground‐based radiometers observed a depletion of ozone and an enhancement of water vapor while NASA's CALIPSO satellite instrument measured a large PSC area over Europe. Ozone depletion in the lower stratosphere is explained by transport of ozone poor air from the cold polar vortex. The depletion of upper stratospheric ozone is caused by a sudden temperature increase of about 50 K. A simulation of a chemical box model confirms that a major fraction of the observed decrease of the ozone mixing ratio at 4 hPa can be explained by the effect of the increasing temperature on the ozone chemistry. The chemical ozone destruction is dominated by a catalytic NOx cycle, which is more efficient at higher temperatures. The water vapor enhancement can be explained by transport processes. The rather unusual occurrence of a PSC and a sudden stratospheric warming at midlatitudes suggest that further monitoring of the Earth's middle atmosphere is required for the timely detection of unexpected problems due to ozone loss and climate change.

Abstract. We use Aura/MLS stratospheric water vapour (H2O) measurements as tracer for dynamics and infer interannual variations in the speed of the Brewer–Dobson circulation (BDC) from 2004 to 2011. We correlate one-year time series of H2O in the lower stratosphere at two subsequent pressure 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 H2O time series at the Equator with the ones at 40° N and 40° S. From these lag coefficients we derive the vertical and horizontal 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 rates are found during the QBO easterly phase. We also find that transport of H2O 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 interannual 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 the variability of the ascent rate at the Equator which is on the average 0.2 mm s−1.