Midstratospheric ozone variability over Bern related to planetary wave activity during the winters 1994-1995 to 1998-1999

Since November 1994, continuous observations of the stratospheric and mesospheric ozone volume mixing ratio (VMR) profiles over Bern (46.95 degreesN, 7.45 degreesE), Switzerland, are performed using the Ground based Millimeter-Wave Ozone Spectrometer (GROMOS), an instrument of the Network for the Detection of Stratospheric Change. We report on large episodic perturbations of the midstratospheric (25-40 km) ozone VMR values observed during the winters 1994-1995 through 1998-1999. Backward trajectory calculations show that the observed episodes are coincident with periods of enhanced meridional transport. Representations of the isentropic potential vorticity field indicate that this transport goes along with significant deformations and southward excursions of the polar vortex in association with strong planetary wave activity. Along the eastern edge of the distorted vortex, northward advection of subtropical air leads to anomalously high ozone VMR values in the midlatitudes middle stratosphere, whereas the passage of polar vortex air over Bern leads to midstratospheric ozone minima. Besides a comprehensive analysis of all extreme episodes detected between November 1994 and June 1999, details are presented for one specific episode. For another episode the influence of photochemical processes is investigated, and it is found that photochemistry acts to damp (rather than to enhance) the effects of planetary-wave-driven meridional transport. It is concluded that the extreme ozone episodes observed over Bern during winter, are primarily a dynamical feature, their amplitude being determined by the meridional ozone VMR gradient rather than by photochemical processes.