Stratospheric transport by planetary wave mixing as observed during CRISTA-2

Planetary waves drive the mean meridional circulation of the stratosphere and at the same time facilitate quasi-horizontal mixing of trace gases. This paper presents significant day-to-day variability of stratospheric trace gas fields associated with large planetary wave activity observed during the second mission of the Cryogenic Infrared Spectrometers and Telescopes for the Atmosphere (CRISTA) experiment. Geopotential height data of the UK Met Office show that the CRISTA-2 observations in the Southern Hemisphere winter were made during a period of extremely large amplitudes of both wave-1 and wave-2. The planetary wave-1, usually a quasi-stationary feature, moved eastward with the traveling planetary wave-2. The large amplitudes of both wave-1 and wave-2 led to a significant displacement of the edge of the polar vortex toward the tropics (down to 30 deg S). As a result of the large wave amplitudes and favorable phase alignment, the anticyclone drawing up tropical air was unusually strong, and thus considerable wave-induced trace gas flux from the tropics toward midlatitudes was observed, mainly in the form of a pronounced planetary-scale tongue advected out of the tropics around the vortex and into the anticyclone. Quantitative transport calculations based on a sequential data assimilation system highlight the importance of such transport events for trace gas eddy-flux in the Southern Hemisphere winter stratosphere.