Mountain-wave-induced record low stratospheric temperatures above northernScandinavia

On 22 January 1997 1200 UT, the routine radiosonde from Sodankyla, Finland, measured a record low temperature of -94.5 degrees C at 26 km. Mesoscale n umerical simulations indicate strong mountain wave activity on this day. Tw o stratospheric temperature minima are simulated: one directly above the Sc andinavian mountain ridge and another minimum in its lee about 500 km to th e east. Both minima are not resolved in the global analyses. The radiosonde profile as well as the mesoscale model indicate that the eastern mesoscale temperature anomaly is caused by orographic inertia-gravity waves, i.e., h ydrostatic mountain waves influenced by Coriolis force. Stratospheric ice c louds were observed visually and by ground-based lidar at Kiruna, Sweden an d Sodankyla, Finland on this day. The formation of these ice clouds require d the cooling in the mountain waves as the temperature according to global analyses was about 3 K above the frost point. The occurrence of additional polar stratospheric ice clouds due to mountain-wave cooling increases the e fficiency of chlorine activation and has implications for the resulting Arc tic ozone depletion. The extraordinary event under consideration occurred d uring a cold air outbreak with a cold front passing over the Scandinavian o rography. This front was associated with strong winds in the lower troposph ere. At the same time, northern Scandinavia was located below the inner edg e of the polar vortex, where low synoptic-scale stratospheric temperatures and a strong polar night jet are found.