Mountain-wave-induced record low stratospheric temperatures above northern Scandinavia

On 22 January 1997 1200 UT, the routine radiosonde from Sodankylaä, Finland, measured a record low temperature of - 94.5°C at 26 km. Mesoscale numerical simulations indicate strong mountain wave activity on this day. Two stratospheric temperature minima are simulated: one directly above the Scandinavian mountain ridge and another minimum in its lee about 500 km to the 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., hydrostatic mountain waves influenced by Coriolis force. Stratospheric ice clouds were observed visually and by ground-based lidar at Kiruna, Sweden and Sodankylaä , Finland on this day. The formation of these ice clouds required 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 efficiency of chlorine activation and has implications for the resulting Arctic ozone depletion. The extraordinary event under consideration occurred during a cold air outbreak with a cold front passing over the Scandinavian orography. This front was associated with strong winds in the lower troposphere. At the same time, northern Scandinavia was located below the inner edge of the polar vortex, where low synoptic-scale stratospheric temperatures and a strong polar night jet are found.