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

Abstract

On 22 January 1997 1200 UT, the routine radiosonde from Sodankylaä, Finland, measured arecord low temperature of - 94.5°C at 26 km. Mesoscale numerical simulations indicate strongmountain wave activity on this day. Two stratospheric temperature minima are simulated: onedirectly above the Scandinavian mountain ridge and another minimum in its lee about 500 kmto the east. Both minima are not resolved in the global analyses. The radiosonde profile as wellas the mesoscale model indicate that the eastern mesoscale temperature anomaly is caused byorographic 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, Swedenand Sodankylaä , Finland on this day. The formation of these ice clouds required the cooling inthe mountain waves as the temperature according to global analyses was about 3 K above thefrost point. The occurrence of additional polar stratospheric ice clouds due to mountain-wavecooling increases the efficiency of chlorine activation and has implications for the resultingArctic ozone depletion. The extraordinary event under consideration occurred during a coldair outbreak with a cold front passing over the Scandinavian orography. This front was associatedwith strong winds in the lower troposphere. At the same time, northern Scandinavia waslocated below the inner edge of the polar vortex, where low synoptic-scale stratospheric temperaturesand a strong polar night jet are found.