INTRUSIONS INTO THE LOWER STRATOSPHERIC ARCTIC VORTEX DURING THE WINTER OF 1991-1992

Investigations of the kinematics of the lower stratospheric Arctic vor tex during the winter of 1991-1992 using the contour advection with su rgery technique reveal three distinct events in which there was substa ntial intrusion of midlatitude air into the vortex, in apparent contra diction of the view that the polar vortex constitutes an isolated air mass. Two of these events, in late January and mid-February, were well documented. They were predicted in high-resolution forecasts by the E uropean Centre for Medium-Range Weather Forecasts, most clearly in exp erimental forecasts with reduced diffusion. Direct confirmation of the presence of the intrusions and of their calculated locations was prov ided by aerosol observations from the airborne differential absorption laser lidar aboard the NASA DC-8, taken as part of the second Airborn e Arctic Statospheric Expedition campaign; aerosol-rich air of midlati tude origin was seen in the expected position of the intrusions. The r eality of the February event was also confirmed by in situ measurement s from the NASA ER-2. Such events may be significant for the chemical processes taking place within the winter vortex. The intrusions were e vidently related to the meteorology of the northern stratosphere durin g this winter and in particular to persistent tropospheric blocking ov er the northeastern Atlantic Ocean and western Europe and concomitant ridging into the lower stratospheric vortex in this region. Neverthele ss, preliminary investigations have indicated that such events are not uncommon in other northern hemisphere winters, although no such event s were found in the southern hemisphere during the Antarctic winter of 1987.