Indications for a long-term temperature change in the polar summer middle atmosphere
Middle atmosphere temperatures have been measured by in-situ and by remote sensing instruments for several decades. Extensive temperature measurements by rocket-borne falling spheres were performed from Andoya Rocket Range in northern Norway form the late 1980s onward. About 90 rockets were successfully launched within eight measurement campaigns and compiled to an empirical temperature statistic. About half of these measurements were in July and August. Since 1997 the Bonn University Rayleigh/Mie/Raman lidar has been operated at Esrange in northern Sweden during winter as well as during summer. 108 night mean temperature profiles were obtained for July and August from this data set and have been compared to the FS-statistics. A systematic difference could be observed, i.e. the weekly-average temperatures taken from the FS-based empirical temperature statistics are up to 10 K warmer than the temperatures measured by lidar, depending on altitude. In particular comparisons during August show larger differences than comparisons with July data. Temperatures were additionally derived from the Rayleigh-scattered light of the Bonn University Na-resonance lidar which was operated during the 1980s at Andoya. No systematic differences between these measurements and the FS-data were found. Gravity waves, tides, volcanic aerosol, and the solar cycle are not likely to cause the oberserved differences, since their influence is minimised either by data selection (gravity waves and tides) or by measurement times (volcanic aerosol, solar cycle). Additionally to the temperature difference a change in the gravity wave activity was observed, in particular during summer 2002 and 2006. During these years also noctilucent clouds occured rather late in the season. The latest unambiguous observation of a noctilucent cloud by the U. Bonn lidar at Esrange was on 24 August 2006. All these observations are indications of a long-term temperature change in the polar summer middle atmosphere as predicted by model calculations. While similar changes have already been observed at middle and low latitudes, temperature trend analyses for the polar atmosphere did not reveal any variation up to now.