Quasi 2 day waves in the summer mesosphere: Triple structure of amplitudes and long-term development

Abstract

[1] Upper mesosphere OH temperature measurements are compared at the stations of Wuppertal (51°N, 7°E) and Hohenpeißenberg (48°N, 11°E) for 2004–2009 in order to form a combined data set which considerably improves the measurement statistics. This allows time analyses near the Nyquist frequency (2 days) which is used for a study of the quasi 2 day wave (QTDW) in summer. The well-known maximum near solstice is observed. In addition, there are two unexpected side maxima about 45–60 days before and after the center peak. A similar triplet is seen in the QTDW analysis of Microwave Limb Sounder temperature data. The triple structure is also found in a very similar form 15 years earlier in the interval 1988–1993 in early Wuppertal data. In these 15 years the time distance between the first and last triple peak has increased by about 22 days. Amplitudes of the QTDW correspond to the meridional gradient of the quasi-geostrophic potential vorticity (from MLS data) and baroclinic instabilities (bc) from radar winds (at Juliusruh, 55°N, 13°E). Parameter bc also shows a triple structure, when mean values 2003–2008 are calculated. The QTDW triplet results from the combination of atmospheric (in)stability and critical wind speed. The widening of the QTDW triple structure suggests a long-term change of mesospheric stability and wind structure. This is found, indeed, in the bc and zonal wind data. The changes likely reflect a long-term circulation change in the middle atmosphere extending up to the mesopause.