Planetary waves in the MLT: Vertical coupling and effects
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- Institutt for fysikk 
One of the major difficulties climate models face is the lack of data on the way in which the atmosphere couples energy, dynamics and composition vertically in the Earth’s climate system. This PhD thesis focuses on planetary wave activity in the mesosphere-lower thermosphere (MLT): How the observed planetary wave activity is coupled to the underlying atmosphere and how it affects its surroundings. A method has been developed to observe planetary wave activity in the northern hemisphere (51-66◦N) MLT using neutral atmosphere winds derived from meteor trail drifts observed by a longitudinal chain of Super Dual Auroral Radar Network (SuperDARN) radars as a ”ground-based” satellite. Using these observations, the PhD thesis investigates the intra and inter-annual variability of planetary wave activity in the MLT, its effect on the atmosphere and coupling to the underlying atmosphere. To investigate the coupling to the underlying atmosphere, reanalysis and model results have been used. The seasonal climatology of nine years (2000-2008) of planetary wave activity in the MLT shows an enhancement of wave activity during winter, mid-summer and autumn. The autumn enhancement has been shown to occur simultaneously with a minimum in gravity wave forcing in the MLT and a poleward perturbation of the meridional circulation, suggesting a temporary forcing through westward momentum deposition by planetary waves. The enhancement in summer results from a year-to-year consistent quasi-stationary planetary wave S1 structure. Investigations of the source of this structure rule out direct propagation and modulation of gravity wave filtering by planetary waves at the tropopause. Finally the winter enhancement has been shown to be triggered by stratospheric sudden warmings and their modification of the underlying wind field.