| dc.description.abstract | 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. | nb_NO |
