High Arctic gravity waves and their impact on middle atmospheric circulation and temperature

The main goal of the project was to characterize gravity waves observed in the mesosphere at approximately 90 km above Svalbard and to find a relationship between the gravity waves and variations in the temperature and wind of the mesosphere. The Kjell Henriksen Observatory (KHO) outside Longyearbyen, Svalbard, houses the world?s largest collection of optical instruments for studies of aurora and airglow. In November 2010 a camera was installed that can image near infrared airglow not visible to the naked eye. Three of the cameras six filters has been chosen to image hydroxyl (OH) airglow, more specifically from the vibrational-rotational band called OH(6-2). Four winter seasons of images have been analyzed in search of signatures of gravity waves propagating through the OH layer, and which lead to areas with varying intensity in the images. By using data from a closely located meteor radar that measure winds in the mesosphere and from a spectrometer that also measures the temperature of the airglow layer by measuring the intensity of the OH(6-2) emissions, the wave characteristics have been found and compared to data from other parts of the World. A total of 99 waves with wavelength <100 km were identified in the images. Since both temperature and wind data are available from the same location, calculations of the waves? vertical characteristics have also been possible by using a so-called dispersion relation. The study shows that 47% of the waves observed at 87 km altitude are freely propagating in the height area 75-95 kilometers, but as much as 39% seem to be ducted, moving in layers that work as wave guides. A few other cameras are located at similar latitudes, both in the southern and northern hemisphere and the data from Longyearbyen have been compared to published data from these. Statistics of the waves observed in sodium airglow from Resolute Bay (75°N), Canada, show similar characteristics to the waves observed over Svalbard. A majority of these are also moving in the western direction. That the waves above High Arctic Canada are relatively similar to those above Svalbard, is not so surprising as the waves? ability to propagate upwards in the atmosphere, strongly depends on the direction and strength of the winds lower in the atmosphere. The polar jets in the stratosphere are directed Eastward and gravity waves can only propagate past this if they move in the opposite direction, or have a higher phase speed than the wind. In addition to using it for observations of gravity waves, the camera can also be used for calculating the temperature of the mesosphere from the relative intensity between two of the filters used to image the OH airglow. These filters image the light from two lines in the band OH(6-2): P1(2) and P1(4) at wavelengths 800.0 nm and 846.5 nm, respectively. The camera lacks a filter for measuring the intensity outside of the lines, so the background has been estimated by using the relative intensity of the same two OH lines from the spectrometer measurements, but this is then only valid for the two instruments? common field of view, which is zenith. The ratio between the airglow intensity for the two filters, has proven to give good estimates for the temperature of the upper mesosphere, but only for atmospheric conditions with low background intensities (i.e. low auroral activity, low solar angle and no moonlight). By installing a background filter in the camera, it can probably be used as a «temperature mapper» during periods of good atmospheric conditions. Comparing temperature and wind data from the winter season 2011/2013 confirms that there is a clear relationship between the temperature of the OH layer and the wind direction and strength in the mesosphere during so-called Sudden Stratospheric Warming events, where large and swift changes of the stratospheric and mesospheric winds happen. When comparing the Svalbard measurements of OH temperatures with similar data from Andøya (69°N), you find very similar variability which is probably controlled by the same wave activity: Large scale planetary waves. In addition to the research, other important activities during the project include: During the visiting fellowship at Utah State University (USU) in 2012 a scientific workshop was arranged: The workshop MLTI Waves and Dynamics at Polar Latitudes focused on the effects of waves and dynamics in the mesosphere-thermosphere-ionosphere (50-100 km). The workshop gathered scientists from 15 different institutions in USA, Japan, Canada and Norway for a 2 ½ day meeting of lectures and discussions on collaboration within polar atmospheric research. Margit Dyrland, Kim Nielsen and Mike Tayolor, USU, were also conveners for a session titled Wave dynamics in the Polar regions at the 2012 CEDAR meeting. Results from the project has been presented at several conferences: 38 AM Helsinki 2011 and 39 AM Sopot 2011, NDMC Oberpfaffenhofen 2011 and 2012, AGU 2012, CEDAR 2012 etc.

Gravity waves (GWs) play an essential role in determining the global circulation and thermal balance of the atmosphere. Knowledge of the seasonal and latitudinal behavior of the GW momentum flux is essential to understand how they interact with the mean c irculation. However, at high latitudes such information is very sparse, mainly due to the inaccessibility of the high-Arctic regions (>75°N). This three year post-doc project will study GWs at a High Arctic location. An OH airglow imager installed at the Kjell Henriksen Observatory (KHO) at Longyearbyen, Svalbard (78°N) and co-located instruments will be used to study the influence of GWs on the middle atmospheric circulation and temperature. Spectral analysis of the images will be used to determine horiz ontal wave parameters of short-period (<1 h) GWs observed at polar night (Oct-Mar) during 2010/2011 and the subsequent years. The vertical propagation characteristics and direction and magnitude of the momentum flux will be derived combining the results w ith upper mesospheric wind measurements from a co-located meteor radar. GW characteristics for longer period waves (1-2 h) will be obtained from cross-sections of the image (keograms). These data will comprise a unique record of wintertime GW characterist ics for such a high latitude. This climatology will be compared to similar observations at other latitudes and to models in order to examine the latitudinal and inter-hemispheric characteristics of the gravity wave forcing. The Network for the Detection o f Mesopause Change (NDMC) will be used as a tool to aid in this comparison. We also aim to confirm whether there is a consistent strong linear relationship between the strength of the ~80 km meridional wind and the OH airglow peak altitude, brightness and temperature for both short- and long period variation.