The equatorial Quasi Biennial Oscillation (QBO) is known to be an important source of inter-annual variability at mid and high latitudes in both hemispheres. Coupling between QBO and the polar vortex has been extensively studied over the past few decades, however, less is known about QBO influences in the mid-latitude mesosphere. One reason for this is the relative lack of instrumentation available to study mesospheric dynamics at mid-latitudes. In this study, we have used the mid-latitude SuperDARN HF radar at Saskatoon (52.16 N, -106.53 E) to study inter-annual variation in mesospheric winds. The specific aim was to determine whether or not a Quasi Biennial signature could be identified in the Saskatoon mesosphere, and if so, to understand its relationship with the equatorial stratospheric QBO. To achieve this goal, a technique has been developed which extracts meteor echoes from SuperDARN near-range gates and then applies least-squares fitting across all radar beam directions to calculate hourly averages of the zonal and meridional components of the mesospheric neutral wind. Subsequent analysis of 13 years (2002-2014) of zonal wind data produced using this technique indicates that there is indeed a significant QBO signature present in Saskatoon mesospheric winds during late winter (Jan-Feb). This mesospheric QBO signature is in opposite phase with the equatorial stratospheric QBO, such that when QBO (at 50 hPa) is in its easterly (westerly) phase, the late winter winds in Saskatoon mesosphere become more (less) westerly. To further examine the source of the signature, we also analyzed winds in the Saskatoon stratosphere between 5 hPa and 70 hPa using the ECMWF ERA-Interim reanalysis data set, and found that the late winter stratospheric winds become less (more) westerly when QBO is easterly (westerly). This QBO signature in the mid-latitude stratospheric winds is essentially the same as that observed for the polar vortex in previous studies but it is opposite in phase to the mid-latitude mesospheric QBO. We therefore conclude that filtering of gravity waves through QBO-modulated stratospheric winds plays a major role in generating the mesospheric QBO signature we have identified in the Saskatoon HF radar data. When the Saskatoon stratospheric winds are anomalously westward during easterly QBO, the gravity waves having westward momentum might be filtered out, depositing a net eastward momentum in the mesosphere as they propagate upwards. This would result in increased westerly mesospheric winds at Saskatoon. The opposite would happen when the equatorial QBO is westerly. / Master of Science

The middle atmosphere is a very important part of the Earth system. Until recently, we did not realize the importance of the structure of this vaporous shell and of the fundamental role it plays in both creating and sustaining life on the planet. Thanks to the development and improvement of new sounding methods and techniques, our knowledge of the composition of the atmosphere has become more detailed than ever before. We have also learned how to reveal complex interactions between different species and how they react to the incoming solar radiation. The middle part of the Earth’s atmosphere serves as a host for the Polar Mesospheric Clouds. These clouds consist of a thin layer of water-ice particles, only exsisting during the summer months and only close to the poles. There are indications that the occurrence of Polar Mesospheric Clouds may be linked to climate change. It has been pointed out that the first sightings coincide with the industrial revolution. Satellite observations have shown that Polar Mesospheric Clouds have become brighter and possibly more widely distributed during the 20th century. The clouds might therefore be suited as indicators of the variability of the climate - a good reason for studying this night-shimmering phenomena. The clouds can also be used as a proxy for middle atmospheric dynamics. In order to fully utilize Polar Mesospheric Clouds as tracers for atmospheric variability and dynamics, we need to better understand their local properties. The Optical Spectrograph and Infra-Red Imager System (OSIRIS) is one of two instruments installed on the Odin satellite. The optical spectrograph of this instrument observes sunlight scattered by the atmosphere and thus the Polar Mesospheric Clouds. This thesis deals with a tomographic technique that can reconstruct both horizontal and vertical structures of the clouds by using observations from various angles of the atmospheric region. From this information, microphysical properties such as particle sizes and number densities are obtained. The tomographic technique presented in this thesis also provides a basis for a new satellite concept - MATS. The idea behind the MATS satellite mission is to analyze wave activity in the atmosphere over a wide range of spatial and temporal scales, based on the scientific heritage from Odin/OSIRIS and the tomographic algorithms presented in this thesis. / <p>At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper3: Submitted. Paper 4: Manuscript.</p>

Submitted by Maria Medeiros (maria.dilva1@ufcg.edu.br) on 2018-11-08T11:09:05Z No. of bitstreams: 1 ROBSON BATISTA DE SOUSA - DISSERTAÇÃO (PPGFísica) 2012.pdf: 22073508 bytes, checksum: 07991b4cdb0e710b093893fcc69708ec (MD5) / Made available in DSpace on 2018-11-08T11:09:05Z (GMT). No. of bitstreams: 1 ROBSON BATISTA DE SOUSA - DISSERTAÇÃO (PPGFísica) 2012.pdf: 22073508 bytes, checksum: 07991b4cdb0e710b093893fcc69708ec (MD5) Previous issue date: 2012-05-16 / Medidas de ventos e de temperatura obtidas por radar meteórico (SKiYMET) em São João do Cariri-PB (7, 4◦S) e em Cachoeira Paulista-SP (22, 7◦S) foram utilizadas para investigar a oscilação de 6, 5 dias na região mesosférica. Os resultados das análises dos dados de vento e de temperatura revelaram a presença da onda de 6, 5 dias em ambas as localidades, em que as atividades máximas ocorreram durante os meses de primavera austral. Os valores das amplitudes e dos comprimentos de onda vertical, determinados a partir das medidas de ventos, para São João do Cariri, foram superiores aos de Cachoeira Paulista. Em ambas as localidades foram verificadas uma modulação da onda de 6, 5 dias pela Oscilação Semi-Anual (SAO) na temperatura mesosférica. De um modo geral, tanto em São João do Cariri, quanto em Cachoeira Paulista, as atividades máximas da onda de 6, 5 dias foram registradas durante a fase para oeste da Oscilação Quase-Bienal (QBO). Entretanto, o conjunto de dados utilizados neste trabalho não são suficientes para estabelecer uma relação conclusiva dos efeitos da QBO na atividade da onda de 6, 5 dias. Em geral, os parâmetros físicos da onda de 6, 5 dias obtidos neste trabalho são compatíveis com os reportados em outras localidades. / Measurements of wind and temperature obtained from SKiYMET meteor radars at São João do Cariri-PB (7.4◦S) and Cachoeira Paulista (22.7◦S) were used to investigate the 6.5-day oscillations in the mesospheric region. The wind and temperature data analysis results revealed the presence of the 6.5-day waves at both sites, in which the maximum activities have occurred during the austral spring months. The amplitudes and vertical wavelength values, estimated from the wind vertical structure phase delay, for São João do Cariri were longer than for Cachoeira Paulista. For the first time, has been observed for both sites that the 6.5-day wave activities display a semi-annual modulation (SAO) in the meteor temperature. In general, the maximum activities of the 6.5-day waves took place during westward QBO wind phase. However, the data series used in this study are not enough to establish a reliable QBO modulation of the 6.5-day wave. In general, the 6.5-day wave parameters obtained in this work are consistent with those reported for other sites.

The deep convection from monsoons is known to be a major source of gravity waves in the Earth's summer troposphere. While propagating through the middle atmosphere, these waves can carry their momentum up to the mesosphere, following either a vertical or an oblique path. This upward and oblique propagation of gravity waves refers to the latitudinal propagation, away from their low-latitude tropospheric source and towards the polar summer mesosphere. Their dissipation in this atmospheric region plays an important role in the global dynamical structure of the middle atmosphere and yet, the oblique propagation of gravity waves is not included in the present global models. Understanding the influence of the obliquely propagating monsoon gravity waves on the polar summer mesosphere, as well as the hemispheric and seasonal variations of this phenomenon, can improve the gravity-wave parameterization schemes used in the global models. My dissertation relies upon the atmosphere theory and the gravity-wave observations, first, to perform an observational analysis of the oblique propagation of gravity waves in the summer hemisphere. In response to temperature anomalies in the winter northern stratosphere, the distribution of the gravity-wave pseudomomentum flux in the opposite summer mesosphere appeared to be altered. This in turn changes the gravity-wave oblique propagation and its influence on the temperature variations seen in the polar mesospheric clouds. After the development of a 4-D non-hydrostatic ray-tracing model for the simulation of the gravity-wave propagation, my dissertation explores the hemispheric and seasonal differences in the propagation and dissipation of more than 40,000 gravity waves from the low-latitude troposphere. These ray-tracing simulations show the southern hemisphere to be more conducive to both the vertical and the oblique propagation of tropospheric to mesospheric gravity waves. This analysis also highlighted a strong wave filtering at the northern tropopause where a significant number of gravity waves were vertically reflected before reaching the stratosphere. / Doctor of Philosophy / The propagation of waves throughout the Earth's atmosphere is a key phenomenon to understanding the global atmosphere dynamics. These atmospheric waves are known to change the temperature, the pressure, the density and the composition of the middle atmosphere. As a wave propagates upward, the density of the atmospheric background exponentially decreases, resulting in an exponential increase in the wave amplitude and thus, an exponential increase in the energy carried by the wave. When the wave breaks, this energy is released and transferred to the background flow. Gravity waves are part of the atmospheric wave spectrum that is of interest to the scientific community. While small-scale gravity waves can form from tropospheric instabilities such as an unbalanced flow over the mountains or a deep convection from monsoon or thunderstorms, they can propagate up to the upper mesosphere where they can break and transfer a significant amount of energy to the background flow. Although the significant role of these gravity waves in the coupling mechanisms between atmospheric regions is without dispute, their horizontal scale is too small to be resolved by most of the global-scale atmospheric models. The deep convection from monsoon regions is known to be a major source of mesospheric GWs and previous studies on summer northern hemisphere have shown that monsoon GWs tend to propagate obliquely from the low-latitude stratopause up to the high-latitude mesopause. We focus the observational study on the summer southern hemisphere and the Inter-Hemispheric Coupling (IHC) between the summer mesopause, where Polar Mesospheric Clouds (PMCs) form, and the winter stratosphere where sudden warmings occur. PMCs are excellent indicators of atmospheric changes. Their correlations with wind, temperature and GW pseudomomentum flux highlight the consequences of anomalies in the winter stratosphere, such as warmings, on the oblique propagation of GWs that influence the PMC formation in the summer southern hemisphere. After the computation of a ray-tracing model for the simulation of the gravity-wave propagation, a hemispheric and seasonal comparison of the tropospheric to mesospheric gravity-wave propagation based on four simulations highlights the spectral nature of this phenomenon.