Variations in characteristics of tropical deep convection are examined for an association with the stratospheric quasi-biennial oscillation (QBO). Eight years (1998-2005) of Tropical Rainfall Measuring Mission (TRMM) Lightning Imaging Sensor (LIS) flash densities and ten years (1998-2007) of TRMM Precipitation Radar (PR) deep convective and stratiform rainfall and convective echo top heights are analyzed. The QBO can be linked to deep convection through two hypothesized mechanisms: 1) modulation of tropopause height, which may affect the altitude that convection can penetrate; and 2) modulation of cross-tropopause shear, which may affect the vertical development of convection via shearing of cloud tops. Tropopause height and cross-tropopause shear is measured by National Centers for Environmental Prediction (NCEP) reanalysis 100 hPa temperatures and 50-200 hPa zonal wind shear, respectively. When partitioned by QBO east and west phases, zonal monthly mean anomalies and anomalous monthly mean difference maps illustrate a QBO signal in lightning flash rates, convective and stratiform rain amounts, and the number of convective echo tops > 12 km. QBO modulation of cross-tropopause shear causes 50-200 hPa shear east (west) phase anomalies to decrease (increase) about the equator and increase (decrease) in off-equator regions. QBO modulation of tropopause height induces a higher/colder (lower/warmer) tropopause near the equator during the east (west) phase. While the expectation was that decreases in cross-tropopause shear and tropopause temperatures at monthly time scales during the QBO would result in an increase of deep convective properties near the equator, observations suggest that deep convective properties may increase or decrease depending on the location and season. Similar to the QBO results, the increase or decrease of deep convective properties with general variations in cross-tropopause shear and tropopause temperatures depends on the location and season.

We investigate the solar cycle modulation of the quasi-biennial oscillation (QBO) in stratospheric zonal winds and its impact on stratospheric ozone with an updated version of the zonally averaged CHEM2D middle atmosphere model. We find that the duration of the westerly QBO phase at solar maximum is 3 months shorter than at solar minimum, a more robust result than in an earlier CHEM2D study due to reduced Rayleigh friction drag in the present version of the model. The modeled solar cycle ozone response, determined via multiple linear regression, is compared with observational estimates from the combined Solar Backscattered Ultraviolet (SBUV/2) data set for the period 1979–2003. We find that a model simulation including imposed solar UV variations, the zonal wind QBO, and an imposed 11-year variation in planetary wave 1 amplitude produces a lower stratospheric ozone response of ∼2.5% between 0 and 20°S and an upper stratospheric ozone response of ∼1% between 45 and 55 km, in good agreement with the SBUV-derived ozone response. This simulation also produces an (enhancement/reduction) in the (lower/upper) stratospheric temperature response at low latitudes compared to the effects of solar UV variations alone, which are consistent with model vertical velocity anomalies produced by the solar-modulated QBO and imposed changes in planetary wave forcing.

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 Quasi-Biennial Oscillation (QBO) is the strongest phenomena influencing the stratopheric (~15-50 km height) circulation over the equator. QBO has two phases of downward propagating easterly and westerly winds, which has a total period of approximately 28 months and the phase is defined by the wind direction between the airpressure 25-50 hPa, which is roughly at a height of 30 km. QBO is induced by atmospheric gravity waves originating from the troposphere (~0-15 km height) and are generated by a plethora of sources, such as tropical convection and wind shear. The winds propagate downward at about 1 km per month through the stratosphere until reaching the tropopause (~15 km height) where they dissipate. The wind speed is at its maximum in the middle of the phase, where the wind shear is at its lowest, and the easterly winds can grow up to 30 m/s whilst the westerly winds reach roughly 15 m/s. Although the QBO is an equatorial phenomena it has a poleward component radiating its signal from the tropics to the higher latitudes where it affects other circulations such as the stratospheric polar vortex on the northern hemisphere (NH). The polar vortex consists of westerly winds around the polar region and is a major influence on the winter climate on the NH and thereby allows the QBO to indirectly affect the tropospheric climate through it. The easterly QBO disturbs and weakens the polar vortex, which results in warm subtropical air penetrating the vortex and warming the Arctic region whereas the polar air is released southward creating a colder winter on the NH. The westerly QBO on the other hand enhances the polar vortex and contains the cool polar air over the Arctic, which results in a milder winter. The correlation between QBO and El Niño Southern Oscillation (ENSO) as well as the tropical cyclones (TC) has either changed (ENSO) or completely disappeared (TC). The ENSO-QBO correlation depends on which phase of ENSO coincide with which phase of QBO, where El Niño coinciding with easterly QBO and La Niña coinciding with westerly QBO results in wind anomalies in the NH stratosphere. If the opposite combination takes place the wind anomalies will instead be situated in the subtropical troposphere, displacing the subtropical jet poleward. To what extent these stratospheric winds exert their influence is to some degree still uncertain, but that they have an effect on the tropospheric climate is unbeknownst to no one. / Cirkulationen i den ekvatoriella stratosfären (ca 15-50 km höjd) domineras av Quasi-Biennial Oscillation (QBO), ett zonalt (parallellt ekvatorn) vindfenomen med två faser bestående av östliga respektive västliga vindar och en period på ca 28 månader. Fasen definieras mellan lufttrycken 25-50 hPa, vilket representerar en höjd på ca 30 km. Drivkraften bakom QBO är ett brett spektrum av atmosfäriska gravitationsvågor som skapas genom bland annat den tropiska konvektionen, vindskjuvning och frontsystem. Vindarna propagerar vertikalt nedåt genom stratosfären med ungefär 1 km per månad tills de når tropopausen (ca 15 km) där vindarna försvagas kraftigt till ett zonalt medelvärde på 0 m/s. Vindhastigheten under östlig QBO uppgår i ca 30 m/s medan västlig QBO uppgår i ca 15 m/s, och är maximal i höga stratosfären samt i mitten av faserna där vindskjuvningen är minimal. QBO sprider sig meridionalt (nord-syd) från tropikerna till högre breddgrader genom stratosfären där andra fenomen som den stratosfäriska polarvirveln kan påverkas på norra halvklotet (NH). Polarvirveln består av västliga vindar i stratosfären runt polarregionen och är en stor influens på vinterklimatet i framförallt Europa och Nordamerika. Genom polarvirveln kan QBO indirekt påverka klimatet i troposfären (ca 0-15 km), där den östliga fasen av QBO försvagar medan den västliga fasen av QBO förstärker polarvirveln. En försvagad polarvirvel innebär en varmare medeltemperatur på Arktis och att kallare polarluft söker sig söderut och orsakar kalla vintertemperaturer. Troposfäriska klimatfenomen som El Niño Southern Oscillation (ENSO) och tropiska cykloner (TC) har uppvisat ett samband till QBO, men sedan förändrats (ENSO) eller helt försvunnit (TC). ENSO-QBO korrelationen förändras beroende på vilken fas QBO respektive ENSO är i relativt varandra. Då El Niño sammanfaller med östliga QBO samt La Niña sammanfaller med västliga QBO uppstår vindanomalier vid höga latituder i NH:s stratosfär, medan vid omvända sambandet förflyttar sig vindanomalierna till subtropikerna i troposfären och kan där förskjuta den subtropiska jetströmmen norrut. Att de stratosfäriska vindarna påverkar troposfären är känt, men hur och till vilken grad är ännu inte uppenbart. På grund av den korta tidsperiod med kontinuerliga och tillförlitliga vindmätningar i stratosfären uppkommer flera hypotetiska effekter av QBO och dess påverkan på klimatet i troposfären.

L'ozone joue un rôle primordial sur l'équilibre photochimique de l'atmosphère et participe au processus d'équilibrage radiatif entre les deux hémisphères (Mecke, 1931). Dans la troposphère, l'ozone détermine la capacité oxydante de la majorité des gaz et absorbe continuellement dans la stratosphère les radiations ultraviolettes nocives (McMicheal et al., 2003). D'où l'intérêt de surveiller la variation de la couche d'ozone de façon régulière. Il a été constaté au début des années 80, une diminution inquiétante et progressive de la colonne totale de l'ozone dûe aux émissions anthropiques des substances riches en chlore, brome et fluor. Ce constat a conduit au Protocole de Montréal en 1987 dont l'objectif est de mettre en place une politique internationale visant à réduire les émissions des substances appauvrissant l'ozone. Dix ans après la signature du dit Protocole, la concentration de ces substances commence à diminuer dans l'atmosphère et la prospection d'un recouvrement progressif de la couche d'ozone demeure aujourd'hui un sujet d'actualité (UNEP/PNUE, 2009 ; OMM, 2010 et 2014). Les besoins d'aujourd'hui sont de réaliser des mesures continues et fiables de l'ozone dont leurs exploitation dans des méthodes et/ ou des modèles bien adaptés à la problématique aideront la communauté à suivre l'évolution de l'ozone et d'estimer les tendances à long terme. Dans ce travail, une variété de produits d'ozone issue de différents instruments a été combinée pour construire des bases des données fiables et homogènes afin d'étudier sa variabilité et d'estimer la tendance de l'ozone dans les régions tropicale et subtropicale sud. L'application de ces bases de données sur les ondelettes a permis d'identifier les principaux forçages qui contrôlent la variabilité de l'ozone et la période de retour associée à chaque forçage. Il s'agit des variations saisonnières du climat, les oscillations quasi-biennales, les oscillations australes El-Niño et l'activité solaire dont le cycle moyen est évalué à 11ans. Le comportement et l'influence de chacun de ces paramètres sur la viabilité de l'ozone sont étudiés. Cette étude est faite en s'appuyant sur des méthodes statistiques et sur le modèle Trend-Run. Avec ce modèle, la part de contribution et la réponse de chaque paramètre sur la variabilité de l'ozone sont quantifiées. Les résultats sur les tendances montrent une augmentation de la couche d'ozone avec un taux variant entre 0 et 2.78% par décade (selon la région et le site) sur la période 1998-2012. Cette amélioration est bien observée au-dessus de 22km, surtout aux subtropiques par rapport à la région équatoriale. / Ozone plays an important role on photochemical equilibrium of atmosphere and participate on radiative balance process between hemispheres (Mecke, 1931). In the troposphere, ozone determines the oxidizing capacity of major species and absorbs continuously in the stratosphere the harmful ultraviolet radiation (McMichael et al, 2003). Based on the above facts, it is important to monitor ozone continuously with consistency and accuracy. Global total column ozone (TCO) has depleted gradually since 1980 with an increase of chlorofluorocarbon concentrations in the stratosphere due to anthropogenic activities. In 1987, the Montreal protocol was formulated in order to regulate the emissions of substances that deplete ozone. Concentrations of these substances are observed to decrease ten years after the Montreal protocol. Thus we have been expecting an increase in ozone by now (UNEP/PNUE, 2009; WMO, 2010 and 2014). The current needs are to achieve consistent and reliable measurements in which their exploitation on adapted methods/models can help scientists to follow the ozone evolution and to estimate long term ozone trend. In this work, a variety of ozone products from different instruments was combined in order to create reliable and homogenous dataset to study the ozone variability and trend over the southern tropics and subtropics. The dataset application on wavelets method allowed to identify the dynamic parameters that control ozone variability and their periodicities. These include seasonal variations of climate, the quasi-biennial oscillations, the El-Niño Southern Oscillation and the 11-years solar cycle. The behavior of each parameter and its influence on ozone variability were analysed based on statistical method and the Trend-Run model. The contribution and response of each variable on ozone variability were quantified from the model. The obtained trends results exhibit an increase of total ozone from 1998 to 2012 with a rate varying between 0 and 2.78% par decade (depending of the site and region). The ozone increase was observed mainly above 22 km and it is more important over the subtropical region with respect to equatorial zone.

The Quasi-Biennial Oscillation (QBO) is the dominant pattern of variability in the tropical stratosphere. Despite a well established theory regarding its generation in the atmosphere, the simulation in global climate models remains difficult. A set of metrics assessing the quality of model simulations is presented in this study. The QBO simulations in models submitted to the CMIP5 and CCMVal-2 intercomparison projects are characterised and compared to radiosonde observations and reanalysis datasets. Common model biases and their potential causes are addressed. As the QBO has a long intrinsic period, knowing its influences on other parts of the climate system can be used to improve long range forecasts. These teleconnections of the QBO in observations are investigated using composite analysis, multilinear regression and a novel approach called causal effect networks (CEN). Findings from these analyses confirm previous results of the QBO modulating the stratospheric polar vortex and subsequently the North Atlantic Oscillation (NAO). They also suggest that it is important to take the equatorial zonal mean zonal wind vertical profile into account when studying teleconnections, rather than the more traditional method of using just one single level. While QBO influences on the Northern Hemisphere winter polar vortex and the NAO are more clearly established, interactions within the tropics remain inconclusive. Regression analysis does not show a connection between the QBO and the MJO, whereas the CEN analysis does. Further studies are needed to understand the interaction mechanisms near the equator. Finally, following the unprecedented disruption of the QBO cycle in the winter 2015/16, the event is described and a model analogue from the MPI-ESM-MR historical simulation is presented. Future implications are unclear, although model projections indicate more frequent QBO irregularities in a warming climate.

Cette thèse est consacrée à l'évaluation des échanges entre le réservoir tropical et les moyennes latitudes au travers de la barrière dynamique stratosphérique subtropicale de l'hémisphère sud. La stratégie consiste à reconstituer le transport sur des surfaces isentropes avec le modèle d'advection de contours à haute résolution MIMOSA. Ce modèle a été alimenté par les champs de vorticité potentielle calculé sur 26 années consécutives (1980-2005) à partir des champs de vents et de température du modèle européen ERA-40. L'algorithme DYBAL a calculé l'intensité des barrières dynamiques (barrière subtropicale et vortex polaire) et du mélange. Couplé à l'algorithme des k-moyennes, il a permis la détection en coordonnées géographiques de la position des barrières et donc le calcul de l'extension méridienne des réservoirs stratosphériques. Cet algorithme a également été utilisé pour reconstituer les épisodes filamentaires caractéristiques du transport isentrope. Enfin, la perméabilité de la barrière subtropicale a été évaluée par le suivi des échanges et les routes privilégiées de ces échanges ont été mises en évidence. L'implication de ces événements sur le transport de l'ozone a été estimée ainsi que l'influence de l'oscillation quasi-biennale (QBO) et du cycle solaire de 11 ans. Les résultats présentent la variabilité dynamique de la stratosphère sur l'isentrope 600 K et montrent une augmentation de la taille du réservoir tropical et des moyennes latitudes en correspondance avec le rétrécissement du vortex polaire. Une diminution de l'intensité des échanges à travers la barrière dynamique subtropicale depuis les années 1980 est également mise en évidence.

The ENSO-monsoon system is known to have a biennial component. Here we show using high resolution satellite data, mainly daily rainfall and sea surface temperature (SST) from the Tropical Rainfall Measuring Mission (TRMM), and daily scatterometer surface winds from QuickSCAT, that there is a clear biennial oscillation (TBO) in summer monsoon rainfall over Central India – Bay of Bengal (Cl-BoB) and the far west Pacific in the period 1999-2005. Summer (JJAS) mean rainfall oscillates between high and low values in alternate years; the rainfall is high in the odd years 1999, 2001, 2003, and 2005, and low in even years 2000, 2002 and 2004. The amplitude of the oscillation is significant, as measured against the long term standard deviation of seasonal rain based on 1979-2005 Global Precipitation Climatology Project (GPCP) data. We find that the TBO in rainfall is associated with TBO of SST over the tropical Indian, west Pacific and Atlantic Oceans in different seasons. There is no TBO in east Pacific SST, and no strong El Nino in this period. The TBO of SST is related to change in evaporation due to TBO of surface wind speed. A TBO of the surface branch of the Walker circulation in the eastern Indian and western Pacific basins is clearest in the autumn season during 1999-2005. There is a clear relation between a large-amplitude TBO of winter surface air temperature over north Asia associated with TBO of the Arctic oscillation (AO), and the TBO of summer monsoon rainfall. High rainfall over CI-BoB lin summer is followed by a relatively high value of the AO Index, and warm air termperature over north Asia in the succeeding winter. The Inter Tropical Convergence Zone(ITCZ) over the central Pacific and Atlantic Oceans shift north by about two degrees when the northern hemisphere is warm, reminiscent of the behaviour of the climate system of ENSO, decadal and palaeoclimate time scales. In this thesis we document the biennial oscillation of monsoon rain and its spatial structure in the recent period, and its relation with biennial oscillation of surface climate over the global tropics and extratropical regions. The existence of TBO in the tropical Atlantic, and its relation with the monsoon, is a new finding. We demonstrate that the interannual variability of the summer monsoon during 1999-2005, including the drought of 2002, is part of a pervasive TBO of global surface climate.