The Lagrangian dynamics of two-dimensional incompressible fluid flows is considered, with emphasis on transport processes in atmospheric and oceanic flows. The dynamical-systems-based approach is adopted; the Lagrangian motion in such systems is studied with the aid of Kolmogorov-Arnold-Moser (KAM) theory, and results relating to stable and unstable manifolds and lobe dynamics. Some nontrivial extensions of well-known results are discussed, and some extensions of the theory are developed. In problems for which the flow field consists of a steady background on which a time-dependent perturbation is superimposed, it is shown that transport barriers arise naturally and play a critical role in transport processes. Theoretical results are applied to the study of transport in measured and simulated oceanographic and atmospheric flows. Two particular problems are considered. First, we study the Lagrangian dynamics of the zonal jet at the perimeter of the Antarctic Stratospheric Polar Vortex during late winter/early spring within which lies the "ozone hole". In this system, a robust transport barrier is found near the core of a zonal jet under typical conditions, which is responsible for trapping of the ozone-depleted air within the ozone hole. The existence of such a barrier is predicted theoretically and tested numerically with use of a dynamically-motivated analytically-prescribed model. The second, oceanographic, application considered is the study of the surface transport in the Adriatic Sea. The surface flow in the Adriatic is characterized by a robust threegyre background circulation pattern. Motivated by this observation, the Lagrangian dynamics of a perturbed three-gyre system is studied, with emphasis on intergyre transport and the role of transport barriers. It is shown that a qualitative change in transport properties, accompanied by a qualitative change in the structure of stable and unstable manifolds occurs in the perturbed three-gyre system when the perturbation strength exceeds a certain threshold. This behavior is predicted theoretically, simulated numerically with use of an analytically prescribed model, and shown to be consistent with a fully observationally-based model.

Die Residualzirkulation entspricht der mittleren Massenzirkulation und beschreibt die im zonalen Mittel stattfindenden meridionalen Transportprozesse. Die Variationen der Residualzirkulation bestimmen gemeinsam mit dem anthropogen verursachten Ozonabbau die jährlichen Schwankungen der Ozongesamtsäule im arktischen Frühling. In der vorliegenden Arbeit wird die Geschwindigkeit des arktischen Astes der Residualzirkulation aus atmosphärischen Daten gewonnen. Zu diesem Zweck wird das diabatische Absinken im Polarwirbel mit Hilfe von Trajektorienrechnungen bestimmt. Die vertikalen Bewegungen der Luftpakete können mit vertikalen Windfeldern oder entsprechend einem neuen Ansatz mit diabatischen Heizraten angetrieben werden. Die Eingabedaten stammen aus dem 45 Jahre langen Reanalyse-Datensatz des "European Centre for Medium Range Weather Forecast" (ECMWF). Außerdem kann für die Jahre ab 1984 die operationelle ECMWF-Analyse verwendet werden. Die Qualität und Robustheit der Heizraten- und Trajektorienrechnungen werden durch Sensitivitätsstudien und Vergleiche mit anderen Modellen untermauert. Anschließend werden umfangreiche Trajektorienensemble statistisch ausgewertet, um ein detailliertes, zeit- und höhenaufgelöstes Bild des diabatischen Absinkens zu ermitteln. In diesem Zusammenhang werden zwei Methoden entwickelt, um das Absinken gemittelt im Polarwirbel oder als Funktion der äquivalenten Breite zu bestimmen. Es wird gezeigt, dass es notwendig ist den Lagrangeschen auf Trajektorienrechnungen basierenden Ansatz zu verfolgen, da die einfachen Eulerschen Mittel Abweichungen zu den Lagrangeschen Vertikalgeschwindigkeiten aufweisen. Das wirbelgemittelte Absinken wird für einzelne Winter mit dem beobachteten Absinken langlebiger Spurengase und anderen Modellstudien verglichen. Der Vergleich zeigt, dass das Absinken basierend auf den vertikalen Windfeldern der ECMWF-Datensätze den Nettoluftmassentransport durch die Residualzirkulation sehr stark überschätzt. Der neue Ansatz basierend auf den Heizraten ergibt hingegen realistische Ergebnisse und wird aus diesem Grund für alle Rechnungen verwendet. Es wird erstmalig eine Klimatologie des diabatischen Absinkens über einen fast fünf Jahrzehnte umfassenden Zeitraum erstellt. Die Klimatologie beinhaltet das vertikal und zeitlich aufgelöste diabatische Absinken gemittelt über den gesamten Polarwirbel und Informationen über die räumliche Struktur des vertikalen Absinkens. Die natürliche Jahr-zu-Jahr Variabilität des diabatischen Absinkens ist sehr stark ausgeprägt. Es wird gezeigt, dass zwischen der ECMWF-Zeitreihe des diabatischen Absinkens und der Zeitreihe aus einem unabhängig analysierten Temperaturdatensatz hohe Korrelationen bestehen. Erstmals wird der Einfluss von Transportprozessen auf die Ozongesamtsäule im arktischen Frühling direkt quantifiziert. Es wird gezeigt, dass die Jahr-zu-Jahr Variabilität der Ozongesamtsäule im arktischen Frühling zu gleichen Anteilen durch die Variabilität der dynamischen Komponente und durch die Variabilität der chemischen Komponente beeinflusst wird. Die gefundenen Variabilitäten von diabatischem Absinken und Ozoneintrag in hohen Breiten werden mit der vertikalen Ausbreitung planetarer Wellen aus der Troposphäre in die Stratosphäre in Beziehung gesetzt. / Due to the variability of tropospheric wave activity, the strength of the residual circulation has a distinct seasonal cycle and significant year-to-year variability. The variability of the residual circulation causes interannual variations of the polar ozone layer in late winter and spring. A reverse domain filling trajectory model based on atmospheric data sets is used to calculate the strength and spatial structure of the polar branch of the residual circulation. The atmospheric data sets (ERA-40 and ECMWF Analysis) emerge from a combined analysis of Reanalysis data and a weather forecast model and are available for a time period of 47 years starting from September 1957. Two different approaches are used in the trajectory routine to calculate the vertical movement of air. The first approach is based on the vertical velocity given by "European Centre for Medium Range Weather Forecast" (ECMWF), a quantity that is derived from the divergence of the horizontal winds and that tends to be noisy. In the second approach a radiation transfer model is used to calculate the diabatic heating rates from the divergence of the net radiation flux. The derived descent from both methods is compared with measured tracer distributions from satellite data and Arctic field campaigns. The comparison shows that the second approach results in a much more realistic vertical transport. The method based on the diabatic heating rates is used to compile a climatology of the diabatic descent, averaged within the polar vortex for the Arctic winters 1957/58-2003/04. Furthermore, the climatology contains information regarding the spatial structure of the diabatic descent. The influence of the diabatic descent in the Arctic polar vortex on the total ozone column is calculated for the recent winters since 1990. It is shown that the interannual variability of the Arctic total ozone column is in equal shares caused by dynamical transport processes and by chemical ozone depletion.

The eastward winds that form each winter in the Arctic stratosphere are intermittently disrupted by planetary-scale waves propagating up from the surface in events known as stratospheric sudden warmings. It is shown here that following roughly half of these sudden warmings, the winds take as long as three months to recover, during which time the polar stratosphere evolves in a robust and predictable fashion. These extended recoveries, termed here Polar-night Jet Oscillation (PJO) events, are relevant to understanding the response of the extratropical troposphere to forcings such as solar variability and climate change. They also represent a possible source of improvement in our ability to predict weather regimes at seasonal timescales. Four projects are reported on here. In the first, the approximation of stratospheric radiative cooling by a linear relaxation is tested and found to hold well enough to diagnose effective damping rates. In the polar night, the rates found are weaker than those typically assumed by simplified modelling studies of the extratropical stratosphere and troposphere. In the second, PJO events are identified and characterized in observations, reanalyses, and a comprehensive chemistry-climate model. Their observed behaviour is reproduced well in the model. Their duration correlates with the depth in the stratosphere to which the disruption descends, and is associated with the strong suppression of further planetary wave propagation into the vortex. In the third, the response of the zonal mean winds and temperatures to the eddy-driven torques that occur during PJO events is studied. The collapse of planetary waves following the initial warming permits radiative processes to dominate. The weak radiative damping rates diagnosed in the first project are required to capture the redistribution of angular momentum responsible for the circulation anomalies. In the final project, these damping rates are imposed in a simplified model of the coupled stratosphere and troposphere. The weaker damping is found to change the warmings generated by the model to be more PJO-like in character. Planetary waves in this case collapse following the warmings, confirming the dual role of the suppression of wave driving and extended radiative timescales in determining the behaviour of PJO events.

The eastward winds that form each winter in the Arctic stratosphere are intermittently disrupted by planetary-scale waves propagating up from the surface in events known as stratospheric sudden warmings. It is shown here that following roughly half of these sudden warmings, the winds take as long as three months to recover, during which time the polar stratosphere evolves in a robust and predictable fashion. These extended recoveries, termed here Polar-night Jet Oscillation (PJO) events, are relevant to understanding the response of the extratropical troposphere to forcings such as solar variability and climate change. They also represent a possible source of improvement in our ability to predict weather regimes at seasonal timescales. Four projects are reported on here. In the first, the approximation of stratospheric radiative cooling by a linear relaxation is tested and found to hold well enough to diagnose effective damping rates. In the polar night, the rates found are weaker than those typically assumed by simplified modelling studies of the extratropical stratosphere and troposphere. In the second, PJO events are identified and characterized in observations, reanalyses, and a comprehensive chemistry-climate model. Their observed behaviour is reproduced well in the model. Their duration correlates with the depth in the stratosphere to which the disruption descends, and is associated with the strong suppression of further planetary wave propagation into the vortex. In the third, the response of the zonal mean winds and temperatures to the eddy-driven torques that occur during PJO events is studied. The collapse of planetary waves following the initial warming permits radiative processes to dominate. The weak radiative damping rates diagnosed in the first project are required to capture the redistribution of angular momentum responsible for the circulation anomalies. In the final project, these damping rates are imposed in a simplified model of the coupled stratosphere and troposphere. The weaker damping is found to change the warmings generated by the model to be more PJO-like in character. Planetary waves in this case collapse following the warmings, confirming the dual role of the suppression of wave driving and extended radiative timescales in determining the behaviour of PJO events.

Land ice is losing mass to the worlds oceans at an accelerated rate. The worlds glaciers contain much less ice than the ice sheets but contribute equally to eustatic sea level rise and are expected to continue to do so over the coming centuries if global temperatures continue to rise. It is therefore important to characterize the mass balance of these glaciers and its relationship to climate trends and variability. In the Canadian High Arctic, analysis of long-term surface mass balance records shows a shift to more negative mass balances after 1987 and is coincident with a change in the mean location of the July circumpolar vortex, a mid-troposphere cyclonic feature known to have a strong influence on Arctic summer climate. Since 1987 the occurrence of July vortices centered in the Eastern Hemisphere have increased significantly. This change is associated with an increased frequency of tropospheric ridging over the Canadian High Arctic, higher surface air temperatures, and more negative glacier mass balance. However, regional scale mass balance modeling is needed to determine whether or not the long-term mass balance measurements in this region accurately reflect the mass balance of the entire Canadian High Arctic. The Canadian High Arctic is characterized by high relief and complex terrain that result in steep horizontal gradients in surface mass balance, which can only be resolved if models are run at high spatial resolutions. For such runs, models often require input fields such as air temperature that are derived by downscaling of output from climate models or reanalyses. Downscaling is often performed using a specified relationship between temperature and elevation (a lapse rate). Although a constant lapse rate is often assumed, this is not well justified by observations. To improve upon this assumption, near-surface temperature lapse rates during the summer ablation season were derived from surface measurements on 4 Arctic glaciers. Near-surface lapse rates vary systematically with free-air temperatures and are less steep than the free-air lapse rates that have often been used in mass balance modeling. Available observations were used to derive a new variable temperature downscaling method based on temperature dependent daily lapse rates. This method was implemented in a temperature index mass balance model, and results were compared with those derived from a constant linear lapse rate. Compared with other approaches, model estimates of surface mass balance fit observations much better when variable, temperature dependent lapse rates are used. To better account for glacier-climate feedbacks within mass balance models, more physically explicit representations of snow and ice processes must be used. Since absorption of shortwave radiation is often the single largest source of energy for melt, one of the most important parameters to model correctly is surface albedo. To move beyond the limitations of empirical snow and ice albedo parameterizations often used in surface mass balance models, a computationally simple, theoretically-based parameterization for snow and ice albedo was developed. Unlike previous parameterizations, it provides a single set of equations for the estimation of both snow and ice albedo. The parameterization also produces accurate results for a much wider range of snow, ice, and atmospheric conditions.

No description available.

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.

No description available.