The South China Sea is a marginal basin with a complex circulation influenced by the East Asian Monsoon, river discharge and intricate bathymetry. As a result, both the mesoscale eddy field and the near-inertial energy distribution display large spatial variability and they strongly influence the oceanic transport and mixing. With an ensemble of numerical integrations using a regional ocean model, this work investigates how the temporal resolution of the atmospheric forcing fields modifies the horizontal and vertical velocity patterns and impacts the transport properties in the basin. The response of the mesoscale circulation in the South China Sea is investigated under three different forcing conditions: monthly, daily and six-hourly momentum and heat fluxes. While the horizontal circulation does not display significant differences, the representation of the vertical velocity field displays high sensitivity to the frequency of the wind forcing. If the wind field contains energy at the inertial frequency or higher (daily and six-hourly cases), then Vortex Rossby waves and near inertial waves are excited as ageostrophic expression of the vigorous eddy field. Those waves dominate the vertical velocity field in the mixed layer (vortex Rossby waves) and below the first hundred meters (near inertial waves) and they are responsible for the differences in the vertical transport properties under the various forcing fields as quantified by frequency spectra, vertical velocity profiles and vertical dispersion of Lagrangian tracers.

Copies of author's previously published articles inserted / Bibliography: leaves 219-226 / xvi, 22l leaves, [5] leaves of plates : ill. (some col.), maps ; 30 cm. / Title page, contents and abstract only. The complete thesis in print form is available from the University Library. / Thesis (Ph.D.)--University of Adelaide, Mawson Institute for Antarctic Research, 1990

Thesis (M.S. in Meteorology)--Naval Postgraduate School, March 2005. / Thesis Advisor(s): Patrick Harr. Includes bibliographical references (p. 91-92). Also available online.

Cette étude a pour objectif d'analyser le rôle des ondes de Rossby dans la variabilité des courants-jets troposphériques des moyennes latitudes à l'aide d'un modèle numérique idéalisé. Elle s'intéresse aux mécanismes dynamiques responsables de la persistance des principaux modes de variabilité : celui de déplacement méridien et celui de pulsation d'amplitude. Le premier est souvent le principal mode de variabilité du fait de sa grande persistance causée par une rétroaction positive des ondes de Rossby.Deux nouveaux types de rétroactions négatives ont été mis en évidence à une échelle de temps plus courte que cette rétroaction classique et dont le mécanisme dépend de la nature des ondes impliquées. Ces différents mécanismes sont aussi retrouvés et ainsi validés dans le contexte plus réaliste des données de réanalyse.Une réflexion sur les conséquences du changement climatique est aussi proposée via l'étude de sensibilité menée sur trois paramètres clés du modèle : le gradient méridien de température, conduisant à un mode de déplacement plus persistant dans le futur, la position moyenne du jet, conduisant à un mode moins persistant pour des jets plus proches du pôle, et enfin la friction dans les basses couches de l'atmosphère, aussi étudiée car étant un paramètre plutôt difficile à évaluer et présentant une forte disparité entre les modèles.Cette thèse a donc permis de mettre en évidence deux nouveaux mécanismes de rétroaction des ondes sur les courants-jets et de développer des diagnostics théoriques qui pourront être plus amplement testés et appliqués dans d'autres contextes, particulièrement des réanalyses et des simulations de climat réalistes. / This study investigate the impact of Rossby waves on the tropospheric midlatitude eddy-driven jets using an idealized numerical model. It focuses on the dynamical mechanisms driving the persistence of the main modes of variability: a shifting mode and a pulsing mode. The shifting mode is often found to be the leading mode of variability due to an enhanced persistence caused by a positive feedback of Rossby waves. Two new kinds of negative feedbacks have been found for a shorter time-scale than the more classical feedback which mechanism depends on wave properties. These new mechanisms have also been found in the more realistic set up of reanalysis. The impact of climate change is also investigated using the sensitivity analysis of the model to three key parameters : the meridional gradient of temperature, which leads to longer lasting phases of the shifting mode in the future, the jet mean position, which leads to less persistent shifting mode for poleward shifted jets, and frictional damping, because it is a parameter difficult to tune and which varies between numerical models. In conclusion, two new feedback mechanisms acting on eddy-driven jets variability have been found and theoretical diagnostics have been developed and could be used to probe more realistic data such as future climate simulations and reanalysis.

Abstract This study presents the characteristics of small-scale gravity waves in the mesosphere region as derived from the imaging riometer data at high altitude (~90 km) over SANAE (72˚S, 3˚W). Wavelet analysis and FFT (Fast Fourier transform) have been applied to extract short period gravity wave parameters for the year 2000. The horizontal wavelength, phase speed and observed period of gravity waves are typically 10-100 km, 5-60 m.s-1 and 3-60 minutes, respectively. The horizontal propagation direction is north-eastward throughout the year. This could probably be due to selective filtering by the zonal wind. Zonal and meridional winds in the region of the MLT (mesosphere and lower thermosphere) have been measured using HF radars at high latitudes in the southern hemisphere. Data from January 2000 to December 2003 have been used with the aim of investigating the characteristics of planetary wave activity at ~90 km. For SANAE and Halley stations, 2-, 5-, 10-, 16- and 20-day planetary waves are dominant in summer and winter. The results show the seasonal variations of the mean winds, which are caused by the internal variability of the quasi stationary planetary waves. Planetary wave coupling processes between UKMO assimilated and mesospheric data have also been investigated. The cross wavelet results show a strong coupling during winter months. The results suggest that planetary waves are generated at lower atmospheric heights and propagate upwards into mesospheric heights. However, not all observed disturbances in mesospheric heights can be explained by the propagation of planetary waves from lower atmospheric heights.

No description available.

Bekanntlich sind atmosphärische Rossbywellezüge (RWTs) Lösungen der Singular Vector Analyse eines gedämpften, barotropen Modells mit Nordwinter Grundströmen. In den SV Basen der verwendeten 40 DJF Grundströme konnten nur wenige wachsende den Rossbywellenzügen ähnliche (RWT Moden) Singulären Vektoren (SVen) gefunden werden. Die RWT Moden kommen nur in wenigen Gebieten der Erde vor. Die instabilste Mode entwickelt sich in der Region des Nordpazifiks (NPACs) innerhalb von 4 Tagen in jedem der verwendeten beobachteten DJF Grundströme. Alle anderen RWT Moden kommen nur bei Verwendung einiger der Grundströme vor. Ihre Entwicklungspfade sind eindeutig für Entwicklungszeiten bis zu 96 h und streuen für längere Zeiten. Die NPAC Mode erklärt zum Optimierungszeitpunkt 96 h bis zu 60 % der atmosphärischen kinetischen Energie (KE) auf der 300 hPa Fläche in der NPAC Region. Es konnte auch gezeigt werden, daß die Zeitreihe des beobachteten Wachstums der NPAC Mode mit dem berechneten Wachstum (den Eigenwerten) konsistent ist. Interessanterweise zeigt die NPAC-KE zum Optimierungszeitpunkt 96 h auch eine schwach signifikante Korrelation mit dem PNA-Index, die für die Optimierungszeit 144 h nicht mehr existiert. Die Ergebnisse legen die Vermutung nahe, daß die verwendeten Grundströme die Entwicklung der RWT Moden bis zu einer Entwicklungszeit von 4 Tagen dominieren und daß die finite Instabilität maßgeblich zur Entwicklung der beobachteten NPAC Rossbywellenzüge in der Atmosphäre beiträgt. Die Ergebnisse geben Hinweise darauf, daß die NPAC mode auch einen Beitrag zur Entwicklung der PNA leistet.

Thesis (S.M.)--Joint Program in Physical Oceanography (Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences; and the Woods Hole Oceanographic Institution), 2002. / Includes bibliographical references (leaf 119). / The linear and nonlinear Rossby wave solutions are examined in homogeneous square basins on the [beta]-plane both with and without a thin meridional barrier. In the presence of the meridional barrier the basin is almost partitioned into two; only two small gaps of equal width, d, to the north and south of the barrier allow communication between the eastern and western sub-basins. Solutions are forced by a steady periodic wind forcing applied over a meridional strip near the eastern side. Bottom friction is present to allow the solutions to reach equilibrium. The linear solution for the basin containing the barrier is determined analytically and the nonlinear solutions for both basins are found numerically. In the linear solution with the barrier present, particular attention was paid to the resonant solutions. We examined the effects of varying the symmetry of the forcing about the mid-latitude, the frequency of the periodic forcing and the strength of the bottom friction. For each solution we focus on how the no net circulation condition, which is central to any solution in a barrier basin, is satisfied. The nonlinear solutions were studied for both basin configurations. In each case the transition from the weakly nonlinear solution to the turbulent solution was examined, as the forcing frequency and forcing strength were varied. Only integer multiples of the forcing frequency are present in the weakly nonlinear solutions. The turbulent solutions were accompanied by the appearance of many other frequencies whose exact origins are unknown, but are probably the result of instabilities. A hysteresis was found for the turbulent solutions of both the barrier-free and barrier basins. In the weakly nonlinear solutions of the barrier basin it was predicted and confirmed that there is never a steady net flow from sub-basin to sub-basin. It was also shown that with a symmetric forcing all modes oscillating with an odd multiple of the forcing frequency are symmetric and all modes oscillating with even multiples of the forcing frequency are antisymmetric. / by Juli Atherton. / S.M.

Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Meteorology, 1981. / Microfiche copy available in Archives and Science. / Bibliography: leaves 210-215. / by William Roy Young. / Ph.D.

Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Meteorology, 1979. / Microfiche copy available in Archives and Science. / Vita. / Bibliography: leaves 143-146. / by Charles Augustin Lin. / Ph.D.