FREE ELECTRONIC LIBRARY - Dissertations, online materials

Pages:   || 2 | 3 | 4 | 5 |   ...   | 21 |

«See discussions, stats, and author profiles for this publication at: ...»

-- [ Page 1 ] --

See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/44249544

Linear and nonlinear stratified spindown over

sloping topography

Article · October 2010

DOI: 10.1575/1912/3635 · Source: OAI


1 author:

Jessica Benthuysen

Australian Institute of Marine Science



Available from: Jessica Benthuysen Retrieved on: 18 October 2016 MIT/WHOI 2010-09 Massachusetts Institute of Technology Woods Hole Oceanographic Institution Joint Program in Oceanography/ Applied Ocean Science and Engineering


Linear and Nonlinear Stratified Spindown over Sloping Topography by Jessica A. Benthuysen June 2010 MITIWHOI 2010-09 Linear and Nonlinear Stratified Spindown over Sloping Topography by Jessica A. Benthuysen Massachusetts Institute of Technology Cambridge, Massachusetts 02139 and Woods Hole Oceanographic Institution Woods Hole, Massachusetts 02543 June 2010


Funding was provided by the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution Academic Programs Office.

Reproduction in whole or in part is permitted for any purpose of the United States Government. This thesis should be cited as: Jessica A. Benthuysen, 2010. Linear and Nonlinear Stratified Spindown over Sloping Topography. Ph.D. Thesis. MIT/WHOI, 2010-09.

Approved for publication; distribution unlimited.

Approved for Distribution:

Robert A. Weller, Chair Department of Physical Oceanography Edward A. Boyle James A. Yoder MIT Director ofJoint Program WHOI Dean of Graduate Studies Linear and Nonlinear Str

–  –  –

Abstract In a stratified rotating fluid, frictionally driven circulations couple with the buoyancy field over sloping topography. Analytical and numerical methods are used to quantify the impact of this coupling on the vertical circulation, spindown of geostrophic flows, and the formation of a shelfbreak jet.

Over a stratified slope, linear spindown of a geostrophic along-isobath flow induces cross-isobath Ekman flows. Ekman advection of buoyancy weakens the vertical circulation and slows spindown. Upslope (downslope) Ekman flows tend to inject (remove) potential vorticity into (from) the ocean. Momentum advection and nonlinear buoyancy advection are examined in setting asymmetries in the vertical circulation and the vertical relative vorticity field. During nonlinear homogeneous spindown over a flat bottom, momentum advection weakens Ekman pumping and strengthens Ekman suction, while cyclonic vorticity decays faster than anticyclonic vorticity. During nonlinear stratified spindown over a slope, nonlinear advection of buoyancy enhances the asymmetry in Ekman pumping and suction, whereas anticyclonic vorticity can decay faster than cyclonic vorticity outside of the boundary layers.

During the adjustment of a spatially uniform geostrophic current over a shelfbreak, coupling between the Ekman flow and the buoyancy field generates Ekman pumping near the shelfbreak, which leads to the formation of a jet. Scalings are presented for the upwelling strength, the length scale over which it occurs, and the timescale for jet formation. The results are applied to the Middle Atlantic Bight shelfbreak.

Thesis Supervisor: Steven J. Lentz Title: Senior Scientist Thesis Supervisor: Leif N. Thomas Title: Assistant Professor Acknowledgments I thank my advisors, Leif Thomas and Steve Lentz, for their scientific guidance and support. I greatly appreciate the respect that they have shown me throughout the past several years. They have allowed me the freedom to explore new ideas and have helped focus and shape these ideas into feasible projects. With patience, open doors, and constructive criticism, they have set true examples of how to do science as well as how to be a scientist.

I thank Raffaele Ferrari who always encouraged me to think about the big picture.

I thank Glen Gawarkiewicz for giving me the opportunity to participate in a cruise to the Middle Atlantic Bight shelfbreak front and the opportunity to learn about autonomous underwater vehicles. Finally, I thank Ken Brink for chairing my defense and helpful discussions ranging from writing to numerical modelling.

Thanks to the students who have helped me along this journey. To my officemates at MIT and WHOI: Shin Kida, Max Nikurashin, Claude Abiven, Christie Wood, Kjetil V˚ and Rebecca Dell. To my PO classmates for their support: Beatriz Pe˜aage, n Molino and Katie Silverthorne as well as Stephanie Waterman, Hristina Hristova, and Rachel Horwitz. Finally, thanks to Melanie Fewings, Greg Gerbi, and Carlos Moffatt for being good mentors who I can always count on for advice.

I thank my parents, Jim and Mei, and my sister, Jackie, for being supportive of all my endeavors. I also thank my undergraduate advisor, Bill Criminale, for introducing me to fluid dynamics and boundary layer theory.

Funding for my research and education was provided by MIT EAPS, the WHOI Academic Programs Office and the MIT Presidential Fellowship. Financial assistance from the Houghton Fund is also acknowledged.


–  –  –

Chapter 1 Introduction Ocean bottom boundary layers are regions adjacent to topography where turbulence mixes heat, momentum and biogeochemical tracers. These regions serve as a dynamical control on the circulation by dissipating energy and shape the local characteristics of the marine environment by redistributing tracers. Tracers, such as sediment and nutrients, are transported by the lateral circulation near the bottom as well as the vertical circulation into and out of these layers. In order to quantify these momentum and tracer fluxes, an understanding of the strength and structure of this circulation is needed.

Friction plays an important role in driving this lateral and vertical circulation.

The boundary exerts a frictional stress on the flow that reduces the near bottom velocity within a frictional boundary layer, the Ekman layer. This frictional force induces an ageostrophic Ekman flow down the pressure gradient through a subinertial balance between the frictional force, the Coriolis acceleration, and the horizontal pressure gradient. The vertically-integrated Ekman flow, the Ekman transport, is directed to the right (left) of the frictional force in the Northern (Southern) Hemisphere. Convergences and divergences in the Ekman transport eject fluid out of, Ekman pumping, or inject fluid into, Ekman suction, the boundary layer. This process drives an ageostrophic secondary circulation that can accelerate or decelerate the geostrophic flow in the interior.

Observational and theoretical studies have examined the role of cross-isobath Ekman advection of buoyancy in setting the structure of the ocean bottom boundary layer as well as the frictionally driven circulation. Over an insulated stratified sloping boundary, downslope (upslope) Ekman advection of buoyancy induces a positive (negative) buoyancy anomaly and tilts the isopycnals near the bottom. This isopycnal tilting leads to vertical shear in the geostrophic flow, reducing the bottom stress and hence weakening the Ekman transport (MacCready and Rhines 1991, Trowbridge and Lentz 1991). An arrested Ekman layer occurs when this buoyancy anomaly is sufficiently large to reduce the bottom stress to zero. This process, known as buoyancy shutdown of the Ekman transport, has important consequences for the interior flow field (MacCready and Rhines 1991). By weakening the Ekman transport, buoyancy shutdown also weakens Ekman pumping and suction. When the Ekman flow is arrested, the interior geostrophic flow can evolve unimpeded by frictional forces.

The purpose of this thesis is to examine how coupling between the frictionally driven flow and the buoyancy field over sloping topography modifies the vertical circulation and the interior geostrophic flow through feedback by secondary circulations.

Analytical and numerical techniques are used to address how this coupling impacts the temporal evolution and spatial characteristics of the flow. The results of this analysis are applied to observations to determine the extent that cross-isobath Ekman advection of buoyancy can explain the structure of flows over stratified sloping topography.

In the following sections, an overview of previous research is presented regarding the significance of cross-isobath Ekman advection of buoyancy on the structure and dynamics of flows along stratified boundaries. This overview addresses notable studies, with a primary focus on observations, that have shaped our current understanding of frictionally driven flows over stratified sloping topography. Then, the goals of this dissertation are presented, with relation to open questions unanswered by previous research, followed by an outline of the thesis chapters.

1.1 Background and Motivation Previous research has identified cross-isobath Ekman advection of buoyancy as a potentially important mechanism influencing currents over stratified shelves and slopes.

Observational and theoretical studies have ranged from examining the one-dimensional bottom boundary layer dynamics to accounting for lateral variations in its structure.

These studies have considered different aspects of this mechanism, which can be categorized into the following four questions.

How does cross-isobath Ekman advection of buoyancy:

• influence the height of the bottom boundary layer?

• impact mixing processes by shear or convective instability?

• couple with the lateral Ekman flow and on what timescales?

• modify the vertical circulation and feedback into the geostrophic flow by secondary circulations?

These questions have been examined in different flow regimes along stratified sloping topography. These regimes include coastal currents along continental shelves and the upper continental slopes off of the west and east coast of the United States as well as the more weakly stratified deep western boundary currents along the lower continental slope in the North and South Atlantic ocean. A particular region of interest is the frontal system along the Middle Atlantic Bight shelfbreak, where the gradually sloping continental shelf intersects the steeply sloping continental slope off of the east coast of the United States. Studies of these regions reveal where cross-isobath Ekman advection of buoyany may or may not be important to the subinertial dynamics over sloping topography.

The overview of past research is presented in three parts. The impact of crossisobath Ekman advection of buoyancy on the structure of bottom boundary layers over the continental shelves and slopes is presented in section 1.1.1. In section 1.1.2, observations of flows near the Middle Atlantic Bight shelfbreak are presented as motivation for studying how these stratified bottom boundary layer processes over slopes feedback into the coastal currents. Finally, observations supporting or refuting the importance of cross-isobath Ekman advection of buoyancy on bottom boundary layers in deep western boundary currents is presented in section 1.1.3.

1.1.1 Bottom boundary layers over continental shelves and upper continental slopes Over continental shelves and upper continental slopes, characteristics in the near bottom flow and tracer fields distinguish bottom boundary layers from the overlying flow.

First, currents tend to veer counterclockwise downward, which is consistent with the direction of Ekman veering predicted by a balance between frictional forces and the Coriolis acceleration (e.g. Weatherly 1972, Wimbush and Munk 1970, Kundu 1976, Mercado and Van Leer 1976). Second, small scale measurements of temperature as well as velocity gradients can be used to distinguish the bottom boundary layer as a region with high levels of turbulent kinetic energy dissipation (e.g. Perlin et al.

2005, Moum et al. 2004). Third, temperature, salinity, and density tend to appear vertically well-mixed within a bottom mixed layer (e.g. Weatherly and Niiler 1974, Weatherly and Van Leer 1977, Pak and Zaneveld 1977). Observations indicate that the Ekman layer thickness, determined from Ekman veering, may (e.g. Mercado and Van Leer 1976) or may not (e.g. Perlin et al. 2005) equal the bottom mixed layer thickness.

Observational and numerical studies have shown that coupling between frictionally driven flows and the density field can impact the thickness of the frictional bottom boundary layer and the bottom mixed layer. Over a flat bottom, Weatherly and Martin (1978) argued on dimensional grounds that stratification reduces the bottom boundary layer height from the unstratified case. By including stratification in the scaling for the frictional boundary layer height, they showed that the revised stratified scale height was qualitatively consistent with previous estimates of bottom boundary layer thickness over the West Florida continental shelf (Weatherly and Van Leer 1977). From examination of near bottom temperature profiles in the Florida current, Weatherly and Niiler (1974) suggested that horizontal advection of buoyancy over sloping topography was key in explaining the formation of bottom mixed layers.

Weatherly and Van Leer (1977) also suggested that patterns of warming (cooling) within the bottom boundary layer may be explained by frictionally driven downslope (upslope) flows due to downwelling (upwelling) favorable along-shelf flows.

Weatherly and Martin (1978) used a numerical model, with the Mellor-Yamada level 2 turbulence closure scheme, to examine how the frictional bottom boundary layer is modified by upslope or downslope Ekman advection of buoyancy. The thickness of the bottom boundary layer is specified by the height at which the turbulence vanishes away from the bottom. For upslope Ekman flows, they showed that the bottom boundary layer height tended to remain constant and approximately equal to their stratified bottom boundary layer height scale. In contrast, for downslope Ekman flows, their model showed a thickening of the bottom boundary layer beyond this scale estimate. Model results compared with Weatherly and Van Leer’s (1977) observations showed qualitative agreement in bottom boundary layer heights.

Pages:   || 2 | 3 | 4 | 5 |   ...   | 21 |

Similar works:

«Classical and Revisionary Theism on the Divine as Personal: A Rapprochement? Elizabeth Burns, Heythrop College, University of London International Journal for Philosophy of Religion, 78 (2), pp. 151-165. First published online 19 December 2014. The final publication is available at Springer via http://dx.doi.org/10.1007/s11153-014-9500-3 Abstract To claim that the divine is a person or personal is, according to Richard Swinburne, ‘the most elementary claim of theism’ (1993, 101). I argue...»

«Palacky University Olomouc Philosophical Faculty Department of English and American Studies English and American Literature Veronika Klusáková Playing in the Dark and Dirt: Tomboyism and its Image in Carson McCullers’s The Heart Is a Lonely Hunter and The Member of the Wedding as Novels and Films Hry v temnotě a špíně: tomboyismus a jeho obraz v románech Carson McCullers Srdce je osamělý lovec a Svatebčanka a jejich filmových verzích Ph.D. Dissertation | Dizertační práce...»

«Tel Aviv University The Lester & Sally Entin Faculty of Humanities The Shirley & Leslie Porter School of Cultural Studies Morpho-phonological and Morpho-thematic Relations in Hebrew and Arabic Verb Formation THESIS SUBMITTED FOR THE DEGREE “DOCTOR OF PHILOSOPHY” by Lior Laks SUBMITTED TO THE SENATE OF TEL AVIV UNIVERSITY May 2011 This work was carried out under the supervision of Prof. Outi Bat-El Prof. Tal Siloni Table of Contents Abstract Acknowledgements Chapter 1. Introduction 1.1....»

«The Concept of Self-Reflexive Intertextuality in the Works of Umberto Eco by Annarita Primier A thesis submitted in conformity with the requirements for the degree of Doctor of Philosophy Centre for Comparative Literature University of Toronto © Copyright by Annarita Primier 2013 ii The Concept of Self-Reflexive Intertextuality in the Works of Umberto Eco Annarita Primier Doctor of Philosophy Centre for Comparative Literature University of Toronto 2013 Abstract Umberto Eco’s novels are...»

«Faculty of Arts and Philosophy Dagmar De Tandt ‘Making Heads Throb Heartlike’: David Foster Wallace’s Oblivion as a Deleuzian Clinical and Critical Project Supervisor: Sarah Posman Master dissertation submitted in fulfillment of the requirements for the degree of “Master in English” Academic year 2012-2013 Acknowledgments ‘(.) thesis, the project that had occupied and defined me for months’ (Here and There)1 In the car reading interviews with David Foster Wallace: ‘[James Brown:...»

«Selectively De-animating and Stabilizing Videos by Jiamin Bai A dissertation submitted in partial satisfaction of the requirements for the degree of Doctor of Philosophy in Computer Science in the Graduate Division of the University of California, Berkeley Committee in charge: Professor Ravi Ramamoorthi, Chair Professor Maneesh Agarwala Professor Marty Banks Fall 2014 Selectively De-animating and Stabilizing Videos Copyright 2014 by Jiamin Bai 1 Abstract Selectively De-animating and Stabilizing...»

«The Middle Voice of Love in 1 Corinthians: Reading Singularity and Plurality from Different Cultures By Meng Hun Goh Dissertation Submitted to the Faculty of the Graduate School of Vanderbilt University in partial fulfillment of the requirements for the degree of DOCTOR OF PHILOSOPHY in Religion December, 2014 Nashville, Tennessee Approved: Daniel M. Patte Fernando F. Segovia Annalisa Azzoni Kathy L. Gaca William Franke Khiok-Khng (K. K.) Yeo TABLE OF CONTENTS Page Acknowledgements.. v-vi...»

«Piedras votivas de Pampacolca Nuevos datos sobre las lajas pintadas del sur del Perú Votivsteine aus Pampacolca Neue Daten über bemalte Steinplatten aus Südperu Inauguraldissertation zur Erlangung des Grades eines Doktors der Philosophie am Fachbereich Geschichtsund Kulturwissenschaften der Freien Universität Berlin vorgelegt von Renata Faron-Bartels M.A. Berlin 2011 1. Gutachter: Prof. Dr. Jürgen Golte 2. Gutachterin: Prof. Dr. Ursula Thiemer-Sachse Tag der Disputation: 10. 11. 2009 2...»

«The Lamont Harp The Healing Harp (From Druid Tradition) By Susa Morgan Black, FSA, Scot Harper, Druid (OBOD) Quote “Music is the principle that unites body, soul, and spirit.” – Boethius, 6th century philosopher.The Healing Musician, Stella Benson Irish Legends The Daghdha (Dagda) was the Chief Druid and King of the mythical Tuatha Dé Danann – the faerie race of Ireland and Scotland. He played an enchanted harp, called Una, which produced three magical strains of music – Geantraí,...»

«Isolated Experiences: Gilles Deleuze and the Solitudes of Reversed Platonism James Brusseau Facultad de Filosofia y Letras Universidad Nacional Autonoma de Mexico Acknowledgement I acknowledge Professor Alphonso Lingis for his contributions to this work. The task of contemporary philosophy has been defined: the reversal of Platonism.Gilles Deleuze, 1968 Difference and Repetition Contents Introduction I Difference 1. Difference As Production And Limitation. 2. The Eternal Return Does Difference:...»

«UNIVERSITETET I BERGEN Coupled surface process and tectonic modelling of extension-inversion tectonics in the Pyrenees Dissertation for the degree of Philosophiae Doctor (PhD) Zoltán Erdős University of Bergen 2014 Preface The research presented in this thesis was conducted at the Department of Earth Science, University of Bergen (Norway), and in parallel at the Institute of Earth Sciences, Joseph Fourier University (Grenoble, France) under the supervision of Prof. Ritske Huismans, Prof....»

«Sabiduria, Vol. 1, 2 Lenke 1 The Transformation of Hazel Quarrier in At Play in the Fields of the Lord1 Madeleine Lenke The plot of Peter Matthiessen‘s novel At Play in the Fields in the Lord revolves around religious, cultural, and philosophical clashes between Christian missionaries, mercenaries, local authorities, and native tribes in the Amazon rain forest. The author has created an array of characters with strong archetypal features and symbolic names which serve a specific purpose in...»

<<  HOME   |    CONTACTS
2016 www.dissertation.xlibx.info - Dissertations, online materials

Materials of this site are available for review, all rights belong to their respective owners.
If you do not agree with the fact that your material is placed on this site, please, email us, we will within 1-2 business days delete him.