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92 CHAPITRE 4. ETUDES DE PROCESSUS OCÉANOGRAPHIQUES Ocean Modelling 12 (2006) 101–111 www.elsevier.com/locate/ocemod Tilted convective plumes in numerical experiments A. Wirth * , B. Barnier LEGI BP 53, 38041 Grenoble Cedex 9, France Received 25 October 2004; received in revised form 8 March 2005; accepted 21 April 2005 Available online 20 June 2005 tel-00545911, version 1 - 13 Dec 2010 Abstract The dynamics of a single convective plume in an isothermal ocean is investigated by numerically integrating the three dimensional Boussinesq equations. Our study emphasizes on the important consequences of a non-vanishing angle between the axis of rotation and the buoyancy force (gravity). Experiments are performed for four different values of the angle corresponding to open-ocean convection at latitude: 90° N, 60° N, 45° N and 0° N. We show that the horizontal component of the rotation vector leads to qualitative and quantitative changes in the convective dynamics of a single plume. Plume structures are aligned along the axis of rotation rather than the direction of gravity (tilted convection), the vertical velocity of the plume is reduced, and the mixing is enhanced by the horizontal component of the rotation vector. These results suggest that in future parametrisations of ocean convection the effect of the horizontal component of the rotation vector should be included. Ó 2005 Elsevier Ltd. All rights reserved. PACS: 65N35; 76M22; 76R05; 86A05 Keywords: Geophysical fluid dynamics; Ocean convection; Traditional approximation * Corresponding author. Tel.: +33 0 6 32 97. E-mail address: achim.wirth@hmg.inpg.fr (A. Wirth). 1463-5003/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.ocemod.2005.04.005
4.4. TILTED CONVECTIVE PLUMES IN NUMERICAL EXPERIMENTS 93 102 A. Wirth, B. Barnier / Ocean Modelling 12 (2006) 101–111 1. Introduction tel-00545911, version 1 - 13 Dec 2010 The important role of deep convection on the overturning circulation at global scale is now well established and its impact on the climate dynamics recognized (Willebrand et al., 2000). Deep convection is also a principal component of the carbon cycle and has a strong influence on the biological dynamics in parts of the worlds ocean (Wood et al., 1999). Convection in the world ocean is a highly intermittent process in space and time. Convective regions, also called convective chimneys, typically span a few hundreds of kilometers in the horizontal directions. The actual convection process, that is, the vertical exchange and mixing of water masses only takes a few days. During this time the convective chimney is composed of a large number of convective plumes measuring up to 1 km in the horizontal directions (Schott and Leaman, 1991). The downward transport of the heavy water within the plumes is almost perfectly compensated by an upward transport in between the plumes (Send and Marshall, 1995). The plumes are the coherent structures governing the convective dynamics. A detailed understanding of the convection process is thus not conceivable without a detailed understanding of the plume dynamics. For a review on open-ocean convection we refer the reader to Killworth (1983) and Marshall and Schott (1999). The plumes can indeed be seen as the ‘‘atoms of convection’’ which, however cannot be explicitly resolved in the numerical models of the ocean circulation at basin scale. Their impact on the resolved dynamics has to be parametrised in such models. Efficient parameterizations are and will be key to the model development. Indeed, the rapid advancement of numerical ocean models and their improvement asks for an increasing accuracy in the representation of several phenomena. Current parameterisations of convection used in state of the art numerical models are inspired by the physics of the convection process and respond to the necessity of removing unstable stratifications in a numerical ocean. Improving parameterisation of the convection process does require a detailed understanding of the dynamics and the important processes and parameters involved. In this paper we demonstrate that the ‘‘traditional approximation’’, that is, neglecting the horizontal component of the rotation vector, is not appropriate when considering the deep convection processes, even at higher latitudes. Convective plumes in an isothermal ocean are investigated by numerically integrating the three dimensional Boussinesq equations. The effects of rotation on the plume dynamics are considered. We not only vary the magnitude of the rotation vector but also its direction. More precisely, in the majority of numerical calculations considering ocean dynamics, the traditional approximation is employed which completely neglects the horizontal component of the rotation vector. Rotation is thus supposed to be colinear with gravity (see e.g. Marshall et al., 1997; for a discussion on the traditional approximation in an oceanic context). The traditional approximation may be justified in instances where vertical velocities are small compared to their horizontal counterparts, that is, when non-hydrostatic terms can be neglected. When, however, the non-hydrostatic terms are essential for the dynamics, as in the case of convection, the traditional approximation has to be relaxed, especially in regions of the world ocean of not too high latitudes. For the here presented numerical investigation we choose an inclination of the rotation vector corresponding to open-ocean convection at latitudes: 90° N, 60° N, 45° N and 0° N a case of no rotation is added for the completeness of the dynamical picture. The angles of 45° N and 60° N correspond to the Gulf of Lions and the Labrador Sea, respectively, two major convection sites of the world oceans (Killworth, 1983; Marshall and Schott, 1999). The angle of 90° N, the North Pole where gravity
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Mémoire de Recherche présenté pa
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Table des matières I Etudes de pro
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Chapitre 1 Avant-propos tel-0054591
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Chapitre 2 Comprendre la dynamique
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2.3. HIÉRARCHIE DE TYPES DE MODÈL
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2.3. HIÉRARCHIE DE TYPES DE MODÈL
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2.5. MA RECHERCHE DANS LE CONTEXTE
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Chapitre 3 Dynamique océanique et
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3.1. LA CIRCULATION OCÉANIQUE À G
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3.1. LA CIRCULATION OCÉANIQUE À G
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3.2. PROCESSUS SUBMÉSO ECHELLES ET
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3.4. RETOUR À LA GRANDE ECHELLE PA
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Bibliographie tel-00545911, version
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33 Curriculum Vitae du Dr. Achim WI
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Colloque bilan LEFE, Plouzané, Mai
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4.7. ON THE BASIC STRUCTURE OF OCEA
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4.8. ESTIMATION OF FRICTION LAWS AN
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177 Contents 1 Preface 5 tel-005459
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179 Chapter 1 Preface tel-00545911,
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181 Chapter 2 Observing the Ocean t
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183 Chapter 3 Physical properties o
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185 3.3. θ-S DIAGRAMS 11 3.3 θ-S
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187 3.6. HEAT CAPACITY 13 tel-00545
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189 3.7. CONSERVATIVE PROPERTIES 15
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191 Chapter 4 Surface fluxes, the f
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193 4.2. FRESH WATER FLUX 19 water.
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195 Chapter 5 Dynamics of the Ocean
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197 5.2. THE LINEARIZED ONE DIMENSI
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199 5.4. TWO DIMENSIONAL STATIONARY
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201 5.6. THE CORIOLIS FORCE 27 Whic
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203 5.8. GEOSTROPHIC EQUILIBRIUM 29
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205 5.10. LINEAR POTENTIAL VORTICIT
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207 5.13. A FEW WORDS ABOUT WAVES 3
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209 Chapter 6 Gyre Circulation tel-
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211 6.1. SVERDRUP DYNAMICS IN THE S
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213 6.2. THE EKMAN LAYER 39 In the
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215 6.3. SVERDRUP DYNAMICS IN THE S
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217 Chapter 7 Multi-Layer Ocean dyn
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219 7.3. GEOSTROPHY IN A MULTI-LAYE
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221 7.5. EDDIES, BAROCLINIC INSTABI
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223 Chapter 8 Equatorial Dynamics t
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225 Chapter 9 Abyssal and Overturni
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227 9.2. MULTIPLE EQUILIBRIA OF THE
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229 9.3. WHAT DRIVES THE THERMOHALI
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231 Chapter 10 Penetration of Surfa
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233 10.2. TURBULENT TRANSPORT 59 If
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235 10.5. ENTRAINMENT 61 instabilit
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237 Chapter 11 Solution of Exercise
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239 65 Exercise 32: The moment of i
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241 INDEX 67 Transport stream-funct
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Annexe A Attestation de reussite au
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Annexe B Rapports du jury et des ra
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251 utilisés avec pertinence. Sur
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