Fundamentals of Geophysical Fluid Dynamics James C. McWilliams Department of Atmospheric and Oceanic Sciences UCLA March 14, 2005 Contents Preface 5 1 The Purposes and Value of Geophysical Fluid Dynamics 6 2 Fundamental Dynamics 13 2.1 Fluid Dynamics . 13 2.1.1 Representations . 13 2.1.2 Governing Equations and Boundary Conditions . 14 2.1.3 Divergence, Vorticity, and Strain Rate . 20 2.2 Oceanic Approximations . 21 2.2.1 Mass and Density . 23 2.2.2 Momentum . 26 2.2.3 Boundary Conditions . 28 2.3 Atmospheric Approximations . 31 2.3.1 Equation of State for an Ideal Gas . 31 2.3.2 A Stratified Resting State . 34 2.3.3 Buoyancy Oscillations and Convection . 37 2.3.4 Hydrostatic Balance . 40 2.3.5 Pressure Coordinates . 41 2.4 Earth's Rotation . 45 2.4.1 Rotating Coordinates . 47 1 2.4.2 Geostrophic Balance . 50 2.4.3 Inertial Oscillations . 53 3 Barotropic and Vortex Dynamics 55 3.1 Barotropic Equations . 57 3.1.1 Circulation . 58 3.1.2 Vorticity and Potential Vorticity . 62 3.1.3 Divergence and Diagnostic Force Balance . 64 3.1.4 Stationary, Inviscid Flows . 66 3.2 Vortex Movement . 72 3.2.1 Point Vortices . 73 3.2.2 Chaos and Limits of Predictability . 80 3.3 Barotropic and Centrifugal Instability . 83 3.3.1 Rayleigh's Criterion for Vortex Stability . 83 3.3.2 Centrifugal Instability . 85 3.3.3 Barotropic Instability of Parallel Flows . 86 3.4 Eddy{Mean Interaction . 91 3.5 Eddy Viscosity and Diffusion . 94 3.6 Emergence of Coherent Vortices . 97 3.7 Two-Dimensional Turbulence . 101 4 Rotating Shallow-Water and Wave Dynamics 108 4.1 Rotating Shallow-Water Equations . 111 4.1.1 Integral and Parcel Invariants . 115 4.2 Linear Wave Solutions . 119 4.2.1 Geostrophic Mode . 122 4.2.2 Inertia-Gravity Waves . 122 4.2.3 Kelvin Waves . 125 2 4.3 Geostrophic Adjustment . 128 4.4 Gravity Wave Steepening: Bores and Breakers . 138 4.5 Stokes Drift and Material Transport . 145 4.6 Quasigeostrophy . 148 4.7 Rossby Waves . 152 4.8 Rossby Wave Emission . 153 4.8.1 Vortex Propagation on the β-Plane . 154 4.8.2 An Eastern Boundary Kelvin Wave . 157 5 Baroclinic and Jet Dynamics 161 5.1 A Layered Hydrostatic Model . 163 5.1.1 2-Layer Equations . 163 5.1.2 N-Layer Equations . 167 5.1.3 Vertical Modes . 170 5.2 Baroclinic Instability . 176 5.2.1 Unstable Modes . 178 5.2.2 Upshear Phase Tilt . 182 5.2.3 Eddy Heat Flux . 184 5.2.4 Effects on the Mean Flow . 185 5.3 A Turbulent Baroclinic Zonal Jet . 187 5.3.1 Posing the Problem . 187 5.3.2 Equilibrium Velocity and Buoyancy Structure . 191 5.3.3 Zonal Momentum Balance . 198 5.3.4 Potential Vorticity Homogenization . 203 5.3.5 Meridional Overturning Circulation and Mass Balance . 204 5.3.6 Meridional Heat Balance . 206 5.3.7 Summary Remarks . 207 5.4 Rectification by Rossby Wave Radiation . 208 3 6 Boundary-Layer and Wind-Gyre Dynamics 212 6.1 Planetary Boundary Layer . 213 6.1.1 Boundary Layer Approximations . 215 6.1.2 The Shear Boundary Layer . 219 6.1.3 Eddy Viscosity Closure . 223 6.1.4 Bottom Ekman Layer . 225 6.1.5 Oceanic Surface Ekman Layer . 229 6.1.6 Vortex Spin Down . 234 6.1.7 Turbulent Ekman Layer . 235 6.2 Oceanic Wind Gyre and Western Boundary Layer . 244 6.2.1 Posing the Problem . 244 6.2.2 Interior and Boundary-Layer Circulations . 251 6.2.3 Application to Real Gyres . 256 6.2.4 Turbulent Baroclinic Wind Gyres . 261 Afterword 267 Glossary of Symbols 268 Bibliography 276 Index 279 4 Preface Earth's atmosphere and oceans exhibit complex patterns of fluid motion over a vast range of space and time scales. On the planetary scale they combine to establish the climate of Earth in response to the forcing by solar radiation inhomogeneously absorbed by the materials that comprise the air, water, and land. Through instabilities of the planetary-scale circulation, spontaneous and energetic variability arises in many different forms such as waves, jets, vortices, boundary layers, and turbulence. Geophysical fluid dynamics (GFD) is the science of all these types of fluid motion. It seeks to identify and analyze the essential dynamical processes that lie behind observed phenomena. As with any other theoretical science of complex, nonlinear dynamics, mathematical and computational modeling are essential research methodologies, and there is a continuing search for more powerful, accurate, and efficient techniques. This book is an introduction to GFD for readers interested in doing research in the physics, chemistry, and/or biology of Earth's fluid environment. It is a product of teaching a first graduate course on GFD in the Department of Atmospheric and Oceanic Sciences at the University of California, Los Angeles (UCLA) for several years. It is only an introduction to the subject; additional, more specialized GFD courses are required to fully prepare for practicing research in the subject. Nevertheless, to stimulate students' enthusiasm, the contents are a mixture of rudimentary mathematical analyses and somewhat complex dynamical outcomes. Students in this course are expected to have some familiarity with physics and mathematics, at the level of general dynamics (mechanics) and partial differential equations. In the present graduate curriculum at UCLA, students are first exposed to a course on basic fluid dynamics and thermodynamics and to another course on the major phenomena and underlying conceptual models for winds and currents. This background comprises the starting point for this book. GFD is a mature subject, having had its adolescence in the middle of the last century. Consequently many meritorious books already exist. Most of them are specialized in their material, but several of the more general ones are usefully complementary to this book, e.g., Cushman-Roisan (1994), Gill (1982), Holton (1992), Pedlosky (1987), Salmon (1998), and Stern (1975). 5 Chapter 1 The Purposes and Value of Geophysical Fluid Dynamics In this book we will address a variety of topics that, taken together, comprise an introduction to geophysical fluid dynamics (GFD). The discussion is intended to be more about the concepts and methods of the subject than specific formulae or observed phenomena. I hope they will be of both present interest and future utility to those who intend to work in Earth Sciences but do not expect to become specialists in the theory of dynamics, as well as to those who do have that expectation and for whom this is only a beginning. Before starting I would like to make some preliminary remarks about the scope, purposes, and value of GFD. The subject matter of GFD is motion in the fluid media on Earth and the distributions of material properties, such as mass, temperature, ozone, and plankton. (By common custom, planetary and astrophysical fluids are also included in GFD, since many of the scientific issues are similar, but it is awkward to use a more accurate title that explicitly includes all of these media. This book will not leave Earth.) So there is some chemistry, and even biology, in GFD, insofar as they influence the motion and evolution of the reactive materials. Nevertheless, for the most part GFD is a branch of physics that includes relevant aspects of dynamics, energy transfer by radiation, and atomic and molecular processes associated with phase changes. Yet GFD is by no means the entirety of ocean-atmosphere physics, much less its biogeochemistry. Within its subject-matter boundaries, GFD is distinguished by its purpose and its methodology. It is not principally 6 concerned with establishing the facts about Earth's natural fluids, but rather with providing them a mathematical representation and an interpretation. These, in my opinion, are its proper purposes. Beyond knowledge provided by basic physics and chemistry, the facts about Earth's fluids are established in several ways: in the laboratory, where the constitutive relations, radiative properties, • and chemical reactions are established, and where some analog simulations of natural phenomena are made; in the field, where measurements are made of the motion fields, • radiation, and material property distributions; by theory, where the fundamental laws of fluid dynamics are well known, • although | primarily because of their nonlinearity | only a small fraction of the interesting problems can actually be solved analytically; and on the computer, where relatively recent experience has demonstrated • that simulations, based upon the fundamental relations established in the laboratory and theory as well as parameterizations of influential but unresolved processes, can approach the reality of nature as represented by the field measurements, but with much more complete information than measurements can provide. In physical oceanography most of the pioneering laboratory work (e.g., the equation of state for seawater) has already been done, and so it is easy to take it for granted. This is also true for physical meteorology, but to a lesser degree: there remain important mysteries about the physical properties of water droplets, aerosols, and ice crystals, especially in clouds since it is difficult to simulate cloud conditions in the laboratory. For many decades and still today, the primary activity in physical oceanography is field measurements. Field measurements are also a major part of meteorology, although computer modeling has long been a large part as well, initially through the impetus of numerical weather forecasting. Field measurements are, of course, quite important as the \measurable reality" of nature. But everyone who does them comes to appreciate how difficult it is to make good measurements of the atmosphere and ocean, in particular the difficulty in obtaining a broad space-time sampling that matches the phenomena.