Tidal Band Current Variability Over the Northern California Continental Shelf

-1- TIDAL BAND CURRENT VARIABILITY OVER THE NORTHERN CALIFORNIA CONTINENTAL SHELF by Leslie Karen Rosenfeld B.S. University of Washington, Seattle, Washington (1978) SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY at the MASSACHUSETTS INSTITUTE OF TECHNOLOGY and the WOODS HOLE OCEANOGRAPHIC INSTITUTION December 1986 @ Lesl i e K. Rosenfel d 1986 The author hereby grants to MIT and WHOI permission to reproduce and distribute copies of this thesis document in whole or in part. Signature of Author Joint Program in Oceanography, Massachusetts Institute of Technology!Woods Hole Oceanographic Inst;tut;on Certified by Robert c. Beardsley « Thesis Supervisor Accepted by J~Ph -~eal osky - · C-;man. Joint Conmittee -for Physical Oceanography, Massachusetts Institute of Technology/Woods Hole Oceanographic Institution -3- ACKNOWLEDGMENTS My interest in oceanographY started long before graduate school, and I have been fortunate to meet along the way a number of people who taught me, encouraged me, and offered me the opportunities that have enabled me to get to this point in my career. My thanks to Bob Graunke, Nikos Orphanoudakis, Mike Rawson, John Thompson, Hal Mofjeld, Bill Boicourt, Oick Whaley, and Bill Ryan for believing in me and for showing me that oceanographY could be fun. I would like to express my appreciation to the scientists of the Physical OceanographY Departments at WHOI and MIT, most of whom I have called upon for assistance, at one time or another, during my years as a graduate student in the Joint Program. In particular, I would like to thank my advisor: Bob Beardsley; my thesis committee members: Ken Brink, Jim Price, Paola Rizzoli, and Bob Weller; and Dave Chapman, Harry Bryden, and Mel Briscoe, with whom I have had many helpful discussions. My thanks also to Carol Alessi and Dick Limeburner, without whom the data collection and processing would never have been achieved. Thanks to Anne-Marie Michael and Veta Green for assistance with the typing, and to all the other support staff who helped me. Dave Aubrey of the Geology Department provided the computer program for the tidal analysis; Brad Butman, John Moody, and Marlene Noble of USGS shared their computer programs, their time and their ideas with me; and John Farrington of the Chemistry Department was a constant source of encouragement. CODE was a multi-institutional experiment involving too many people for me to thank individually each person who contributed to its success and to my dissertation work, so I thank the CODE Group collectively for allowing me to use this data set. Bill ~rant fit into many of the categories above. He was a member of my thesis committee, a CODE participant, and someone who helped and encouraged me. Most importantly, he was not afraid or embarrassed to be openly supportive, and his enthusiasm was unflagging. I am sorry I did not finish this dissertation in time for him to see the completed version. Without the constant support of my fellow Joint Program students (and their spouses), I could not have completed this undertaking. They were there in the good times and the bad. Thanks guys. And to my many friends around the country who 'phoned in their moral support long-distance -I really appreciated it. Finally, thanks go to my parents who, although they thought I was crazy, didn't try to stop me, and to my sister, Alison, who stepped in at the eleventh hour and kept me from falling apart. While in graduate school, I received financial support from the WHOI Education Office, a NASA traineeship, and NSF grants OCE 80-14941 and OCE 84-17769. -4- TABLE OF CONTENTS Page ABSTRACT 2 ACKNOWLEDGMENTS 3 CHAPTER 1. INTRODUCTION 7 References 13 CHAPTER I1. DIURNAL PERIOD WIND STRESS AND CURRENT FLUCTUATIONS OVER THE CONTINENTAL SHELF OFF NORTHERN CALIFORNIA 15 Abstract 16 l. Introducti on 17 2. Historical Review 18 3. Data 21 4. Description of Diurnal Wind Stress and Heating Cycles 27 A. Wi nd Stress 27 i. Frequency Domain 28 ii. Time Domain 34 B. Heat Fl ux 3B 5. Description of Diurnal Period Currents 40 6. Correlation Between Wind Stress and Currents 51 7. Di urnal Ti dal Cu rrents 54 8. Forced Response 59 A. Interior Forced Response 64 B. Frictional Forced Response 67 9. Concl usions 85 Appendix A: Complex Demodulation 87 Appendix B: High-Pass Filter 90 Appendix C: Time-Dependent Ekman Layer 91 Appendix D: Mixed Layer Parameters 97 Acknowledgments 99 References 100 CHAPTER II 1. BAROTROPIC SEMIDIURNAL TIDAL CURRENTS OFF NORTHERN CALIFORNIA DURING THE COASTAL OCEAN DYNAMICS EXPERIMENT (CODE) 105 Abstract 106 l. Introducti on 107 2. Data 109 3. Method of Analysis 113 4. Observations 118 5. Discussion 122 -5- 6. Theory for a Kel vi n Wave Along a Bumpy Boundary 136 7. Conclusions 145 Appendix 147 Acknowledgments 148 References 149 CHAPTER IV. BAROCLINIC SEMIOIURNAL TIDAL CURRENTS OVER THE CONTINENTAL SHELF OFF NORTHERN CALIFORNIA 151 Abstract 152 List of Symbols 153 1. Introduction 156 2. Observations of Internal Tides on Continental Shelves 157 3. Data 165 4. Methods of Analysis 169 5. Observations 173 6. Comparison with Internal Wave Theory 191 7. Generati on 210 8. Effects of Shear in the Low Frequency Flow 220 9. Conclusions 223 Appendix: Application of ~aine's Theory of Interna1 Ti de Generati on 225 Acknowledgments 227 References 228 CHAPTER V. CONCLUSIONS 233 BIOGRAPHICAL NOTE 237 -6- -7- CHAPTER I INTRODUCTION -8- The rise and fall of sea level with fixed periodicity, known as the tide, is probably one of the earliest physical oceanographic phenomena observed by man. The tidal currents associated with this process have also received man's attention for centuries, particularly in coastal areas. Newton (1687) provided an explanation for the astronomical tide-generating force (ATGF), but systematic study of the dynamics of tidal currents awaited the work of Laplace (1776) who formulated the fundamental equations governing long waves in a barotropic ocean. Since that time, an enormous body of literature dealing with both barotropic and baroclinic tides has accumulated. Nearly every physical oceanography textbook includes a discussion of tides, and there have been numerous review articles written on the subject. While some of these references are cited in the succeeding chapters, the lists given there are by no means comprehensive. More references were consulted as background reading during the preparation of this dissertation than are cited in the text. Two of these which I have found to be particularly useful, and which deserve specific mention, are LeBlond and Mysak (1978) and Hendershott (1981). In spite of all the effort spent on the study of tides and tidal currents, not everything about them is clearly understood. Although the equilibrium tide that would result if the earth were covered with a homogeneous fluid of uniform depth that could respond instantaneously to the ATGF is well known for every point on the earth, local geometric, bathymetric, hydrographic, and meteorological conditions cause the observed tide to deviate substantially from this. Global numerical models can predict fairly well sea level variations on a coarse grid. Finer resolution local models have been applied to some -9- coastal areas, again with good results for sea level. Tidal current predictions are usually not as accurate. The focus of this dissertation is on the description and dynamical interpretation of the tidal band current fluctuations over the continental shelf off northern California. The term "tidal band" is '< used here to denote fluctuations with periods from about one-half to one day, including all the major diurnal and semidiurnal tidal constituents. The semidiurnal frequency is super-inertial, and the diurnal frequency sub-inertial, at this mid-latitude location. Kinetic and potential energy are strongly peaked at the diurnal and semidiurnal frequencies. Although inertial currents are occasionally observed, particularly during the winter when internal wave energy in general is elevated in this locale, they do not contribute significantly to the current variance. Consequently, the treatment here is divided into discussion of the diurnal and semidiurnal variability. Each chapter emphasizes a process which can cause the tidal currents to deviate from what would be anticipated based solely on observations of sea level. In Chapter II, the diurnal current variability is discussed, and the role played by atmospheric forcing is examined in detail. In Chapter III, the barotropic semidiurnal tidal currents over the shelf are described, and the effect of small-scale bumps in the coastline is evaluated. The baroclinic semi diurnal tidal currents, which are dependent upon the local time-varying hydrographic conditions, are examined in Chapter IV. The term barotropic means that pressure is constant on constant density surfaces, while baroclinic means that pressure is not constant on constant density surfaces. In a homogeneous fluid, all flow is -10- barotropic. In a stratified fluid, the flow may be considered to be made up of two components, one of which is independent of stratification and, in an inviscid fluid, is constant throughout the water column, and another which varies with depth due to the influence of the stratified density field on the pressure gradient. The first component, which is due to the deflection of the sea surface, is commonly referred to as the barotropic component of flow, and the second component is referred to as the baroclinic. There is no practical way to completely separate the barotropic and baroclinic components of flow in the data. In regard to the tidal currents analyzed here, two approaches were used together to estimate the barotropic component. First, the currents were vertically integrated over the depth of the water column, to take advantage of the fact that the baroclinic horizontal currents can be expressed as a sum of modes, each of Which, in an inviscid fluid, integrates to zero under the assumptions of a flat bottom and a rigid lid.

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