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On the East Australian Current: U pwelling and Separation Moninya Roughan 151 E 152 E 153 E 154 E 151 E 152 E 153 E 154 E A t hesis s u bmitted for the degree of D octor of P hilosophy at The University of New South Wales Sydney, Australia December 2001 Do not tempt me to beat my chest and say this is what I have done. Nelson Mandela Declaration I hereby declare that this submission is my own work and to the best of my knowl­ edge it contains no material previously published or written by another person, nor material which to a substantial extent has been accepted for the award of any other degree or diploma at UNSW or any other educational institution, except where due acknowledgement is made in the thesis. Any contribution made to the research by others, with whom I have worked at UNSW or elsewhere, is explicitly acknowledged in the thesis. I also declare that the intellectual content of this thesis is the product of my own work, except to the extent that assistance from others in the project's design and conception or in style, presentation and linguistic expression is acknowledged. Monmya Roughan There's a song in the wind and a kiss in the rain, There's a thrill in the sound of the sea, There is joy in the grey of a stormy today, There's a song in the heart of me. Molly Hall circa 1930 Dedication For my mum, for her ceaseless love, support and encouragement. 11 Friends help you move. Real friends help you move bodies. The very best friends help you bury them, then forget where. Anonymous Acknowledgements I would like to thank my supervisor Jason Middleton for his encouragement, support and advice. Throughout my PhD Jason has been an inspiration in how to combine work and play. Thanks go to Greg Nippard, Iain Suthers, Richard Piola, Jocelyn Dela Cruz, David Ghisolfi, Ann Marie Wong, Augy Syahailatua, the Master and crew of the RV Franklin, as well as the shore crew for their assistance, dedication and genuine hard work in the field component of this research. Thanks to George Cresswell, Stuart Godfrey, and Scott Condie for the insightful discussions during my visits to the CSIRO division of Marine Research in Hobart. Also from the Labs, Kim Badcock for his tireless efforts in providing me with satellite data on many occasions, in many different formats, and °Jeff Dunn for supply of the CARS data. Lixin Qi provided the GRADS graphics in Figure 3.16. Nathan Bindof and Steve Rintoul must be thanked for helping make my Antarctic dream come true, (even after I chose sub-tropical oceanography as the subject of my research). Both trips were unforgettable for completely different reasons, and anyone who has ever been stuck in the ice on a burning ship, or has flown by helicopter over iii ACKNOWLEDGEMENTS iv icebergs and glaciers will know what I am talking about! My colleagues at UNSW Pedro and Mahk have been full of good ideas, and even subjected themselves to the proof reading. And thanks of course to my predecessors in the lab who made an impression in the early days of my career and have now moved on to bigger and better things: Mark, the Witchdoctor, Brad, and the Professor. My friends and family have been a constant source of encouragement and support, their unfailing love and understanding despite months of neglect, both astounded me and inspired me. Finally Rob 'BC' Brandez must be thanked for his tireless efforts proof reading what to a non-oceanographer, are essentially boring manuscripts! Not to mention the R and R at Bronte and Coogee and the unlimited supply of banana smoothies. Abstract A multidisciplinary experimental program was undertaken during the 1998 - 1999 Austral summer to investigate upwelling processes on the continental shelf of New South Wales, Australia. Oceanographic time series data were obtained from two arrays of current meters and thermistors moored across the continental shelf at Smoky Cape (30°55'8) and Diamond Head (31 °44'8) for a 2 month period. These shore-normal arrays spanned the point where the East Australian Current (EAC) normally separates from the coast. Two intensive hydrographic surveys were also conducted aboard the RV Franklin during mooring deployment and retrieval. The combination of the time series measurements with the hydrographic data comprise the basis of the observational data set examined in this study. The observations show that the EAC dominates the physical processes across the narrow continental shelf at Smoky Cape. Analysis of the dynamical balances up­ stream and downstream of the EAC separation point reveals inherent variability in the system. Upstream of the separation point the along-shore pressure gradient drives the flow, and the processes of advection and bottom stress are important, whereas the variability in the across-shore currents and local acceleration tend to dominate the flow downstream. Generally the influence of wind stress on the flow is found to be minimal in the separation region. V ABSTRACT vi The EAC is responsible for driving colder nutrient rich water from the continental slope at Smoky Cape into the near surface waters in the coastal region south of Smoky Cape by means of Ekman transport through the bottom boundary layer. Current driven upwelling occurs concurrently with the encroachment of the EAC across the continental shelf. The acceleration of the current increases the bottom friction which allows water to be transported shoreward through the bottom bound­ ary layer for extended periods, resulting in upwelling downstream of Smoky Cape. Current driven upwelling occurs on a more massive scale than that driven by local wind forcing by an order of magnitude. A configuration of the Princeton Ocean Model is initialised using climatological measurements of temperature and salinity, which is used to investigate the mean current field off the NSW coast. The modelled velocity field closely resembles the observed currents and is used as a basis for a series of Lagrangian particle track­ ing experiments designed to investigate the source and sink of upwelled water and nutrients. Advection of particles both forward and backward in time reveals that upwelled waters originate offshore at depth, well to the north of Smoky Cape. In­ shore, coastal recirculations form and surface waters flowing northward toward the separation point are more often than not entrained into the southward flowing EAC jet. This implies that phytoplankton blooms, which have historically been observed in this region, could have been advected northward inshore towards Smoky Cape or southward along the EAC front. Supporting Publications Journal Articles: Roughan, M. and Middleton, J.H. 2002: A comparison of current driven upwelling mechanisms: Observations off the coast of NSW Australia. Accepted: Continental Shelf Research Roughan, M. Oke, P.R. and Middleton, J .H. 2002: A modelling study of the climato­ logical current field and the trajectories of upwelled particles in the East Australian Current. Submitted: J. Physical Oceanography Conference Proceedings: Roughan, M. and Middleton, J.H. 2002: Current driven upwelling about the separa­ tion point of the East Australian Current. Eos. Trans. AGU, 83(4), Ocean Sciences Meet. Suppl., Abstract OS51E-13, 2002. Roughan, M. and Middleton, J.H. 2001: Observations and comparisons of upwelling mechanisms on the NSW shelf. In Proceedings, Eighth National Australian Meteo­ rological and Oceanographic Society Conference, Hobart, Australia, 2001. Roughan, M. and Middleton, J.H. 2000: Nutrient enrichment on the NSW Shelf. In Proceedings, Australian Marine Sciences Association Annual Conference, Sydney, Vll SUPPORTING PUBLICATIONS viii Australia, 2000. Roughan, M. and Middleton, J.H. 1999: Three dimensional structure of the East Australian Current on the NSW shelf: Upwelling and nutrient enrichment. In Pro­ ceedings, Sixth National Australian Meteorological and Oceanographic Society Con­ ference, Canberra, Australia, 1999. Contents Declaration ii Dedication iii Acknowledgements iv Abstract vi Supporting Publications viii 1 Introduction 1 1.1 Motivation . 2 1.2 Overview .. 6 1.3 Scope of Study 9 1.4 Objectives . 10 IX CONTENTS X 1.5 Outline . 11 1.5.1 Chapter 2: The Field Project . 11 1.5.2 Chapter 3: Experimental Observations . 12 1.5.3 Chapter 4: Variability about the EAC separation . 12 1.5.4 Chapter 5: Current Driven Upwelling ...... 12 1.5.5 Chapter 6: A Modelling Study about the EAC Separation 13 1.5.6 Chapter 7: Recapitulation . 13 2 The Field Project 14 2.1 The Experimental Program . 15 2.1.1 The Hydrographic Survey . 15 2.1.2 The Current Meter Array . 18 2.1.3 Mooring Construction . 21 2.2 Other Data Sources . 23 2.2.1 Wind. 23 2.2.2 Sea Level Elevation 25 2.2.3 Atmospheric Pressure at the Sea Surface 26 2.2.4 Satellite Imagery 26 2.2.5 Climatology of Australian Regional Seas 27 CONTENTS xi 2.3 Summary . 27 3 Experimental Observations 28 3.1 The Velocity Field . 29 3.2 The Temperature Field . 33 3.3 The Wind Field . 37 3.4 Sea Level Elevation .................. 38 3.4.1 The Inverse Barometer Effect . 40 3.5 Variability of the Coastal Currents . 42 3.5.1 EAC Events . 44 3.5.2 Wind Event . 48 3.5.3 Reversal Event . 48 3.5.4 Current Event . 53 3.5.5 Mean Current Field. 54 3.5.6 Mean Temperature Field . 59 3.6 Summary . 59 4 Variability about the EAC separation 62 4.1 Temporal Variability . 64 4.1.1 Frequency Analysis . 64 CONTENTS XU 4.1.2 Time Series Correlations . 69 4.2 Spatial Variability . 71 4.2.1 Vertical Structure . 73 4.2.2 Horizontal Structure . 76 4.3 Depth Averaged Velocity Field .

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