The Interannual Variability of The Leeuwin Current Ryan Warrington i - The Interannual Variability of The Leeuwin Current Ryan Warrington Abstract Previous numerical modeling studies have replicated the mean flow characteristics of the warm poleward current off the coast of Western Australia, known as the Leeuwin Current and subsequently raised questions about the current’s interannual variability. This thesis used the Regional Ocean Modeling System (ROMS) to investigate the effect that pronounced Southern Oscillation events have on the mean flow of the Leeuwin Current. The modeling consisted of three ROMS simulations running from July-December to compare current behavior during the strong 1997 El Niño, the 1998 La Niña response and 1996, a neutral year. ROMS was forced using temperature, salinity and baroclinic velocity data from the OCCAM global ocean model. The critical decision of deciding when to perform the run was based on OCCAM plots, satellite altimetry and southern oscillation composite images. The timing of the run (July - December) was intended to incorporate both the latter phase of the current’s peak and the peak phase of Southern Oscillation events. A comprehensive analysis of OCCAM’s reproduction of both Southern Oscillation events and mean flow characteristics of the Leeuwin Current revealed that both interannual and seasonal forcing of the Leeuwin Current were adequate. However it was revealed that 1996 showed weak to mild La Niña characteristics in the forcing data, which reduced the effectiveness of the comparison between 1996 and 1998. After a stable ROMS solution was generated the reproduction of the Leeuwin Current’s mean flow characteristics was verified using satellite altimetry. An analysis of model results revealed that Southern Oscillation events had a significant impact on both the North West Shelf source waters and the Leeuwin Current itself. It was also found it took approximately one month before the effect of warm, low salinity inflow onto the North West Shelf was felt by the Leeuwin Current. ii - The Interannual Variability of The Leeuwin Current Ryan Warrington The overall climate of the domain was significantly affected by El Niño as reflected by cooler temperatures and higher salinity. The El Niño Leeuwin Current was observed to be distinctly cooler and more saline then the other two years by approximately 1-1.5 ° C and 0.1 - 0.2 PSU on average, which was attributed not only to a reduced geopotential gradient along the Western Australia coast, but a lack of pooling in the North West Shelf. The La Niña Leeuwin Current was distinctly warmer then the El Niño year but was very similar to the ‘neutral’ year. This was put down to the likeness of the neutral year to a La Niña event. The increase in temperature in the latter part both 1996 and 1998 reflected the behaviour of warm Indonesian Throughflow discharge onto the North West Shelf. Whilst the difference between a La Niña and neutral year could not be fully quantified, the dynamic behaviour of the Indonesian Throughflow-Leeuwin Current interaction was sufficient to suggest that La Niña year produces a warmer Leeuwin Current then normal. iii - The Interannual Variability of The Leeuwin Current Ryan Warrington Acknowledgements There any many people to thank generously providing their time, resources and support all throughout the year and without who this project would not have gone ahead. A big thankyou to Greg Ivey for overseeing the project and for all the guidance and support he provided. I am indebted to Michael Meuleners, who generously gave his time all throughout the year and provided answers to my endless questions about modelling. Without his help this project would have taken substantially longer, possibly in the order of years. I would also like to thank iVEC for their generous use of their facilities, without which the project would have been unable to go ahead. Also thanks to Darren Carey the systems administrator for helping me to gain access to the system. I would also like to thank to Geosciences Australia for providing the bathymetry data. Also thankyou to David Griffin from CSRIO for providing satellite imagery and suggesting relevant work to look at. Finally I would like to thank my family and extend a special thankyou to my girlfriend Jessica for all of her support and for understanding. iv - The Interannual Variability of The Leeuwin Current Ryan Warrington Table of Contents 1 INTRODUCTION.........................................................................................................................1 2 GLOSSARY AND ABBREVIATIONS.......................................................................................1 3 LITERATURE REVIEW.............................................................................................................3 3.1 INTRODUCTION.......................................................................................................................3 3.2 DRIVING FORCE .....................................................................................................................5 3.3 THE SOUTHERN OSCILLATION................................................................................................6 3.4 PREVIOUS STUDIES.................................................................................................................9 3.4.1 Observational Studies.......................................................................................................9 3.4.2 Modelling Studies ...........................................................................................................13 3.5 EDDY GENERATION..............................................................................................................15 3.6 ROMS STUDIES ...................................................................................................................15 4 METHODS..................................................................................................................................18 4.1 ROMS MODEL DESCRIPTION...............................................................................................18 4.1.1 Primitive Equations ........................................................................................................18 4.1.2 Transformed Co-ordinates..............................................................................................20 4.1.3 Method of Solutions and Forcing ...................................................................................21 4.2 OCCAM MODEL DESCRIPTION ...........................................................................................22 4.3 VISUALISING DATA..............................................................................................................23 4.4 MODEL DOMAIN ..................................................................................................................24 4.5 EVALUATING RUN TIMEFRAME............................................................................................25 4.6 SATELLITE IMAGERY............................................................................................................26 4.7 OCCAM VALIDATION: SOUTHERN OSCILLATION ...............................................................26 4.8 OCCAM’S REPRODUCTION OF THE LEEUWIN CURRENT SIGNATURE ..................................27 4.9 MODEL SETUP......................................................................................................................28 4.10 MODEL RUN.........................................................................................................................30 4.11 MODEL ANALYSIS................................................................................................................32 4.12 VERIFICATION ......................................................................................................................34 5 RESULTS ....................................................................................................................................34 5.1 SATELLITE IMAGERY............................................................................................................34 5.1.1 AVHRR PATHFINDER ..................................................................................................34 5.1.2 TOPEX/POSIDEN ERS ½ ..............................................................................................36 5.1.3 Summary of Observations...............................................................................................37 5.2 OCCAM VALIDATION: OUTHERN OSCILLATION..................................................................38 5.2.1 Pacific Signature ............................................................................................................39 5.2.2 Indonesian Signature......................................................................................................40 5.3 OCCAM’S REPRODUCTION OF THE LEEUWIN CURRENT SIGNATURE ..................................40 5.4 ROMS MODEL RESULTS......................................................................................................44 5.4.1 Run 1: Tides....................................................................................................................44 5.4.2 Run 2: No Tides ..............................................................................................................45 5.4.3 Run 3: No Western Boundary or Tides...........................................................................46 6