A Quantitative Analysis of the Leeuwin Current System Using the Bluelink Model Solution
Bradley Smith
Supervisors: Michael Meuleners Greg Ivey
This page has been left blank intentionally Faculty of Engineering
The University of Western Australia
35 Stirling HWY
Crawley WA 6009
Attention: The Dean
Dear Sir,
It is with great pleasure that I submit this thesis, entitled “A Quantitative Analysis of the Leeuwin Current System Using the Bluelink Model Solution”, as a partial fulfillment of the requirements for the degree of
Bachelor or Engineering (Environmental) with Honours.
Yours Sincerely,
Bradley Smith
This page has been left blank intentionally
Abstract
The Leeuwin Current system off the Western Australian coastline consists of the surface, poleward flowing Leeuwin Current and the subsurface (300-600m), equatorward flowing Leeuwin Undercurrent (Thompson, 1984). Countless studies have investigated the properties of the Leeuwin Current and its influences on the mesoscale oceanic conditions in the region yet very little is known about its companion, the Leeuwin Undercurrent. Using Bluelink’s global ocean circulation model, the Leeuwin Current system is investigated, with focus on a quantitative analysis of the Leeuwin Undercurrent.
A qualitative model validation revealed that the Bluelink model solution accurately models the mesoscale oceanic conditions surrounding Western Australia. A number of cross-shore transects were set-up to investigate the transport, width, depth, and temperature and salinity signatures of the Leeuwin Current. These values were then compared to results from other studies on the Leeuwin Current to determine the effectiveness of this methodology at extracting the Leeuwin Current information, with promising results.
This validated methodology along with horizontal spatial plots of current speed and direction were then used to define the Leeuwin Undercurrent boundary and extract information on the Leeuwin Undercurrent, with ground-breaking results. This study found that the Leeuwin Undercurrent has its origins between Cape Leeuwin and Cape Naturaliste and does not exist as a shelf edge current on the south coast. The Flinders Current was identified flowing offshore along the southern coastline with a transport of 8Sv, as seen in other studies (Middleton and Cirano, 2002). A climatology analysis of the Naturaliste Plateau revealed a component of the Flinders Current diverges south of Cape Leeuwin, tending to the north-west where a series of persistent, clockwise eddy-like structures rotate the Flinders Current water 180º. Subsequently, a consistent onshore flow exists along the coastline between Cape Leeuwin and Cape Naturaliste, producing an alongshore pressure gradient, as seen in other studies (Woo et al., 2005, Meuleners et al., 2007b) which is believed to drive the Leeuwin Undercurrent northwards at around 1-2Sv throughout the year. This study will provide a foundation for future research into the Leeuwin Undercurrent, as it reveals the Naturaliste Plateau as the region of Leeuwin Undercurrent formation.
i
Acknowledgements
I would like to take this opportunity to thank a number of people whose help was integral to the completion of this project:
Most importantly, I would like to thank my two supervisors, Mike Meuleners and Greg Ivey, who have provided me with expert guidance and endless support throughout this project. Mike, thanks for your countless meetings with me and always having your door open, it was greatly appreciated and I couldn’t have finished this project without you. To Greg, thanks for your wealth of experience into the subject and for your enthusiasm into a number of my findings. We’ll have to get the ball rolling on naming that eddy the “Meuleners Meander”.
Nicole Jones for providing the Matlab code for the cross-shore transect analysis, without it I could not have analysed the Leeuwin Current system as accurately as I have.
The Bureau of Meteorology, Royal Australian Navy and CSIRO for providing access to the Bluelink model solution; it is an amazing model that should serve the oceanographic community well.
Finally to my mates outside of uni, for persuading me to relax a bit on the weekends and to wind down with a kick of soccer and a few beers, you can’t think about uni 24/7 otherwise you’ll go mad.
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Table of Contents
1.0 Introduction ...... 1 1.1 Objectives of this Study ...... 1 2.0 Literature Review...... 3 2.1 Australia’s West Coast...... 3 2.1.1 The Leeuwin Current ...... 3 2.1.2 The West Australian Current...... 5 2.1.3 Coastal Currents...... 6 2.2 Australia’s South Coast...... 7 2.2.1 Physical Characteristics...... 7 2.2.2 The Leeuwin Undercurrent ...... 9 2.2.3 Flinders Current...... 11 2.3 Summary ...... 11 3.0 Methodology ...... 12 3.1 Model Description...... 12 3.1.1 Model Stability...... 13 3.1.2 Why Bluelink?...... 14 3.1.3 Limitations of the Model...... 15 3.2 Model Domain and Setup...... 16 3.2.1 Model Domain...... 16 3.2.2 Time Period...... 17 3.3 Model Validation...... 18 3.3.1 Leeuwin Current Properties ...... 18 3.3.2 Eddy Shedding Characteristics...... 19 3.4 Model Output Analysis ...... 20 3.4.1 Cross-Shore Transects...... 20 3.4.2 Matlab Programs ...... 21 3.4.3 Leeuwin Current...... 24 3.4.4 Leeuwin Undercurrent...... 25 3.4.5 Volume Flux...... 28 3.4.6 Temperature and Salinity ...... 28 3.4.7 Width-Depth...... 29 3.4.8 Core Position...... 29 3.5 Summary of Methodology ...... 29 iii
4.0 Model Results...... 30 4.1 Model Validation...... 30 4.1.1 Leeuwin Current Properties ...... 30 4.1.2 Eddy Shedding Characteristics...... 31 4.1.3 Summary of Model Validation...... 33 4.2 Leeuwin Current...... 34 4.2.1 Volume Fluxes...... 34 4.2.2 Temperature Signatures...... 38 4.2.3 Salinity Signatures...... 40 4.2.4 Width-Depth to Slope...... 42 4.3 Leeuwin Undercurrent...... 44 4.3.1 Volume Fluxes...... 45 4.3.2 Location of Core...... 47 4.3.3 Other results...... 48 4.4 Capes Current...... 50 4.5 Summary of Results ...... 50 5.0 Discussion...... 52 5.1 Model Validation...... 52 5.1.1 Eddy Shedding Characteristics...... 52 5.2 Leeuwin Current...... 55 5.2.1 Volume Fluxes...... 55 5.2.2 Temperature Signature...... 56 5.2.3 Salinity Signature...... 58 5.2.4 Width-Depth to Slope...... 59 5.3 Leeuwin Undercurrent...... 60 5.3.1 Volume Fluxes...... 60 5.4 Capes Currents...... 69 5.5 Limitations of the Study...... 69 5.5.1 Model Validation...... 69 5.5.2 Leeuwin Current Analysis...... 71 5.5.3 Leeuwin Undercurrent Analysis...... 71 6.0 Conclusions ...... 74 7.0 Recommendations for Future Research...... 76 7.1.1 Bluelink ...... 76 7.1.2 Leeuwin Current...... 76 iv
7.1.3 Leeuwin Undercurrent...... 77 8.0 References ...... 79
v
List of Figures
Figure 2.1 – The complex nature of the oceanic currents around Australia (CSIRO, 2000)..... 3 Figure 2.2 – Schematic of the Leeuwin Current system (Woo et al., 2005)...... 9 Figure 2.3 – Plot of bathymetry highlighting the Naturaliste Plateau’s location...... 10 Figure 3.1 – Schematic showing the initialization procedure used in BRAN (Oke et al., 2007b)...... 13 Figure 3.2 – Depth profiles of the RMSE between the observed temperature profiles from Argo, BRAN and climatology. (Oke et al, 2007b)...... 14 Figure 3.3 – Spatial plot of the model domain used in this study...... 17 Figure 3.4 – Cross-shore transect locations ...... 21 Figure 3.5 – Vertical profile of velocity at Transect 2 - 1/4/03 ...... 22 Figure 3.6 – Vertical profile of velocity with Leeuwin Current boundary in pink...... 22 Figure 3.7 – Vertical profile of Leeuwin Current velocity with extracted points shown as X. 23 Figure 3.8 – Leeuwin Current Volume Flux at Transect 2, April 2003...... 24 Figure 3.9 – Horizontal plot of current speed and direction - 9/2/2003...... 26 Figure 3.10 – LUC velocity profile on 9/2/2003 at Transect 5 (left plot) and Transect 7 (right plot)...... 27 Figure 3.11 – LUC velocity profile on 9/2/2003, with eddy included (left plot) and without eddy (right plot)...... 28 Figure 4.1 – Eddy generating regions and typical eddy migratory paths along the W.A. coastline...... 33 Figure 4.2 – Seasonal response of LC during summer (left) and winter (right)...... 35 Figure 4.3 – Mean seasonal variation in Transport of LC along the coastline ...... 35 Figure 4.4 – Seasonal and annual transport of LC ...... 36 Figure 4.5 – Geostrophic inflow along the Western Australian coastline (Pearce and Phillips, 1988)...... 37 Figure 4.6 – Transport of LC relative to Shark Bay...... 38 Figure 4.7 – Temperature Profile of Leeuwin Current: Transect 1 June 7th...... 39 Figure 4.8 – Leeuwin Current Mean Temperature Signature ...... 40 Figure 4.9 – Salinity Profile of Leeuwin Current at Transect 1 June 7th...... 41 Figure 4.10 – Leeuwin Current Mean Salinity Signature ...... 42 Figure 4.11 – Leeuwin Current Depth: Seasonal average (left), annual average (right) ...... 44 Figure 4.12 – LUC velocity profile at Transect 4 and Transect 6 - 11/6/2003 ...... 45 Figure 4.13 – LUC seasonal (left) and annual (right) transport...... 46 Figure 4.14 – The proposed divergent flow of the Leeuwin Undercurrent at Cape Leeuwin . 47 vi
Figure 4.15 – Offshore distance of the LUC core...... 48 Figure 4.16 – LUC temperature (left) and salinity (right) profiles at Transect 4, 1/5/2003 .... 49 Figure 4.17 – Temperature (left) and salinity (right) signatures of the LUC...... 49 Figure 4.18 – Velocity profiles of Capes Current at Transect 2 and 3 – 25/2/2003 ...... 50 Figure 5.1 – Meander in LC prior to anticlockwise eddy formation...... 53 Figure 5.2 – Velocity profile of the Leeuwin Current System during eddy creation...... 54 Figure 5.3 – Ekman Spiral, depicting the wind and current directions (National Aeronatical Space Administration, 2006)...... 61 Figure 5.4 – Seasonal (left plot) and Annual (right plot) transport of the LUC...... 62 Figure 5.5 – Divergent flow of Flinders Current at Cape Leeuwin ...... 64 Figure 5.6 – Proposed Leeuwin Undercurrent formation (Meuleners et al, 2007b) ...... 65 Figure 5.7 – Schematic of the Climatology of the Naturaliste Plateau over 2002 and 2003 at - 372m underlaid with current speed ...... 66 Figure 5.8 – Horizontal spatial plot of mean temperature over 2002 and 2003...... 68 Figure 5.8 – Current Speed horizontal plot - 27/12/2002 ...... 70 Figure 5.9 – Current Speed horizontal plot - 1/1/2003 ...... 70 Figure 5.10 – LUC boundary before (left) and after (right) coding change...... 72 Figure A1 - Satellite and BRAN v2.1 output - 6/2/2003 ...... ii Figure A2 - Satellite and BRAN v2.1 output - 7/4/2003 ...... iii Figure A3 - Satellite and BRAN v2.1 output - 6/6/2003 ...... iv Figure A4 - Satellite and BRAN v2.1 output - 7/8/2003 ...... v Figure A5 - Satellite and BRAN v2.1 output - 6/10/2003 ...... vi Figure A6 - Satellite and BRAN v2.1 output - 7/12/2003 ...... vii
vii
List of Tables
Table 3.6 – Leeuwin Current Cross-shore transect properties...... 25 Table 3.7 – Leeuwin Undercurrent Cross-shore transect properties...... 25 Table 4.1 – Leeuwin Current properties at Shark Bay throughout 2003 ...... 30 Table 4.2 – Leeuwin Current properties at Perth throughout 2003...... 30 Table 4.3 – Leeuwin Current properties at Albany throughout 2003 ...... 31 Table 4.4 – Eddy observations from satellite images...... 32 Table 4.5 – Eddy observations from Bluelink v2.1 output ...... 32 Table 4.6 – Seabed slopes at LC transect locations ...... 43 Table 4.7 – Correlation Coefficients and LOBF slope for LC width vs. depth ...... 43 Table 5.1 – Comparison of peak transport of LC...... 56 Table 5.2 – Leeuwin Current Temperature signature comparison with previous studies...... 57 Table 5.3 - Leeuwin Current Salinity signature comparison with previous studies ...... 59
viii Introduction
1.0 Introduction
The ocean circulation off the coast of Western Australian (W.A.) is dominated by the Leeuwin Current system, consisting of the surface, poleward flowing Leeuwin Current (hereby referred to as LC) and the subsurface, equatorward flowing Leeuwin Undercurrent (LUC). Countless studies have investigated the properties of the L.C. (Thompson, 1984, Rochford, 1984, Batteen and Butler, 1998), revealing a strong connection between a number of endemic, tropically derived ecosystems and the existence of the warm, L.C. waters along the W.A. coastline (Caputi et al., 1996, van Hazel, 2001). These ecosystems support a number of lucrative industries, including rock lobster fisheries and tourism in the Recherche Archipelago. Surprisingly, very little is known about the other half of the LC system; the LUC, or the important role it may play in the ocean circulation off W.A.
The LUC is a sub-surface (300-600m), shelf-edge current that transports relatively high salinity, low temperature waters from the south-west to the north-west along the W.A. coastline (Thompson, 1984). Early modeling studies described the LUC as a countercurrent, flowing northwards in a conservation of mass response as the LC propagates down the coast (Thompson and Veronis, 1983). Field studies by Thompson (1984) found that a positive geostrophic pressure gradient exists at the depth of the LUC. This subsurface pressure gradient was also observed by Woo et al (2005) and modeled by Meuleners et al. (2007b) and is believed to drive the LUC northwards. Meuleners et al. (2007b) hypothesised that the LUC is a divergent flow, created via interactions of the Flinders Current with topographic irregularities around Cape Leeuwin. All of these studies propose possible LUC formation processes, none of which have been confirmed because of a lack of quantitative research on the LUC.
1.1 Objectives of this Study
This study aims to provide the first quantitative analysis into the LUC’s formation, migration and physical properties. Bluelink’s global ocean circulation model was used to accurately model the LC and LUC, allowing several cross-shore transects to extract information such as temperature, salinity, transport and length scales of the currents. The modelling was performed over 2003, a La Nina year, which should provide a stronger LC response due to the influences of ENSO on the LC (Feng et al., 2003). Particular attention was paid to the region 1 Introduction surrounding Cape Naturaliste, an area of topographic irregularity, that may influence the migration or possibly even drive the formation of the LUC (Meuleners et al., 2007b).
Previously, studies of the LUC have been limited, relying on observations made from shipboard and coastal measurements (Thompson, 1984, Woo et al., 2005), or as a side note to modelling studies into the LC (Batteen and Butler, 1998). While observations provide insight for the basic nature of features of the LUC, process-oriented studies are useful for investigating the characteristics and dynamical forcing mechanisms for the LUC system. To date, no modelling studies have effectively simulated and analysed the formation and migration of the LUC along the W.A. coastline.
Subsequently, the primary objective of this modelling study is to determine the origins of the LUC and examine some of the physical properties such as salinity, temperature, transport and length scales of the LUC system, providing a foundation for future studies into the LUC and its impact on the W.A. coastline. This modelling study aims to employ the validated Bluelink model solution to analyse the LC system, via a cross-shore transect methodology. Validation of this methodology, for use in the LUC analysis, will then be completed by comparing the LC transport, salinity and temperature signatures with previous studies(Rochford, 1986, Smith et al., 1991), allowing the LUC to be quantitatively analysed for the first time.
2 Literature Review
2.0 Literature Review
This section will provide an extensive review of the literature related to the ocean circulation around W.A., to identify the knowledge gaps that motivate this study.
2.1 Australia’s West Coast
The west coast of Australia is dominated by the LC system, with a number of smaller currents contributing to the complex nature of the oceanography. The complexity of the region is shown in Figure 2.1.
Figure 2.1 – The complex nature of the oceanic currents around Australia (CSIRO, 2000). The Leeuwin Current system is visible along the length of the W.A. coastline, with its origins as Indonesian Throughflow from the north. The turbulent nature of the LC system is also apparent with a large number of eddy structures generated along the western and southern coastlines. Other currents presented here are the West Australian Current and the Capes Current.
2.1.1 The Leeuwin Current
The LC is an anomalous, poleward flowing eastern boundary current which exists along the continental shelf edge off W.A. (Weaver and Middleton, 1989). The LC differs markedly in its characteristics and dynamics from other eastern boundary currents, i.e., Peru, California, 3 Literature Review which are characterized by weak surface flow towards the equator. Cresswell and Golding (1980) highlighted the existence of this strong, poleward current flowing down the W.A. coast, around Cape Leeuwin and into the Great Australian Bight. The LC transports warm, low salinity, and nutrient-depleted water of tropical origin at speeds of up to 1.5m/s around Cape Leeuwin in a shallow (<300m) and narrow (<100km) band (Cresswell and Golding, 1980). Further investigations have been carried out on the effect of the LC on the marine ecosystems and fisheries of the west and south coast. The LC, being the dominant oceanographic feature off W.A., has a major influence on the abundance of many species with high abundances of invertebrate species (such as Western Rock Lobster) compared with finfish (Caputi et al., 1996). It is suggested that the strength and high temperatures of the LC are responsible for the existence of tropical fauna on the south coast and Great Australian Bight as well as the existence of the unique Ningaloo Reef (Maxwell and Cresswell, 1981).
Formation
Subsequent studies have included the modelling of the current by Batteen and Butler (1998) and by Boedeker (2001), focussing on the forcing mechanisms of the LC. These studies revealed that the LC is formed through a combination of geostrophic inflow from the Indian Ocean and a meridional pressure gradient, created by throughflow from the Indonesian Archipelago and Pacific Ocean (Batteen and Butler, 1998, Boedeker, 2001). The southeast trade winds in the Pacific Ocean drive the South Equatorial Current westwards, advecting warm surface waters towards the west and creating a large pool of very warm water in the Indonesian Archipelago (Godfrey and Ridgway, 1985). The relative height of the Pacific Ocean water causes it to flows into the equatorial Indian Ocean, through the Indonesian Archipelago. A meridional pressure gradient forms between the low-density, high sea level waters off north-western Australia and the cool, denser water in the Southern Ocean, which induces the LC flow (Godfrey and Ridgway, 1985). These inflows overwhelm the equatorward wind stress, which is the main forcing for most eastern boundary currents (Batteen and Butler, 1998). The current is strongly seasonal, with its maximum flow observed off W.A. in winter, believed to be caused by a reduced wind stress with the alongshore pressure gradient being seasonally independent (Smith et al., 1991). A relationship between the El Nino Southern Oscillation (ENSO) in the Pacific Ocean and the LC strength has also been observed, with stronger flows during La Nina years (Feng et al., 2003). 4 Literature Review
Eddy Generation
The LC is a highly energetic and meandering flow, spawning anticlockwise, warm-core eddies that propagate offshore for some distance and time (Waite et al., 2007, Andrews, 1983). Cyclonic, cold-core eddies are often associated with these warm-core LC eddies as cold, open ocean water upwells to the surface creating large productivity pulses (Waite et al., 2007). Andrews (1983) and Batteen et al (1992) found that these eddies exist at two distinct length scales, 150km and 300km, with eddy generation occurring on the offshore side of the core of the LC. Larger scale eddies were created when all three forcing mechanisms (ie. The Indian Ocean temperature fields, the NW shelf waters and equatorward wind forcing) were apparent (Batteen et al., 1992). Meuleners et al. (2007a) found that the eddies are generated by the LC as a result of two instabilities: barotropic and baroclinic. The eddies are often found in dipole pairs, with the clockwise, cold-core eddies propagating from offshore, causing the LC to meander and eventually spawn an anti-clockwise, warm-core LC eddy (Meuleners et al., 2007a). A study by Fang and Morrow (2003) tracked the movement of anti-clockwise LC eddies over a period of 5 years using satellite images. The study focused on the W.A. coastline and found three major sites of eddy generation: at 20–21.5º S (North-west Cape), 24–25 º S (Shark Bay) and 28–31 º S(Abrolhos Islands) (Fang and Morrow, 2003). Eddies were found to have a mean velocity of 5-10km/day and the majority of eddies migrated in a westerly direction (Fang and Morrow, 2003).
Leeuwin Current Decay
The LC is believed to decay along W.A.’s south coast as it widens and extends into the Great Australian Bight. Limited research has been carried out on the decay dynamics of the LC in this region, although it is hypothesised that the alongshore pressure gradient driving the LC slackens along the south coast and offshore transport of eddies in the region removes energy from the system (Griffiths and Pearce, 1985).
2.1.2 The West Australian Current
Andrews (1983) identified the West Australian Current during four summer cruises off the southwestern coast of Australia. The West Australian Current is a hypothesized 100-200km wide cyclonic stream that extends some 800km from the Naturaliste Plateau to the coastline, where it interacts with the LC and turns poleward. Speeds of 0.5m/s were recorded at the 5 Literature Review surface of the current, with depth scale around 350m and a transport up to 10 Sv (Andrews, 1983). Limited research has been carried out on the West Australian Current but it is thought to form as an offshore Eastern Boundary Current, driven by the predominant south-easterly winds during summer (Andrews, 1983).
2.1.3 Coastal Currents
A number of recent, extensive studies have led to an improved understanding of the local circulation in the continental slope inner shelf region (< ~50 m). This research has identified two coastal currents: the Capes Current and the Ningaloo Current. In summer, the stronger equatorward wind causes water to move in the opposite direction to the LC within the inner shelf region (Cresswell et al., 1989, Gersbach et al., 1999, Woo et al., 2005). The general equatorward flows were first observed in a study by Rochford (1969) off of Perth. The experiment used a drifter bottle to track the flow, indicating an equatorward flow of water during summer, which turned south during winter.
Capes Current
Pearce and Pattiaratchi (1999) defined the Capes Current as a cool inner shelf current, originating between Cape Leeuwin and Cape Naturaliste. This current was found to move equatorward along the southwestern Australian coast in summer and is probably linked with the general northward shelf current which has been observed previously along most of the W.A. coastline. The current is cooler (21.0ºC–21.4ºC) and higher in salinity (35.37mg/L– 35.53mg/L) than the LC. The origin of this water makes the CC unique—its relative coolness and seasonality are from upwelling (Gersbach et al., 1999). Under the influence of Ekman transport, the LC moves offshore, allowing colder water to upwell onto the continental shelf and forcing the LC to move farther offshore. The dominant southeasterly wind stress on the west coast causes the water to upwell, bringing cold water to the surface, and later advecting, with the assistance of wind stress, to travel along the continental shelf to the north (Cresswell and Peterson, 1993).
6 Literature Review
Ningaloo Current
The Ningaloo Current is another coastal current, formed under the same conditions and processes as the Capes Current. It is an equatorward current, flowing counter to the LC and is prevalent in the northwest region of W.A. between 21ºS and 24.5ºS. CTD and ADCP data indicated the coastal flow consisted of colder (< 23ºC) saline (34.92) water when compared with offshore waters (Woo et al., 2005). Field measurements from Woo et al (2005) showed the surface water mass, with a depth of 50 m, moved northward with the prevailing wind. Furthermore, higher nutrient concentrations, reflected in a higher phytoplankton biomass, confirmed the existence of upwelling in connection with this current, which originated from an approximate depth of 100 m (Hanson et al., 2004). Further research into the Ningaloo Current upwelling events is being undertaken by Megan Wills at the School of Environmental Systems Engineering.
2.2 Australia’s South Coast
The remote nature of the Great Australian Bight has lead to it being largely overlooked by oceanographic scientists even though it has a number of interesting oceanic circulation patterns supporting a number of unique ecosystems. The southern Australian marine macro algal flora has the highest levels of species richness and endemism of any regional macro algal flora in the world (Phillips, 2004) and the Recherche Archipelago contains a number of endemic, tropically derived fish species (Maxwell and Cresswell, 1981). The region is also of considerable interest oceanographically: the LC was found to influence the oceanic conditions from the west (Cresswell and Golding, 1980), while the Eastern Australian Current was found to extend through the Bass straight into the eastern portion of the Bight (CSIRO, 2000). The complexity of the region can be seen in Figure 2.1.
2.2.1 Physical Characteristics
Godfrey et al (1986) undertook one of the first studies into the oceanic circulation patterns of the south coast, which highlighted the existence of a narrow shelf-edge current flowing eastwards along the entire distance, most probably an extension of the LC. This study involved an oceanographic cruise along the continental shelf from Fremantle, W.A. to Portland, Victoria (Godfrey et al., 1986). The analysis of more cruise data collected within
7 Literature Review the Great Australian Bight (seven cruises spanning the period 1961-1982) by Rochford (1986) revealed that a warm water mass existed within the Great Australian Bight, which could be distinguished from the LC water by its very high salinity, known as GAB water. The presence of this GAB water suggests that the warm band across the GAB in winter may not be solely attributable to a LC extension (Rochford, 1986).
A modelling investigation into this subject by Herzfeld and Tomczak (1997) demonstrated that the warm, eastern plume observed in the GAB in summer and autumn can be generated by local processes independent of the LC. During winter the observed warm band of water is the combination of the LC extension and remnants of the GAB water (Herzfeld and Tomczak, 1997). Cresswell and Griffin (2004) collected data from a research vessel cruise in late 1994 and several years of satellite observations, which revealed complex interactions of ocean features off south-western Australia. Of importance was the interaction of anti-cyclonic eddies of LC origin interacting with cyclonic eddies of sub-Antarctic water origin (Cresswell and Griffin, 2004). Satellite thermal and topographic measurements showed that cyclonic eddies accelerated the LC along the southern upper continental slope, whereas anticlockwise eddies diverted it out to sea and then back again (Cresswell and Griffin, 2004). Further research into the LC along the southern coastline is required if the impact of the LC on the ocean circulation patterns of the south coast is to be known.
Upwelling
Large, seasonal coastal upwellings have also been observed along Australian southern shelves (Kaempf et al., 2004). During each summer period there are about 2-3 wind-driven upwelling events, each lasting about 1 week (Kaempf et al., 2004). Upwelling of cold deep water in this region was also observed in recent studies from the Recherché Archipelago and adjacent waters (van Hazel, 2001). Recent studies showed the Ekman transport from the Leeuwin and Flinders currents induced upwelling from the permanent thermocline (Cirano and Middleton, 2004). These upwellings and the creation of warm-core eddies by the LC along the southern coast adds to the complexity of the region, making the oceanic conditions interesting yet difficult to study.
8 Literature Review
2.2.2 The Leeuwin Undercurrent
Shipboard observations made by Thompson (1984) in May 1982 showed a definite poleward surface flow (the Leeuwin Current) over the West Australian shelf. There was also a subsurface current at a few hundred meters depth which was salty, high in oxygen concentration and low in nutrients: the Leeuwin Undercurrent (Thompson, 1984). The LC system is shown in Figure 2.2 with the LUC hugging the shelf edge below the LC. This undercurrent has also been noted in the modelling study by Batteen and Butler (1998) with maximum undercurrent velocities occurring between Cape Leeuwin and Cape Naturaliste. No real quantitative studies have focussed on the LUC, which means the formation, flow characteristics and physical properties are still largely unknown.
Figure 2.2 – Schematic of the Leeuwin Current system (Woo et al., 2005) The surface LC (red) flows against the dominant wind stress (yellow) towards the South Pole, while the surface coastal current (purple) and the subsurface LUC (green) have an equatorward flow.
Formation
It is widely recognised that the LUC exists, however very little is known about its formation along Australia’s south coast. Early modeling studies described the LUC as a countercurrent, flowing northwards in a conservation of mass response as the LC propagates down the coast 9 Literature Review
(Thompson and Veronis, 1983) but this theory has never been confirmed or refuted. Field studies by Thompson (1984) found that a positive geostrophic pressure gradient existed at the depth of the LUC. This subsurface pressure gradient was also observed by Woo et al (2005) and modeled by Meuleners et al. (2007b), believed to drive the LUC but it does not explain the origins of the LUC. Recently, experts have hypothesised that it is a divergent flow created from coastal irregularities around Cape Leeuwin, with the strong, westerly Flinders Current as its original source (Meuleners et al., 2007b). Preliminary research by Meuleners et al. (2007b) found that the LUCs apparent bifurcation at Cape Leeuwin with transport in both westerly and northerly directions, could be due to topographic features such as the Naturaliste Plateau. The true impact of these bathymetry and coastal irregularities on the currents migratory path needs to be determined. Figure 2.3 highlights the Naturaliste Plateau’s predominance off of Cape Leeuwin.
Naturaliste Plateau
Figure 2.3 – Plot of bathymetry highlighting the Naturaliste Plateau’s location. The Naturaliste Plateau is a 500 x 1000 km plateau rising 1500m from the sea floor off the south-west corner of W.A., shown here in pink. The contour plot is of temperature and is of no real interest at this stage.
If the Leeuwin Current system is to be understood completely, the LUC and its interactions with the LC must first be examined quantified. Determining the source of the LUC and some of its physical properties would provide a foundation for future research into this potentially important current.
10 Literature Review
2.2.3 Flinders Current
Work by Middleton and Cirano (2002) demonstrated the existence of a westward flowing current, called the Flinders Current, which lies to the north of the continental shelf edge and is believed to result from the wind stress curl and Sverdrup dynamics. An upwelling favorable western boundary current was found in the OCCAM results, with speeds of up to 16 cm/s off the shelves south of Australia (Middleton and Cirano 2002). These results match those found in observations of the region made by Callahan (1972) and Hufford et al.(1997) with values between 8 and 17 Sv respectively, matching the OCCAM values of 10 and 16 Sv for the same sections. The current is quite distinct from other major current systems of the region. For example, the LC is a seasonal shelf break current that enters from the west, while the near- coastal currents are driven by surface Ekman transport and change direction with season (Middleton and Cirano 2002). The Flinders Current interacts with the LC near the shelf break, where the Flinders Current flows beneath the eastward LC, similar to the Undercurrent observed on the west coast (Church et al., 1989). This behaviour shows indirectly it is likely the Flinders Current acts to feed the LUC (Church et al., 1989).
2.3 Summary
This extensive literature review on the mesoscale oceanic conditions of W.A.’s coastlines has revealed a lack of quantitative knowledge on the LUC. Only a handful of studies have identified the existence of the LUC, either through field observations (Thompson, 1984) or as an aside to models investigating the LC (Batteen and Butler, 1998). The uncertainty in the origins and migratory path of the LUC needs to be rectified if the true impact of the LUC on the ocean circulation of the W.A. coastline is to be determined. This knowledge gap relating to the LUC is the main motivation behind this report as discussed in the Objectives section 1.1.
11 Methodology
3.0 Methodology
Outlined in this section are the methods that will be used to reach the objectives for this study, described in section 1.1. The methodology can be classified into four main components. These include an analysis of the model itself, a description of the model setup, a validation procedure to test the suitability of the model and the methods used to analyse its output.
3.1 Model Description
This modeling study will be one of the first to utilize a new model provided to the SESE, called Bluelink. Bluelink is an Australian collaboration of the CSIRO, the Bureau of Meteorology (BoM) and the Royal Australian Navy, whose primary objective is to develop a forecast system for the mesoscale ocean circulation in the Australian region. To do this, the Ocean Forecasting Australia Model (OFAM), a global ocean circulation model, has been configured to develop the Bluelink Ocean Data Assimilation System (BODAS). BODAS is an ensemble optimal interpolation (EnOI) system that uses the Bluelink ReAnalysis (BRAN) as its primary testbed (Oke et al., 2007b). BODAS uses model-based, multivariate background error covariances (BECs), and are the means by which an observation of some variable is projected onto the full model state, including all model grid points and all model variables (Oke et al., 2007b).
BRAN is a multi-year assimilation of OFAM with data assimilation. Data such as sea-levels from satellite altimetry and in situ measurements, satellite-derived sea-surface temperature (SST) and in situ temperature and salinity are compared to the Bluelink output every 6 days (Oke et al., 2007b). This output is then nudged or assimilated back to the measured data over a one day period to ensure the model is producing accurate results (Oke et al., 2007b). Conceptually, BRAN is intended to be a three-dimensional time-varying analysis of oceanic observations that uses OFAM as a dynamic interpolator. At the moment BRAN is in the test or hindcast phase with Bluelink v1.5 recently being superseded by Bluelink v2.1. Future aims are for Bluelink to begin forecast modelling for use in research and defence roles using the process shown in Figure 3.1, which is similar to the process carried out in the BRAN testing.
12 Methodology
Figure 3.1 – Schematic showing the initialization procedure used in BRAN (Oke et al., 2007b) The forecast assimilation cycle is presented over three weeks. Each step involves a 7 day forecast period, which is then compared to observations for the same 7 days, called Analysis. The cycle is then reset but with the model output being nudged towards the observed/analysis data, called nowcast. This process is repeated to determine the BECs and to keep the results as accurate as possible.
3.1.1 Model Stability
Initial testing of Bluelink v1.5 output by Oke et al (2007b) compared reanalysed fields to satellite-derived and in-situ observations, yielding the following, positive results:
• sea surface height within 6-10cm of altimetric observations and within 4-7cm of observed coastal sea levels,
• sea surface temperature within 0.4-0.9° of observed SST,
• subsurface temperatures within 1° of Argo temperatures,
• subsurface salinity within 0.15 psu of observed Argo salinity, and
• near-surface currents within 0.2m/s of Argo.
A draft report by Oke et al (2007b) found that Bluelink v1.5 is an appropriate tool for constraining an eddy-resolving ocean model that with appropriate tuning could prove to be a valuable resource for the oceanographic community. Figure 3.2 highlights the accurate results of Bluelink v1.5 data (blue) when compared to climate data (green).
13 Methodology
Figure 3.2 – Depth profiles of the RMSE between the observed temperature profiles from Argo, BRAN and climatology. (Oke et al, 2007b) Observed temperatures (climatology) are shown in green for 5 locations around Australia. The BRAN output is presented in blue and matches the climatology accurately. The model output with no data assimilation shown in red is 1-2ºC less accurate.
Several updates were made to Bluelink v1.5 following suggestions made by Oke et al., (2007b), which resulted in the creation of Bluelink v2.1. These changes were:
• an increase in the reanalysis time span from 4 years (2003 to 2006) to 14 years (1993 to 2006),
• the use of SST only in waters deeper than 200m. Originally, BRAN v1.5 used satellite altimetry in both shallow and deep regions, and
• a technical difference in the way that the model is updated.
3.1.2 Why Bluelink?
For the purposes of this study the hindcasting properties of Bluelink v2.1 were used. Bluelink v2.1 has a resolution of 10km around the Australian coastline (90-180ºE, south of 17ºN) and a resolution of 90km and 200km covering the rest of the globe. The model has a vertical depth scale of around 4500m, using a non-linear depth profile, with a resolution of 10m within the top 200m extending to a resolution of around 1000m at 4000m depth. This high resolution in 14 Methodology the horizontal and vertical planes makes Bluelink suitable for modelling and analysing the LC and LUC.
Results from Oke et al (2007b), described in section 3.1.1 of this study, reveal that Bluelink v1.5 produces accurate results for mesoscale oceanic modeling. Further model validation of Bluelink v2.1, carried out in section 3.3 of this study; produces similar results with a high degree of accuracy. Data from Bluelink v2.1 and a number of associated plotting and analysis tools were also provided free to SESE to encourage the wider research and validation of this new model. With accurate results and hindcast data easily obtained from the CSIRO, Bluelink provides a simple yet powerful tool for modeling the mesoscale oceanic conditions along Australia’s western and southern coastlines.
3.1.3 Limitations of the Model
Bluelink v2.1 is still in the test phase and is not yet available to the wider scientific community. However, through projects such as this study, any issues in the Bluelink v2.1 data will be identified and rectified by the Bluelink team. One of the updates from Bluelink v1.5 to v2.1 was a lengthening of the reanalysis time period from 4 years to 14 years. Bluelink v1.5 began data assimilation in 2003, while Bluelink v2.1 began data assimilation in 1993. Data for this study was originally obtained for 2003 in v1.5 format but was quickly changed to v2.1 data when the updates were made. The main issue arises when comparing 2003 data with 2002 data, as the years do not appear to be linked. This is a slight limitation in the model, creating an issue for this project, as a number of eddies identified in the model validation were apparent in January 2003 but were not there in December 2002. Subsequently, a number of eddy durations could not be determined as there was no inter- annual consistency between 2002 and 2003. Further discussion on this limitation is presented in section 5.5.1.
Bluelink is a global ocean model, with a set horizontal resolution of 10km around Australia. This resolution is sufficient for the purposes of this study, as the majority of length scales are in the tens of kilometers. However, if fine resolution water bodies or dynamic interactions needed to be resolved, a model such as Regional Ocean Modelling System (ROMS) would be more appropriate as ROMS resolution can be modified to suit the oceanic conditions being investigated (Shchepetkin and McWilliams, 2004). Bluelink is also limited in its flexibility.
15 Methodology
It is not possible to change the resolution or to modify the bathymetry of Bluelink, making it unsuitable for modeling studies with multilevel domains or for studies where changes need to be made to the bathymetry.
There are also a number of fundamental limitations associated with using ensemble optimal interpolation (EnOI) systems. For example, a straight forward implementation of EnOI does not typically preserve water mass properties during the assimilation step (Oke et al., 2007a). This makes EnOI inappropriate for some climate applications, where preservation of water masses may be regarded as essential. The estimates of the background error co-variances (BEC) for an EnOI scheme are only an approximation to the true BECs. Therefore EnOI is not an optimal method of assimilation (Oke et al., 2007a). Additionally, EnOI does not provide time varying estimates of analysis errors, such as those derived from an Ensemble Kalman Filter (Oke et al., 2007a).
3.2 Model Domain and Setup
The use of Bluelink v2.1 data in this study meant that no significant model setup or model runs were required. CSIRO had already completed modeling of the Australian coastline from 1993 to 2006 via the BRAN data assimilation process. Access to Bluelink v1.5 and v2.1 model data was provided by Bluelink through the CSIRO website. Daily data such as u and v velocities, temperature, salinity and sea surface height were downloaded in ASCII format. This data was then converted to NetCDF format for periods of 2-4 days, which were of manageable size and could be easily analysed.
3.2.1 Model Domain
The model domain for this study is rectangular in shape and covers the south-west corner of W.A., from Shark Bay in the north-west to Spencer Gulf in the south-east as shown in Figure 3.3. This model domain was chosen to cover the main areas of interest for this study, including the west Australian coast where the LC is most obvious and the Great Australian Bight where the LUC is thought to have its origins (Thompson, 1984). The model domain is parallel to the latitude with exact coordinates at 39.9ºS, 100ºE and 24.9ºS, 100ºE along the western boundary and 39.9ºS, 135ºE and 24.9 ºS, 135 ºE along the eastern boundary. This created a model domain with 151 points on the edges parallel to the latitude and 351 points on
16 Methodology the edges parallel to the longitude. All 3-dimensional data was obtained to the maximum depth of 4500m to allow detailed analysis of the whole water column to be undertaken.
Model Domain
Figure 3.3 – Spatial plot of the model domain used in this study The model domain extends from Shark Bay in the north-west to Spencer Gulf in South Australia. The model domain is parallel to the latitude with exact coordinates at 39.9ºS, 100ºE and 24.9ºS, 100ºE along the western boundary and 39.9ºS, 135ºE and 24.9 ºS, 135 ºE along the eastern boundary. The colour plot is of surface temperature.
The topography for Bluelink is a composite of a range of different topography sources, including dbdb2 and GEBCO (Oke et al., 2007b). Secondary bathymetry data was obtained from Geoscience Australia’s 1km resolution dataset for the south-west corner of Australia. This secondary data was used in horizontal spatial plots to display the bathymetry information within the model domain.
3.2.2 Time Period
This study is focused on the LC system, which is hypothesized to be stronger during La Nina events (Feng et al., 2003). Subsequently, 2003 was selected as the focus year as it is a recognized La Nina year and is likely to yield more obvious results (Feng et al., 2003). Data
17 Methodology was obtained once every 4 days during January, February, November and December and once every 2 days throughout the rest of 2003. This was done to get a higher temporal resolution during the period of greater LC activity through winter and autumn (Smith et al., 1991, Rochford, 1986). Data was also obtained once every 10 days over the whole of 2002 to provide additional data for the model validation and for inter-annual comparisons.
3.3 Model Validation
Bluelink v2.1 was created as a result of the Bluelink v1.5 model validation described in Oke et al (2007b). The modeling study described in this report utilizes data from Bluelink v2.1, which has model validation results pending. Subsequently, to be confident in the Bluelink v2.1 output, a simple model validation of the mesoscale oceanic conditions was undertaken. This was done by comparing Bluelink model output, for 2002 and 2003, with satellite images for the same period. These satellite images were obtained from the CSIRO website and contained sea surface temperature, sea surface height and geostrophic velocity. In particular, the strength, temperature and salinity signatures of the LC were examined as were the formation and migrational dynamics of the anticlockwise eddy structures.
It may appear erroneous to complete a model validation using satellite data, which has been used to nudge or assimilate the model output. However, the satellite data is only one component of the BRAN data assimilation process, which also includes ARGO float data and coastal data. The satellite data provides an accurate and simple comparison tool, suitable for the scope of this project. Further validation of the subsurface components of Bluelink v2.1 will need to be completed in the future for complete confidence in the model results.
3.3.1 Leeuwin Current Properties
The LC provides a useful focus for the model validation as it is a complicated system with a number of easily identifiable characteristics. The properties that will be investigated include velocity and temperature signatures along the coastline as well as the waxing and waning of the current due to seasonal effects.
The temperature and velocity values of the LC were estimated from the satellite images once every 2 months during 2003. This was done at three locations along the W.A. coast; at Shark
18 Methodology
Bay, Perth and Albany. The temperatures and velocities for the same days and locations were then estimated from the Bluelink v2.1 horizontal plots. These values were then compared to determine the accuracy of the model solution. The plots used in the analysis can be found in Appendix A, while the results of this part of the validation process are contained in section 4.1.1.
The seasonal response of the LC is also a large scale characteristic that can be validated through the use of satellite data. Satellite images in the summer and winter periods of 2002 and 2003 were compared to their modeled equivalent to determine whether the model recreated the phenomenon observed in the satellite images. The plots used in the analysis can be found in Appendix A, while the results of the comparison are shown in section 4.1.1.
3.3.2 Eddy Shedding Characteristics
The LC is a highly energetic and meandering flow, spawning anti-clockwise, warm-core eddies that propagate offshore. These eddies play a pivotal role in the dynamics of the LC, influencing its direction and transporting energy off-shore (Waite et al., 2007). Subsequently, the validation process was extended to compare eddies seen in the satellite data with those present in the Bluelink model solution. All anticlockwise eddies that were generated throughout 2002 and 2003 were observed and details such as generating region, migratory speed, direction and duration as well as horizontal scales were collated. This was done for both the satellite images and the Bluelink model output. Results of this analysis are contained in the section 4.1.2 of the Results.
19 Methodology
3.4 Model Output Analysis
The following methodology was used to analyse the data output from the Bluelink model, with the main focus of this based on the objectives of the study, outlined in section 1.1.
Throughout this project horizontal plots were used to visualize important information. These spatial plots were created using the Matlab toolbox ODVT (Oceanographic Data Visualisation Tool), which had been reconstructed to specifically focus on Bluelink output. Horizontal plots of temperature, salinity, density, swirl, u and v velocities and current speed and direction could be produced for any of the 47 vertical layers. This system was particularly useful for defining the LUC boundary during the LUC analysis, described in section 3.4.4 of the Methodology.
3.4.1 Cross-Shore Transects
A series of horizontal plots of temperature, current speed and direction were produced and then analysed to determine the extent of the LC over the model domain. Subsequently, a number of cross-shore transects were created along the western and southern coastlines to cover this extent and to ensure that the LC and LUC were being adequately examined. Seven transects were used in the analysis of the LC, while an additional transect was included for the LUC (in pink) as shown in Figure 3.4. The transects are aligned perpendicular to the coastline, as the LC and LUC generally flow parallel to the coastline. All transects were created with a changeable length due to the meandering nature of the LC and LUC so that offshore movements of the currents could be examined. Details of the angle (from north) and the length of the transects are contained in Tables 3.6 and 3.7 in the following sections.
20 Methodology
Transect 1
Transect 2
Transect 3 Transect 7 Transect 4 Transect 8 Transect 6
Transect 5
Figure 3.4 – Cross-shore transect locations The 8 cross-shore profiles used in the LC (blue) and LUC (blue and pink) analysis overlay a horizontal plot of temperature. The temperature plot highlights the extent of the LC along the W.A. coastline. Details of locations, angles and lengths of the transects are contained in tables 3.6 and 3.7
These cross-shore transects provide the foundation for all model data analysis throughout this study. Details such as velocity, volume flux, temperature, salinity, width and depth of the LC and LUC can be extracted from these transects using a suite of Matlab functions described in the next section.
3.4.2 Matlab Programs
A number of Matlab programs were created to analyse the data contained along the cross- shore transects, using the velocity, temperature or salinity to define the LC and LUC boundaries. See Appendix C for the full Matlab code. Several steps were completed for each time step at each transect to define the current boundary and extract the necessary information. These steps are as follows.
21 Methodology
1. The program displays a vertical plot for the property being examined along the transect for the first time step as shown below.
Figure 3.5 – Vertical profile of velocity at Transect 2 - 1/4/03 This is a plot of velocity with the depth (m) on the y axis and distance (km) on the x axis. The velocity colorbar is in m/s and the white region to the right is the seabed. The LC is visible as a large negative (southerly) surface flow off the shelf edge. It has a width of approximately 50km and a depth of around 250m
2. A series of questions are asked to establish the focus of the analysis and to define the boundary of the current under examination as shown below.
Figure 3.6 – Vertical profile of velocity with Leeuwin Current boundary in pink
22 Methodology
3. Once the boundary is defined, all temperature, salinity and velocity values are extracted (shown as x in Figure 3.14) and the minimum, maximum and average values are identified and saved in ASCII format. Other data such as volume flux, depth and width are calculated using a series of smaller in-built functions and saved in ASCII format.
Figure 3.7 – Vertical profile of Leeuwin Current velocity with extracted points shown as X.
The program saves all vertical profile plots, which are contained on a CD accompanying this thesis (See Appendix D for details). It also creates a plot of the data for the specified transect and month as shown below. This was completed for both the LC and LUC at all transects for every month in 2003.
23 Methodology
Figure 3.8 – Leeuwin Current Volume Flux at Transect 2, April 2003 This is simply an example plot of volume flux of the LC at Transect 2. All vertical profile plots and volume flux graphs are contained on the data CD accompanying this report as outlined in Appendix D.
3.4.3 Leeuwin Current
The LC is being analysed to provide confirmation that the cross-shore transect methodology can accurately define the boundary of the LC and thus the LUC. The LC’s boundary was most accurately defined using a velocity limit of 0.3m/s. This value was determined through an extensive review of literature and preliminary transect runs. Under turbulent conditions, where eddies were disrupting the LC, the current boundary was defined by the temperature signature as this produced more realistic results. The temperature and salinity signatures are season and site specific and are shown in Figures 4.8 and 4.10 in the Results section. At all transects the depth profile was extended to a depth of approximately 500m, to ensure that the entire LC was being examined. Table 3.6 illustrates the transect properties used throughout the LC analysis. Note the negative velocity limit is only used to define the LC boundary on the west coast of Australia. This is because the Leeuwin Current travels south along the coast, decreasing in latitude and producing a negative alongshore velocity. On the south coast the LC heads in an easterly direction, increasing in longitude and therefore it has a positive alongshore velocity.
24 Methodology
Table 3.6 – Leeuwin Current Cross-shore transect properties
Angle (from Velocity Limit Transect Location Length (km) north) (m/s) 1 28S, 114.1E 315 260 -0.3 2 30.5S, 115E 206 260 -0.3 3 33.65S, 115E 194 270 -0.3 4 34S, 114.9E 195 270 -0.3 5 34.36S, 115.15E 219 180 0.3 6 35.2S, 117.8E 221 170 0.3 7 33.9S, 121.3E 221 170 0.3
3.4.4 Leeuwin Undercurrent
The LUC’s boundary was most accurately defined using a velocity limit of 0.125m/s. This value was determined through an extensive review of literature and preliminary transect runs. At all transects the depth profile extended from the surface to a depth of 1100m or 1400m, to ensure that all of the LUC was being examined. As mentioned in section 3.4.1, an eighth transect was added at the south end of Cape Leeuwin with a westerly orientation for the LUC analysis (shown in pink in Figure 3.4). This transect was added to provide additional information on the origins and migratory properties of the LUC. Table 3.7 illustrates the transect properties when defining the LUC boundary. Again note the change in negative and positive boundary limits along the west and south coastlines depending on the alongshore component of the velocity.
Table 3.7 – Leeuwin Undercurrent Cross-shore transect properties
Angle (from Velocity Limit Transect Location Length (km) north) (m/s) 1 28S, 114.1E 319 260 0.125 2 30.5S, 115E 313 260 0.125 3 33.65S, 115E 299 270 0.125 4 34S, 114.9E 282 270 0.125 5 34.36S, 115.15E 336¹, 448² 180 -0.125 6 35.2S, 117.8E 352¹, 466² 170 -0.125 7 33.9S, 121.3E 353¹, 467² 170 -0.125 8 34.36S, 115.15E 281 270 0.125 Notes ¹ January, February, March and April lengths ² Rest of the year lengths
25 Methodology
Preliminary analysis of the LUC on the southern coastline necessitated a change in methodology for defining the boundary of the current. Unlike the LC, the LUC is highly dynamic and disjointed along the southern coastline, making it hard to identify its boundary. A large majority of the westerly flowing water was not associated with the LUC but with eddies occurring in the area. Subsequently, every vertical profile was compared to a horizontal profile of current speed and direction, similar to Figure 3.9, to distinguish between this entrained eddy water and the LUC water, providing a more realistic examination of the LUC on the southern coastline. The LUC’s position on the west coast was consistent, making it easier to distinguish from the eddy effects, thus not requiring the use of horizontal plots in the analysis.
The 9/2/2003 will be used as an example to explain the process and to identify some of the issues involved in defining the LUC boundary on the south coast.
Transect 5 Transect 7 Transect 6
Eddy Disrupting Flow
Figure 3.9 – Horizontal plot of current speed and direction - 9/2/2003. A typical horizontal plot of current speed and direction used in the LUC analysis. The southern transects are shown in pink and the large, westerly flow is shown as the black arrow. The offshore westerly flow is consistent at Transects 5 and 7. At Transect 6 an eddy/meander structure is disrupting the westerly flow, making it hard to undertake the LUC analysis.
Using Figure 3.9, the boundary of the LUC was easy to define at Transects 5 and 7 due to the lack of coastal eddies. Figure 3.10 contains the vertical velocity profile of Transect 5 (on the left) and Transect 7 (on the right).
26 Methodology
Figure 3.10 – LUC velocity profile on 9/2/2003 at Transect 5 (left plot) and Transect 7 (right plot) The LUC is visible as a negative offshore flow of around 0.1-0.2 m/s at both transects. The LUC is a sub-surface flow at Transect 5, extending from 200-800m depth, with the LC on the surface, adjacent to the continental shelf. The LUC is further offshore at Transect 7, with a vertical scale of 0-800m. The LC is not visible at Esperance during February.
Figure 3.9 highlights the existence of a large, anti-clockwise eddy formation at Transect 6, making it hard to distinguish between the LUC flow and the eddy water. This scenario occurred frequently throughout the analysis, with a stable LUC at Transects 5 and 7 and an eddy disrupted LUC at Transect 6. Just to the west of Transect 6, the LUC continues its migration but it is almost impossible to separate this from the eddy water. Figure 3.11 presents the two options for calculating the volume flux; including all of the westerly flowing water (left plot) or excluding all of this water (right plot).
27 Methodology
Figure 3.11 – LUC velocity profile on 9/2/2003, with eddy included (left plot) and without eddy (right plot) The left plot depicts the boundary of the LUC when the eddy water is included in the analysis. This water body extends to 1400m and has a maximum velocity of 0.4m/s, resulting in a large transport. The right plot does not include the eddy in the analysis, resulting in a transport of 0Sv.
This inability to distinguish between the LUC water and the eddy water can cause a number of overestimation and underestimation issues when extracting the LUC properties. Unlike the LC, the temperature and salinity signatures of the LUC have never been determined and are likely to be too varied to help define the boundary of the LUC. The implications of this methodology limitation are presented in section 5.3.1 of the Discussion.
3.4.5 Volume Flux
The volume flux gives an idea of the strength of the LC and LUC and provides a good method for determining the seasonal and decay dynamics of the currents. The volume flux was calculated by integrating the velocity with respect to the boundary depth. This value is then integrated with respect to the horizontal distance to produce a volume flux for the LC or LUC.
3.4.6 Temperature and Salinity
The temperature and salinity values were obtained for the LC and LUC. These properties can be used to define the boundary of the current using the Matlab programs and following the steps in section 3.4.2. The temperature and salinity signatures for the LC will be compared to
28 Methodology those found in previous studies (Godfrey and Ridgway, 1985, Smith et al., 1991, Rochford, 1986), to provide validation of the methodology used in this study and to further validate Bluelink’s model solution.
3.4.7 Width-Depth
The width and depth of the LC was estimated at each of the transects using a simple function in the Matlab programs. The depth was calculated by subtracting the shallowest depth from the greatest depth. The width was calculated by subtracting the greatest horizontal point from the smallest horizontal point within the same horizontal plane. This was done to ensure that the width of skewed LC boundaries was not over-estimated. The slope of the seabed was also calculated at each of the transects to determine the relationship between width/depth to slope. These slope values are shown in Table 4.6 in the results section.
3.4.8 Core Position
The core position is the approximate location of the centre of the LC or LUC and is useful for investigating the movement or meandering of the currents. This value is determined by finding the maximum velocity within the current boundary and then extracting the vertical and horizontal position of this value.
3.5 Summary of Methodology
The methods described in this section provide the basis for model validation and data analysis throughout this study. Using this methodology it is possible to determine the volume fluxes, temperature, salinity and size characteristics of the LC and LUC along the W.A. coastline.
29 Model Results
4.0 Model Results
This section presents the results from the modeling and data analysis outlined in the methodology. The results from the model validation are presented first, followed by the LC and LUC analysis results.
4.1 Model Validation
4.1.1 Leeuwin Current Properties
The methods described in section 3.3.1 were carried out every 2 months for 2003, using the plots found in Appendix A. The results of the full analysis are shown in Tables 4.1 to 4.3.
Table 4.1 – Leeuwin Current properties at Shark Bay throughout 2003
Satellite Images BRAN v2.1 Output Date Temperature (ºC) Velocity (m/s) Temperature (ºC) Velocity (m/s) 6/2/2003 25 0.3 24 0.3 7/4/2003 27 0.4 26 0.45 6/6/2003 25 0.3 23 0.4 7/8/2003 24 0.5 23 0.4 6/10/2003 22 0.3 21 0.35 7/12/2003 22 0.3 22 0.3
Table 4.2 – Leeuwin Current properties at Perth throughout 2003
Satellite Images BRAN v2.1 Output Date Temperature (ºC) Velocity (m/s) Temperature (ºC) Velocity (m/s) 6/2/2003 22 0.3 22 0.4 7/4/2003 23 0.3 22 0.35 6/6/2003 23 0.5 21 0.4 7/8/2003 20 0.5 19 0.45 6/10/2003 19 0.3 19 0.4 7/12/2003 20 0.3 20 0.35
30 Model Results
Table 4.3 – Leeuwin Current properties at Albany throughout 2003
Satellite Images BRAN v2.1 Output Date Temperature (ºC) Velocity (m/s) Temperature (ºC) Velocity (m/s) 6/2/2003 21 n.a. 20 n.a. 7/4/2003 22 0.2 21 0.3 6/6/2003 22 0.4 20 0.4 7/8/2003 20 0.3 18 0.4 6/10/2003 19 0.2 18 0.3 7/12/2003 18 n.a. 18 n.a. Notes n.a. indicates that the LC was not apparent at this location during this timestep.
Tables 4.1 to 4.3 show that Bluelink v2.1 accurately models the velocity and temperature characteristics of the LC along the west Australian coastline. The LC temperature is within 1- 2ºC and the velocities are within 0.1m/s for all comparisons.
The investigation into the seasonal response of the LC was also completed using the figures from Appendix A. The summer extension of the LC is presented in Figure A1 of Appendix A. The Bluelink output and the satellite image reveal that the LC extension is limited to the south of Cape Leeuwin. Numerous studies have found this weakening of the LC during the summer and spring seasons (Thompson, 1984, Smith et al., 1991). Quantitative results of this phenomenon are also produced by this study, presented in section 4.2.1 of this report.
Figure A3 in Appendix A presents the LC during the winter when it is at its peak flow. In contrast to summer, the LC during the autumn and winter months can be observed to the east of Esperance, entering the Great Australian Bight. The seasonal response of the LC has been documented in a number of scientific studies (Thompson, 1984, Smith et al., 1991) and can be observed in both the satellite image and the BRAN v2.1 horizontal plot.
4.1.2 Eddy Shedding Characteristics
The results of the LC eddy analysis are shown in Tables 4.4 and 4.5 for the satellite images and BRAN v2.1 model output respectively.
31 Model Results
Table 4.4 – Eddy observations from satellite images
SSH Distance Speed Season Location Anomoly Width (km) Travelled Direction (m) (km) (km/day) Abrolhos Autumn 0.5 250 1400 4.2 W Islands Autumn Shark Bay 0.35 200 1400 4.2 W Year 2002 2002 Year SW then Winter Albany 0.25 200 1300 3.1 NW Spring Esperance 0.2 200 721 3.33 SW Autumn East Albany 0.25 100 412 3.4 WSW West Winter 0.2 200 806 2.8 WNW Albany Winter Shark Bay 0.45 200 894 2.1 WNW
Year 2003 2003 Year Abrolhos Winter 0.2 100 971 4.5 WSW Islands Autumn Perth 0.25 150 400 5.0 W
Table 4.5 – Eddy observations from Bluelink v2.1 output
SSH Distance Speed Season Location Anomoly Width (km) Travelled Direction (m) (km) (km/day) Autumn East Albany 0.25 150 781 4.6 WSW West Winter 0.2 200 412 3.7 W then NW Albany
Year 2002 2002 Year Winter Shark Bay 0.3 100 500 7.1 W Abrolhos Winter 0.3 100 300 3.8 W Islands Autumn Perth 0.35 250 566 2.5 WNW Autumn East Albany 0.2 100 721 4.7 SW West Winter 0.25 100 500 4.0 W then NW Albany 03 03 Winter Shark Bay 0.25 100 200 6.1 W Winter Shark Bay 0.3 200 500 6.8 W Year 20 Abrolhos Winter 0.3 100 608 5.1 WNW Islands Autumn Perth 0.45 200 640 2.6 W
The following general results were observed in both the satellite images and the model solution: • each year around 5-6 eddies are generated, predominantly in winter and autumn when the LC is most active,
32 Model Results
• 5 main eddy generating regions were identified at Shark Bay, Abrolhos Islands, Perth, Albany and Esperance, with each region having a distinct migratory path as shown in Figure 4.1,
• eddies had two distinct length scales of 100 and 200km, • average observed migration speeds were 4km/day for the satellite images and the model output. The migration distance and duration were smaller in the Bluelink output due to the limited time and model domain.
Shark Bay
Abrolhos Islands
Perth
Esperance
Albany
Figure 4.1 – Eddy generating regions and typical eddy migratory paths along the W.A. coastline. This spatial plot presents the current speed and direction, overlaying the temperature during winter. Five anti- cyclonic eddy generating regions were identified, shown as circles, at Shark Bay, Abrolhos Islands, Perth, Albany and Esperance. The arrows depict the general migratory path of eddies spawned at each of these eddy spawning regions. Each generating regions in this plot has recently spawned an eddy, which are shown as a higher temperature and swirling currents in this plot.
4.1.3 Summary of Model Validation
Results from the model validation are promising; with the Bluelink model output recreating the LC properties observed in the satellite images. As mentioned previously this validation 33 Model Results was concerned primarily with the general oceanic conditions around W.A. and was qualitative in nature. The results observed in the LC eddy analysis are satisfactory for this study but would need to be extended for multiple years if they are to be used for more detailed research.
4.2 Leeuwin Current
The following section presents the results from the Leeuwin Current analysis described in section 3.4.3. The majority of the results presented in this section have already been documented in a number of previous studies (Thompson, 1984, Smith et al., 1991, Woo et al., 2005). However, the results shown here provide further validation of both the model solution and the methodology, allowing more confident conclusions to be made on the LC’s and more importantly the LUC’s temporal and spatial characteristics.
4.2.1 Volume Fluxes
The volume fluxes were calculated using the method described in section 3.4.5 and provide an insight into the strength of the LC over the year.
Seasonal Response
The seasonal response of the LC has been widely documented and was immediately apparent through visualisation of horizontal plots and vertical profiles during the LC analysis. Horizontal plots of the LC seasonal response can be seen in Appendix A, while vertical profiles are shown in Figure 4.2.
34 Model Results
Figure 4.2 – Seasonal response of LC during summer (left) and winter (right). The left hand plot of Figure 4.6 depicts the velocity profile of the LC at Esperance in summer. The velocity of the LC at this time step does not exceed the 0.3 m/s limit and so no LC boundary is found. The right hand plot of Figure 4.6 reveals the velocity profile of the LC at Esperance in winter. The LC dominates the velocity profile, with a depth of 250m and a velocity in excess of 0.7m/s.
These two contrasting plots, reinforce the observations made in Figures A1 and A3 in Appendix A. A more quantitative approach analysed the collated volume fluxes to determine the seasonal response of the LC. Figure 4.3 presents the LC’s annual volume flux along the west and south coasts.
Figure 4.3 – Mean seasonal variation in Transport of LC along the coastline 35 Model Results
The mean transport of the LC along the west coast throughout 2003 is shown in the left hand plot. Most transects have peaks in transport of around 4Sv and summer transports of between 0.5-2 Sv. Transect 2 appears to have much higher summer and spring transports of around 2.5Sv. The right hand plot presents the mean transport of the LC along the south coast throughout 2003. Maximum transports of 3-4Sv also occur during winter at Transects 5-7, with varying spring and summer transports around 0-1Sv.
The seasonality of the LC can be seen along the western (left hand plot) and southern (right hand plot) coasts, with peaks during autumn and winter and lower transports during summer and spring. An interesting feature observed in the right hand plot is the lag in the peak flow as you move eastwards, with Transect 5 peaking in April, Transect 6 peaking in May and Transect 7 peaking in June.
The seasonal response of the LC can also be seen in the left hand plot of Figure 4.4, a plot of the transects during each of the seasons. The mean annual transport was determined by simply taking the averages of the seasonal volume fluxes, presented in the right hand plot of Figure 4.4.
Figure 4.4 – Seasonal and annual transport of LC This is a plot of the transect location vs the transport throughout 2003. The left hand plot depicts the seasonal transport of the LC down the coast. Winter and autumn transports are much higher, averaging around 2.5Sv. Summer and spring transports are smaller in magnitude, averaging around 1Sv. The Annual transport of the LC is presented in the right hand plot. There is a general negative trend in the transport as you move from north to south-east.
36 Model Results
The seasonality is still obvious in the left hand plot of Figure 4.4. Autumn and winter have around twice the transport of the summer and winter transports. The annual average also reveals a general decrease in LC transport down the west coast and along the southern coastline.
It has been hypothesized that the LC increases in strength between Shark Bay and Cape Leeuwin due to the occurrence of geostrophic onshore waters (Godfrey and Ridgway, 1985). This is shown diagrammatically in Figure 4.5.
Figure 4.5 – Geostrophic inflow along the Western Australian coastline (Pearce and Phillips, 1988) The geostrophic inflow is shown as arrows, entering the LC system from the west.
This hypothesis can be tested by setting Shark Bay (Transect 1) as a reference point and viewing all volume fluxes in relation to Shark Bay. The results of this analysis can be seen in Figure 4.6.
37 Model Results
Figure 4.6 – Transport of LC relative to Shark Bay The left hand plot displays the seasonal transport of the LC relative to Transect 1. The right hand plot displays the annual transport of the LC relative to Transect 1. Most transects decrease in transport with the exception of Transect 2 which appears to increase by approximately 0.5Sv.
The LC appears to increase in strength between Transect 1 and 2 but then decrease in strength along the rest of the transects. Further investigation of this result will be carried out in the Discussion section.
4.2.2 Temperature Signatures
The temperature signature of the LC was collected using the methodology described in section 3.4.6. Generally, the regions of higher temperature matched the LC boundary defined by the velocity limit, with a slight onshore movement of the high temperatures as seen in Figure 4.7.
38 Model Results
Figure 4.7 – Temperature Profile of Leeuwin Current: Transect 1 June 7th. The boundary of the LC (defined using the velocity limit) is shown as a pink line, with all extracted data points displayed as X’s. The temperature core appears to match the boundary defined by the velocity accurately, with a slight onshore movement of the temperature.
Figure 4.7 shows the LC core around 2-3 ºC warmer than the ambient temperature. As mentioned previously the LC boundary defined by the velocity does not exactly match the region of higher temperature, which will be investigated further in section 5.2.2 of the Discussion. All of these temperature signatures were collated and are presented in Figure 4.8 for the western and southern coastlines.
39 Model Results
Figure 4.8 – Leeuwin Current Mean Temperature Signature The left hand plot presents the temperature signature for the LC on the west coast throughout 2003. The temperature ranges from 24ºC at Shark Bay in April to 17ºC at Cape Leeuwin in September. The right hand plot presents the temperature signature for the LC on the south coast throughout 2003. The temperature ranges from 21ºC at Cape Leeuwin in April to 16ºC at Esperance in September.
Two main trends can be observed in these temperature plots. The first is that the seasonality of the LC is visible in the temperature signatures, with a definite temperature peak in early winter and a dip in spring for both the western and southern coasts. The second pattern is the decrease in temperature as you move from Shark Bay in the left hand plot, down to Esperance in the right hand plot. These patterns observed in the temperature signature will be investigated further in section 5.2.2.
4.2.3 Salinity Signatures
The salinity signature of the LC was collected using the methodology described in section 3.4.6. On most occasions the regions of lower salinity matched the LC boundary defined by the velocity limit. However, a slight onshore movement of these low salinities, similar to the process seen in the temperature analysis, was apparent as seen in Figure 4.9.
40 Model Results
Figure 4.9 – Salinity Profile of Leeuwin Current at Transect 1 June 7th. The boundary of the LC (defined using the velocity limit) is shown as a pink line, with all extracted data points displayed as X’s. The low salinity core appears to match the boundary defined by the velocity accurately, with a slight onshore movement of the lower salinity.
Figure 4.9 shows the LC’s salinity around 0.4-0.7 mg/L lower than the ambient salinity. As mentioned previously the LC boundary defined by the velocity does not exactly match the region of lower salinity, which will be investigated further in section 5.2.3 of the Discussion. All of these salinity signatures were collated and are presented in Figure 4.10 for the western and southern coastlines.
41 Model Results
Figure 4.10 – Leeuwin Current Mean Salinity Signature The left hand plot presents the salinity signature for the LC on the west coast throughout 2003. The salinity ranges from 35.9mg/L at Cape Leeuwin in March to 35.3mg/L at Shark Bay in May. The right hand plot presents the salinity signature for the LC on the south coast throughout 2003. The salinity ranges from 35.9mg/L at Albany in March to 35.5mg/L at Esperance in September.
The salinity signature of the LC displays the same patterns as the temperature signature, only with more variability. In general the lower latitude transects such as Shark Bay and Cervantes have lower salinities than transects on the south coast such as Esperance and Albany. The west coast also shows a dip in the salinity during the winter months and peaks in summer. The south coast has a dip in the salinity at the end of winter and a peak at the start of autumn. The increased variability of the salinity signature compared to the temperature signature means the temperature is a more suitable substitute for defining the LC boundary when the velocity profiles are disrupted by eddies. Further examination of the trends displayed in Figure 4.10 will be carried out in section 5.2.3.
4.2.4 Width-Depth to Slope
The width and depth details of the LC along the W.A. coastline were collected using the methodology described in section 3.4.7. The seabed slopes adjacent to the LC were also calculated for all transects and are presented in Table 4.6.
42 Model Results
Table 4.6 – Seabed slopes at LC transect locations
Transect Seabed Slope 1 0.00725 2 0.01 3 0.00680 4 0.00659 5 0.00377 6 0.00388 7 0.00467
The average widths were then plotted against the average depths and compared to the slopes at each of the locations as shown in Appendix B. The lines of best fit (LOBF) and corresponding correlation coefficients were also calculated for the width vs. depth plots, which are presented in Table 4.7.
Table 4.7 – Correlation Coefficients and LOBF slope for LC width vs. depth
Transect Correlation Coefficient Line of Best Fit Slope 1 0.7567 0.0016 2 0.8030 0.0013 3 0.9313 0.0022 4 0.9495 0.0025 5 0.7941 0.0017 6 0.7848 0.0018 7 0.9253 0.0031
The width and depths show a strong linear relationship for all transects on the west coast, with correlation coefficients between 0.75 and 0.95. However, these values do not appear to be related to the slope, shown as the red lines in the figures in Appendix B. Along the southern coastline, the width and depth of the LC appear to be linearly related with high correlation coefficients between 0.78 and 0.92. The seabed slope appears to match the LOBF more closely along the south coast.
The vertical length scales of the LC were also collated and plotted to determine the seasonal influence, as shown in Figure 4.11.
43 Model Results
Figure 4.11 – Leeuwin Current Depth: Seasonal average (left), annual average (right) The LC has a longer vertical length scale in winter and autumn, averaging around 150m. During summer and spring this decreases to around 100m. This indicates a seasonality response of the LC depth. The annual depth reveals a slight negative trend in the LC vertical length scale as the LC moves from Transect 1 to Transect 7.
4.3 Leeuwin Undercurrent
The following section contains the results from the LUC analysis as described in section 3.4.4. Along the west coast the LUC was observed in a consistent water depth, close to the continental shelf edge as seen in the left hand plot of Figure 4.12. However, along the south coast the LUC was rarely observed on the shelf edge and was regularly seen as a much larger, meandering westerly flow as shown in the right hand plot of Figure 4.12.
44 Model Results
Figure 4.12 – LUC velocity profile at Transect 4 and Transect 6 - 11/6/2003 The left hand plot shows the general position of the LUC on the west coast. The LUC was consistently found on the shelf edge at a depth of 200-800m, with a positive alongshore velocity. The right hand plot reveals the general position of the LUC on the south coast. The LUC was consistently found offshore, often with a surface expression to a depth of 1000m and a negative alongshore velocity.
This qualitative observation of the LUC’s position and strength on the southern coastline is reinforced by the quantitative results of the analysis presented in the next sections.
4.3.1 Volume Fluxes
The volume fluxes were calculated using the methodology described in sections 3.4.4 and 3.4.5. Figure 4.13 presents the strength of the LUC for each of the seasons along the 8 transects.
45 Model Results West West South
Flow
Figure 4.13 – LUC seasonal (left) and annual (right) transport The left hand plot reveals the seasonal transport of the LUC. Transects 1 to 8 (on the west coast) have a small flow of around 1-2Sv. Transects 5 to 7 (on the south coast) have a much larger flow of around 5-10Sv. The right hand plot reveals the mean annual transport of the LUC, with 1Sv on the west coast and 8Sv on the south coast.
The results presented in Figure 4.13 are ground-breaking as they show a definite distinction between the LUC strength along the south coast and the LUC strength on the west coast. A large decrease in transport occurs at transect 8 (the edge of Cape Leeuwin) when the LUC turns from a large westerly flow to a more consistent and lower northerly flow. There is also a seasonal component to the LUC transport along the south coast with autumn maintaining the highest transport, then winter, summer and spring, as seen in the left hand plot of Figure 4.13. The summer and winter points at Transect 6 appear to be erroneous as they do not follow the pattern seen at the other transects or for the other seasons. The possible cause of this error will be investigated in section 5.3.1 of the Discussion. The results of the volume flux analysis can be summarized diagrammatically as shown in Figure 4.14.
46 Model Results
Cape Naturaliste
Cape Leeuwin
Figure 4.14 – The proposed divergent flow of the Leeuwin Undercurrent at Cape Leeuwin The LUC volume flux or transport along the south coast is around 8 Sv, while it separates and decreases to around 1 Sv on the west coast.
This finding focuses our analysis on the Cape Leeuwin region of W.A., as it is now hypothesized that the LUC has its formation here and not on the southern coastline as was originally believed. Further analysis of this region and the interactions between the large westerly flow and the bathymetry will be carried out in the Discussion section, which will aim to determine the origins of the LUC.
4.3.2 Location of Core
The location of the LUC core was calculated using the methodology described in section 3.4.8. The offshore distance of this core was then plotted for all of the seasons (left hand plot) and as a yearly average (right hand plot) in Figure 4.15, to investigate the meandering nature of the LUC.
47 Model Results West West South
Figure 4.15 – Offshore distance of the LUC core. The left hand plot shows the seasonal offshore distance of the LUC core. The right hand plot reveals more, presenting the annual offshore distance of the LUC (blue) along with the shelf edge at -427m depth (red). In general the LUC core is closer to the shelf edge along the west coast than the LUC on the south coast.
The right hand plot of Figure 4.15 provides a nice summary of what is occurring to the LUC at all of the transects. Along the west coast, the offshore distance of the LUC core matches the continental shelf edge very well, with the core in a consistent location close to the shelf edge. However, on the south coast the LUC core is found much further offshore and varies much more than on the west coast. This variation in core location reinforces the use of the horizontal plots, described in the methodology section 3.4.4, when defining the LUC boundary on the south coast.
4.3.3 Other results
The LUCs temperature and salinity signatures were also calculated but the results were too variable to produce any discernable results. A review of the temperature and salinity profiles reveals limited correlation between these physical properties and the LUC boundary as shown in Figure 4.16.
48 Model Results
Figure 4.16 – LUC temperature (left) and salinity (right) profiles at Transect 4, 1/5/2003 The LUC boundary overlying the temperature at Transect 4 is shown in the left hand plot, showing no relationship between the LUC and an expected region of lower temperature at the LUC core. The LUC boundary overlying the salinity at Transect 4 is shown in the right hand plot, which also shows no relationship between the LUC and an expected region of higher salinity at the LUC core.
This limited relationship between the LUC velocity boundary and the temperature and salinity profiles is reflected in Figure 4.17, which shows the LUC temperature and salinity signatures varying dramatically over the year.
Figure 4.17 – Temperature (left) and salinity (right) signatures of the LUC The left and right hand plots reveal inconsistent results due in part to the difficulty in defining the LUC boundary.
49 Model Results
The temperature and salinity values do not appear to show any trends. The inconsistency of the LUC necessitated the exclusion of a number of points in the salinity and temperature plots. Further studies could be carried out to determine the signatures for the LUC, if the current could be found consistently.
4.4 Capes Current
Throughout the LC and LUC analysis, the Capes Current was observed adjacent to the coastline and extending for some distance up Cape Leeuwin to Transect 2 at Cervantes. Although no quantitative evaluation of this current was undertaken, it is encouraging to see the Bluelink model solution produce the Capes Current in the right location at the right time of the year. Figure 4.18 shows the Capes Current at Transect 2 and 3 in February.
Figure 4.18 – Velocity profiles of Capes Current at Transect 2 and 3 – 25/2/2003 The Capes Current is shown as a positive (equatorward) flow of around 0.3 m/s, with the boundary highlighted in pink. The Capes Current was observed from Cape Naturaliste to Cervantes on the 25/2/2003 and on a number of other dates throughout summer.
4.5 Summary of Results
The BRAN v2.1 model validation produced promising results. A review of the temperature, velocity, seasonal and eddy shedding properties of the LC reveals a strong correlation between the satellite images and the Bluelink output. This model validation, although qualitative in nature, provides a suitable foundation for this study as the model can now be 50 Model Results used to confidently model the mesoscale oceanic conditions around W.A.. The quantitative analysis of the LC also produced promising results. The LC volume flux, temperature and salinity signatures reflected those already presented in other scientific studies (Godfrey and Ridgway, 1985, Rochford, 1986, Smith et al., 1991) and provided further model validation and methodology assurance, allowing confidence to be placed in the results we obtained in the LUC analysis. The first quantitative analysis of the LUC reveals the likely location of LUC formation. The LUC volume flux calculations reveal a strong separation of the LUC at the bottom of Cape Leeuwin as well as a strong seasonal component of the LUC on the southern coastline, which will be thoroughly investigated in the Discussion section.
51 Discussion
5.0 Discussion
The discussion will be focused on examining the possible origins of the LUC around the Cape Leeuwin region as a result of the findings in section 4.3. The results of the model validation and LC analysis will also be discussed to provide further justification that Bluelink and the methodology described in section 3.1 and 3.4 can be confidently used to investigate the LUC.
5.1 Model Validation
The qualitative model validation undertaken at the beginning of this study produced a strong correlation between the LC properties observed in the satellite images and in the Bluelink v2.1 model solution. This was anticipated to some degree because Bluelink v2.1 uses satellite SST and altimetry as part of the data assimilation process in regions of water depth greater than 200m. The validation was still necessary as a number of other data sources are used in the assimilation process and the continental shelf region could be erroneous. Bluelink performs 3D data assimilation by determining which of an ensemble of 72 runs, best represents the surface profile of the data to be assimilated (Oke et al., 2007a). It then uses the ensemble run with the highest correlation to data assimilate vertically and model the 3D components of the model solution (Oke et al., 2007a). Accurate vertical data assimilation means that the 2D model validation completed for this study is sufficient and a subsurface field comparison is not necessary.
5.1.1 Eddy Shedding Characteristics
Eddies are generated by the LC as a result of two instabilities: barotropic and baroclinic. Meuleners et al. (2007a) modeled the eddy-generating characteristics of the LC at two locations using the Regional Ocean Modelling System (ROMS). Development of eddies at both locations followed a similar pathway, where an offshore flow created a clockwise eddy on the shelf edge, adjacent to the LC (Meuleners et al., 2007a). The clockwise eddy redirected the LC flow, creating an offshore meander in the LC, which continued to grow and eventually disconnects in the form of an anti-clockwise eddy. Similar processes were observed throughout LC analysis, with an eddy-dipole pair being formed in Figure 5.1.
52 Discussion
Clockwise Eddy
Anti-Clockwise Eddy
Figure 5.1 – Meander in LC prior to anticlockwise eddy formation The horizontal plot of current speed and direction underlaid with temperature reveals the initial steps in the creation of an anti-clockwise LC eddy just north of Perth. The offshore, clockwise eddy has pushed up against the LC, creating a meander as the flow is forced offshore. The anti-clockwise eddy will soon close and detach from the LC, migrating to the west.
The baroclinic instability source in most dipole pairs is the available potential energy stored within the mean lateral density gradient between the eddy dipole (Meuleners et al., 2007a). The source of the barotropic instability is the LC’s southward flow interacting with the LUC’s northward subsurface flow, generating horizontal shear. When barotropic instability is apparent, the LUC is found to rise from the continental shelf and increase the transfer of energy with the LC via horizontal shear (Meuleners et al., 2007a). An example of this process is presented in Figure 5.2, observed during the LUC analysis.
53 Discussion
Figure 5.2 – Velocity profile of the Leeuwin Current System during eddy creation Barotropic instability between LC and LUC causes the LUC to rise (left plot) and an eddy to form (right plot). When the LUC rises, the LC is pushed offshore until the eddy is formed. The eddies formed in this manner are generally deep and often extend 1000m into the water column.
The length scales of the eddy generating processes may be too fine to be resolved by Bluelink, however identification of these eddy formation processes adds to the model validation and thus the confidence in the model solution.
Fang and Morrow (2003) used satellite images to track warm core LC eddies and found that eddies are generated at 3 major locations on the West Australian coastline, at 20–21.5º S (North-west Cape), 24–25 º S (Shark Bay) and 28–31 º S(Abrolhos Islands). These eddies had a general westerly direction with a drift speed of 5-10 km/day (Fang and Morrow, 2003), similar to the results found in the model validation section of this report. The model validation carried out for this study also found 2 eddy generating regions along the southern coastline, a region which was not investigated by Fang and Morrow (2003). Using satellite images, Griffiths and Pearce (1985) found that warm-core LC eddies are formed along the southern coastline between Cape Leeuwin and Cape Arid. However, only the nearshore formation dynamics of the eddies were investigated, thus the eddy migratory paths observed in this study can not be compared to previous literature.
Figure 4.1 presents the eddy migratory paths observed during the LC eddy study. Of particular interest is the migratory path of the eddies generated at Albany, which tend south-
54 Discussion west then turn to the north-west as they reach the Naturaliste Plateau. This process could be explained as an interaction with the seabed. When a parcel of fluid is squeezed laterally, its vorticity must increase to conserve circulation (Cushman-Roison, 1994), resulting in a clockwise vorticity response and a north-westerly change in direction. The eddy could also be interacting with a northerly flow in the region, such as the West Australian Current, which is believed to exist in the region (Andrews, 1983). Further research into this region is proposed in section 7 of this report.
5.2 Leeuwin Current
The extensive analysis of the LC revealed a number of interesting yet well studied results. These results have been included and examined to provide validation of the Bluelink model solution as well as the cross-shore transect methodology used to define the LC and LUC boundaries. One general observation made throughout the LC analysis is that the LC is highly turbulent, at times meandering several hundred kilometers offshore and spawning large, anticlockwise eddies that disrupt the poleward flow. Subsequently if a similar study was to be undertaken it is recommended that an El Nino or less turbulent year is used in the analysis to avoid the possible boundary definition errors created by these meanders and eddies.
5.2.1 Volume Fluxes
The seasonality of the LC is visible in Figures 4.3 and 4.4. The LC has a transport peak in autumn and winter with a relaxation in flows during summer and spring. The seasonality of the LC is believed to be caused by a reduced, opposing wind stress during winter with the alongshore pressure gradient being seasonally independent (Smith et al., 1991). The average peak value for the LC transport was calculated as 4 Sv, while the average relaxed transport during summer and spring was calculated as 1 Sv. This compares well to the list of transports calculated from other studies, contained in Table 5.1.
55 Discussion
Table 5.1 – Comparison of peak transport of LC Study Peak Transport (Sv)
This study 4 – 4.5 Thompson (1984) 4 (During May 1983) Godfrey and Wridgway (1985) 3 – 5 Smith et al (1991) 4 – 5
The right hand plot of Figure 4.3 highlights another interesting feature of the LC as it flows along the southern coastline. The peak LC flow occurs during subsequent months for transects 5, 6 and 7, with the peaks occurring in April, May and June respectively. This lagging of the LC peak is due to the seasonality of the LC, as it takes approximately a month for the LC to increase in strength from one transect to the next along the southern coastline.
The right hand plot of Figure 4.4 shows a general negative trend in the LC transport from Transect 1 at Shark Bay to Transect 7 at Esperance. The seasonal response of the LC is likely to cause this trend. The LC is apparent at Shark Bay for the entirety of the year, while the LC is barely visible at Esperance during summer and spring as shown in the left hand plot of Figure 4.2. This means that on average the LC is observed for more days and with a higher transport at Shark Bay compared to Esperance.
Figure 4.6 was produced to determine whether the results of the LC analysis supported the hypothesis that offshore geostrophic inflows increase the LC strength down the western coast (Godfrey and Ridgway, 1985). The results of this analysis reinforce this hypothesis, as the transport at Transect 2 is around 0.5Sv higher than the reference point (Transect 1). However, care should be taken when making any conclusions from this graph as only two points exist within the area of hypothesized onshore flow; Shark Bay and Cervantes. The methods used in this study would provide an accurate set of results if the number of transects between Shark Bay and Cape Leeuwin was increased.
5.2.2 Temperature Signature
On most continental west coasts, equatorward flowing surface currents exist due to wind and salinity interactions. However, the LC is unique as it transports warm, tropical water towards the poles (Cresswell and Golding, 1980). From observations, the LC core temperature in this study was approximately 2-3 ºC higher than ambient temperatures. The occurrence of a 56 Discussion number of endemic, tropically derived ecosystems along the western and southern coastline has been linked to the existence of the warm-core LC. The LC flow is fast enough and of high enough temperature to transport a number of tropical fauna such as oriental bonito, asteroid species and sea turtles to latitudes they would not normally be found (Maxwell and Cresswell, 1981). Tropically derived fish species have also been observed in the Recherche Archipelago, most probably due to the poleward transport of warm water by the LC (van Hazel, 2001).
The seasonal response of the LC is apparent in the temperature signature plot presented in Figure 4.8. A peak in LC temperature occurs in winter while a dip occurs in spring. The cause of this seasonality is the decrease in opposing southerly winds during winter, as discussed in section 5.2.1. The seasonality of the LC also drives the puerulus settlement, which are the larvae of the Western Rock Lobster (Caputi et al., 1996). This settlement occurs during September and January, when the LC is weakest and favorable winds transport puerulus towards the shore (Caputi et al., 1996). The LC is also believed to influence the recruitment of scallops, western king prawns, pilchards, whitebait and herring (Caputi et al., 1996).
The temperature ranges from a maximum of 23 ºC at Shark Bay in winter to a minimum of 16 ºC at Esperance in spring. A comparison of these values to those found in previous literature is contained in Table 5.2.
Table 5.2 – Leeuwin Current Temperature signature comparison with previous studies
Temperature (ºC) Rochford D J Transect This Study Smith et al (1991) (1986)
1 20 – 23.5 20.87– 23.77 – 2 19 - 22 19.22 – 21.78 – 3 17.6 – 21.3 19.12– 21.08 – 4 17.7 – 21.3 19.95 – 21.02 – 5 17.2 – 20.5 – – 6 16.3 - 20 17.21 – 17.99¹ – 7 16.2 - 20 – 16 – 19.6 Notes ¹ Only over the summer months
57 Discussion
The range of temperature values from the Smith et al (1991) study were calculated as the average temperature plus and minus one standard deviation. A strong correlation exists between the temperature values calculated in this study with those observed in previous field studies. In general, the maximum temperatures in the range are equivalent, while the minimum temperature is 1-2ºC lower than the observed values. This lower minimum temperature is most likely due to the way the LC boundary was defined using the velocity limit. Figure 4.7 highlights the onshore movement of the warm temperature core, all of which is not picked up using this boundary method. Subsequently, only part of the higher temperature core is extracted using the LC methodology, lowering the temperature range. The temperature values calculated in this study are close enough to previous observations to provide some confidence in the cross-shore transect methodology that is utilised throughout the LC and LUC analysis.
5.2.3 Salinity Signature
The LC is recognized as a flow that transports relatively low salinity water from the tropics to the temperate latitudes (Cresswell and Golding, 1980). This study found that the LC has a low salinity core compared to the surrounding waters. This difference was less obvious along the higher latitude transects at Esperance and Albany due to a number of reasons. Firstly the LC does not influence the southern regions as much, so the salinity signature is likely to be closer to the ambient salinity for more of the year. Secondly, entrainment of high salinity water into the LC occurs along the length of the western coast as a result of geostrophic inflows (Woo et al., 2005), resulting in a smaller difference between ambient and LC core salinities.
The variability in Figure 4.10 highlights the impact that geostrophic flows have on the LC along the west coast. The left hand plot, which depicts the west coast, has a high variability in salinity. This is due to the entrainment of high salinity offshore waters into the LC as it moves from Transect 1 in the north to Transect 4 at Cape Leeuwin. High salinity waters mix with the low salinity waters of the LC to produce salinities between 35.6 and 35.9 mg/L at Cape Leeuwin. Along the south coast the salinity signature has much less variability, with all three transects ranging from 35.5 to 35.9 mg/L, which is very similar to the salinity range found at Cape Leeuwin. This reveals that geostrophic flows influence the LC on the west coast, while limited entrainment of offshore waters occurs on the south coast. 58 Discussion
The seasonality is also apparent in the salinity signature of the LC, with peaks and troughs in salinity occurring in opposite seasons to the temperature signature. The LC is a low salinity, high temperature current, so the stronger the LC influences in a region, the lower the salinity. A comparison of the salinity signature found in this study (shown in Figure 4.10) with the salinity signature found in previous studies is presented in Table 5.3.
Table 5.3 - Leeuwin Current Salinity signature comparison with previous studies
Salinity (mg/L) Smith et al (1991) Rochford D J Transect This Study (March) (1986)
1 35.27 – 35.5 35.0 – 35.6 2 35.35 – 35.76 35.4 – 35.8 3 35.59 – 35.88 4 35.57 – 35.87 35.6 – 35.8 5 35.61 – 35.85 6 35.59 – 35.86 35.4 – 35.6 7 35.51 – 35.77 35.5 – 35.75
Table 5.3 highlights the limited LC salinity data that is available along the W.A. coast. Subsequently, the comparison was undertaken for March, as this was when Smith et al (1991) undertook the LUCIE cruises. For the 5 transects with comparable results, the salinity values were very similar. This highlights the accuracy of the cross-shore transect methodology at extracting valuable information on the LC and LUC.
5.2.4 Width-Depth to Slope
In general the width was directly related to the depth as shown in the figures in Appendix B. Only Transect 7 appeared to have a slight correlation between the LC width/depth and the seabed slope. At Transect 7, the seabed had a slope of 0.0046 and the line of best fit had a slope of 0.0031. This result is surprising as the slope of the continental shelf should be the most important factor in determining the shape of the LC. A reason for this lack of relationship could be that the LC on the west and south-west coast sits far enough offshore to be impervious to the influences of the bathymetry.
The linear relationship between the width and depth allows a number of general rules to be formulated for the LC. If the width is known then the approximate depth of the LC can be 59 Discussion estimated and vice versa using the figures in Appendix B. For example, if you have a satellite image of the LC at Shark Bay and you want to know the approximate depth of the LC at this point, you would use the line of best fit in the first figure of Appendix B to estimate the depth from the width. This relationship is likely to transfer to other years because the LC strength varies more throughout the year than it does inter-annually, so inter-annual differences are unlikely to cause any real change in the strong linear relationship found in this study.
5.3 Leeuwin Undercurrent
The LUC analysis revealed a number of ground-breaking results that help to explain the origins of the LUC. This is one of the first quantitative studies into the LUC and so the results can not yet be compared to any other studies. However, it provides a focus for further research into the Naturaliste Plateau, the region believed to spawn the LUC.
5.3.1 Volume Fluxes
As mentioned in the results section, Figure 4.13 presents a definite distinction between the LUC strength along the south coast and the LUC strength on the west coast. A large decrease in transport occurs at transect 8 (the edge of Cape Leeuwin) when the LUC turns from a large westerly flow to a more consistent and weaker northerly flow.
The results from this study suggest that the LUC does not exist as a shelf edge current on the south coast. Instead the LUC on the west coast, observed in a number of previous studies (Woo et al., 2005, Thompson, 1984) is now believed to form somewhere around Cape Leeuwin. Subsequently, the westerly flowing water extracted in the LUC analysis along the south coast, is simply an extension of the Flinders Current, which exists much further offshore, as seen in Figures 4.12 and 4.15. Middleton and Cirano (2002) found that the Flinders Current intensifies from Victoria to W.A. and had a mean flow of 8Sv, extended to a depth of 800m and was present in both summer and winter. Similar results were also determined in this study, with a mean annual transport of 8Sv on the south coast and a depth extending to 800m. This adds strength to the idea that the LUC is not apparent on the southern coastline and the westerly flow is indeed the Flinders Current.
60 Discussion
Ekman pumping is believed to drive the Flinders Current, with a positive wind stress curl in the region, creating downwelling and a northwards transport of 5-10Sv (Middleton and Cirano, 2002). In the southern hemisphere the Coriolis effect acts to the left on moving bodies, which forces the northwards flow to the northwest as shown in Figure 5.3 (Cushman- Roison, 1994). The Flinders Current then intensifies as it approaches W.A. as more surface flow is entrained into the current (Middleton and Cirano, 2002).
Figure 5.3 – Ekman Spiral, depicting the wind and current directions (National Aeronatical Space Administration, 2006) In the southern hemisphere a current is formed at 45º to the wind direction in an anti-clockwise direction.
An interesting trend visible in the LUC transport plot is the seasonality of the Flinders Current on the south coast. Figure 5.4 shows that the Flinders Current strength is 10Sv in autumn, 8Sv in winter, 7Sv in summer and 5Sv in spring. This seasonality is true for all transects except at Transect 6 (Albany) in summer and winter. The likely reason for the sharp variability in these two points is that Albany is one of the eddy generating regions observed in the model validation and in many instances it was too hard to distinguish between the eddy water and LUC water. Subsequently, a choice similar to the one in Figure 3.11 had to be made between including all of the eddy flow or no flow at all. For the purposes of this study the two erroneous points at Albany in summer and winter can be neglected, as the eddy disruptions do not reflect the true Flinders Current flow. Subsequently, the larger Flinders Current transport in winter (8Sv) and autumn (10Sv) could be explained by the relative proximity of the westerly winds, the Roaring Forties, to the coastline as the sub-tropical highs, move north. When the fetch zone of the westerly winds are closer to the south coast 61 Discussion the northerly transport of water, set up by Ekman downwelling, has less distance to cover, thus increasing the strength of the Flinders Current. During summer and spring, the fetch zones are much further away, causing the associated northerly transport to release much more energy to surrounding waters before reaching the coastline as the Flinders Current. Unfortunately, the opposite seasonality was observed in the study by Middleton and Cirano (2002), adding some speculation to the results and possibly highlighting the difficulty in defining the Flinders Current boundary. West West South
Figure 5.4 – Seasonal (left plot) and Annual (right plot) transport of the LUC. The left hand plot reveals the seasonal transport of the LUC. Transects 1 to 8 (on the west coast) have a small flow of around 1-2Sv. Transects 5 to 7 (on the south coast) have a much larger flow of around 5-10Sv. The right hand plot reveals the mean annual transport of the LUC, with 1Sv on the west coast and 8Sv on the south coast.
A number of properties of the LUC transport displayed in Figure 5.4 combine to reject the long-held hypothesis that the LUC is a return flow created to balance the LC water moving south and east (Thompson and Veronis, 1983). The first is that the LUC on the west coast shows no distinct seasonality, which would be expected if the LUC waters were to balance out the LC waters. All four seasons have a LUC transport of around 1Sv, while the LC transport exceeds 4Sv at some transects. Additionally, the LUC on the south coast has average winter transports of around 10Sv while the average winter transports for the LC on the south coast is approximately 4Sv. So the return flow is too high along the south cost and it has no seasonality along the west coast.
62 Discussion
Several studies have found that a subsurface pressure difference exists along the W.A. coastline, which acts to drive the LUC (Woo et al., 2005, Meuleners et al., 2007b). Both Woo et al., (2005) and Meuleners et al. (2007b) found the gradient to be positive with values around ×101 −7 , indicating a northwards flow. This process may explain the migrational forcing mechanism on the LUC but it does not explain the origins of the LUC between Cape Leeuwin and Cape Naturaliste. So what drives the formation of the LUC? Firstly, the process that drives the formation of the LUC must be seasonally independent along the west coast and prevalent throughout the year. This statement can be justified by looking at the LUCs transport up the west coast in Figure 5.4. The LUC shows no seasonality and it exists at most transects throughout the year. With this in mind, a number of hypotheses were proposed.
The first theory is that the LUC is a divergent flow created from interactions of the Flinders Current with coastal and bathymetric irregularities around Cape Leeuwin. The Flinders Current is persistent throughout the year and its large volume flux may allow a steady flow of water to deviate to the north, forming the LUC. On several occasions the Flinders Current appeared to split into two south of Cape Leeuwin, with a majority of flow continuing west and a smaller flow heading north as shown in Figure 5.5. When a parcel of fluid is squeezed laterally, its vorticity must increase to conserve circulation (Cushman-Roison, 1994), resulting in a clockwise rotation in the southern hemisphere. This dynamic response of fluids could explain why a sharp decrease in sea bottom depth south of Cape Leeuwin results in a clockwise rotation of some of the Flinders Current, creating the LUC. However, this was only observed on a few occasions and may not be the dominant forcing mechanism in the LUC formation pprocess.
63 Discussion
Divergent Flow
Figure 5.5 – Divergent flow of Flinders Current at Cape Leeuwin The Flinders Current appears to undergo rotation south of Cape Leeuwin, resulting in a divergent flow. The majority of the Flinders Current continues west but a small component appears to flow and form the LUC west of Cape Leeuwin.
Another theory is that the Flinders Current diverges south of Cape Leeuwin but the divergent flow continues west for some distance before turning back on itself and heading towards the coastline between Cape Leeuwin and Cape Naturaliste. This reversal of flow was observed in an OCCAMS modeling study by Meuleners et al. (2007b), which used long-term averages of flow to determine the climatology of the region. A possible reason for this observation is the interaction of the divergent Flinders Current with the Naturaliste Plateau, creating a permanent subsurface eddy like structure that turns the flow as shown in Figure 5.6 (Meuleners et al., 2007b). The divergent flow could also interact with the onshore geostrophic flow, which has been documented widely in literature (Godfrey and Ridgway, 1985, Pearce and Phillips, 1988), forcing the high-density Flinders Current water against the coast and creating a positive, alongshore pressure gradient.
64 Discussion
Figure 5.6 – Proposed Leeuwin Undercurrent formation (Meuleners et al, 2007b) A plot of climatology adjacent to the south-west corner, with a schematic of the proposed LUC formation process. The Flinders Current diverges south of Cape Leeuwin, resulting in a smaller flow heading north-west. The clockwise eddy (CE) rotates the flow back onto the coastline, creating the LUC.
The final theory is that the Flinders Current is not involved in the creation of the LUC at all. Instead, the onshore geostrophic flow collects against the shelf edge at depth, creating an alongshore pressure gradient that forces the LUC water northwards. The geostrophic flow observed further north is believed to persist throughout the year, with no seasonality (Thompson, 1984), which upholds the conditions believed to form the LUC. A south- westerly flow of water was sometimes observed throughout the analysis of the Naturaliste Plateau, which could be a subsurface component of the northern geostrophic flow. However, this south-westerly flow was only observed for a number of timesteps during winter and may not be a persistent feature.
The climatology of the region was calculated for 2002 and 2003, in an attempt to determine if any of these theories could account for the LUC formation at Cape Leeuwin. This was done by calculating the long-term temperature and velocity averages over the whole dataset, giving an idea of the dominant dynamics, by removing short-term features such as eddies. The climatology of the Naturaliste Plateau over 2002 and 2003 is shown in Figure 5.7
65 Discussion
LUC
Step 3
Step 2
Step 1 Flinders Current
Figure 5.7 – Schematic of the Climatology of the Naturaliste Plateau over 2002 and 2003 at -372m underlaid with current speed
Results of the climatology analysis are very promising. The LUC formation can be separated into three processes: the diverging flow of the Flinders Current (Step 1), the rotation of the Flinders Current waters by the eddy-like structures (Step 2) and the onshore flow of this water against the coastline (Step 3).
Step 1. The Flinders Current is apparent along the majority of the south coast, as an offshore, meandering flow of 0.1 to 0.15 m/s. The climatology shows that the Flinders Current consistently diverges to the south of Cape Leeuwin, with a small component of water heading to the north-west, while the rest of the Flinders Current continues west. The topography of this region steepens dramatically, which may cause the Flinders Current to rotate and diverge. The exact location and reasons for this split would need to be investigated more thoroughly using a finer resolution model focusing on the Naturaliste Plateau region.
Step 2. A series of eddy-like structures located on or to the north of the Naturaliste Plateau, rotate the divergent Flinders Current water. The climatology of 2002 showed a large eddy structure to the north of the Plateau, rotating the flow towards the coast, while the 2003 climatology showed an eddy just offshore in the centre of the Naturaliste Plateau. It appears
66 Discussion that these eddy structures are persistent enough to continuously divert the Flinders Current water back on to the coastline. A number of processes could explain the existence of these eddies around the Naturaliste Plateau. The rotation could be a clockwise vorticity response, created as the Flinders Current water approaches the Naturaliste Plateau from the south, interacting with the seabed (Cushman-Roison, 1994). However, this is unlikely as the water column extends around 2000m along the Naturaliste Plateau, which could be too deep to influence the flow at 300-600m depth. The rotation could also be due to interactions of the Flinders Current with an easterly geostrophic flow, similar to the flow seen interacting with the LC to the north in Figure 4.9. Results from this climatology analysis do not show any offshore geostrophic influences. Subsequently, a much longer climatology analysis, covering 4-5 years, would need to be completed to remove all short-term instabilities and to allow a quantitative analysis of the offshore influences to be determined.
An analysis of the long-term, mean temperature reveals the most probable cause of these persistent eddies. The average temperature of the Naturaliste Plateau is 1-2ºC lower than the ambient temperature, shown in Figure 5.8, creating a temperature and density difference across the region. This density difference increases the Available Potential Energy (APE), which is the fluctuating or eddying component of potential energy, from the equation:
(Winters et al., 1995) = − zyxPEzyxPEtzyxAPE ),,(),,(),,,( RDS
The RDS is the modelled mean density state attained over a long-term climatology analysis and is independent of time (Winters et al., 1995). Clockwise eddies are formed through a process called baroclinic conversion, which involves the transfer of energy from the APE state to Eddy Kinetic Energy (EKE) (Winters et al., 1995). Subsequently, the clockwise eddy present over the Naturaliste Plateau is probably formed as a result of the density gradient that exists across the Naturaliste Plateau, increasing the APE, resulting in a baroclinic conversion of energy to the EKE state. The most likely source of this colder, denser water is the Flinders Current, which is formed as an upwelling response (waters of high density) to the south of Australia (Middleton and Cirano, 2002). This consistent supply of dense water to the Naturaliste Plateau could sustain the eddies, explaining their persistent nature. However, care should be taken when postulating on the origins of the low temperature waters of the Naturaliste Plateau. Clockwise eddies cause upwelling to occur, bringing cold waters from
67 Discussion the subsurface to shallower depths (Waite et al., 2007). The climatology of the region reveals that these eddies exist for at least 2 years, allowing a substantial amount of cold water to be upwelled and trapped over the Naturaliste Plateau. Further analysis of the Flinders Current at the point of bifurcation could indicate the source of this dense water.
Figure 5.8 – Horizontal spatial plot of mean temperature over 2002 and 2003. The temperature climatology of the Naturaliste Plateau reveals a temperature difference of 1-2ºC between the coast and the plateau. The Flinders Current around the point of divergence also appears to have a lower temperature, revealing a possible continuous source of high-density water from the south-east.
Step 3. The final step in the creation of the LUC is the onshore movement of water adjacent to the coastline between Cape Leeuwin and Cape Naturaliste. High density Flinders Current waters are pushed against the continental shelf at around 300-600m depth. This creates a positive, meridonial pressure gradient along the W.A. coastline as the density of the Flinders Current water is greater than the water to the north. Several studies have observed or calculated this subsurface, pressure gradient (Thompson, 1984, Woo et al., 2005, Meuleners et al., 2007b), which acts to drive the LUC northwards.
Only one other study has examined the possible origins of the LUC, producing similar results to this study. Meuleners et al. (2007b) found a similar three step formation process, with a split of the Flinders Current south of Cape Leeuwin, a rotation of this water towards the coast and a build up of these waters along the coastline. In that study, the OCCAMS model was used to complete a climatology analysis of the Naturaliste Plateau, shown in Figure 5.6. Bluelink has a much higher resolution than OCCAMS; 10km compared to 25km, and so is likely to produce a more accurate result. 68 Discussion
5.4 Capes Currents
The Capes Current was observed at a number of transects during the summer period of this study. Figure 4.18 shows the northward flowing Capes Current close to the shoreline in shallow waters. The origin of the Capes Current makes it unique; its relative coolness and seasonality are from upwelling (Gersbach et al., 1999). Under the influence of Ekman transport, the water moves offshore, allowing colder water to upwell onto the continental shelf and forcing the LC to move farther offshore. As local winds are southerly, it is believed advection of cold water from the southern coast also causes the presence of cold water in the region (Cresswell and Peterson, 1993). Because of dominant south-easterly wind stress on the southern coast, the water may upwell, bringing cold water to the surface, and later advecting, with the assistance of wind stress, to travel along the continental shelf to the north for a period of a few days (Cresswell and Peterson, 1993). This process was observed throughout summer between Cape Leeuwin and Cervantes, with similar, short time periods and a shifting of the LC offshore, as shown in Figure 4.18.
5.5 Limitations of the Study
This section outlines some of the issues and limitations experienced throughout the completion of this study and in some cases, the steps that were taken to rectify these problems.
5.5.1 Model Validation
The model validation completed at the beginning of this study relied on the comparison between satellite imagery observations and output from Bluelink v2.1. The validation was qualitative in nature because no hard data was extracted from either resource and only the general oceanic conditions such as LC temperature and seasonality were compared. Due to time constraints and limited field data access, this qualitative review of the BODAS solution provides a suitable testbed for the use of Bluelink v2.1 output in this study. However, further quantitative validation of the model output and of the methodology was provided through the comparison of the LC temperature and salinity signatures calculated in this study with previous literature on the subject.
69 Discussion
Another issue experienced during the model validation was that the total migratory duration and distance could not be calculated for a number of eddies. This was due to a lack of connectivity in the Bluelink data between years 2002 and 2003. These were the two years chosen for analysis in this study, which corresponded to the time period extension of BRAN v1.5 to BRAN v2.1. Figures 5.8 and 5.9 present the model solution and lack of connectivity between 27/12/2002 and 1/1/2003.
Figure 5.8 – Current Speed horizontal plot - 27/12/2002 Note the eddy shown in red at position 28S 110E at the end of 2002.
Figure 5.9 – Current Speed horizontal plot - 1/1/2003 The eddy that was apparent in Figure 5.8 is not recreated at the start of 2003, 4 days later.
The two figures above highlight the lack of connectivity between the end of 2002 and the start of 2003. The eddy shown in red at 28S 110E in Figure 5.8 was created at the end of 2002 but
70 Discussion was not apparent at the start of 2003. This occurred for a number of eddys, limiting the eddy study completed during the model validation.
5.5.2 Leeuwin Current Analysis
The analysis techniques used to define the LC boundary were much simpler than those used in the LUC analysis. The consistency of the LC made it possible to identify its boundary with ease and the influences of eddy generation were identified, then neglected to maintain accurate results. However, the regions of higher temperature and lower salinity did not always match the LC core defined by the velocity limit. The warm, low salinity core was often found onshore of the velocity boundary and so some of the temperature and salinity values were not extracted. This may have resulted in an under-estimation of the LC temperature and an over-estimation of the salinity. Changes could be made for future studies by shifting the boundary 2 or 3 points onshore, so that the LC core matches the temperature and salinity boundaries more accurately.
5.5.3 Leeuwin Undercurrent Analysis
A number of issues were identified throughout the analysis of this project. The first issue was the proximity of the LUC to the continental shelf edge, whose boundary could not be defined due to coding issues in the Matlab program. The contourc command in Matlab does not pick up all of the LUC at the edge of the shelf as seen in the left hand plot of Figure 5.10. This meant an under-estimation of the LUC volume flux and other properties was occurring. This problem was rectified by allowing the user to extend any of the boundaries right to the edge, encircling the whole LUC as shown in the right hand plot of Figure 5.10.
71 Discussion
Figure 5.10 – LUC boundary before (left) and after (right) coding change The left hand plot reveals the limitation in the contourc function, leaving out a number of points adjacent to the shelf edge. The code was changed and now all of the points were extracted as shown in the right hand plot.
The choice of velocity limit for the LUC boundary definition was also a possible limitation or source of error in this methodology. The LUC typically has a lower velocity than the LC, so a lower velocity limit was required to define the boundary of the LUC. Originally, the velocity limit was set at 0.075m/s, which picked up the LUC on all occasions. However, when there was more water movement due to eddies or offshore movements, this low velocity limit picked up too much non-LUC water or background velocities. Subsequently, a higher value of 0.125m/s was used to define the LUC boundary, which may cause an over-estimation of the LUC strength. On a number of occasions it was obvious that the LUC was apparent on the continental shelf edge but it did not exceed the 0.125m/s velocity limit and therefore its information could not be extracted. This means on average a higher LUC velocity and volume flux is being extracted, resulting in a slight over-estimation of the LUC volume flux.
As discussed in section 3.4.4, the LUC on the south coast was hard to distinguish from eddy water and it was not possible to define the boundary using the temperature and salinity signatures as these are yet to be determined. This required a change in analysis technique, where each vertical profile is viewed as part of a larger horizontal plot to determine what water was westerly (Flinders Current) flow and what water was involved in unrelated eddies. Subsequently, the change in methodology introduced a qualitative aspect into the study as choices had to be made into whether the water was included as Flinders Current water or
72 Discussion discounted as unrelated, eddy water. Some of the possible errors produced by this limitation in the methodology are investigated in section 5.3.1. The methods used to define the LUC boundary are considered to be the most accurate given the time constraints and the lack of previous quantitative studies on the LUC.
The scope of this project necessitated a relatively high time resolution, resulting in a timestep of 2-4 days for all of 2003 and a timestep of 10 days for 2002. Subsequently, only two years of data could be analysed due to time and hard drive space limits. This only becomes an issue when a long-term analysis of the climatology needs to be completed, as only two years of data may not be long enough to remove all short term instabilities and thus get a true estimate of the climatology. Figure 5.7 reveals that a number of transient eddies are still apparent after two years of averaging, which could influence some of the assumptions made on the LUC formation processes. However, for the purposes of this study the climatology analysis still provides a useful insight into the mean oceanic conditions around south-west W.A.
73 Conclusions
6.0 Conclusions
This study has utilized the Bluelink model solution to demonstrate that the Leeuwin Undercurrent has its origins between Cape Leeuwin and Cape Naturaliste and does not exist as a shelf edge current on the south coast. Using a number of cross-shore transects, the Flinders Current was identified flowing offshore along the southern coastline with a transport of 8Sv, as seen in a previous study (Middleton and Cirano, 2002). A climatology analysis of the Naturaliste Plateau revealed a component of the Flinders Current diverges south of Cape Leeuwin, tending to the north-west, where a series of eddy-like structures rotate the Flinders Current waters. These persistent, eddy-like structures create an onshore flow against the coastline between Cape Leeuwin and Cape Naturaliste, producing an alongshore pressure gradient, observed in other studies (Woo et al., 2005, Meuleners et al., 2007b) which is believed to drive the LUC northwards at around 1-2Sv throughout the year.
A simple model validation compared the mesoscale oceanic conditions produced by the model with satellite images obtained from CSIRO, revealing an accurate model solution for the purposes of this study. A two year investigation of the warm-core eddy shedding characteristics of the LC, highlighted the existence of five major zones of eddy generation, each showing a distinct migratory path once detached. Three regions along the west coast were found at Shark Bay, Abrolhos Islands and Perth, producing eddies that migrated west at an average 4 km/day. The migratory paths, speeds, longevity and the generating regions closely matched those observed in a previous study into LC eddies by Fang and Morrow (2003). The migratory paths of eddies spawned at the southern eddying regions, Albany and Esperance, had never been tracked until this study. A general south-easterly path for both regions was found, with the exception of a number of eddies at Albany which tracked south- west until Cape Leeuwin, tending north-west over the Naturaliste Plateau. The processes driving this rotation may also be responsible for the turning of the Flinders Current that is believed to create the LUC.
The cross-shore transect analysis of the LC produced a series of widely documented results, adding to the confidence in the Bluelink model and to the methodology utilized throughout this study. The LC shows a strong seasonality, peaking in winter and autumn at 4Sv and weakening to 1Sv in summer and spring, similar to the results from previous studies (Smith et al., 1991, Thompson, 1984, Godfrey and Ridgway, 1985). The temperature and salinity 74 Conclusions signatures of the LC found in this study closely matched the signatures produced in other field studies with ranges of 16.2 – 23.8ºC and 35.27 – 35.9mg/L (Godfrey and Ridgway, 1985, Smith et al., 1991, Rochford, 1986). A set of general equations were also created, relating the width of the LC to the depth at each of the transects. These equations and their associated graphs are presented in Appendix B. The accurate results produced by the LC analysis provided the testbed for the LUC analysis, which utilized the same method to define the boundary of the LUC.
Quantification of the temperature and salinity signatures of the LUC yielded unreliable results as the boundary of the LUC was often hard to distinguish from turbulent eddy structures. However, identification of the region of LUC creation provides a focus for future scientific research into the Naturaliste Plateau region and its influences on the Leeuwin Current system.
75 Recommendations
7.0 Recommendations for Future Research
This section will detail several recommendations for future research into the origins of the LUC and the properties of the LC system. A number of recommendations are also presented that promote further use of this study’s methodology or the Bluelink model.
7.1.1 Bluelink
A quantitative validation of the Bluelink v2.1 model solution should be completed to allow future studies to confidently use the current data. This quantitative study should be carried out in a similar manner to the Bluelink v1.5 model validation undertaken by Oke et al. (2007b), with comparisons of model output to field data along the Australian coastline. This process is most probably underway at CSIRO but it is still important to highlight that Bluelink is still in the test-phase and undergoing finetuning.
7.1.2 Leeuwin Current
The Bluelink v2.1 model output could provide a suitable dataset for the wide scale analysis of LC eddy generation around W.A. Multiple years of data, with a high resolution domain covering all of W.A., are available through the CSIRO website. This data can be downloaded in fortnightly timesteps to reduce space requirements and to reduce analysis times. The advantage of using the BODAS model solution to observe the properties of eddies is that accurate data can be extracted for both surface and sub-surface eddies. Previous studies used satellite images to track and investigate the LC eddies (Griffiths and Pearce, 1985, Fang and Morrow, 2003). This method has its limitations as satellite data can only be used to determine sea-surface temperatures and heights. Using the Bluelink model solution allows the sub- surface properties of the eddies to be investigated, as well as providing alternative information such as salinity and u and v velocities. Bluelink is a suitable platform for studying the migratory paths of LC eddies as the length scales of the eddies are 10-20 times the horizontal resolution of the model. A general rule of thumb is that the length scales of the dynamics being resolved must be at least ten times the resolution of the model. In this case the eddies are 100-200km wide while Bluelink has a horizontal resolution of 10km, so the paths should be accurately resolved using Bluelink.
76 Recommendations
The cross-shore transect methodology applied in this study could be applied to a variety of studies into the LC along the W.A. coastline. One such study is the identification and quantification of the geostrophic onshore flow between Shark Bay and Cape Leeuwin, which is believed to strengthen the LC down the coast (Thompson, 1984, Meuleners et al., 2007b). This study found the existence of the geostrophic inflow, by comparing all transects to Transect 1. However, only two transects were within the recognised zone of onshore waters, resulting in a positive, yet unreliable result. If the number of transects were increased within this region, the onshore water could easily be quantified using the Bluelink v2.1 output and the Matlab programs utilised in this study.
The cross-shore transect methodology could also be applied to an inter-annual investigation of the LC, with particular focus on La Nina and El Nino events. Bluelink v2.1 provides an easy system to obtain data with large timesteps over several years. The transects could then be set up to investigate the transport of the LC during El Nino and La Nina events and to extract other information such as temperature and salinity of the LC during these events.
7.1.3 Leeuwin Undercurrent
A general recommendation when studying the LUC is to focus on the region between Cape Leeuwin and Cape Naturaliste. This region showed the most consistent LUC adjacent to the shelf edge, mainly due to the limited creation of eddies along this stretch of coast. The lack of eddies reduces the chance that the LUC will be disrupted or forced offshore by turbulent waters, making it easier to define the boundary of the current.
One of the major findings of this study was the identification of the LUC formation process around the Naturaliste Plateau. The climatology analysis revealed the LUC formation involves three steps: the diverging flow of the Flinders Current, the rotation of the Flinders Current waters by the eddy-like structures and the onshore flow of this water against the coastline. This analyisis was qualitative in nature, so the cause of the divergent flow and the reasons for the existence of the persistent eddy-like structures is still unknown. Understanding these processes is vital if the LUC is to be properly examined. Two studies completed in unison could investigate the dynamics behind the LUC creation.
77 Recommendations
First, BLuelink should be used to complete a 4-5 year climatology analysis of the Naturaliste Plateau region, to remove any short-term instabilities, allowing accurate quantification of the flows associated with the LUC creation. The climatology analysis used in this study was completed over two years, which still contained a number of anticlockwise LC eddies that were not ‘smoothed’ over, as shown in Figure 5.7. Averaging the velocities and temperature over a longer time period may reveal some underlying oceanic processes, such as an onshore geostrophic flow, that are overpowered by the turbulent eddy structures. Bluelink provides an appropriate model solution for this analysis, as multiple years of data can be obtained with a relatively high horizontal resolution. Bluelink is suitable for resolving these larger scale ocean dynamics as the horizontal resolution of Bluelink is around 1/10th the horizontal scale of the dynamics being investigated.
The second modeling study should utilise a finer-scale, eddy resolving model such as Regional Ocean Modeling System (ROMS) to investigate the possible creation mechanisms of the LUC adjacent to Cape Leeuwin. The horizontal resolution of ROMS can be adjusted to suit the scales of the processes being investigated; allowing an analysis into the dynamics of the bifurcation of the Flinders Current as well as the eddy-like structures to be completed (Shchepetkin and McWilliams, 2004). ROMS can also be used to determine the topographic influence of the shelf edge and the Naturaliste Plateau on the LUC formation. Firstly, ROMS would need to be run with the natural bathymetry intact, to recreate the LUC formation process found in this study. Then the bathymetry along the shelf edge south of Cape Leeuwin would be smoothed, to determine if the bifurcation of the Flinders Current is topographically influenced. The topographic influence of the Naturaliste Plateau can also be investigated by removing the Plateau from the bathymetry and observing if the eddy structures form. If the LUC forms without the topographic irregularities of the shelf edge or the Naturaliste Plateau then the bifurcation and rotation are independent of the bathymetry. The Bluelink climatology study and the ROMS modelling should be completed in tandem to greatly increase the scientific knowledge on the creation of the LUC.
78 References
8.0 References
Andrews, J. C. (1983) Ring structure in the poleward boundary current off Western Australia in summer. Australian Journal of Marine and Freshwater Research, 34, 547-561. Batteen, M. L. & Butler, C. L. (1998) Modeling Studies of the Leeuwin Current off Western and Southern Australia. Journal of Physical Oceanography, 28, 2199-2221. Batteen, M. L., Rutherford, M. J. & Bayler, E. J. (1992) A Numerical Study of Wind- and Thermal-Forcing Effects on the Ocean Circulation off Western Australia. Journal of Physical Oceanography, 22, 1406-1433. Boedeker, S. (2001) A Fine Resolution Model of the Leeuwin Current System. Naval Postgraduate School. Monterey. Callahan, J. E. (1972) Velocity structure and flux of the Antarctic Circumpolar Current south of Australia. Journal of Physical Oceanography, 76. Caputi, N., Fletcher, W. J., Pearce, A. & Chubb, C. F. (1996) Effect of the Leeuwin Current on the Recruitment of Fish and Invertebrates along the Western Australian Coast. Marine and Freshwater Research, 47, 147-155. Church, J. A., Cresswell, G. R. & Godfrey, J. S. (1989) Poleward flows along eastern ocean boundaries. Coastal and Estuarine Studies, 34, 230-252. Cirano, M. & Middleton, J. F. (2004) Aspects of the mean wintertime circulation along Australia's southern shelves: Numerical studies. Journal of Physical Oceanography, 34, 668-684. Cresswell, G. R., Boland, F. M., Peterson, G. L. & Wells, G. S. (1989) Continental shelf current near the Abrolhos Islands, Western Australia. Australian Journal of Marine and Freshwater Research, 40. Cresswell, G. R. & Golding, T. J. (1980) Observations of a south-flowing current in the southeastern Indian Ocean. Deep Sea Research, 27, 449-466. Cresswell, G. R. & Griffin, D. A. (2004) The Leeuwin Current, eddies and sub-Antarctic waters off south-western Australia. Marine and Freshwater Research, 55, 267-276. Cresswell, G. R. & Peterson, J. L. (1993) The Leeuwin Current south of Western Australia. Australian Journal of Marine and Freshwater Research, 44, 285-303. CSIRO (2000) The East Australian Current. Accessed on: 25/8/07,
Hanson, C. E., Pattiaratchi, C. B. & Waite, A. M. (2004) Sporadic upwelling on a downwelling coast: phytoplankton responses to spatially variable nutrient dynamics off the Gascoyne region of Western Australia. Continental Shelf Research, (in press). Herzfeld, M. & Tomczak, M. (1997) Numerical modelling of sea surface temperature and circulation in the Great Australian Bight Progress in Oceanography, 39, 29-78. Hufford, G. E., McCartney, M. S. & Donohue, K. A. (1997) Northern boundary currents and adjacent recirculations off southwestern Australia. Geophysical Research Letters, 24, 2797-2800. Kaempf, J., Doubell, M., Griffin, D. A., Matthews, R. L. & Ward, T. M. (2004) Evidence of a large seasonal coastal upwelling system along the southern shelf of Australia Geophysical Research Letters, 31. Maxwell, J. G. H. & Cresswell, G. R. (1981) Dispersal of tropical marine fauna to the Great Australian Bight by the Leeuwin Current. Australian Journal of Marine and Freshwater Research, 32, 493-500. Meuleners, M. J., Ivey, G. N. & Pattiaratchi, C. B. (2007a) A numerical study of the eddying characteristics of the Leeuwin Current System. Journal of Deep-Sea Resarch I. Meuleners, M. J., Pattiaratchi, C. B. & Ivey, G. N. (2007b) Numerical modelling of the mean flow characteristics of the Leeuwin Current System. Deep Sea Research II. Middleton, J. F. & Cirano, M. (2002) A northern boundary current along Australia's southern shelves: The Flinders Current. Journal of Geophysical Research, 107, 12.1-12.11. National Aeronatical Space Administration (2006) Ocean Motion: Ekman Transport Background. Accessed on: 23/10/2007,
Thompson, R. & Veronis, G. (1983) Poleward boundary current off Western Australia. Australian Journal of Marine and Freshwater Research, 34, 173-185. van Hazel, J. (2001) The climate and physical oceanography of the Recherche Archipelago and adjacent waters. School of Water Research, UWA. Waite, A. M., Thompson, P. A., Pesant, S. & Feng, M. (2007) The Leeuwin Current and its eddies: An introductory overview. Deep Sea Research, 54, 789-796. Weaver, A. J. & Middleton, J. H. (1989) On the Dynamics of the Leeuwin Current. Journal of Physical Oceanography, 19, 626-648. Winters, K. B., Lombard, P. N., Riley, J. J. & D'Asaro, E. A. (1995) Available potential energy and mixing in density stratified fluids. Journal of Fluid Mechanics, 289, 115- 128. Woo, M., Pattiaratchi, C. B. & Schroeder, W. (2005) Hydrography and water masses off the West Australian Coast. Ocean Dynamics.
81 Appendices
Appendices
Appendix A – Model Validation Results
Appendix B – Width-Depth Results
Appendix C – Matlab Program Code
Appendix D – Contents of Data CD
i Appendices
Appendix A – Model Validation Results
Figure A1 - Satellite and BRAN v2.1 output - 6/2/2003
ii Appendices
Figure A2 - Satellite and BRAN v2.1 output - 7/4/2003
iii Appendices
Figure A3 - Satellite and BRAN v2.1 output - 6/6/2003
iv Appendices
Figure A4 - Satellite and BRAN v2.1 output - 7/8/2003
v Appendices
Figure A5 - Satellite and BRAN v2.1 output - 6/10/2003
vi Appendices
Figure A6 - Satellite and BRAN v2.1 output - 7/12/2003
vii Appendices
Appendix B – Width-Depth Results
Depth = 0.0016*Width + 48.05
Depth = 0.0013*Width + 94.37
viii Appendices
Depth = 0.0022*Width + 24.68
Depth = 0.0025*Width + 18.56
ix Appendices
Depth = 0.0017*Width + 40.63
Depth = 0.0018*Width + 62.57
x Appendices
Depth = 0.0031*Width + 12.68
xi Appendices
Appendix C – Matlab Program Code
Master_Calc.m Matlab Code
%Takes all of the inputs, loads the correct netCDF file, calls ID_LC_Master and saves the ASCII %files and vertical profiles function Master_Calc(type, bound, current,year, month, location) close all locationsave = regexprep(location, ' ', '_');
%Sets the directory path directory=('H:\Thesis\BradsBluelinkData\2.1')
%Assigns the correct cross-shore transect properties for each transect if issame(location, 'Transect 1') Trans=[-28, 114.1]; %South Shark Bay theta=260; %angle from N at Cervantes if issame(current, 'LC') L=3; else L=3; end U_tran = [Trans(1) Trans(2)] L_tran=[Trans(1)+L*cos((theta)*pi/180) Trans(2)- L*abs(sin((theta)*pi/180))] elseif issame(location, 'Transect 2') Trans=[-30.5, 115]; %Cervantes theta=260; %angle from N at Cervantes if issame(current, 'LC') L=2; else L=3; end U_tran = [Trans(1) Trans(2)] L_tran=[Trans(1)+L*cos((theta)*pi/180) Trans(2)- L*abs(sin((theta)*pi/180))] elseif issame(location, 'Transect 3') Trans=[-33.65, 115]; %Cape Naturaliste theta=270; %angle from N at Cape Naturaliste if issame(current, 'LC') L=2; else L=3; end U_tran = [Trans(1) Trans(2)] L_tran=[Trans(1)+L*cos((theta)*pi/180) Trans(2)- L*abs(sin((theta)*pi/180))] elseif issame(location, 'Transect 4') Trans=[-34, 114.9]; %Cape Leeuwin theta=270; %angle from N at Cape Leeuwin xii Appendices
if issame(current, 'LC') L=2; else L=3; end U_tran = [Trans(1) Trans(2)] L_tran=[Trans(1)+L*cos((theta)*pi/180) Trans(2)- L*abs(sin((theta)*pi/180))] elseif issame(location, 'Transect 5') Trans=[-34.36, 115.15]; %South Cape Leeuwin theta=180; %angle from N at South Cape Leeuwin if issame(current, 'LC') L=2; else L=3; end U_tran = [Trans(1) Trans(2)] L_tran=[Trans(1)-L*abs(cos((theta)*pi/180)) Trans(2)+L*sin((theta)*pi/180)] elseif issame(location, 'Transect 6') Trans=[-35.2, 117.8]; %Albany theta=170; %angle from N at Albany if issame(current, 'LC') L=2; else L=3; end U_tran = [Trans(1) Trans(2)] L_tran=[Trans(1)-L*abs(cos((theta)*pi/180)) Trans(2)+L*sin((theta)*pi/180)] elseif issame(location, 'Transect 7') Trans=[-33.9, 121.3]; %Esperance theta=170; %angle from N at Esperance if issame(current, 'LC') L=2; else L=3; end U_tran = [Trans(1) Trans(2)] L_tran=[Trans(1)-L*abs(cos((theta)*pi/180)) Trans(2)+L*sin((theta)*pi/180)] elseif issame(location, 'Transect 8') Trans=[-34.36, 115.15]; %South Cape Leeuwin East theta=270; %angle from N at South Cape Leeuwin L=3; U_tran = [Trans(1) Trans(2)] L_tran=[Trans(1)+L*cos((theta)*pi/180) Trans(2)- L*abs(sin((theta)*pi/180))] else disp('Not a location') end
%Sets the time parameters for loading the NetCDF files if issame(month, 'January') xiii Appendices
int = 4; ncfile = 4; kk=2; startdate = 1; dateorder=1; elseif issame(month, 'February') int = 4; ncfile = 4; kk=2; startdate = 32; dateorder=2; elseif issame(month, 'March') int = 2; ncfile = 2; kk=8; startdate = 60; dateorder=3; elseif issame(month, 'April') int = 2; ncfile = 4; kk=4; startdate = 91; dateorder=4; elseif issame(month, 'May') int = 2; ncfile = 4; kk=4; startdate = 121; dateorder=5; elseif issame(month, 'June') int = 2; ncfile = 4; kk=4; startdate = 152; dateorder=6; elseif issame(month, 'July') int = 2; ncfile = 2; kk=8; startdate = 182; dateorder=7; elseif issame(month, 'August') int = 2; ncfile = 2; kk=8; startdate = 213; dateorder=8; elseif issame(month, 'September') int = 2; ncfile = 2; kk=8; startdate = 244; xiv Appendices
dateorder=9; elseif issame(month, 'October') int = 2; ncfile = 2; kk=8; startdate = 276; dateorder=10; elseif issame(month, 'November') int=4; ncfile=2; kk=4; startdate = 305; dateorder=11; elseif issame(month, 'December') int=4; ncfile=2; kk=4; startdate = 337; dateorder=12; elseif year == 2002 int=10; ncfile=2; kk=19; startdate=1; dateorder=1; end phi=(0)*pi/180;% angle of anticlockwise rotation from E
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% n=1; month_day=1;
%Loops through month for jj=0:(int*ncfile):(int*ncfile*(kk-1))
%Loads in the NetCDF files Aname = [directory, '\yr', num2str(year),'_',num2str(startdate+jj),'_to_',num2str(startdate+jj+(int*(n cfile-1))), '_int',num2str(int), '.nc'] %Aname=[directory,'\July\Bluelink_NWS_',num2str(kk),'.nc']; ncload(Aname,'BluelinkU','BluelinkV','LONGITUDE_T','LATITUDE_T','DEPTH','v3 d_time', 'BluelinkTemp', 'BluelinkSalt');
[Snapshots,layers,row,col]=size(BluelinkTemp); for rr=1:Snapshots
timestep=rr;
BluelinkU(find(BluelinkU==-10))=NaN; BluelinkV(find(BluelinkV==-10))=NaN; BluelinkTemp(find(BluelinkTemp==-10))=NaN; BluelinkSalt(find(BluelinkSalt==-10))=NaN;
xv Appendices
[Y,I_Ulat]=min(abs(LATITUDE_T-U_tran(1))); [Y,I_Ulong]=min(abs(LONGITUDE_T-U_tran(2))); [Y,I_Llat]=min(abs(LATITUDE_T-L_tran(1))); [Y,I_Llong]=min(abs(LONGITUDE_T-L_tran(2)));
[row,col]= DeterminePointsAlongSection(I_Ulat,I_Llat,I_Llong,I_Ulong);
%Extracts the data for the given lat and long along the transect Distance=[0 cumsum(sw_dist(LATITUDE_T(row), LONGITUDE_T(col),'km'))'];
U=NaN*zeros(47, length(row)); %U velocity data V=NaN*zeros(47, length(row)); %V velocity data tempSalt=NaN*zeros(47, length(row)+1); %Salinity data tempTemp=NaN*zeros(47, length(row)+1); %Temperature data
for ii=1:length(row) U(:,ii)=squeeze(BluelinkU(timestep,:,row(ii),col(ii))); V(:,ii)=squeeze(BluelinkV(timestep,:,row(ii),col(ii))); tempSalt(:,ii)=squeeze(BluelinkSalt(timestep,:,row(ii),col(ii))); tempTemp(:,ii)=squeeze(BluelinkTemp(timestep,:,row(ii),col(ii))); end
Temp = tempTemp(:, 2:(length(row)+1)); Temp(find(isnan(U)))= NaN; Salt = tempSalt(:, 2:(length(row)+1)); Salt(find(isnan(U)))= NaN;
% Rotate velocity vectors into AS and CS components
if issame(type, 'Velocity')
AS=NaN*zeros(size(U)); CS=NaN*zeros(size(U));
for ii=1:size(U,1) for ll=1:size(U,2) H=[cos(phi) sin(phi); -sin(phi) cos(phi)]*[U(ii,ll); V(ii,ll)]; if issame(location, 'Transect 5') || issame(location, 'Transect 6') || issame(location, 'Transect 7') CS(ii,ll)=H(1); else CS(ii,ll)=H(2); end end end
% Set AS and CS components to temperature values elseif issame(type, 'Temperature')
AS=NaN*zeros(size(Temp)); CS=NaN*zeros(size(Temp));
CS = Temp; xvi Appendices
AS = Temp;
% Set AS and CS components to Salinity values elseif issame(type, 'Salinity')
AS=NaN*zeros(size(Salt)); CS=NaN*zeros(size(Salt));
CS = Salt; AS = Salt; end
%Sets the boundary type if issame(bound, 'Velocity')
%Sets the velocity limits if issame(current, 'LC') defineLC=-0.3; defineLCC=0.3; else defineLC=-0.125; defineLCC=0.125; end Boundary=NaN*zeros(size(U)); for ii=1:size(U,1) for ll=1:size(U,2) H=[cos(phi) sin(phi); -sin(phi) cos(phi)]*[U(ii,ll); V(ii,ll)]; if issame(location, 'Transect 5') || issame(location, 'Transect 6') || issame(location, 'Transect 7') Boundary(ii,ll) = H(1); else Boundary(ii,ll) = H(2); end end end elseif issame(bound, 'Temperature') %Sets the temperature limits defineLC=20.8; %need to be changed for location and time of year defineLCC=20.8; %need to be changed for location and time of year
Boundary=NaN*zeros(size(Temp)); Boundary = Temp;
elseif issame(bound, 'Salinity') %Sets the salinity limits defineLC=35.5; %need to be changed for location and time of year defineLCC=38; %need to be changed for location and time of year
Boundary=NaN*zeros(size(Salt)); Boundary = Salt; end
%Calls Plotting function TF=0; while TF~=1 xvii Appendices
[IN, max_min]=ID_LC_Master(current, Distance, DEPTH, CS, Boundary, defineLC, defineLCC, location, month_day, month, type); button = questdlg('Happy with selection?'); TF = strcmp('Yes', button); end
%find width and depth width = max_min(1,1) - max_min(1,2); depth = max_min(2,1) - max_min(2,2); width_depth(n,1) = width; width_depth(n,2) = depth;
%find Volume Flux (average, max and min) values if issame(type, 'Velocity') boundary_core=IN'.*CS; boundary_core(find(isnan(boundary_core)))=0;
NonZeros = nonzeros(boundary_core); if length(NonZeros) ==0 velocity_max = 0; velocity_min = 0; velocity_mean = 0; else velocity_max = max(NonZeros); velocity_min = min(NonZeros); velocity_mean = mean(NonZeros); end velocity_core(n,1) = velocity_max; velocity_core(n,2) = velocity_min; velocity_core(n,3) = velocity_mean;
integrate_depth=trapz(DEPTH,boundary_core); integrate_distance(n)=trapz(Distance*1000,integrate_depth);%m^3/s
%find centre of core [Y,Imax]=min(boundary_core,[],1); [X Idis]=min(Y); centre_core(n,:)=[Distance(Idis) DEPTH(Imax(Idis))];
%find Temperature (average, max and min) values elseif issame(type, 'Temperature') boundary_core=IN'.*CS; boundary_core(find(isnan(boundary_core)))=0; NonZeros = nonzeros(boundary_core); if length(NonZeros) ==0 temp_max = 0; temp_min = 0; temp_mean = 0; else temp_max = max(NonZeros); temp_min = min(NonZeros); temp_mean = mean(NonZeros); end temp_core(n,1) = temp_max; temp_core(n,2) = temp_min; temp_core(n,3) = temp_mean;
xviii Appendices
%find centre of core [Y,Imax]=min(boundary_core,[],1); [X Idis]=min(Y); centre_core(n,:)=[Distance(Idis) DEPTH(Imax(Idis))];
%find Salinity (average, max and min) values elseif issame(type, 'Salinity') boundary_core=IN'.*CS; boundary_core(find(isnan(boundary_core)))=0; NonZeros = nonzeros(boundary_core); if length(NonZeros) ==0 salt_max = 0; salt_min = 0; salt_mean = 0; else salt_max = max(NonZeros); salt_min = min(NonZeros); salt_mean = mean(NonZeros); end salt_core(n,1) = salt_max; salt_core(n,2) = salt_min; salt_core(n,3) = salt_mean;
%find centre of core [Y,Imax]=min(boundary_core,[],1); [X Idis]=min(Y); centre_core(n,:)=[Distance(Idis) DEPTH(Imax(Idis))]; end
x(n) = month_day; n=n+1; month_day=month_day+int;
%Saves the current vertical plot file = ['H:\Thesis\Volumes\', current,type,'_',bound, locationsave, month, num2str(startdate +month_day-int-1)] saveas(gcf, file, 'jpg'); %Save the Figure end end
%Plots up volume flux over the month if issame(type, 'Velocity') figure set(gcf,'Position',[100 100 600 600]) plot(x, integrate_distance, '--bx', 'LineWidth',1,... 'MarkerEdgeColor','b',... 'MarkerFaceColor', 'b',... 'MarkerSize',10)
xlabel([month,' ', num2str(year)], 'fontsize',12); ylabel('Volume m^3/s', 'fontsize',12); title(['Volume Flux of ', current,' at ', location], 'fontsize',14,'fontweight','b'); file = ['H:\Thesis\Volumes\', current,type, '_', bound, locationsave, month, num2str(dateorder),'graph'] saveas(gcf, file, 'tif'); %Save the Figure
xix Appendices
%save the volume flux and velocity values in ASCII format save(['H:\Thesis\Volumes\Ascii_files\',current,type, '_', bound, num2str(year), locationsave, num2str(dateorder), 'volume_flux'], 'integrate_distance', '-ASCII'); save(['H:\Thesis\Volumes\Ascii_files\',current,type, '_', bound, num2str(year), locationsave, num2str(dateorder), 'velocity_core'], 'velocity_core', '-ASCII'); elseif issame(type, 'Temperature') figure set(gcf,'Position',[100 100 600 600]) plot(x, temp_core(:,3), '--bx', 'LineWidth',1,... 'MarkerEdgeColor','b',... 'MarkerFaceColor','b',... 'MarkerSize',10)
xlabel([month,' ', num2str(year)], 'fontsize',12); ylabel('Temperature (C)', 'fontsize',12); title(['Average Temperature of ', current,' at ', location], 'fontsize',14,'fontweight','b'); file = ['H:\Thesis\Volumes\', current,type, '_', bound, locationsave, month, num2str(dateorder),'graph'] saveas(gcf, file, 'tif'); %Save the Figure
%save the temperature values in ASCII format save(['H:\Thesis\Volumes\Ascii_files\',current,type, '_', bound, num2str(year), locationsave, num2str(dateorder), 'temp_core'], 'temp_core', '-ASCII'); elseif issame(type, 'Salinity') figure set(gcf,'Position',[100 100 600 600]) plot(x, salt_core(:,3), '--bx', 'LineWidth',1,... 'MarkerEdgeColor','b',... 'MarkerFaceColor','b',... 'MarkerSize',10)
xlabel([month,' ', num2str(year)], 'fontsize',12); ylabel('Salinity mg/L', 'fontsize',12); title(['Average Salinity of ', current,' at ', location], 'fontsize',14,'fontweight','b'); file = ['H:\Thesis\Volumes\', current,type, '_', bound, locationsave, month, num2str(dateorder),'graph'] saveas(gcf, file, 'tif'); %Save the Figure
%save the salinity values in ASCII format save(['H:\Thesis\Volumes\Ascii_files\',current,type, '_', bound, num2str(year), locationsave, num2str(dateorder), 'salt_core'], 'salt_core', '-ASCII'); end
%Save Centre core and width_depth data in ASCII format save(['H:\Thesis\Volumes\Ascii_files\',current,type, '_', bound, num2str(year), locationsave, num2str(dateorder), '_centre'], 'centre_core', '-ASCII'); save(['H:\Thesis\Volumes\Ascii_files\',current,type, '_', bound, num2str(year), locationsave, num2str(dateorder), '_width_depth'], 'width_depth', '-ASCII');
xx Appendices
ID_LC_Master.m Matlab Code
%Takes input from Master_Calc.m and plots up and calculates the boundary of %the current, using a series of questions. function [IN, max_min]=ID_LC_Master(current, Distance, DEPTH, CS, Boundary, defineLC, defineLCC, location, index, month, type) if issame(current, 'LC') depth1 = 1; depth2 = 25; else depth1 = 1; depth2 = 39; end
%Plots the variable being investigated h0=figure; pcolor(Distance, DEPTH(depth1:depth2),CS(depth1:depth2,:)); axis ij %28 is default title([current , ' ', type, ' Profile at ', location, ': ',month, ' ', num2str(index)], 'fontsize',14,'fontweight','b'); xlabel('Distance (km)', 'fontsize',12); ylabel('Depth (m)', 'fontsize',12);
hold on shading flat %axis([200 400 0 500]) if issame(current, 'LC') caxis([-0.8 0.8]) else caxis([-0.5 0.5]) end colorbar set(gcf,'Position',[100 100 600 600]) % h2 = gca % posn = get(h2) % text(300,posn-20, 'LAND', 'color', [0.9 0.4 0.3]);
%Asks the first question on what current is being examined button = questdlg('LC source water flow?'); if strcmp('Yes', button)==1 cut=defineLC; elseif strcmp('No', button)==1 cut=defineLCC; elseif strcmp('Cancel', button)==1 IN=zeros(size(CS))'; max_min(1,1) = 0; max_min(1,2) = 0; max_min(2,1) = 0; max_min(2,2) = 0; return end xxi Appendices
%Brings the boundary of the current to the edge if all of the current is %not being extracted, as mentioned in Section 5.5.3 of the thesis True = 0; C=contourc(Distance, DEPTH(depth1:depth2),Boundary(depth1:depth2,:),[cut cut]); indstart=[find(C(1,:)==cut) length(C)+1]; contour(Distance, DEPTH(depth1:depth2),CS(depth1:depth2,:),[cut cut],'k'); for ii=1:length(indstart)-1 ra=indstart(ii)+1:indstart(ii+1)-1; plot(C(1,ra),C(2,ra),'b','LineWidth',2) if True == 0 button = questdlg('Bring contour to the edge'); TF = strcmp('Yes', button); if TF==1
Point1 = C(:,indstart(ii)+1); indice_x_start = 1; for jj = 1:length(Distance) if (Point1(1,1) >= Distance(jj)) && (Point1(1,1) <= Distance(jj+1)) indice_x_start = jj; break end end indice_y_start = 1; DEPTH2 = DEPTH'; for jj = 1:length(DEPTH2) if (Point1(2,1) >= DEPTH2(jj)) && (Point1(2,1) <= DEPTH2(jj+1)) indice_y_start = jj; break end end CS(indice_y_start, indice_x_start+1)
if isnan(CS(indice_y_start, indice_x_start+3))|| isnan(CS(indice_y_start, indice_x_start+2))|| isnan(CS(indice_y_start, indice_x_start+1)) || isnan(CS(indice_y_start, indice_x_start))
C_start = C(:,1:indstart(ii)); C_end = C(:, indstart(ii)+1:end); C_point = [ Point1(1,1) + 20; Point1(2,1)]; C_start = [C_start C_point]; C = [C_start C_end]; True =1; end
if ii ~= (length(indstart)-1)
Point2 = C(:,indstart(ii+1)); else Point2 = C(:,end); end indice_x_end = 1; for jj = 1:length(Distance)
xxii Appendices
if (Point2(1,1) >= Distance(jj)) && (Point2(1,1) <= Distance(jj+1)) indice_x_end = jj; break end end indice_y_end = 1; DEPTH2 = DEPTH'; for jj = 1:length(DEPTH) if (Point2(2,1) >= DEPTH2(jj)) && (Point2(2,1) <= DEPTH2(jj+1)) indice_y_end = jj; break end end
if isnan(CS(indice_y_end, indice_x_end+3)) || isnan(CS(indice_y_end, indice_x_end+2)) || isnan(CS(indice_y_end, indice_x_end+1)) || isnan(CS(indice_y_end, indice_x_end)) if ii ~= length(indstart)-1 C_start = C(:,1:indstart(ii+1)); C_end = C(:, indstart(ii+1)+1:end); C_point = [ Point2(1,1) + 15; Point2(2,1)]; C_start = [C_start C_point]; C = [C_start C_end]; True =1; else C(:, end+1)= [ Point2(1,1) + 20; Point2(2,1)]; True =1; end
end end
clear C_start C_point C_end indice_x_end indice_y_end indice_x_start indice_y_start Point1 Point2 indstart=[find(C(1,:)==cut) length(C)+1];
end end %finds the boundary of the current contour(Distance, DEPTH(depth1:depth2),CS(depth1:depth2,:),[cut cut],'k'); rapoly=NaN*ones(length(indstart)-1,150);
%Plots the boundary and steps through each polygon, asking whether the %polygon should be included in the analysis for ii=1:length(indstart)-1 ra=indstart(ii)+1:indstart(ii+1)-1; plot(C(1,ra),C(2,ra),'m','LineWidth',2) button = questdlg('include this contour in polygon'); TF = strcmp('Yes', button); if TF==1 plot(C(1,ra),C(2,ra),'m','LineWidth',2) rapoly(ii,1:length(ra))=ra';
else plot(C(1,ra),C(2,ra),'red','LineWidth',2) xxiii Appendices
end clear ra end rapoly(all(isnan(rapoly),2),:) = []; if size(rapoly,1)==1 ra_poly=rapoly(find(~isnan(rapoly))); else %make polynomial with parts of contour in the correct order ct=1; rapoly(all(isnan(rapoly),2),:) = []; for ii=1:size(rapoly,1) clear ind ra_sub; ind=find(~isnan(rapoly(ii,:))); ra_sub=rapoly(ii,ind); if sum(ind)~=0 min_dist(ii,:)=[C(1,ra_sub(1)); C(1,ra_sub(end))]; min_depth(ii,:)=[C(2,ra_sub(1)); C(2,ra_sub(end))]; end end
if size(min_dist,1)==2 [M I1]=sort(min_depth,2); rapoly_new_sub1=rapoly(1,find(~isnan(rapoly(1,:)))); rapoly_new_sub2=rapoly(2,find(~isnan(rapoly(2,:)))); if I1(1,1)==I1(2,1)
ra_poly=[rapoly_new_sub1 fliplr(rapoly_new_sub2)]; else ra_poly=[rapoly_new_sub1 (rapoly_new_sub2)]; end
else
% [M I1]=sort(min_dist,2); % [M I2]=sort(M(:,1)); [M I1]=min(min_dist,[],2); [M I2]=sort(M(:,1)); rapoly_new=NaN*ones(length(indstart)-1,100); rapoly_new=rapoly(I2,:); % initial_pt=[0 0]; ra_poly=0;
for ii=1:size(rapoly_new,1) rapoly_new_sub=rapoly_new(ii,find(~isnan(rapoly_new(ii,:))));
if ii==1 dif=C(2,rapoly_new_sub(1))- C(2,rapoly_new_sub(end)); if dif>0 ra_poly=[ra_poly fliplr(rapoly_new_sub)]; else ra_poly=[ra_poly (rapoly_new_sub)]; end else rms_diff1=sqrt((initial_pt(1,1)-C(1,rapoly_new_sub(1)))^2 ... + (initial_pt(1,2)-C(2,rapoly_new_sub(1)))^2);
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rms_diff2=sqrt((initial_pt(1,1)-C(1,rapoly_new_sub(end)))^2 ... + (initial_pt(1,2)-C(2,rapoly_new_sub(end)))^2); if rms_diff2>rms_diff1 ra_poly=[ra_poly rapoly_new_sub]; else ra_poly=[ra_poly fliplr(rapoly_new_sub)]; end end initial_pt=[C(1,ra_poly(end)) C(2,ra_poly(end))];
end ra_poly=ra_poly(2:end); end end C=C(:,ra_poly); [B,I]=sort(C(1,:)); Csort=C;
%find points inside polygonal region Distance_all=Distance'*ones(1,length(DEPTH)); DEPTH_all=(DEPTH*ones(1,length(Distance)))'; IN = inpolygon(Distance_all,DEPTH_all,Csort(1,:),Csort(2,:));
%Calculate width and depth max_min(1,1) = max(C(1,:)); max_min(1,2) = min(C(1,:)); max_min(2,1) = max(C(2,:)); max_min(2,2) = min(C(2,:));
%Plot the points that have been extracted as 'x' plot(Distance_all(IN),DEPTH_all(IN) ,'xk') clear True
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Appendix D – Contents of Data CD
The CD that accompanies this Thesis project contains the vertical profiles at all transects for the LC and LUC analyses as well as the horizontal plots used to define the Flinders Current on the southern coastline. Below is a file directory map to aid in the navigation of the data on the CD.
Leeuwin Current Analysis January Transect 1 Transect 2 Transect 3 Transect 4 Transect 5 Transect 6 Transect 7 February Transect 1 …………. ………….
Leeuwin Undercurrent Analysis January Transect 1 Transect 2 Transect 3 Transect 4 Transect 5 Transect 6 Transect 7 February Transect 1 …………. …………. South Coast Horizontal Plots
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