The surface circulation of the Coral Sea and Great Barrier Reef

Choukroun, Séverine (2010) The surface circulation of the Coral Sea and Great Barrier Reef. PhD thesis, James Cook University.

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The Coral Sea, located to the Northeast of Australia (8ºS to 30ºS and 142ºE and 170º E) covers an area of a million square kilometres and harbours the largest Coral Reef system in the world. The Coral Sea has a very complex bathymetry, great temporal variability and very strong and narrow currents which renders it difficult for the implementation of an adequate observing system. A better understanding of this region’s circulation patterns is important because of its location at a major ocean pathway from the equator to the subtropics that may potentially influence the ENSO cycle (Gu and Philander 1997; Kleeman et al. 1999; Schneider et al. 2002; Schneider 2004).

Existence of jet like structures in the southwest Pacific was first identified by Webb (2000). Since Webb’s discovery, a number of studies have been carried out to observe these jets and understand their influence on the circulation of the Coral Sea (Stanton et al. 2001; Hughes 2002; Ridgway et al. 2002; Kessler and Gourdeau 2006; Kessler and Gourdeau 2007; Ganachaud et al. 2008; Gourdeau et al. 2008; Schiller et al. 2008). Numerical models have so far provided most of our knowledge of the Coral Sea circulation but have been calibrated and validated against very few in situ measurements. Moreover, model results have been incapable to realistically represent the seasonal variability of the jets (Qu and Lindstrom 2002; Kessler and Gourdeau 2007) nor the SEC bifurcation along the Great Barrier Reef (Ganachaud 2007). This thesis aims to investigate:

• The mean and seasonal variation of the surface circulation of the Coral Sea in terms of Eulerian and Lagrangian statistics derived from surface drifters.

• The interaction of the Coral Sea jets with the complex bathymetry of the Coral Sea and their contribution to the Western Boundary Currents (WBCs).

• The Coral Sea inflow and residence times in the Great Barrier Reef from in situ measurements,

• And finally, the East Australian Current (EAC) flow interaction with the complex southern Great Barrier Reef (GBR) bathymetry.

The spatial structure of the surface currents in the Coral Sea were described, at mesoscale, in terms of Eulerian and Lagrangian statistics computed from the low passed drifter velocities (36 hour cut-off period) obtained between September 1981 and December 2009. Pseudo-Eulerian maps of the surface mean flow, mean kinetic energy and eddy kinetic energy were produced to study the circulation at meso-scale in the region. The surface mean flow depicts well the presence of the North Vanuatu Jet as a large (over 2 degrees of latitude) and strong jet (velocities greater than 20 cm s-1) and the North Caledonia Jet which is much narrower (0.5 degrees of latitude) and relatively strong (velocities greater than 15 cm s-1). The jets enter the Coral Sea between the Solomon Archipelago and New Caledonia, and flow westward toward the Australian Shelf where they contribute to the formation of the southward flowing East Australian Current and the northward flowing North Queensland Current. Interestingly, part of the NCJ appears to be deflected to the north by the presence of the Queensland Plateau at ~152°E, where it joins the NVJ at ~ 150°E. A novel surface current structure was also observed in the Solomon Sea, flowing to the southeast along the western side of the Solomon Islands.

The seasonal variability of the surface circulation of the Coral Sea show that most of the variability is contained in the northern Coral Sea, north of 15ºS. This is defined by a strengthening of the North Coral Sea gyre, during the winter, which can clearly be depicted in our mean flow map. The NQC surface transport doubled and the NGCC strengthens. Highest MKE values were observed during the winter, and are concentrated along the NVJ path and the western boundary currents. Surface circulation and residence time of the Great Barrier Reef were derived from drifters that had entered the GBR. The Coral Sea inflow in the GBR is a key parameter to understand the connectivity of coral reef fauna. Knowledge of connectivity patterns is important to enable the formulation of realistic management strategies to ensure successful conservation of reef biodiversity, especially under the looming spectre of climate change. Secondly the data were used to resolve the flushing time of waters in the GBR which has important implications for flushing time of pollutants of terrestrial origin. These results, including diffusion and kinetic energy quantification, are also invaluable for hydrodynamic model calibrations, providing for the first time, an evaluation of the parameters measured in situ Finally, data from a long term mooring array deployed along the Capricorn Bunker shelf and slope, combined with satellite imagery and outputs from a global ocean model, is used to quantify the influence of the EAC to the circulation in the Capricorn Channel. A North westward flow can be observed along the shelf which was observed to strengthen and to reach a maximum of 10 cm s-1during the summer months and a minimum of ~0 cm s-1during the winter months. High energy was observed in the velocity; temperature and sea level between 20-40 days and 70-140 days periodicities were observed.

The results suggest the formation of a cyclonic eddy, over the Marion Plateau, periodically every 20-40 days, modulated by a 70-140 days periodicity. This eddy may drive the northwest ward flow along the shelf and therefore allowing intrusions of oceanic water over the shelf. This has very important implications for the ecosystem health as it provides inflow of cooler and rich in nutrient to the shelf, especially during the summer months. The processes controlling the eddy formation remain unclear but appear to be related to the presence of an anti cyclonic anomaly at 25S. This anomaly appears to affect the flow of the EAC forcing it to bifurcate along the CB shelf.

Item ID: 24024
Item Type: Thesis (PhD)
Keywords: Coral Sea; EAC; East Australian Current; eddies; eddy formation; fluid dynamics; GBR; Great Barrier Reef; mesoscale currents; ocean currents; ocean gyres; physical oceanography; surface circulation; surface-water hydrology; WBCs; western boundary currents; zonal jets
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Publications arising from this thesis are available from the Related URLs field. The publications are:

Andrefouet, S., Ouillon, S., Brinkman, R., Falter, J., Douillet, P., Wolk, F., Smith, R., Garen, P., Martinez, E., Laurent, V., Lo, C., Remoissenet, G., Scourzic, B., Gilbert, A., Deleersnijder, E., Steinberg, C., Choukroun, S., and Buestel, D. (2006) Review of solutions for 3D hydrodynamic modeling applied to aquaculture in South Pacific atoll lagoons. Marine Pollution Bulletin, 52 (10). pp. 1138-1155.

Choukroun, Séverine, Ridd, Peter V., Brinkman, Richard, and McKinna, Lachlan I.W. (2010) On the surface circulation in the western Coral Sea and residence times in the Great Barrier Reef. Journal of Geophysical Research: oceans, 115 . pp. 1-13.

Campbell, Hamish A., Watts, Matthew E., Sullivan, Scott, Read, Mark A., Choukroun, Severine, Irwin, Steve R., and Franklin, Craig E. (2010) Estuarine crocodiles ride surface currents to facilitate long-distance travel. Journal of Animal Ecology, 79 (5). pp. 955-964.

Date Deposited: 03 Dec 2012 00:30
FoR Codes: 04 EARTH SCIENCES > 0405 Oceanography > 040503 Physical Oceanography @ 100%
SEO Codes: 96 ENVIRONMENT > 9699 Other Environment > 969902 Marine Oceanic Processes (excl. Climate Related) @ 33%
96 ENVIRONMENT > 9609 Land and Water Management > 960903 Coastal and Estuarine Water Management @ 33%
96 ENVIRONMENT > 9603 Climate and Climate Change > 960304 Climate Variability (excl. Social Impacts) @ 34%
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