Chapter 15 Merging scales in models of water circulation: perspectives from the great barrier reef

doi:10.1016/S0422-9894(03)80132-0

Elsevier Oceanography Series

Volume 67, 2003, Pages 411-429

https://doi.org/10.1016/S0422-9894(03)80132-0 Get rights and content

Publisher Summary

This chapter describes the technique chosen to include the effect of the oceanic circulation in a 2-D model of the large-scale circulation on the continental shelf. This circulation is found to be modulated at an intermediate spatial scale by the interaction of the tidal circulation with individual reefs, and this process is modeled by merging large-scale and reef-scale 2-D circulation models. The oceanography of the Great Barrier Reef (GBR) is made particularly complex by the extraordinary complex bathymetry. In the GBR, the obstruction of the flow by the presence of reefs steers and modifies, even at these large scales, the oceanic inflow and the longshore currents. Obstruction by large reefs or a reef matrix steers prevailing currents toward areas of low reef density. This provides the modeler the challenge to merge the large-scale oceanic circulation with the shelf-scale general distribution of reefs over the shelf. The resulting currents through a reef matrix, and the deflection of the prevailing currents around a reef matrix, are modulated by the tides.

References (36)

AndrewsJ.C. et al.
Coral Sea circulation and transport from modal information models
Deep-Sea Research
(1989)
AndrewsJ.C. et al.
Subsurface intrusions of Coral Sea water into the central Great Barrier Reef—I. Structures and shelf-scale dynamics
Continental Shelf Research
(1986)
BrinkmanR. et al.
Oceanic inflow from the Coral Sea into the Great Barrier Reef
Estuarine, Coastal and Shelf Science
(2002)
DeleersnijderE. et al.
A three-dimensional model of the water circulation around an island in shallow water
Continental Shelf Research
(1992)
OnishiS.
Study of vortex structure in water surface jets by means of remote-sensing
WolanskiE. et al.
Three-dimensional island wakes in the field, laboratory and numerical models
Continental Shelf Research
(1996)
WolanskiE. et al.
Tidal jets, nutrient upwelling and their influence on the productivity of the algal Halimeda in the Ribbon Reefs, Great Barrier Reef
Estuarine, Coastal and Shelf Science
(1988)
WolanskiE. et al.
Sticky waters in the Great Barrier Reef
Estuarine, Coastal and Shelf Science
(2000)
AndrewsJ.C.
Water masses, nutrient levels and seasonal drift on the Outer Central Queensland Shelf (Great Barrier Reef)
Australian Journal of Marine and Freshwater Research
(1983)
BerkelmansR. et al.
Seasonal and local spatial patterns in the upper thermal limits of corals on the inshore Central Great Barrier Reef
Coral Reefs
(1999)

BlumbergA.F. et al.

A description of the three-dimensional coastal ocean circulation model

ChurchJ.A.

East Australian Current adjacent to the Great Barrier Reef

Australian Journal of Marine and Freshwater Research

(1987)

ChurchJ.A. et al.

A permanent under current adjacent to the Great Barrier Reef

Journal of Physical Oceanography

(1983)

FischerH.B.E. et al.

Mixing in Inland and Coastal Waters

(1979)

FurukawaK. et al.

Shallow-water frictional effects in island wakes

Estuarine, Coastal and Shelf Science

(1998)

GallowayD. et al.

Modelling eddy formation in coastal waters: A comparison between model capabilities

HessK.W.

MECCA Programs Documentation

Hoegh-GuldbergO.

Climate change, coral bleaching and the future of the world's coral reefs

Marine and Freshwater Research

(1999)

Cited by (21)

How fine is fine enough? Effect of mesh resolution on hydrodynamic simulations in coral reef environments
2023, Ocean Modelling
Coral reef environments are biodiversity hotspots that provide many services to coastal communities. They are currently facing an increasing anthropogenic pressure that jeopardises their survival. Numerical ocean models are an important tool to understand the functioning of coral ecosystems and the mechanisms ensuring their resilience. However, simulating the water circulation through reef systems is challenging because of their naturally complex topography and bathymetry. Many ocean models used for such applications have a spatial resolution coarser than the scale of individual reefs, which puts into question their suitability for the task. Here, we assess the sensitivity of a coastal ocean model’s outputs to its spatial resolution when simulating the water circulation in the entire Great Barrier Reef (Australia), the largest coral reef system in the world. We consider the same model with five different resolutions near reefs, ranging from 250 m to 4 km, and compare the model outputs at different locations around reefs and in the open ocean. We also simulate the transport of passive particles released from those different locations. Our results show that the simulated tidal signal is similar for all five resolutions. However, strong discrepancies ( $>$ 10 cm/s) in the current velocity are observed near the reefs and along the rugged coastline. When using a coarse-resolution model, the amplitude of the currents is overestimated over reefs, and underestimated between them. We find that validating the model at deep water mooring sites is not sufficient to ensure it performs well close to reefs. Discrepancies in currents lead to more directional and uniform tracer dispersal patterns on coarse-resolution meshes that contrast with the more dispersive patterns observed on fine-resolution meshes. Those differences at the reef level have a large cumulative effect when simulating transport processes over several weeks. Our results suggest that ocean circulation and transport simulations in coral reef environments should be based on model resolutions finer than the reef scale, which generally means a maximum resolution of about 250–500 m.
An ocean reanalysis of the western Coral Sea and Great Barrier Reef
2019, Ocean Modelling
This article describes a 10 year regional ocean reanalysis of the western Coral Sea and Great Barrier Reef (GBR) from 2006–2015. Here we use the Regional Ocean Modelling System (ROMS) at 4 km resolution and EnOI (Ensemble Optimal Interpolation) data assimilation. We also account for river freshwater discharge at the coast using hydrological stream gauge observations. The system appears to constrain features of the deep ocean circulation that are important for cross-shelf exchanges such as the spatio-temporal locations of mesoscale eddies and boundary currents. Accuracy is evaluated with forecast innovation errors respecting available observations. A four-dimensional climatological atlas of water mass properties and currents, representing 10 years of synthesis between model and data is then presented. This illustrates seasonal and climatic processes driving changes in the region, such as the El-Nino Southern Oscillation (ENSO) and its influence on sea temperature and freshwater flux to the shelf from rivers. On the shelf where dense observation coverage is limited to satellite sea surface temperature (SST), we find where SST forecast errors are low and correlations between SST and bottom temperatures are significant and take this as a reliable predictor of bottom temperatures. Differences and correlations outside these parameters suggest areas where measurement of bottom temperature is likely to be important for a long-term and comprehensive monitoring and prediction system. The reanalysis provides a realistic physical and dynamic picture of the ocean at its given resolution that may be amenable to a variety of marine and environmental studies.
Modelling the fate of marine debris along a complex shoreline: Lessons from the Great Barrier Reef
2015, Estuarine, Coastal and Shelf Science
The accumulation of floating anthropogenic debris in marine and coastal areas has environmental, economic, aesthetic, and human health impacts. Until now, modelling the transport of such debris has largely been restricted to the large-scales of open seas. We used oceanographic modelling to identify potential sites of debris accumulation along a rugged coastline with headlands, islands, rocky coasts and beaches. Our study site was the Great Barrier Reef World Heritage Area that has an emerging problem with debris accumulation. We found that the classical techniques of modelling the transport of floating debris models are only moderately successful due to a number of unknowns or assumptions, such as the value of the wind drift coefficient, the variability of the oceanic forcing and of the wind, the resuspension of some floating debris by waves, and the poorly known relative contribution of floating debris from urban rivers and commercial and recreational shipping. Nevertheless the model was successful in reproducing a number of observations such as the existence of hot spots of accumulation. The orientation of beaches to the prevailing wind direction affected the accumulation rate of debris. The wind drift coefficient and the exact timing of the release of the debris at sea affected little the movement of debris originating from rivers but it affected measurably that of debris originating from ships. It was thus possible to produce local hotspot maps for floating debris, especially those originating from rivers. Such modelling can be used to inform local management decisions, and it also identifies likely priority research areas to more reliably predict the trajectory and landing points of floating debris.
Numerical modelling and graph theory tools to study ecological connectivity in the Great Barrier Reef
2014, Ecological Modelling
Citation Excerpt :
The GBR is a region with a particularly complex bathymetry and a correspondingly complex water circulation (Wolanski, 1994). Small-scale circulation at the reef-scale has been shown to interact significantly with large-scale circulation, for instance through the ‘sticky water’ effect (Wolanski and Spagnol, 2000; Wolanski et al., 2003; Andutta et al., 2012). In order to capture all major scales of motion, it is therefore necessary to resolve currents down to the reef scale: 100 m to 1 km.
The process of coral larval dispersal is important for coral reef ecosystems, but remains poorly understood and hard to gauge. Better knowledge of inter-reef connectivity patterns would be useful in enabling better management of coral reef waters however. By employing a spatially explicit numerical modelling approach, we simulate larval dispersal through the central section of the Great Barrier Reef (GBR), comprising over 1000 reefs, and identify spatial patterns in the inter-reef connectivity network using a community detection method from network science. This paper presents the modelling approach used and discusses the significance of the results.
Inter-reef connectivity networks were estimated for 4 different coral species, and significant differences between them were found. We show how we can partition reefs into clusters, or “communities”, that are sparsely connected with each other, and therefore identify important barriers to larval dispersal. By fine-tuning a resolution parameter in the community detection method, we can find dispersal barriers of varying strength. Finally, we show that the average connectivity length scale varies significantly across the different reef communities, and suggest that this may have repercussions for the optimal placement of marine protected areas (MPAs) to maximise connectivity with surrounding reefs.
Subinertial and tidal currents on the Abrolhos Bank shelf
2013, Continental Shelf Research
Citation Excerpt :
Wolanski and Benett (1983) have stressed the importance of large scale Continental Shelf Waves for the circulation around the Great Barrier Reef region. The complex topography in coral reef regions plays an essential role in determining the sea water response to the forcing mechanisms for the circulation – the wind stress, the tides, and the influence of the offshore circulation – as pointed out for the Great Barrier Reef by Wolanski et al. (2003). The southward flowing BC (Stramma and England, 1999) transports Tropical Water (TW, temperature (T)>22 °C, salinity (S)>36) in its mixed-layer.
Analysis of simultaneous current measurements in the middle (point PA1) and outer (point PA2) Abrolhos Bank shelf showed different characteristics and forcing mechanisms between the two shelf regions. The mean currents at both positions point generally southwestward but are weak due to the high time variability; at PA1, the mean along-shelf currents are almost one order of magnitude larger than the mean cross-shelf currents, while at PA2, both mean current components exhibit the same order of magnitude. At both points, the tidal currents are more important in the cross-shelf direction than they are in the along-shelf direction. The subinertial currents are mainly barotropic and along-shelf at PA1, while at PA2, the baroclinic first mode dominates the subinertial variability, especially in the cross-shelf direction, though the currents are also more energetic in the along-shelf direction. Correlations are significant between the along-shelf subinertial currents and synoptic winds at PA1, showing a local response (almost in phase) to the wind stress. At PA2, the correlations between winds and currents are generally not significant, showing that the subinertial current variability near the shelf break is forced by other mechanisms; the mesoscale variability of the Brazil Current is most likely important at this position. A simple frictional balance model allows a dynamical interpretation of the significant correlations between winds and currents at PA1.
The age and the flushing time of the Great Barrier Reef waters
2013, Continental Shelf Research
Citation Excerpt :
The resulting feedbacks between scales are complicated. The large scale oceanic circulation influences the small scales circulation around individual reefs, while small scale circulation in and around a reef matrix influences the circulation at large scales by the sticky water effect (Wolanski et al., 2003; Andutta et al., 2012). Dight et al. (1990) used a 9.26 km mesh size to simulate currents and larval dispersion in the GBR.
A numerical model of the Great Barrier Reef (GBR) was verified using water current data from twenty sites, and applied to estimate the flushing time and age of waters. These timescales were calculated under different wind and oceanic inflow conditions. The age of oceanic waters intruding in the GBR was estimated to be between 3 and 5 months on leaving the GBR, depending on the location; the largest residence time prevailed in the southern GBR matrix where the age was the highest within the high density reef matrix, indicating veering of the mean currents around the GBR matrix. The flushing time depends on the size of the domain, and was estimated to be 67 days for the whole central GBR. For the flushing of coastal waters the wind had two effects. Firstly, it increased the flushing times by generating wind-driven currents, transporting some water back to the source; this process is comparable to that of an estuary, where water that leaves the estuary at ebb tide may return at flood tide, a process parameterized by the return coefficient. The return coefficient in the GBR due to wind reversals may be as large as 50% at the timescale of the wind. Secondly, in the southern and central regions of the GBR, the southeasterly tradewind deflected the southward flowing oceanic inflow seaward, away from the inner shelf and towards the outer shelf, making room for a wind-driven current of opposite direction on the inner shelf. Thus the intrusion of oceanic water in the GBR depends on the wind over the GBR. The veering of the mean currents around the GBR reef matrix and the wind over the shelf influencing the oceanic circulation demonstrate small scales (the GBR shelf) influencing the large scale oceanic circulation.

View all citing articles on Scopus

View full text

Chapter 15 Merging scales in models of water circulation: perspectives from the great barrier reef

Publisher Summary

Deep-Sea Research

Continental Shelf Research

Estuarine, Coastal and Shelf Science

Continental Shelf Research

Continental Shelf Research

Estuarine, Coastal and Shelf Science

Estuarine, Coastal and Shelf Science

Water masses, nutrient levels and seasonal drift on the Outer Central Queensland Shelf (Great Barrier Reef)

Australian Journal of Marine and Freshwater Research

Seasonal and local spatial patterns in the upper thermal limits of corals on the inshore Central Great Barrier Reef

Coral Reefs

A description of the three-dimensional coastal ocean circulation model

East Australian Current adjacent to the Great Barrier Reef

Australian Journal of Marine and Freshwater Research

A permanent under current adjacent to the Great Barrier Reef

Journal of Physical Oceanography

Mixing in Inland and Coastal Waters

Shallow-water frictional effects in island wakes

Estuarine, Coastal and Shelf Science

Modelling eddy formation in coastal waters: A comparison between model capabilities

MECCA Programs Documentation

Climate change, coral bleaching and the future of the world's coral reefs

Marine and Freshwater Research