About the Models

About the models

The eReefs Marine Modelling component has delivered a modelling suite capable of predicting the hydrodynamic, sediment transport and biogeochemical characteristics of the Great Barrier Reef. Hydrodynamic models are capable of predicting circulation, mixing, transport, connectivity, temperature and salinity. Sediment transport models predict the transport and fate of suspended sediments, and exchange of sediments between water column and bed. Biogeochemical models supply predictions of the cycling of nutrients, carbon and dissolved oxygen through plankton and benthic primary and secondary producers. The models address multiple spatial scales, ranging from whole of GBR (thousands of kilometres) to individual reefs and estuaries (hundreds of metres). To achieve this, the models are implemented within a nested suite, whereby a regional 4 km resolution model is nested within a global general circulation model, and a 1 km resolution model is subsequently nested within the 4 km model. These models provide outputs of sea level, currents, temperature, salinity, suspended sediment, primary and secondary production and nutrients throughout the GBR domain. The models operate in near real-time, such that current conditions may be monitored, and a hindcast archive exists back to September 2010. The hindcast is kept up-to-date by continuously appending the near real-time outputs. Furthermore, a Relocatable Coastal Ocean Model (RECOM) has been developed which is an automated re-locatable modelling system capable of generating high resolution models of hydrodynamics, waves, sediment transport and biogeochemistry that are nested within the 4 km or 1 km regional models. This package is designed for non-specialist users, whereby the user simply and intuitively interacts with the models via a graphical workflow interface.

The eReefs models have the ability to deliver information that addresses the scientific issues surrounding the GBR lagoon and reef; i.e. delivery of nutrients from the catchment, processing of those nutrients by estuaries, eutrophication potential, sediment transport, hypoxia, reef acidification and coral bleaching. Most of these issues require coupled benthic-pelagic sediment transport and biogeochemical (BGC) models to deliver the required system understanding and information. In order to successfully address these issues, the models are required to operate in near real-time and hindcast mode. The approach is to append near real-time results to an ever growing archive so that the models always remain current. Currently the 4km model archive begins in September 2010 and the 1 km model archive in December 2014.

Modelling framework

The modelling framework adopted for this study involves nesting a regional hydrodynamic model within a global circulation model, in order to provide accurate forcing data along the boundary within the Coral Sea, where mesoscale oceanographic processes strongly affect exchange between the Coral Sea and the GBR Lagoon. The regional model covers the continental shelf from the NSW border to the mainland of Papua New Guinea, extending eastwards into the Coral Sea Territories a sufficient distance to avoid the topographical complexities of the Queensland and Marion plateaus. The regional model is intended to simulate the dominant oceanographic processes impacting on the GBR shelf, including the significant mesoscale features of the South Equatorial Current (SEC) and the current jets that result from the interactions of the SEC with the Queensland and Marion Plateaus. The average resolution of the regional model is ~4 km. Within this regional grid, a more finely resolved model (the shelf model) with resolution of ~1 km is nested to cover the continental shelf areas and the adjoining tract of the Coral Sea. Output from the regional model is used to force the boundaries of the shelf model.

Due to computational constrains it is not possible to run sediment transport and biogeochemistry directly coupled to the hydrodynamics model. A transport model is used to achieve the same objective; this model uses currents and mixing variables stored offline from the hydrodynamic model to advect and diffuse the sediment transport and biogeochemical variables. Since this model uses an unconditionally stable semi-Lagrangian advection scheme, the time-step can be significantly increased over that used in the hydrodynamic model, leading to orders of magnitude increases in runtime that allow long term simulations to be achieved.

Coastal locations or individual reefs may be modelled at high resolution (< 1km) to capture important small-scale processes and dynamics using the relocatable coastal model (RECOM). This model can be deployed anywhere within the domain with minimal user input, making it useful for the development of site-specific coastal models by non-specialist users. RECOM includes sediments and biogeochemistry, and allows for the construction of more complex curvilinear grids. RECOM is currently one-way nested into the regional models.