Modelling circulation and connectivity on Australias North West Shelf.

Ocean circulation on the North West Shelf (NWS) influences nearly all aspects of the ecosystem, including sediment transport and turbidity patterns, primary production in the water column and bottom sediments, and recruitment patterns for organisms with pelagic phases in their life cycles. Current patterns are also of direct interest to most industries operating on the NWS, particularly those associated with fisheries, shipping, and offshore structures and operations. This study is the first attempt to describe the water circulation and transport patterns across the region on time scales from hours to years, and space scales from 10 km over the entire shelf to one kilometre in a selected focus area around the Dampier Archipelago. It has also provided a framework for embedded models describing processes such as sediment transport, nutrient cycling, and primary and secondary production (described in accompanying reports). A series of nested circulation models have been developed with forcing by realistic winds, tides, and large-scale regional circulation. The simulations cover a period of more than six years, allowing the tidal, seasonal, and interannual characteristics to be investigated, as well as the response to episodic events such as tropical cyclones. Connectivity patterns throughout the shelf have also been characterised by forcing a particle transport model by the modelled circulation. Model results demonstrate that the instantaneous current patterns are strongly dominated by the barotropic tide and its spring neap cycle. However, longer-term transports over the inner and mid shelf were mainly controlled by wind-driven flow, which followed the seasonal switch from summer monsoon winds to southeasterly trades in winter. Over the outer shelf and slope the large-scale regional circulation, provided by the global model, had a major influence. Results were shown to be relatively insensitive to adjustable model parameters and submodel structures. However, model performance was strongly dependent on the quality of the forcing fields. For example, the prediction of low frequency inner shelf currents was improved substantially when the relatively coarse resolution global winds where replaced by locally observed winds in the Dampier model. Lower skill in predicting low frequency currents on the outer shelf can be largely attributed to errors in the global circulation model. Results from the connectivity modelling have been summarised in statistical form and can be accessed through a web-based user-interface developed as part of the project and referred to as the Connectivity Interface or ConnIe (http://www.per.marine.csiro.au/connie). This tool is expected to find applications in areas such as larval dispersion and recruitment studies, and the development of scenarios and risk assessments for contaminant dispersion.