North West Shelf Joint Environmental Management Study:InVitro Inputs - Statistical extrapolation of currents

The application of the MSE modelling approach to the North West Shelf required three scenarios, for currents and contaminants, each spanning four decades to be simulated for many model runs. These scenarios were loosely defined as: 'high winds' to simulate the potential effect of stronger than normal winds; 'normal' where the distributional property of the winds were the same as those experienced currently; 'low winds' where winds were weaker than current conditions. Three-dimensional simulations of currents and the distributions of contaminants for such periods, at temporal resolutions of at least one hour, would require many months of computer time and, in addition, would generate large quantities of current data which would be impractical to incorporate into the agent-based simulation system InVitro. An alternate approach was required that was: 1) relatively fast, 2) made efficient use of limited computational resources and, 3) accurately represented the modelled currents throughout the study region. We chose to statistically model the currents by recognizing that near the coast the currents were primarily driven by the tides and elsewhere from a combination of tides, winds and boundary forcing. Tide and wind data were available but unfortunately we did not have ready access to the boundary forcing information - this is required in nesting the higher-resolution North West Shelf model within a coarse global circulation model. This was not considered to be a major deficiency as the boundary forcing (derived from a global General Circulation Model) was of coarse resolution with inadequate local representation of winds, bathymetry and water mass structure (Scott Condie, pers. Comm.). Our analyses were therefore focused on statistically modelling the currents and contaminants forced by local tides and winds.