The age and the flushing time of the Great Barrier Reef waters

Abstract

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.