Coral Reefs (1985) 4:47-57 Coral Reefs 9 Springer-Verlag 1985 Long-term observations of currents on the central Great Barrier Reef continental shelf E. Wolanski and G.L. Pickard * Australian Institute of Marine Science, P.M.B. No. 3, Townsville M.S.O., Queensland 4810, Australia Accepted 12 December 1984 Abstract. Time-series are presented of wind, sea levels, currents and temperature in the central region of the Great Barrier Reef continental shelf over the period 1980 to 1982 (inclusive). Except for occasional periods of calm weather or during the passage of tropical cyclones, the wind was alternately southwestward or northwestward, with periods of several days to several weeks. The fluctu- ations of the low-frequency sea level, longshore current and wind components, but not temperature, were highly coherent over distances > 400 km longshore and over the shelf width. The temperature and the atmospheric pres- sure controlled much of the seasonal changes in the sea level. A simple model for wind-driven circulation using a non-linear bottom friction law is able to account for most of the variance of the currents with periods <20 days. An additional poleward base flow was present at all sites, probably driven by the circulation in the Coral Sea. There were also large interannual variations, possibly associ- ated with the E1 Nifio Southern Oscillation phenomena. These observations imply that the advection and disper- sion over the Great Barrier Reef of fish eggs and larvae, coral planulae etc., and hence reef recruitment, experi- ence considerable variability at time scales of days of years. Introduction The topography of the central region of the Great Barrier Reef (GBR) continental shelf is complex (Fig. 1). The iso- baths and the coastline are parallel and oriented S.E.- N.W. The shelf is roughly 60-120 km wide, the outer 30 km in the southern section comprising the GBR ma- trix of widely scattered coral reefs with wide and deep (depth >40 m) passages between them. Between the shore and the GBR matrix is the GBR lagoon where the * Permanent address." Department of Oceanography, University of Brit- ish Columbia, Vancouver, B.C., Canada V6T 1W5 water depth increases from roughly 10-20 m nearshore tO 40 m near the western edge of the GBR matrix. The con- tinental slope offshore from the GBR matrix is very steep. The tidal currents on the shelf are generally small (peak currents <0.2 m s -a) and generally smaller than the low-frequency currents (periods _>_2 days). The low- frequency currents on the shelf appear to be controlled by at least two agencies. Firstly, low-frequency longshore currents on the shelf are also generated by the wind which, in the southeast trade-wind season (typically March-October) blows over the shelf primarily longshore northward, being coherent and stationary over distances > 1,000 km (Wolanski 1982; Wolanski and Bennett 1983). Using 6 months (June-November 1980) of wind, sea level and current data at a number of sites including those shown in Fig. 1 but excluding the mooring sites added in 1981 near Liz- ard Island and Linnet Reef, it was shown (Wolanski and Bennett 1983) that, for the period of observation (when the longshore wind component was always equator- ward), 9 the low-frequency longshore current fluctuations are large (peak to trough < 1 m s- 1) and in geostrophic bal- ance with the cross-shelf sea level slope; 9 the low-frequency disturbances of longshore currents and sea level (the latter are < 0.35 m peak to trough) are highly correlated with the longshore-wind component and propagate longshore equatorward at a speed ~- 450 km day- 1; 9 the spectra suggested that most of the variance, which could not be resolved, was due to events of periods > 14 days. Secondly, the poleward East Australian Current (EAC), with speeds typically of order 0.3 m s-1 as infer- red from geostrophic calculations, prevails on the conti- nental slope from the surface down to 200 m everywhere south of 14~ latitude, with an equatorward return flow