LONG-TERM SEA-LEVEL VARIATIONS IN THE CENTRAL GREAT-BARRIER-REEF

Low frequency sea-level variations and associated geostrophic currents in the central Great Barrier Reef(GBR) region near Townsville are stu died using optimally-lagged multivariate regression. The analyses show that pressure-adjusted coastal sea levels and mid-shelf geostrophic c urrents are influenced predominantly by local along-shelf wind stress at the weather time-scale, and by climatic variables, such as atmosphe ric pressure and temperature, at seasonal and interannual time-scales. These forcing variables can specify sea levels over annual and inter- annual time-scales with a forecasting skill of 0.53 and 0.22, respecti vely (where 1.0 is perfect skill). Associated along-shelf geostrophic currents can be forecast with a skill of 0.57 over an annual time scal e. If, instead, absolute coastal sea levels or offshore sea-level diff erences are used to specify the along-shelf geostrophic current, the f orecasting skill is 0.75. A characteristic El Nino/Southern Oscillatio n (ENSO) response is detected for time periods up to 25 years in month ly sea-level both at Townsville and at western Pacific island sea-leve l stations. This spatially coherent response varies in intensity and p hase within the Coral Sea. Sea-level differences show a pattern which characterizes known features of the large-scale circulation of the Cor al Sea. These very low frequency sea-level variations in the Coral Sea must be taken into account to obtain accurate predictions of along-sh elf geostrophic current variations on seasonal and inter-annual time s cales. Regression analysis and a diagnostic river plume model show tha t the influence of the major rivers can produce sea-level changes due to buoyancy of order 5 cm. The corresponding errors in geostrophic vel ocities estimated using pressure-adjusted Townsville sea-level data al one are of order 5 cm s(-1) rms.