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.