Australia: An Unstable Platform for Tide-Gauge Measurements of Changing Sea Levels

Abstract

Twenty-five tide-gauge records non-uniformly distributed along the coast of Australia were used to evaluate the stability of Australia as a platform from which to measure changes in sea levels. The spatial and temporal variabilities of relative sea levels were defined using a multivariate analytical tool (eigenanalysis) on 13 of these stations. Low-frequency (period 20 years and greater) relative sea levels are believed to reflect the combined effects of subsistence due to sediment and water loading, subsidence due to thermal cooling of oceanic crust, and emergence along a convergent plate boundary. Global eustatic sea-level rise cannot be identified unambiguously from these data, but clearly it is not responsible for the observed patterns of relative sea level. The straight average rate-of-rise of relative sea level around Australia is 1.3 mm/yr. (submergence), with general submergence in the south (at an average rate of 1.8 mm/yr.) and emergence in the north (0.5 mm/yr., the latter an average of two stations). Australian tide-gauge data cannot define unambiguously past eustatic sea-level history because of this considerable spatial non-homogeneity in relative sea-level rise, but they may be useful for monitoring possible future increases in rate of global sea-level rise. Higher-frequency fluctuations in relative sea levels show relative peaks at 20, 12, and 6 years, with significant energy at higher frequencies as well. Higher frequency fluctuations correlate with patterns of El Niño/Southern Oscillation. During El Niño events, yearly-averaged sea levels of Australia fall, although not all local minima in relative sea levels correspond with El Niños. The resultant drop in relative sea levels represents a balance between lower sea levels due to raised atmospheric pressure with coincident lowered water temperatures, and higher steric sea levels due to increased precipitation and runoff (for those stations located in estuaries).