Quality of Shelf Water Oscillations at Macquarie Island in the Antarctic Southern Ocean: The Anomalous Six minute Wave

Concerns about rising sea-levels due to global warming have increased the demand for a better understanding of coastal environments. The effects of local phenomenon such as shelf trapped waves may bias mean sea-level measurements and affect circulation and exchange processes important for coastal water biology. Historic observations have recorded oscillations of around 6 minutes with a beat of 3 hours at Macquarie Island in the Southern Antarctic Ocean (lat 54 deg 37’ S, long 158 deg 52’ E); these were theorised to result from two counter-rotating edge-waves of slightly different frequencies. This study provides an alternative hypothesis suggesting the oscillations correspond to the fundamental ‘natural’ shelf period, with an envelope of period’s characteristic of local coastal topography and bathymetry. New data collected over the month of June 2004, from a shore-mounted pressure transducer, was analysed using conventional Fourier and Wavelet techniques. The results agree with earlier observations and show the phenomenon to be highly variable contained within periods of about 7.4 to 5.5 minutes. The forcing mechanism is not clear with the oscillations occurring in bursts every 1 to 4 hours. Any bias to mean sea-level is thought to be insignificant. Local morphology is important for understanding the impact shelf anomalies may have on coastal water environments.