THE PANORAMA OF THE TIDES

It is important to appreciate fully the synchronization of the tidal action around Australia's coast which subjects many shores in turn to high and low waters. Hourly values of tidal heights recorded at the principal stations between 0000 hours GMT on 1. January and 1100 hours GMT on 18. January, 1970 characterize this phenomenon apart from seasonal variations which are due to mean sea level changes caused by atmospheric pressure variations and winds. The following observations refer to this time interval.

In order to focus attention on to pronounced features of this phenomenon, it is convenient to group stations according to tidal ranges and their semi-diurnal and diurnal types . For a complete understanding of the tidal phenomenon, one finds in greater resolution the Neap Tides and Spring Tides. During the Neaps, the sea levels along substantial sections of the Gulf of Carpentaria, around the South-West, along the Southern part of the East Coast experience relatively small oscillations, while the pulse of the tide continues to beat strongly in the North-West as far as Darwin, around Mackay and in Bass Strait. On the other hand, only in the South-West does the tide fail to rise substantially during the Springs.

The Spring Tides readily permit an assessment of the differences in the times of occurrence of high and low waters. Note that these curves refer to Greenwich Mean Time (GMT), so that local times must be taken into account. Eastern Standard Time (EST) is 10 hours, Central Standard Time (CST), which applies in South and North Australia, is 9 hours 30 minutes and Western Standard Time (WST) is 7 hours ahead of GMT. These times may be changed during the Summer by introduction of Summer time.

Apart from the approximately three hours delay in the times of occurrence of high and low waters around Mackay (MK), the entire East Coast has high water simultaneously on 8. January at 1100 GMT or 2100 EST. The only other major exception in this pattern occurs at Brisbane (BB), located deeply inside Moreton Bay and up the Brisbane River. Through Bass Strait and into St. Vincent and Spencer Gulfs high water was delayed progressively until it occurred at Thevenard (TV) on 8. January 1970 at 1500 GMT or on 9. January 1970 at 0300 CST. Pursuing the same high water through the diurnal zone of the South-West and up the West Coast, one finds that beyond Broome (BM) large changes take place. High water at Darwin (DN) was either 5 hours after or 7 hours before high water at Brome (BM). The diurnal region of the Gulf of Carpentaria separates the tides of the East and West Coasts and complicates the process of tracing corresponding times.