MEAN SEA LEVELS

At anyone tide recording station arises invariably the question regarding the reference level of the measurements. Especially in outlying regions, in particular on islands, the datum of a tide gauge is quite arbitrary. Once the data have been harmonically analyzed and mean sea levels computed, one can find the gauge's Indian Spring Low Water (ISLW).

When mean sea levels are evaluated over shorter periods, for example, over a month, their values differ from the longer period averages. Through Earth's geological past, mean sea levels computed over a year or longer would also have varied, for example during ice ages due to accumulation of ice on land. These means exhibit seasonal waves, due to the presence of Meteorological Tides. Along the South Coast, the curves for the three years are similar. Their features change gradually through Bass Strait and around the West Coast. In the North, the largest changes occur in the Gulf of Carpentaria.

Two processes affect these mean sea levels: Barometric pressure and wind. The former can be compensated for by subtracting or adding 1 cm for every rise or fall of 1 millibar, when there arises immediately the question regarding the location where the pressure should be recorded. Unfortunately, there are around Australia's coast many areas where such corrections still do not account for all mean sea level changes.

As a second step, attempts are often made to compute a further correction based on Wind Stress, i.e, the action of local wind on the sea surface. Again there arises the difficulty that the wind distribution over the sea is unknown; at times, the Geostrophic Wind is computed from weather charts.

Another source of information are Daily Filtered Mean Sea levels, i.e., levels from which all astronomical level variations up to and including diurnal periods have been removed. It is remarkable that these sea level variations, which in winter may exceed 1 m, extend along the entire South Coast and up the West Coast as far as Carnarvon (CN) as well as to Hobart (HT). They have also been detected recently in records from Macquarie Island, halfway between New Zealand and Antarctica. In Summer, the periods of these oscillations range up to 8 days, while they reduce in Winter to as little as 5 days. It is also evident that the oscillations along the West Coast are approximately out of phase with those along the Eastern South Coast.

The low and high pressure systems, shown on the weather charts above, are readily associated with the sea level variations during that period along the West and South Coasts. It is also clear that these Daily Filtered Mean Sea Levels are not explained simply by barometric pressure variations.In fact, global rather than local phenomena are involved which must be sought in the Roaring Forties and which also generate the Surf. Their storm systems generate long waves which have even been detected all the way up to Alaska many years ago by Walter Munk of the Institute of Geophysical and Planetary Physics of the University of California at La Jolla.

Along the North Coast, overall short period variations in the sea level are much less pronounced. In January, during the global Monsoon Season, the prevailing winds come from the North and drive for several months water into inshore regions including the Gulf of Carpentaria and Bonaparte Gulf. As a consequence the Daily Filtered Mean Sea Levels at Weipa (WP) are consistently above the longer period mean value. In Winter, during the dry season, when the prevailing winds come from the South-East, water is driven out of the northern gulfs and the sea levels drop below their long term means.

It can now be concluded that the variations in the Monthly Mean Sea Levels along the North and South Coasts of Australia are generated by different mechanisms. In the North, for about half a year, Mean Sea Levels are high, for the other half low, maxima and minima occurring early and in the middle of the year, respectively. Along the South coast, the oscillations in the Mean Sea Level accelerate as Winter approaches and their amplitudes are higher in Winter than in Summer, whence the Monthly Mean Sea Levels have a minimum and maximum in Summer and Winter, respectively. The transition from the conditions along the South Coast to those along the North Coast is gradual.

The East Coast is not subject to as strong oscillations in the Mean Sea Level as the South and North Coasts, probably, because the oscillations generated in the Roaring Forties reach the Tasman Sea through Bass Strait. as well as around Tasmania, but with delays which cause attenuation.

These sea level variations affect strongly the width of the intertidal zone, which, in particular, is of great concern to yachtsmen and motorboat owners, when it comes to launching their crafts. Their magnitude can be assessed from the difference between locally predicted and observed tides. Although the maxima of these changes in sea level do not often occur during the Spring tides, engineers must allow for this possibility.

Extreme conditions occur during cyclones which occur frequently along the North Shore. Their effects are short-lived, but may be disastrous. At Townsville (TL), during Cyclone Althea on 24. December 1971, the predicted tide level was exceeded by more than 1 metre almost at high tide.

In South Australia's St. Vincent Gulf and even along the Gold Coast of the East Coast, the meteorological tides are such that during winter storms with high sea levels may occur frequently and denude beaches of their sand. It is very unlikely that such effects can be eliminated by breakwaters, groins or shore walls, largely because these conditions widen the intertidal zone. Developers should take good note of this fact and respect the need for wider shore zones bare of housing and other facilities, as they may demand expensive maintenance.

Independent evidence in support of these statements can be found along the South Coast, where, in the absence of settlements, the ocean has locally penetrated massive dunes and temporarily flooded the land behind them. Many of the salt lagoons along the eastern part of the Great Australian Bight may have been formed in this manner.

A favourable aspect of these mean sea level changes extending over several days is that it leads to renewal of inshore waters. In general, during ordinary wave motion, a volume of water moves to and fro with the period of the wave and little net transport occurs in the direction of wave propagation. On the other hand, as the period of a wave motion grows, water moved by it remains in new locations long enough for mixing mechanisms to occur. This is what happens in such land-locked regions as Port Phillip and Westernport Bays, the South Australian Gulfs, Bonaparte Gulf and the Gulf of Carpentaria. In the southern basins, the meteorological tides raise and lower mean sea levels over periods from 5 to 8 days; local diurnal land and sea breezes then can mix the injected water.

Similarly, in the northern gulfs, the maintenance of high sea levels for longer periods during the monsoons assures water renewal. The variations in salinity and the accompanying rainfall provide evidence that this process is at work in the Gulf of Carpentaria.