INTRODUCTION
There is little information on the size and type of swells being generated in the Southern Ocean. This is because of the vastness of the ocean accompanied by low shipping numbers and a generally small coastline population. extending across the Great Australian Bight and down Tasmania's southern coast. However there is a need for a greater understanding into the nature of the swells developed over one of the world's longest fetches, and the establishment of a forecasting system for these swells. This information will benefit fishing and shipping industries as well as water sport such as surfing and in the long term could lead to the exploitation of the power of the ocean.

SWELL MECHANICS

Solar radiation leads to an increase in the air temperature which in turn results in pressure systems forming and thus the generation of winds. As well a positive feedback occurs between the wind and the temperature of air masses resulting in the general circulation of the atmosphere. Ocean swells are generated from the wind blowing over vast areas (ie. fetch). The wind's energy is accumulated into choppy waves, which further from the storm centre form regular swell lines. The size of the swell is dependent upon the length of fetch and the duration and intensity of the wind. A number of models have been proposed for wave generation by wind. Two complementary models are the resonance model and the shear flow model. The resonance model is based on pressure fluctuations and the irrotationability and turbulent nature of wind (Dr. O M Phillips 1957). The shear flow theory states that wave motion is induced by tangential shears and in this case vorticity is implicit.

The weather pattern in the Southern Ocean leads to a high swell regime due to winds roaring in an easterly direction around Antarctica embedded with storm systems and polar fronts. For increased accuracy this generality must be replaced with concise data so the wave climate can be assessed leading eventually to swell forecasting. Currently the CSIRO is deploying ocean wave measurement buoys off the West Coast of Tasmania.

FORECASTING SWELL

The establishment of the wave climate at sea is notoriously difficult. To measure the periods is the most straight forward. Wavelengths can be calculated from a ship or the shore. However to measure wave heights using a buoy, damping discs must be attached to the buoy at a level below the surface where there is no wave induced motion. Buoys are situated off Tasmania to allow the swell conditions to be employed in coastal weather services. By repeating this system over a vast grid system across the Southern Ocean and using coastal ports as well, a definitive model could be made of a high and low swells on a particular day.

Having established a data base for collecting swell information and understanding the physical mechanism of ocean wave generation, propagation, and attenuation, the next step is to relate all these aspects to other meteorological tools. It is proposed that current synoptic charts and satellite information are correlated with the present swell data. This would illustrate which low pressure systems were generating the largest swells and how a region of low swell may be due to a blocking high. For example one of the largest swells ever recorded on the Californian coast was due to a twin storm system in the North Pacific off the Aleutian Islands.

Once recorded data is related to meteorological information, the important phase is to correlate these two sources in respect to the pace of systems moving across the Southern Ocean and the rate at which the swell is increasing or decreasing. The buoys relaying the swell size and wavelength must do so consistently over a time period. By tracking many low pressure systems using satellite data and other means, it would be possible to show what type of lows at particular latitudes sustained swells. Different types of swells generated could be matched to cold fronts or deep lows. With enough data collected, long term averages could be established and seasonal fluctuations accounted for. by observing increased ocean swells in winter and a higher frequency of cold fronts in Southern latitudes and the northerly migration of high pressure systems.

Satellite imagery is a most important tool for defining lows and pools of very cold air which can be responsible for large swells. These cold air pools appear as a collection of white dots on infra-red imagery representing cumulus cloud, while lows are swirls of white cloud often with an eye present. Jet streams also require analysis for their effect on swell production. Over the Southern Ocean, jet streams occur at the 30-40s latitudes, varying from day to day. A single jet stream may last for a number of weeks. Analysis of these winds occurring at 500 mB altitude has recorded wind strengths up to 300 knots. The presence of jet stream may alter the pace of high pressure systems and in turn dictating the swell size by allowing cold fronts to slide away or to move through faster.

With adequate research to correlate ocean swell with observed lows, the major step of forecasting the generation of swells and their duration is possible. It would be feasible to establish a swell prognostic chart based on all the available data and research. Like prognostic synoptic maps, a certain amount of personal skill would be required by the forecaster using evidence available and previous experience.

Another potential method is correlative forecasting of ocean swells. This is based on the notion that swells of the magnitude and frequency are generated similar to ones observed beforehand when a similar synoptic situation was present. This method requires the matching of a current weather pattern and swell size to previous occasions where a similar sequence of events occurred. This method is employed roughly by a lay surfer using his own recollection of past events when trying to forecast swells from a meteorological chart. By employing a computer with the correlated data base of ocean swell, synoptic charts and satellite information over a few years it would be possible to achieve quite a degree of accuracy.

The feasibility of forecasting ocean swells is dependent on the accuracy of gathering data. Here monetary considerations come into play, but the benefits would be substantial. With increase awareness of ocean swells and their behaviour, the shipping and fishing industry could be made safer. The practicality of using the ocean as a viable energy source in the long term could also be investigated. Beach erosion prevention could be aided with the knowledge of the wave climate. Finally the surfing fraternity would also reap the rewards of an ocean swell forecast.

ADDENDA

The Significant Wave Height reading of the Cape Sorell buoy in meters equates to the the swell size in feet at Bells Beach 9-12 hours later. This theory is most accurate for medium swell conditions ie Sorell reading between 1.5 and 5 meters.

On the ocean beaches the best sets appear when there is little or no swell. This theory is more relevant in the warmer months.
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