What is the "Second Swell"?
Swells travel over large distances towards the Australian coastline, generated from weather systems located well away from Australia. Generally, there is a dominant or primary swell affecting the coast at any one time, however, at times two swells can be present along the coast. The Bureau's coastal waters forecast have traditionally focused on the larger primary swell. However, recent improvements in wave observations and computer wave models have enabled the Bureau to include the second swell in the forecast if relevant.
One way of grasping the concept of two swells is to think of two children throwing large rocks into either end of a pond. The waves will travel out from where the rock impacts the water through to the other side of the pond. Eventually all parts of the pond will have waves caused by both children. The two wave trains will interact with each other where their paths cross, generating higher peaks where the peaks of the wave trains intersect, and lower troughs where the two troughs meet. Even after the last rock has been thrown, the waves generated will continue to exist and interact for some further period until they decay.
Similarly, two (or more) different weather systems can generate different wave trains that then independently travel to your location. If the wind associated with each weather system has been blowing from the same direction and for long enough, then the waves become 'organised'. Once this occurs the wind that initiated the waves can cease (for example, if a low pressure system decays, or a front moves away from the area) but the waves that the wind has generated will continue propagating out from the source area as a swell. In this way, two swell wave trains generated by two separate weather systems can then cross paths at some location between the two systems. These swell waves were generated entirely separate to each other, but can exist within the same area.
An example
In the following example from 1 May 2011, there are two swell generating weather systems that are influencing the swell patterns along the Southeast coast of Tasmania. The first weather system is the low to the north of New Zealand that would be generating an easterly swell into the Tasmanian area. The second weather system is located along the bottom of the map highlighted by the succession of deep lows to the north of the Antarctic continent. These lows would be generating the strong west to southwest winds and the resulting southwest swells that are prevalent in the Tasmanian area.
The Coastal Waters forecast for this day indicated that two swells were present, a southwest swell that was expected to increase from 2 to 4 throughout the day, and an easterly swell of 1.5 metres. In the past when there were two swell wave trains meeting the coastal waters forecast would have been for a confused swell.
A second swell may be important when choosing your anchorage. For example, for the forecast conditions above, when choosing your anchorage you will need to seek an area that is firstly sheltered from the larger southwesterly swell, whilst still bearing in mind that the easterly swell, although smaller, may give you a rocky night if you are not sheltered from it also.
Second Swell on the forecast
There will not always be a second swell on the forecast. To be included as a second swell, the wave train will have to be a metre or greater. Additionally, if the height of the primary swell is significantly larger than the height of the second swell, then the second swell will not be included.
Rules used by the Bureau of Meteorology for including second swell.
- When the primary swell is less than 4 metres, second swell is included if it is greater than 1 metre and from a different direction.
- When the primary swell is between 4 and 6 metres, second swell is included if it is greater than 1.5 metres and from a different direction.
- When the primary swell is between 6 and 8 metres, second swell is included if it is greater than 2 metres and from a different direction.
- When the primary swell is between 8 and 10 metres, second swell is included if it is greater than 2.5 metres and from a different direction.
