Swellnet Dispatch Mythbusting: Cyclones and Swell

In: Swellnet Dispatch by Stu Nettle 16 Comments Mon 21st Mar '11

Tags: cyclones , meteorology , surf forecasting , tropical cyclone sose , severe tropical cyclone zelia

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OK, it's mythbusting time!

Last October various Australian weather forecasters predicted that there would be 15 tropical cyclones in our waters during summer. That's five more, or an increase of 50%, than an average year. Their predictions were based upon the formation of the la nina weather phenomena that affected eastern Australia during the summer. During la nina years warmer than average water pools off the east coast of Australia providing conditions conducive to tropical cyclone creation.

As soon as the news story was published it went viral in the surf world. It was tagged and posted on many surfers' Facebook walls and was given column space on almost all Australian surf sites. The reason the story was picked up with such fervour is the connection that surfers make between cyclones and good waves. Due to the connection it became accepted knowledge that summer 2010/2011 would be an above average swell season.

And this is where the mythbusting begins...

While it's true that cyclones may create great waves the odds of them doing that are actually very low. Simply having a cyclone in the swell window doesn't ensure good waves. For that to happen there has to be a set of associated weather phenomena also occurring and the cyclone has to be acting in a particular manner.

A cyclone, acting in isolation from any other weather system, is an extremely small area of strong winds. The fetch (the distance the wind blows over water to create swell) can be as little as 50 kms, which makes it a veritable pinprick in the ocean.

The best chance a cyclone has of creating a significant swell is when it butts up against a high pressure system and creates a pressure gradient with winds blowing over a large section of ocean. During summer and autumn high pressure systems can form and stall in the Tasman Sea, the positioning of them influences the easterly trade wind belt that supplies much of the background swell to SE QLD and NE NSW during these seasons.

The ideal scenario is for a cyclone to form in the Coral Sea while there is a strong high in the Tasman Sea, the combination of the two creating a pressure gradient directing strong winds toward the coast. When this happens (as it did during TC Jasper last March) it is usually the cyclone that gets all the credit, however the high pressure system is equally important for the creation of the swell.

During the 2010/2011 season none of the cyclones worked in conjunction with a high pressure system as described above.

Another aspect that can prevent cyclones from creating good waves is their speed and track. As mentioned above, cyclones in isolation are very small areas of strong winds. If a cyclone stalls and remains in position for a time then it can theoretically create a reasonable swell, albeit for a small stretch of coastline. Most cyclones however, are perpetually moving which reduces their singular ability to create a swell. The faster the cyclone moves the less chance there is for it to create a swell.

A good example of this was Tropical Cyclone Zelia. In fact Zelia was a Severe Tropical Cyclone, such was the strength of its winds. Zelia tracked from north of New Caledonia to New Zealand's North Island passing through all of Queensland's and NSW's swell window yet it didn't create a wave over three feet. The reason for that was its speed. During the two days it was a classified STC, and hence at its strongest, it travelled 2000 km's and moved too quick to build up any significant swell.

As of March 21 there have been 10 cyclones in Australian waters, equal to the amount in an average year. While the cyclone season still has a few weeks to run – one of the biggest swells to ever hit the Gold Coast was from Tropical Cyclone Sose in mid-April 2001 - it is fair to say that whatever happens the season has come in well under expectations. It also busts the myth that cyclones always create great waves.

Postscript: For a shining example of how cyclones can turn normal surfers into blithering fools read this article.

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