The Mechanics of a Swell: Why We've Been Bathed in South Swell

Stu Nettle picture
Stu Nettle (stunet)
Swellnet Analysis

If ever there was an argument that significant low pressure systems be named similarly to tropical cyclones then recent events in south-east Australia have provided it.

Early last week a large low pressure system stalled approximately 2000 kilometres south of Tasmania with a corresponding high developing in the Great Australian Bight. The systems remained almost stationary for the next seven days. While in position the low pressure system spawned multiple cold fronts that pushed northward toward South Australia, Tasmania, Victoria and New South Wales providing waves to all their exposed coastlines.

Since the weather pattern was established arguably more surfers have ridden waves created by the low than any Coral Sea cyclone. Surfers across the whole south-east of the country have been united by the same incredible weather event, yet without a name to reference it the event will slip from our collective memory. Which is a shame considering the incredible proportions of the swell.

The swell began last Wednesday although it wasn't until the weekend when it kicked into gear. On Saturday morning the Cape Sorell buoy on the west coast of Tasmania peaked at 18 metres. Around lunchtime the Hobart points were as big as they ever get. On the same day there were reports of 25 foot plus waves west of Cape Otway while the Bells / Winki stretch saw perfect 12 foot waves just before dark. By Sunday the the swell had pushed up the Tasman Sea and refracted toward the NSW coast. The Central Coast and Hunter regions benefitted most with 12 foot sets, although the whole NSW coast experienced significant waves combined with an ideal westerly wind pattern.

Seven days after its effects were first felt Victorian and NSW coastlines were still experiencing long-period 4-6 foot waves borne from the same weather system. It was a rare swell indeed.

For those asking why the swell was so rare the answer lies in the position of the Long Wave Trough (LWT). The mechanics of the LWT are explained in this article, however what needs to be understood is that the current position of the LWT corresponds with the weather system that made the swell. And very importantly, the LWT stalled and therefore the weather system stalled.

To get a graphic understanding of this take a look at the two synoptic charts: Chart 1 is from the beginning of the swell, Chart 2 from the end. Although they are six days apart the high in the Bight and the low south of Tasmania have made little easterly progress. The isobars in the Southern Ocean - the generators for this swell event - are pointing almost exactly the same direction.

For six days the wind blew the same direction over the same stretch of water, and therein lies the answer to the broad and sustained nature of the swell. Unfortunately for those surfers getting used to having south swell on tap the inevitable is occurring: the Long Wave Trough is now moving east and with it the weather systems that created the incredible event. //STUART NETTLE and CRAIG BROKENSHA