Roaring Against The Forties
Here at Swellnet, we’ve been referring to the weather system that created the recent south swell in the Tasman Sea as the ‘Southern Gyre’ - note the capitals.
The system deserved a title as, along with spawning a week-long south swell, it displayed other idiosyncrasies, such as a very slow rate of travel, and a multi-centred configuration. Each of these aspects aided in creating the many pulses of swell that struck the whole Tasman basin, from Eden, to New Caledonia, Fiji, and Raglan
However, in a final act of showmanship, the Southern Gyre sent swell far beyond the basin, not onwards into the Pacific, as you might expect, but unexpectedly backwards towards unusual nooks of Tasmania and the Australian mainland.
Weather systems travelling through the Roaring Forties move west to east, and as there are no landmasses to slow them down, they move relatively fast in that direction. Hence, most swells borne from those weather systems also move west to east, with diffraction causing the larger swells to travel NE into the various ocean basins: the Pacific, Indian, and Atlantic, plus the Tasman Sea basin.
These are the classic southwest swells of the southern hemi winter - the bread and butter swells of all west and south facing coasts in this hemisphere. In winter, these coasts can go months without the swell needle deviating from SW, and even then it’ll only be by a few degrees this way or that.
Back to the Southern Gyre…
The largest swell from the system - which struck the East Coast late last Sunday - was created while the centre of the low was triangulated equidistant between Tasmania, NZ’s South Island, and the polar shelf. Four days later the Southern Gyre had inched just 2,000km east, poised for one last burst of swell.
Wind graphs from Tuesday 14th June show a broad fetch of wind skirting the polar ice shelf blowing southeast, against the grain of Roaring Forties traffic, back towards Australia (see image below).
This is one of the more unusual swell windows for Australia, simply because it’s so rare.
This is the swell that struck the East Coast on Friday the 17th and delivered the last barrage of a prolonged campaign. Compared to the previous week of swell, it arrived at a more southeasterly direction, allowing a greater spread of size into southerly corners that had largely missed out during the bulk of the swell.
However, because of its unusual origin, the swell also sent waves into long-dormant corners of Tassie’s east coast and the same with Bruny Island. It then bounced through the Bass Strait islands much as long-period northwest swells pass between California’s Channel Islands. Swellnet heard reports of southeast swell on Victoria’s Mornington Peninsula and the Surf Coast. Both coasts get occasional summertime wind swell from the southeast but long period lines are almost unheard of.
It would've been an ideal swell to chase down those quiet corners of Bass Strait. The strait most commonly gets swell from one direction but on this rare occasion the direction was reversed.
Around the same time the swell was hitting the East Coast of NSW, it was registering on the Cape Sorell buoy offshore from Macquarie Harbour on Tasmania’s west coast - see image below. With no clear line between Sorell and the source, the long-period swell must’ve refracted around Tasmania’s South West Cape before moving northwest towards Cape Sorell and the mainland.
24 hours later, on Saturday the 17th, the swell appeared on South Australia’s Cape du Couedic buoy, tracking from the southeast at a period of 16 seconds. That’s an extremely rare combination of direction and period for that part of the world, and the swell was captured by local photographer Andy Smyth.
By that stage the swell had travelled approximately 5,000km from the source. Australia’s southern coastline has a sparse network of waverider buoys, so the next available place the swell could make itself known was Esperance, a further 1,500km west. Despite being a weekend, the Swellnet office was mildly abuzz tracking the swell across the Bight, yet for reasons unknown the swell failed to materialise on the Esperance waverider buoy.
Why it didn't show was a mystery, which is a fitting end for a mysto swell.