The curious case of Tropical Cyclone Winston
As a surfer growing up in the 80s I often heard stories about the Gold Coast's famous 23 day swell of 1975. It was lore up there in Queensland, a tale that kept getting retold, yet as I got older I began to suspect that maybe it had a touch of the porkies about it. I mean, 23 days from one single swell source?
Most tropical cyclones, at least the ones that form in the southern Coral Sea and provide the east coast with swell, are flash in the pan events. If there's no other atmospheric interference a cyclone will head south and then very quickly be absorbed into the mid-latitude westerly airflow and begin to decay. The opportunity to provide swell, therefore, is minimal.
Which is why Tropical Cyclone Winston is a curious fish. Formed on February 11th, TC Winston has been circling around the far reaches of Queensland and northern NSW's swell window for a fortnight, and except for a short pause behind New Caledonia it's provided waves the whole time.
Fourteen days of swell with the likelihood of many more to come? All of a sudden that 23 days swell doesn't sound so ludicrous...
To get a bit of context I spoke to Professor Ian Goodwin about the curious case of Cyclone Winston.
For the first week of its life Winston behaved normally. It formed near Vanuatu and moved south into our swell window, the first lines making landfall on the 15th of February. "The really interesting thing," says Ian, "is that Winston was taking the track of a normal cyclone. We'd normally expect it to keep heading south and disspate in the mid-latitude westerlies, except it got captured by an extremely strong high pressure system."
It's the influence of this high pressure system that's caused Winston to behave in an unusual manner. The high was centred over New Zealand with strong easterly winds blowing across the top of it.
These winds are a "classical easterly dip" says Ian. "Think about the formation of an easterly trough low, or an ECL [East Coast Low], they form in that dip, except they often occur in the middle of the Tasman Sea. However, this time a sub-tropical high pressure system has an easterly dip." The result is that Winston was captured just before it was lost to the mid-latutide westerlies. It then got a second wind, so to speak.
"The fuel for this one is not coming out of the tropics, it's coming out of the sub-tropics. It's a weird hybrid of a tropical and sub-tropical storm system."
Caught by the strong high pressure system, Ian says the "moist easterly airstream continued to fuel it, and because the high pressure system wasn't going anywhere the only exit for Winston was a path back toward Australia." Although cyclones are notoriously unpredictable that's the exact opposite of what you'd expect to happen.
The path of Winston, first moving south, then quickly north-east, before being captured and sent back west
"It's what we call a retrograde system," says Ian of the east-to-west movement of Winston, "and this is retrograding a long way." As Winston retrogrades a 'captured fetch' scenario develops whereby the storm moves in the same direction as the wind creating a compounding effect with swell production.
Interestingly, as big wave surfers in the North Pacific bow down at the feet of El Nino, Ian believes it's the little boy's sister who we have to thank.
"Yeah, La Nina is the wildcard. A strong sub-tropical easterly," such as what captured Winston and caused it to retrograde, "is a signature of La Nina."
"Everyone was waiting for this big El Nino signal which really wasn't there," says Ian "And it's slipping back into this persistent La Nina mode and that's what's caught a lot of people off guard."
"The Pacific is slipping back into La Nina because of the Decadal La Nina pattern. This system is potentially a precursor of moving back into La Nina."
When we spoke, Ian was clearly excited by the nature of Winston though not necessarily surprised. "Yeah, these kind of systems do happen" said Ian when I asked if it was an anomaly. "In fact, they happened more when we were last locked into a La Nina background phase which was the 60s through to early 70s."
Maybe the old boys were telling the truth all along?
STOP PRESS: Phil Jarratt, a man whose memory should never be doubted, said his "recollection of the 23-day swell is that it was back to back TCs, but they overlapped and there was no day off. Kirra was off and on a bit but Burleigh just pumped."