The Tale Of The Endless Swell
The Tale Of The Endless Swell
The comments from disgruntled new arrivals to the Gold Coast during the horrendous El Niño / Black Summer period of 2018 - 2020 still resonate for me. I'm paraphrasing, but after months of barely a wave over two feet approaching the coast it went something like, “how the fuck does anyone ever get good at surfing around here?”
Those comments seem almost perverse now after a three week run of swell coming at the tail end of a triple dip La Niña. Surf has been abundant and while crowds have been almost apocalyptic at times it's still justified Rabbit Bartholomew's description of legendary surf episodes as a “war of attrition”. Eventually the body can't take another day of paddling against the current at one of the points.
Why is this so? How does this mild-mannered lee shore transform at times into a surf machine of unparalleled duration?
Other east facing-coastlines: Brazil, the US from Florida north to Maine, South Africa, New Zealand, don't get the same payload. Their swell events are much more discreet and sporadic; sometimes dynamic yet equally fleeting and short-llived.
The short answer is that you need a few things need to come together; you need an anvil and you need a hammer.
The longer answer? Well, here goes...
Our anvil comes in the form of the high pressure belt which helps to equalise the movement of warm air towards the poles. Individual high pressure cells can become extremely slow moving as they enter the Tasman Sea and become almost stationary as they straddle New Zealand. Sydney surfers may fantasise about removing New Zealand so they receive more east swell, yet under these atmospheric conditions the very presence of the Land of the Long White Cloud benefits surfers from the Mid North Coast to Noosa.
The pressure force along the top of the high is often enhanced by areas of low pressure which form along the western edge of the West Pacific Warm Pool. This is our hammer. The West Pacific Warm Pool bleeds into the South Pacific Convergence Zone (SPCZ), creating a vast area of warm water, unstable air, and enhanced low pressure development focussed around our near island neighbours in New Caledonia, Vanautu, Fiji, and Tonga.
We know that during La Niña years stronger than usual tradewinds pile up warm water in the western Pacific. Another effect is that the SPCZ is pushed south-westwards, towards the East Coast swell window. From there, the warm water further enhances instability as it rises, carrying water vapour aloft.
In layman's terms: During La Niña years, there's more low pressure development, from simple easterly dips, to troughs, to low pressure depressions and tropical cyclones. And best of all, it's happening deeper into our swell window.
When the hammer of warm water and unstable air, meets the anvil of a large, creeping high pressure system, the result is a slowly building pressure gradient and a deep easterly fetch aimed directly at the coastline from Noosa to Nambucca. Such a system can last for days, weeks even, as reinforcing high pressure cells can move in without the overall pattern breaking down.
With our current knowledge and access to data we can see the resulting swell cycles aren't one single swell, as is sometimes reported, but instead a series of overlapping swells.
In our recent case, which started back on the Easter long weekend, a pair of lows in the Northern Tasman drifted between Lord Howe and Norfolk Islands, followed by a strong blast from the south. Those were the tasty entrees. If that's all it was, then we would've had a few days of great waves and called it another good Easter. However, before it dropped a classic blocking pattern established itself with a long trough, that extended from up near Vanuatu, positioned in place as, underneath it, a succession of monster highs moved in from the Bight.
The cherry on the pie was a broad tropical low which formed near New Caledonia and took the inside track to break the blocking pattern while spraying the entire East Coast with quality groundswell.
Modern technology greatly assists our understanding of these systems. You see, each individual snapshot - think of the old black and white weather map in the newspaper - of the blocking pattern looks underwhelming. It betrays one of the key features of how these events generate such quality surf.
Unlike so many other ocean basins, it's not the windspeeds which are the key ingredient, but the duration. The continuing wind, blowing in the same direction, at the same strength, even if not particularly strong, leads to a 'fully developed sea state'. That is, the windfield reaches its theoretical maximum amount of wave energy.
So, looked at in snapshot these fetches reveal nothing amazing in terms of size and quality. Yet when viewed in an animated wave model, keen-eyed weather watchers will note the sheer persistence of the weather system; how it stays in place as systems around it come and go.
Once a fully developed sea state is achieved double-overhead vortexes behind the rock at Snapper become a reality. These fully developed sea state swells also have an incredible consistency which marks them out from longer range groundswells. The contrast between the sub-tropical East Coast and Australia's southern states is stark. Rather than delivering high period yet inconsistent sets, we see an endless conveyor belt of waves pinwheeling down the points with no relent.
Even so, swells of such unremarkable periods - 8-10 seconds is typical, 11-12 is substantial - still seem to over-perform across the sub-tropical points.
That mystery is solved below the surface. Bathymetry is altered on a seasonal basis by the arrival of vast quantities of sand carried by longshore transport mostly in the near-shore zone. This sand - mostly fine-grained quartz - moves in slugs, and its most mobile during swell events.
No-one yet, neither man nor Mother Nature, has developed a superior base material for generating perfect waves. The sand moves north along the coast then rounds the many protruding headlands, filling the lee side bays. Once the rocky irregularities are filled in, excess sand is groomed by prevailing currents and the resulting hard-packed substrate creates an incredible inducement for incoming swells to break in a hollow and perfect fashion. It forms, even in lower period swells, what George Greenough calls “bottom tension”, an intangible but importance characteristic of the way these sand bottom points create quality surf.
Data from the Tweed River Entrance Sand Bypassing Project (TRESBP) estimates the average sand transport to be around 500,000 cubic metres/year and the most active months for sand slugs is April and May. Which is just what we saw in the recent run of swells. Sand slugs being transported and laid down, from behind the rock at Snapper then all the way down the Superbank.
Comparisons with the last triple dip La Niña, back in '74-'76, are inevitable but likely now lost in the mists of time and myth-making from the pre-digital era. The famous 28 day swell of 1975 is impossible to find hard data on. In fact, the most tuned in map watcher I know, Thornton Fallander, swore blind to me it was in 1974.
1978 World Champion Wayne 'Rabbit' Bartholomew, in his youthful prime at the time, places the event at April 1975 and ascribes the 28 day swell to, "a very slow moving low pressure system crawling along about 1,500 kms out in the South Pacific between Noumea and the North Island."
"It was stationary for days at a time," says Rabbit, "most likely held in place by a series of high pressure systems ridging from the Great Australian Bight”.
Which sounds fair enough, even if experience tells us the single swell was most likely a series of over-lapping pulses.
We know when the ENSO cycle tilts towards La Niña and the sand stacks up on the points, the swell can seem endless. We also know that what goes up must come down, and that periods of wave drought will inevitably follow the good times.
// STEVE SHEARER