Roaring Against The Forties

Stu Nettle picture
Stu Nettle (stunet)
Swellnet Analysis

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.

(Andy Smyth)

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.

(Andy Smyth)

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.

The swell hit the Cape Sorell buoy (at left) on Friday just shy of true SE, while on Saturday it hit Cape de Couedic (at right) closer to SE, both buoys registering periods around 16 seconds when it struck

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.

Comments

zenagain's picture
zenagain's picture
zenagain Monday, 20 Jun 2022 at 7:54pm

Informative, well written and entertaining.

I reckon you would be a good teacher Stu.

stunet's picture
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stunet Monday, 20 Jun 2022 at 8:14pm

Thanks, Zen.

I imagine there'd be some cosmic comeuppance if I tried to control a classroom full of kids.

Sprout's picture
Sprout's picture
Sprout Monday, 20 Jun 2022 at 8:38pm

Great stuff stu.

seeds's picture
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seeds Monday, 20 Jun 2022 at 8:59pm

Really interesting and amazing the directions the swell headed. Learnt something there. Good article Stu

Tooold2bakook's picture
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Tooold2bakook Monday, 20 Jun 2022 at 9:19pm

Interesting! Great article

Major kong's picture
Major kong's picture
Major kong Monday, 20 Jun 2022 at 9:25pm

Very cool.

Roystein's picture
Roystein's picture
Roystein Monday, 20 Jun 2022 at 9:38pm

So cool, what a sick system. I still awe at the days before computers, imagine the surfers with some weather nouse, they would have been wondering what the fuck was happening in some of these rare events

quokka's picture
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quokka Tuesday, 21 Jun 2022 at 12:01pm

I yearn for those days when people weren't hand fed forecasts, you used to be able to snag many uncrowded surfs, even in metro Perth!

Island Bay's picture
Island Bay's picture
Island Bay Tuesday, 21 Jun 2022 at 4:29am

The ones with some nous would have looked at the newspaper synoptic chart and had a very good idea.

stunet's picture
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stunet Tuesday, 21 Jun 2022 at 8:45am

As Roystein says below, this swell would've been far outside the window of Australian newspaper MSLP charts. Same goes for tropical lows tracking near Fiji during summer, and swells developing SW of Tassie.

Then there were the tricky, hard to pick swells such as westerlies exiting Bass Strait (and doing a near enough 180 degree turn), and gales exiting Cook Strait with very narrow and short-lived fetches yet manage to make an impression on the East Coast when they arrive.

Island Bay's picture
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Island Bay Tuesday, 21 Jun 2022 at 11:16am

That's a good point.

Roystein's picture
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Roystein Tuesday, 21 Jun 2022 at 7:04am

But did they publish that area in an Aussie paper?

Paul McD's picture
Paul McD's picture
Paul McD Tuesday, 21 Jun 2022 at 7:25am

So the big question....does Bells turn into a Shark Island type slab off the bowl in a SE groundy?! (haven't as yet seen a single photo to support this)

SI's picture
SI's picture
SI Wednesday, 22 Jun 2022 at 10:03am

No.

Paul McD's picture
Paul McD's picture
Paul McD Wednesday, 22 Jun 2022 at 11:11pm

Gee really?

garyg1412's picture
garyg1412's picture
garyg1412 Tuesday, 21 Jun 2022 at 8:40am

I love the way Swellnet turns us all into "closet meteorologists". Great article and makes you think that in this modern age with this type of information we can still be surf pioneers for a day hunting down that elusive once in a lifetime surf spot!!
The other interesting thing if you rewind a week when the system was at it's SSW peak is that certain north facing coasts received some small fun offshore surf. I'm no expert but I'm assuming that this swell was so powerful it had the ability to refract much further than your run of the mill roaring forties systems.

stunet's picture
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stunet Tuesday, 21 Jun 2022 at 8:56am

Though I didn't see said north facing beaches, I wouldn't be surprised as various pulses from the swell had periods in excess of 15 seconds. In a swell of that strength, there's enough underwater torque to straighten up, perhaps even refract right around, a direct south swell.

I occasionally hear people talk about an 'easterly swell' when the truth is the swell is simply high period from the south. The higher period has it refracting earlier and arriving as an apparent easterly swell.

Lot of that depends on offshore bathymetry - i.e shallow(er) water offshore, means more refraction, while deeper water offshore, means less refraction and it'll arrive closer to its true direction.

Stu2d2's picture
Stu2d2's picture
Stu2d2 Tuesday, 21 Jun 2022 at 9:33am

Nice article. If the southern states had the same spectral chart as the MHL you might have seen a faint line down in southern WA. Someone down there probably surfed it and is either keeping it hush or has no idea what they were actually surfing.

wiseautogas's picture
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wiseautogas Tuesday, 21 Jun 2022 at 11:41am

gone are the days where you go somewhere sit and wait, it's all strike missions now, suppose nobody has time to sit and wait a week or so for a good swell.

wiseautogas's picture
wiseautogas's picture
wiseautogas Tuesday, 21 Jun 2022 at 11:41am

gone are the days where you go somewhere sit and wait, it's all strike missions now, suppose nobody has time to sit and wait a week or so for a good swell.

Island Bay's picture
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Island Bay Tuesday, 21 Jun 2022 at 11:52am

Not a fan of strike missions - at least not chasing a big swell. Too much froth and nervous laughter.

I will wait for good winds, and then go somewhere and hope the swell cooperates.

Seaweed's picture
Seaweed's picture
Seaweed Tuesday, 21 Jun 2022 at 2:46pm

Thanks for an inspiring read it’s got me wondering about the origins of the Gyre. Did it start from a seemingly normal low splitting into several or did several meet up to do a dance for a few days?

stunet's picture
stunet's picture
stunet Tuesday, 21 Jun 2022 at 2:58pm

Not sure about that.

Steve?
Ben?
Craig?
Maybe even IB..?

You guys know?

Island Bay's picture
Island Bay's picture
Island Bay Tuesday, 21 Jun 2022 at 3:53pm

From memory a massive upper low over warm Tasman Sea, spawning a large surface low.
Polar Jet feeding tons of cold air into it.
Upper troughs within this cold airstream, probably aided by poleward exit region of Subtropic Jet driving separate vortices within the big low/gyre.
Would have to go back to work and check details, but that's the typical development of these big unstable winter events.

freeride76's picture
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freeride76 Tuesday, 21 Jun 2022 at 5:38pm

Yep, it's more common in the North Pacific with Aleutian lows getting backed up as they emerge from the Siberian high pressure belt and rapidly deepen via the trophospheric temperature gradient along the Kuroshio current then get steered by the sub-polar jet into that huge rotating gyre.

ruckus's picture
ruckus's picture
ruckus Tuesday, 21 Jun 2022 at 6:00pm

Nice IB! Where’s old mate Southey on this one too? I like to get confused by technical terms and paraphrases that are well over my head

Sega na leqa

Island Bay's picture
Island Bay's picture
Island Bay Tuesday, 21 Jun 2022 at 6:22pm

Maybe I was too quick with the jet exit thing. Here goes:
Jet streaks are little wind maxima along the jet core. Where the air exits a jet streak and slows down, it induces divergence on the poleward side, convergence on the equatorward side. Divergence aloft at jet level in turn induces convergence at the surface, which aids cyclogenesis (development of a low).
And there you are!

Edit: the same thing happens near the equatorward jet entry region.

McQuartzie's picture
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McQuartzie Tuesday, 21 Jun 2022 at 3:41pm

The big question is did the "second" most famous secret left south of radelaide break in the se swell?

warddy's picture
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warddy Wednesday, 22 Jun 2022 at 6:35pm

Funny I was thinking the same ….
Long time away from SA but saw it break the Simon Bells Easter Day …
That’s a long time now …
Doubt anyone would report if it did LoL

jetson.rover's picture
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jetson.rover Tuesday, 21 Jun 2022 at 3:54pm

That swell would of butted heads with the oncoming south westerly weather front out in the bight and died out before making any significant impact in WA.
Would have been getting too far away from it's power source,and no swell is going to keep marching west from the east before the predominant east moving southern indian ocean weather patterns nip it in the bud at that time of the year.

stunet's picture
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stunet Tuesday, 21 Jun 2022 at 3:59pm

Ocean swells don't lose energy when they pass through each other.

They decay due to travel but not from meeting another swell.

jetson.rover's picture
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jetson.rover Tuesday, 21 Jun 2022 at 5:28pm

I don't know what the exact swell situation was at the time coming from the other direction in that instance,but i would presume if there was one,they wouldn't meet,shake hands then pass on their merry ways through each other unimpeded.
It'd have to mess with the period and i would imagine the size too.
Otherwise out in the bight was far enough from it's source for the decay factor to have become apparent i guess.
I've seen here and there over the years people blame 2 swells from different directions for conditions that didn't get as good as what was expected to arrive according to surf reports.
Maybe we were wrong in our presumptions?

stunet's picture
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stunet Tuesday, 21 Jun 2022 at 5:39pm

"It'd have to mess with the period and i would imagine the size too."

Not one iota. Each swell passes through the other as if it wasn't there.

Pretty sure it's the law of the conservation of energy, though one of the eggheads will correct me if I'm wrong.

Pops's picture
Pops's picture
Pops Wednesday, 22 Jun 2022 at 11:42am

You'll get constructive and/or destructive interference as they pass through each other, but yes, once the swell trains have passed each other it would be as if they had not met.

donweather's picture
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donweather Wednesday, 22 Jun 2022 at 12:42pm

Has anyone looked at the great circle path from the fetch to SW WA to see if Tassie was in the road?

old-dog's picture
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old-dog Tuesday, 21 Jun 2022 at 5:46pm

I'm a tad skeptical, those pics just look like a standard small south swell day at that place, shouldn't there be lefts peeling all along the beach or waves hitting the banks at a different angle to normal or something. Any first hand accounts?

Craig's picture
Craig's picture
Craig Tuesday, 21 Jun 2022 at 8:24pm

With 16s periods it would come in looking fairly standard, not like a peaky swell running down the beach at 9-10s.

Fatnuts's picture
Fatnuts's picture
Fatnuts Tuesday, 21 Jun 2022 at 5:50pm

Loved the article. I always love to look at the southern hemisphere MSLP chart in the bom weather maps section.( http://www.bom.gov.au/australia/charts/index.shtml) being in Tassie its nice to see Lows brewing up under South Africa or off South America and watch as they progress eastwards. Currently (ffs) they have stopped marking fronts and pressures for everywhere except the quadrant that is over OZ. The other week there was a 940 under South Africa ( it may have got swell to g-land after the event) It went to 947 and then combo'ed into the peanut ( double centre ) and marched through giving us days of points sized swells ( shame about the high low tides during the swell event though) and then kept giving with the SE afterglow. Is the low you're talking about now mid pacific and has dropped again to 948? looks like it on todays chart. Combined with a look at the synoptic I go to free website called earthnullschool (https://earth.nullschool.net/#current/wind/surface/level/orthographic=17...) looking at the wind at the heights of SFC ( surface) and then click the overlays up to 250HPA ( approx 10,000M) and see what the polar jet streams are doing ( yes for free, not sure how accurate they are) the other week the jet stream bent North over the eastern end of the bight and I thought that the front was able to run up the eastern flank allowing Southern Oz to get the brunt of the storm rather than is often the case that they shear away under Tassie and across under NZ. (ps please complain to bom to mark all of the maps if you're annoyed that their products are regressing!)

Clam's picture
Clam's picture
Clam Tuesday, 21 Jun 2022 at 6:32pm

there was a blip on the Esperance buoy.. hardly proof that it arrived but it was spot on the time. just one red dot deviated from the sw dots and it showed south direction .. those wa buoys are not as good as the TAS and SA buoys.. maybe the swell just didn't reach the buoy . it's inside many islands . maybe it's shadowed.
I have never seen such a south swell at my local spot it was fascinating !

peabo's picture
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peabo Wednesday, 22 Jun 2022 at 11:37am

Can think of a couple of spots in Vic i've seen doing interesting things in the aftermath of a strong SE windswell. Wish I was down there to see this swell.

Seaweed's picture
Seaweed's picture
Seaweed Wednesday, 22 Jun 2022 at 1:12pm

Thanks, I just read the south by southwest article that goes into the Gyres origins though it really just told me how much there is to learn. Thanks again

Seaweed's picture
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Seaweed Wednesday, 22 Jun 2022 at 2:19pm

Most mornings I check the conditions from the top of a hill about one kilometre inland with a view of around 30Ks of coastline in SW vic and noticed no suspicious characters or any swells looking lost or travelling in the wrong direction on any of the days in question though I do recall some very thick ocean fogs around the time.

PeteH's picture
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PeteH Wednesday, 22 Jun 2022 at 4:38pm

I surfed that place in the pics on the same morning they were taken and the size was well above the forecast although still only about 4ft on the sets. They were coming in fairly straight so only good shape on a few banks on a beach that stretches a few km. They did however have quite a bit of punch them, so a good,strong groundswell with light off-shores.
It'a a fickle spot at the best of times!

Sas's picture
Sas's picture
Sas Wednesday, 29 Jun 2022 at 3:35pm

I surfed the beach next door. Although you tell there was a different angle to normal, I agree with you in that it was just a normal long period, straight swell. Would have been nice to have the time to drive around and check a few spots.

Sheepdog's picture
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Sheepdog Wednesday, 22 Jun 2022 at 7:36pm

Damn!!! I spent SIX years around the coast south of Dover of Tassie. These kind of swells are soooooooo rare. I saw one particular left point break maybe 4 times in 6 years. This swell looks like it would've been Bali in neoprene. So jealous right now.

scrotina's picture
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scrotina Wednesday, 22 Jun 2022 at 8:02pm

i was hoping for more pics of novelty spots, not the same beach with 2 different angles

mitchvg's picture
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mitchvg Wednesday, 22 Jun 2022 at 10:14pm

Since Southey hasn't chimed in yet I'm gonna say he woulda say it deflected counterclockwise and away from WA by the time it got that far.

southey's picture
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southey Thursday, 23 Jun 2022 at 12:00am

Hahhaha ..... These swells are more frequent than you would believe ( atleast in these parts ) , but they are usually lost in a sea of other swells . Apart from Cape Sorell most buoys West and slightly Nth of there are in shallower waters .
So as previously discussed the true angle that the swell is approaching from is not reflected . Sorell only saw it due to the longer shelf south of Tassie refracting swells up in this case opposite to what they usually do . WA = GCP ( there ya go Ruckus ) & the tail end of the Leeuwin would have made the angle too obtuse to the shelf . Remembering that the period would be quite bigger if it reached there . What made this stand out was the blocking highs , so there was very little other energy in the ocean to see this swell get lost in the noise of the stereo typical winter WSW energy . Unfortunately i was in the far west of our state , i did see the swell and due to where i was it was energy that came around the bottom of Tassie , lots of long lulls ( which is unusual for a south swell in that neck of the woods ) . For Homework @ Stu see if you can see any difference to the Swell angle during the tide shifts and report back if its opposite to what we normally see . I know where i wanted to be , but alas work spoiled that .

ruckus's picture
ruckus's picture
ruckus Thursday, 23 Jun 2022 at 9:10am

Ahhhhh the great circle path

Craig's picture
Craig's picture
Craig Thursday, 23 Jun 2022 at 9:12am

Nice!

southey's picture
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southey Thursday, 23 Jun 2022 at 12:03am

PS . @Clam i think you have had someone hack your normal form of communication . I keep getting weird ( cyber ) requests from your account .

Craig's picture
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Craig Thursday, 23 Jun 2022 at 11:54am

It's quite incredible to see how close to the edge of the swell window the Fleurieu Peninsula was to this rare SE groundswell.

Just glancing the South East and Tasmania.

Fliplid's picture
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Fliplid Thursday, 23 Jun 2022 at 1:13pm

So, did Chicken Run roar to life!?

Craig's picture
Craig's picture
Craig Thursday, 23 Jun 2022 at 1:13pm

That's the burning question!!

Major kong's picture
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Major kong Friday, 24 Jun 2022 at 1:59pm

That's gold

Major kong's picture
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Major kong Friday, 24 Jun 2022 at 2:00pm

That's a wicked few pics... Wow