Mythbusting: Cyclones and Swell
OK, it's mythbusting time!
Last October various Australian weather forecasters predicted that there would be 15 tropical cyclones in our waters during summer. That's five more, or an increase of 50%, than an average year. Their predictions were based upon the formation of the la nina weather phenomena that affected eastern Australia during the summer. During la nina years warmer than average water pools off the east coast of Australia providing conditions conducive to tropical cyclone creation.
As soon as the news story was published it went viral in the surf world. It was tagged and posted on many surfers' Facebook walls and was given column space on almost all Australian surf sites. The reason the story was picked up with such fervour is the connection that surfers make between cyclones and good waves. Due to the connection it became accepted knowledge that summer 2010/2011 would be an above average swell season.
And this is where the mythbusting begins...
While it's true that cyclones may create great waves the odds of them doing that are actually very low. Simply having a cyclone in the swell window doesn't ensure good waves. For that to happen there has to be a set of associated weather phenomena also occurring and the cyclone has to be acting in a particular manner.
A cyclone, acting in isolation from any other weather system, is an extremely small area of strong winds. The fetch (the distance the wind blows over water to create swell) can be as little as 50 kms, which makes it a veritable pinprick in the ocean.
The best chance a cyclone has of creating a significant swell is when it butts up against a high pressure system and creates a pressure gradient with winds blowing over a large section of ocean. During summer and autumn high pressure systems can form and stall in the Tasman Sea, the positioning of them influences the easterly trade wind belt that supplies much of the background swell to SE QLD and NE NSW during these seasons.
The ideal scenario is for a cyclone to form in the Coral Sea while there is a strong high in the Tasman Sea, the combination of the two creating a pressure gradient directing strong winds toward the coast. When this happens (as it did during TC Jasper last March) it is usually the cyclone that gets all the credit, however the high pressure system is equally important for the creation of the swell.
During the 2010/2011 season none of the cyclones worked in conjunction with a high pressure system as described above.
Another aspect that can prevent cyclones from creating good waves is their speed and track. As mentioned above, cyclones in isolation are very small areas of strong winds. If a cyclone stalls and remains in position for a time then it can theoretically create a reasonable swell, albeit for a small stretch of coastline. Most cyclones however, are perpetually moving which reduces their singular ability to create a swell. The faster the cyclone moves the less chance there is for it to create a swell.
A good example of this was Tropical Cyclone Zelia. In fact Zelia was a Severe Tropical Cyclone, such was the strength of its winds. Zelia tracked from north of New Caledonia to New Zealand's North Island passing through all of Queensland's and NSW's swell window yet it didn't create a wave over three feet. The reason for that was its speed. During the two days it was a classified STC, and hence at its strongest, it travelled 2000 km's and moved too quick to build up any significant swell.
As of March 21 there have been 10 cyclones in Australian waters, equal to the amount in an average year. While the cyclone season still has a few weeks to run – one of the biggest swells to ever hit the Gold Coast was from Tropical Cyclone Sose in mid-April 2001 - it is fair to say that whatever happens the season has come in well under expectations. It also busts the myth that cyclones always create great waves.
Postscript: For a shining example of how cyclones can turn normal surfers into blithering fools read this article.
Comments
Great article Stu but TC Vania sat against a strong high and provided both the initial slowly strengthening tradewind swell and then the cyclone swell proper as the cyclone tracked into the swell window.
We got a week of strong surf from that combination.
But yeah, all in all bit of a disappointment as far as cyclone seasons go.
I'm still holding out hope for a late season burst though.
Spot on Stu, Ive had better swells from pressure gradiants up here than the cyclones, and I didn't read Yasi properly and bummed out! Good read though.
Water is still warm enough for more!
Yeah Steve, but I guess I'm benchmarking against the 'classic' cyclone swells that have been created by buttressing a high pressure system: Jasper, Sose, et al.
Interestingly Vania gave the East Coast the best cyclone swell of the summer but it isn't counted as one of the 15 predicted cyclones as it didn't enter Australian waters.
I've still got a keen eye on the Coral Sea, Fitzy....
Well Sose didn't have a buttressing high: it was a one day wonder of epic proportions.
I think is a conversation to be had over quite a few beers.
A few classics cyclone swells: Bola, Betsy, Betty, Justin, Rewa, Roger, Damon, Nancy, Dinah (before my time). Oh and not forgetting Wati.
methinks your correct about conjunction etc ... only it is the pressure differential that creates 'ground' swell rather than wind swell... low pressure lifts the water surface relative to high pressure and the water 'falling off' the lift creates ground swell ....
Way to tar every surfer with the same brush! Not everyone of us are that stupid...
Say what Nomad :?
Good article stu. The simple formula I adhere to is wind times fetch. Distance of fetch and main generating energy can allow for swell organisation which will give the 'ground' swell effect. All swells are wind swells.
Steve's right about Vania, apart from it creating a week of swell. Vania created a very interesting case study which I wrote to Craig about at the time. Most interesting, it was clear that there was two distinct swells in the water at the same time, from similar directions, based on how the swell at its peak was highly tide affected where I was. (slightly in the 'shadow' of the Hunter region.)
Nomad, the pressure differential is what creates the wind, which creates the swell. Ground swell has nothing to do with the lifting of the water from low pressure.
Where did you get that malarkey?
Tucky, surfers, as a group, do not impress for their intellectual prowess. Many don't seem to understand these fundamentals. Witness nomad's comments. :-)
Anyway, just for the record: QLD and NENSW surfers aren't going to listen to some Sydney-siders prattle on about cyclones.
It's pure jealousy really.....c'mon, come clean Stu.
No time for jealousy Steve, it's 4-6 and smoking out front. So....how big is it in Ballina this morn?
One more bit of prattle: Sose did have a buttressing high. Check the first two images in the story.
Hahah....well played sir.
You'd have to give this summer a C or B- at best I reckon.
The problem was Yasi which punched such a huge hole in the atmosphere/ocean dynamics we're only now starting to regain form in the CS/South Pac.
Still feels mighty monsoonal though.
Sose had a supporting ridge in it's early manifestation (when it was behind New Cal) but it was a weak ridge which didn't provide much in the way of precursor tradewind swell (unlike the glorious July 2001 swell which followed it).
By the time Sose tracked SW from Grand Terre it was a free agent and it was this SW movement(at the time of maximum intensity) which supplied the burst of massive groundswell.
It very quickly became absorbed into a mid-lat cold front and Tasman low.
Looks pretty shabby down there today Stu.
Terribly shabby Steve. That's why I didn't bother with it, twice. And Craig's predicted an arvo pulse which will make it even worse.
Bune hey Steve!
Tropical Cyclone Bune is about to validate a point I made in the article, or it's about to make me look a bit silly.
Bune has been almost stationary in the SW Pacific Ocean, in both QLD's and NSW's swell windows, for the last 48 hours. If Bune formed with the associated weather features mentioned (buttressing high pressure system, embedded trade winds etc.) it would provide a very significant swell for the east coast. As it is however, Bune is an isolated system so, although being a slow-moving Cat 3 cyclone, is unlikely to provide waves over three feet anywhere on the coast.
In addition to Stu's comment..
Bune will turn extra-tropical during tomorrow afternoon and squeeze against a strong high in the Tasman Sea, producing a secondary more significant pulse of E/SE groundswell for the Gold Coast on the weekend.
Just on this topic, and with particular reference to last week's magic swell.
I'm now leaning towards fetch being a much bigger variant of swell height on the beaches then wind speed.
Cyclones have wind speed but can produce very little. Last week's swell, at least looking at the charts that I was, had tremendous fetch while most of the really powerful winds were running down the coast rather than into the coast.
I reckon last week's swell was all about fetch and much less to do with wind speed.
Fetch also allows for swell organisation. Damned straight was last week's swell, damned straight.
So what's with the general media (both surf and non-surf) attributing the Goldy's pumping surf from Wednesday thru' Friday as a "cyclone swell" courtesy of TC Marcia?
As per a couple of Facebook posts:
And via a quick Google search:
Surfing Life wrote: "Cyclone Marcia lights up the Gold Coast points with classic Kirra, Snapper and Greenmount serving it up for the masses".
Liquify Mag wrote: "... as solid swells from the cyclone Marcia system continue to hit the Gold Coast".
The Australian wrote: "Marcia provides Golden swell".
The New Daily ran with the headline: "Surfers embrace Marcia’s sublime swell" (which was itself a syndicated AAP story).
The Border Mail wrote: "Tropical Cyclone Marcia brought big swells with her to southern Queensland on Friday".
And on, and on...
Now we know that the surf built steadily all week with Kirra really started to pump on Wednesday afternoon. Excellent, sizeable waves then held through Thursday and Friday (and also over the weekend, however all of the articles/videos above were referencing the Wed thru' Fri time frame).
However, at 10pm Wednesday, TC Marcia had only just formed (Cat 1) and was positioned roughly 1000km N/NE from the Gold Coast (see chart below), almost completely out of the swell window.
As a rough guide, 15 second swell periods generated from this source at this time would have taken a full 24 hours or more - i.e. technically not arriving until overnight Thursday (therefore visible Friday morning at the earliest). But the early stages of TC Maria weren't favourable for generating long range groundswell.
And in any case, TC Marcia was never in a favourable swell generating position during that crucial intensification period - over the following days it tracked inside the swell shadow of the Capricornia Coast (see second chart below). Therefore we did not see ANY swell from TC Marcia on Wednesday, Thursday or Friday.
So where did this extended run of pumping Gold Coast surf originate from?
Quite simply, a broad enhanced trade flow stretching from the mainland to longitudes well east of New Zealand (as per the first chart below).
But definitely not TC Marcia.
oh well. every media outlet likes to over-egg the omelette with a sexy headline.
Hang on a sec ...
Are ya now gunna tell us that all the rain the goldy and northern nsw got over that same late wed into thurs & friday period was also NOT from TC Marcia?
Bingo!!! Media hype bullshit. And my work even got suckered in and closed our Brisbane office on Friday afternoon due to the impending "cyclonic weather conditions". I just laughed as I took Friday afternoon off and walked out the door!!!
Wingy, that bad weather including the period of gale force winds at Cape Byron Fri night was from a deepening surface trough lying NW/SE off the far north coast that probably contained small scale circulations within the vorticity.
Wingnut, it wasn't directly marcia... But it sure was associated with marcia..... If Marcia did not exist, The associated trough and fetch east of the gold coast would've delivered waves 2/3s of the size experienced at best....
SD, the size would of been there regardless.
The uptick in size and pumping point surf seen through Wednesday and Thursday was generated by the strengthening trades directly east off the coast as well as a small embedded surface trough right off the coast.
Ben's pointed this out above.
Size jumped 3-4ft early Wednesday to a solid 6ft. Marcia was declared a TC late Wednesday night.
We'll agree to disagree.... At 18.00utc 17/2, pre marcia at 997hpa was enabling a trough to push below New Cal, which boosted the fetch east of SEQ.... No pre marcia, no trough, no fetch of that strength...... At 0600utc 18/2, pre marcia was drifting ssw, enhancing the fetch with a high positioned at about 165/37s... ... Due to this a break off trough reformed below New Cal, again enforcing the fetch....
00.00utc 19/2, an offshore trough formed out of TC marcia, running the length of the coast from marcia to NNSW... This again boosted wave size..
None of the above would've occured to the extent it did if it wasn't for pre marcia, marcia, and ex marcia...
As I said, it wasn't direct cyclone swell, but it was swell "associated" with the system..
Run through from 1800utc 17/2 here.... You'll see what I mean....
http://www.bom.gov.au/australia/charts/synoptic_col.shtml?unit=p7
Okay SD.
Eliminate the pre-existing, week-and-a-half enhanced trade flow, and keep TC Marcia as it developed (but as a solitary, cut off system).
How much surf would we have seen on the Gold Coast?
I'm with Sheepy on this one. Just looking at the synoptic posted above, you can see them interacting together. But point taken anyway, for Marcia. What I would like to know, is where does Cyclone Bola sit in this argument? No one remembers the great trade swell of 86. I get that trades are a reliable, dependable source of steady long term swell manufacturing. But the thing with trades is, due to their shape / nature, you usually get an easterly surface wind flow hangover associated with them. However I've never lived up there, so correct me if I'm wrong, please. With a well positioned cyclone, sitting off the coast somewhat, you have your swell generation but also an area of slack isobars overland. I feel trades can be seen as bread and butter, great for any local, while cyclones are the gravy. Trades might make up 17 of the best 20 days of the year, but cyclones will take the top 3. (August last year not so*) Another thing, it's not just hype, it's part of surf culture. We all love tracking a storm with a name right?
Anyway. I'm on a plane, inbound. Hope there's some left for me. Yew!
Just read the article. Haha looks like you already made the points I was trying to make. Here's a pic of a pic from Jasper. Drove up in my car, slept in my car, didn't leave that zone, (the water or the hill) for 62 hours. Watched a lot of good waves, even scored one or two for myself, and did it all without dropping in once.
Except for the captured fetch scenario when the TC comes straight at you, then turns away before it hits the coast....