Why The Swell Train Is Often Late
What's the biggest swell period you've ever seen recorded at your local wave buoy?
On Tuesday morning the Point Nepean wave buoy, located in Bass Strait, recorded an incredible peak period of 25 seconds. The impressive swell period was associated with the arrival of a long-range groundswell generated by an intense storm in the deep Southern Ocean.
The Cape du Couedic wave buoy in South Australia also registered this swell at a similar time with peak periods of 22 seconds. The interesting thing regarding this swell is that a peak in wave energy was not seen until 24 hours after the initial arrival of the swell when periods had dropped to 15-16 seconds.
So why a 24 hour time lag between the peak wave period and peak of the swell?
We're about to get a bit technical here so try and stay with me...
Storms in the Southern Ocean often have large variations in wind speeds which result in numerous swell trains, each with unique characteristics, travelling out and away from the storm. The general rule of thumb is that the stronger the wind speed, the duration and the fetch length, then the larger the period of the resulting swell.
The storm responsible for this weeks swell produced maximum wind speeds in excess of 60kts initially, but for the majority of its life winds were between 40 and 50kts.
The initial 22-25 second period component of the swell was generated by a short burst of 60kt+ winds, yet due to the storm blowing at 40-50kts for the majority of its life cycle the main body of the swell was of a lesser period. These waves registered in the 15-16 second bracket and provided the bulk of the storms wave energy.
That explains the creation of different swell trains but why such a lag in time between the peak wave period and peak of the swell?
The large time difference is a product of swell trains with higher periods travelling through the ocean faster than their lower period counterparts. This is a common occurrence but in this case the time difference was greatly amplified because the swell had to travel 5000 km's to reach the Southern Australian coast. Therefore there was a significant time difference – 24 hours – between the arrival of the fast-moving, long-period forerunners and the peak in wave energy.
So next time you see a jump in swell periods on the buoy, it may be worth just holding out for a while as the swell train may be running late. //CRAIG BROKENSHA
Comments
The refraction shown by that swell was amazing. Places that usually only open up on a Sth swell were on and I am pretty sure that swell was quite SW. Must have been the extra power it was carrying.
If you've ever spent time in Hawaii and paid attention to the swells you'll see this sort of thing happen pretty often. In just a couple of hours, and during the same swell event, you can see the waves change in frequency, size and consistency. Think of those Eddie Aikau comps when some heats (45 mins) barely get any sets while the next heat will be non-stop.
I think it's just burst of winds inside the same storm cell creating different characterisitics (as you explained). The thing is in Hawaii you dont have the continental shelf to slow down and create uniformity in the swell so it's raw and you see every fluctuation.
Yes spot on trolleyboy.
Due to there being no major bottom obstructions or land masses to influence the swell as it approaches Hawaii the swell would eb and pulse in sync with bursts of stronger winds during the storms life.
Sunbay, where you based? Vicco?
First time I read this I thought, "ahah, not sure I get it, but I'll put that in the memory bank for later on."
The more I read it the more I think it either hides or simplifies incredible intricacies that make thinking about it too much a little less fruitful.
Re wave bouys, exactly how far out are they and how deep is the water. While the connection between period and swell height is fairly predictable at the beach (very shallow water), I'm not convinced that the connection at sea is the same. Again, the question is how far out are the bouys and how deep the water.
Now I have read other stuff from you Craig where you explain that waves 'feel' the bottom at quite significant depths, based on some formula. While accepting these formulae as they would be grounded in empirical science, I'm not sure that the phrase 'feel bottom' means the same thing to you as it does to me, and I'm not convinced that open ocean waves would have a significant difference in height dependent on the period. i.e. the relationship between period and wave height really only applies at comparatively shallow water! (my conjecture)
So while it all sounds good on the surface, all I am left with is questions, and doubts about the veracity of the theory.
And then we come back to the whole question of how waves are actually formed, and how 'period' is created. Nick Carroll has a standard response to those questions which tries to answer the question 'why does long period swell form out of the chaos of storm swell'.
However his explanation, although I think fervently believed by his good self, doesn't conform with basic laws of physics, and I have read much better theories on wave formation from a storm than what he is peddling. In all it is a wholly uinsatisfying explanation which leads to more questions than you started with. In other words, it is no explanation at all.
I wish I could find that original article which was the most comprehensive analysis of wave formation from storms that I have ever read. Alas, past attempts to find if have been unsuccessful.
In any case, perhaps this is fodder for a future story rather than a response here. Suffice to say that there is a hell of a lot about wave creation, period and wave height that I have read from very learned people, and I include yourself and Ben in that group which is;
- either certainly conjecture and not particularly imaginative conjecture at that
- or a gross simplification to allow communication with the layman (myself)
And often mutually exclusive - i.e. it can only either be this force creating it or the other, but not both, and adherence to one theory cancels any possibility of adherence to another.
Go figure. :-)
Batfink, this article is fundamentally about differences in wind speed within a storm generating different swell trains which arrive at different times at a selected location during a swells lifespan.
Without going into the mechanics of wave formation and growth the usual rule of thumb is that stronger winds produce larger period swell trains.
This gets more complicated when introducing fetch length and duration, but again the longer the wind blows over a similar area, the bigger the resulting period swell from that area than say a shorter lived fetch.
So with big storms that produce large variations in wind speeds over their lifetime we'll see a large span of swell trains approaching the coast ranging from 18 seconds say at the head of the swell to 8-10 seconds at its tail.
Where the bulk of the swell is sitting along this sliding scale of periods depends on what strength the majority of winds were.
If the majority of winds were say 50-60kts then the bulk of the swell will be contained towards the 18 second end of the scale.
If the majority of winds are between 40-50kts as was the case with this storm then the bulk of the swell will be contained back in the 15-16 second bracket.
This can be seen in the chart above with the bulk of the swell not arriving until nearly 24 hours after the initial forerunners of the swell were picked up by the buoy.
SA.
Just came across this article by accident and couldn't help myself ....
The Southern Ocean is a lot different to the NW Pacific .
Due to shear distance , Swell intervals ( period ) can reach numbers close to 22 Seconds quite often when a storm is a long way from Aust and in between the storm and landfall is a very long singular or series of Broad High pressure systems . With little winds within the central portion of these Highs Swell can and will last unaffected through them very similar to Indo and or Mexico . But seeing that residing Circum Polar Current tends to foster swells in the same Easterly direction as said long period swells then these swells can peak at under 4 ft . So swell period doesn't always correlate to swell strength or even Wind speed at origin . Currents and opposing surface winds can also effect this .
The typical Aleutian Swell or even the More West North Westerly Japan - Kamchatka Peninsula ( Pipe Swells) tend to more so be :- closed cell ,Broad, cold core? / subtropical lows . These will rotate through and often one follows the other but Each's Fetch will not interact as much on each other . The only variation on this is one that stalls and as described the sheer size of unevenly round/circular systems can lead to various uneveness of rotation and Symmetry . As their end path is at right angles to swell fetch direction ( atleast to where I'm talking of Hawaii ) . Whilst in the Southern ocean similar conditions are found for Mexico , and to lesser extent Indo . But there is far more interaction with other systems for Mexico , and often due to sheer distance this place can match southern Australia , Chile and South Africa for Swell trains leap frogging , combining or redirected by ; Each other .......
West Coast of Tassie , Far West Vic , can see multiple variations in Swell produced from sometimes Riculously long Fetches and even unexplainable swell events that can seem to be totally out of Step with what appears to be on the charts even up to 3-4 days Prior . With Circumnavigation of currents that can be sped up with various Pulses of the LWT , Swells can travel and be carried forward with Extremely Fast moving / multiple Fronts .
I'm of the Opinion that the Pt Nepean Buoy ( as Batfink descibed ) , will give a false/misleading account especially when it comes to Period , and Even Direction . As for exactly why , that I will keep in my Pocket for obvious reasons .
Its all part of what can be termed "close to Chaos theory " for Western Facing Shores at or Below 32 deg S . Lets just say , alot of Victoria is sheltered from many Observable Idiosyncracy's . ( Its Filtered ) .
Living in the Pac NW now, and having watched swells progress far earlier than swell prediction sites ( i used to call the local airport in SLO CA for best updates) this info. is great!
Thanks CB for posting this story.
San Luis Obispo ?
Another great example of this occuring with the current long-range W/SW groundswell impacting SA and Victoria.
This swell was generated between a position south-east of South Africa, and south-west of WA, in SA/Vic's far swell window and with peak core wind speeds of 50-60kts a large long-period swell in the 22s range was created.
The bulk of the wind speeds around the polar low were in the 45kt range though, generating swell in the 17s range, and this has arrived nearly 24 hours after the inital 22s forerunners Monday evening:
Had to laugh at the forecast for tuesday 29th , wich basically said the surf would be( tiny, and only worth a look at the western end of the mornington penisula) . I rocked up to my local wich was aleady solid overhead at 9 am , no one was out , i watched a mate get in , he got half way out only to be flogged by a big closeout set , i went up to the far end of the beach and opted for the rip out the back only to watch the channel disappear under the next onslaught of solid waves, a dozen duckdives later i got out the back and into a rapidly rising swell , my mate got flushed out waveless , i didnt fair much better with 3 waves that just held on the bank , i hung out there to watch the whole bay close out end to end before i bailed an hour or so later . That swell train wasnt late at some beaches , or tiny .
Strawbs, the tiny comment was in regards to the Surf Coast, which it was, and the reference to the western end of the MP was due to the strong forecast N'ly winds.
I only expected maybe 2ft+ of swell early on the MP. The swell did kick earlier than expected though in Vic and as you said got solid through the day.
But this article is pointing out, that even though a large spike in swell period is seen on the buoy, the bulk/peak of the swell doesn't arrive until well after this.
this is going to sound contentious , but it ties into what i've said before .
These larger swell periods really act differently further east into bass strait than say outside of it .
The Cape du Couedic wave buoy is far closer to the shelf than Pt Nepean , and somewhere east of PI is further again .
There is science missing for shallower bathymetry and tidal flows that can and do affect swell trains . Not only does the sea bed interaction make some swells refract harder , it also dissects the swell mechanics itself . Reducing the period or atleast the speed but also improving the frequency or consistancy of sets .
I swear that the swells power / peak intensity is seen at differing times at places far apart like well inside Bass Stait and outside .
I agree. It's a main research interest of mine at the moment.. hoping to look into this in more detail over the next year or two.
Southey and Ben ,
I spent some time in victoria , on occasion i would make the trek to phillip island to surf quite regularly . One time i went down in the late stages of a swell event and the waves were forecast to be 4 to 6 foot(at woolamai). When i arrived and checked and there was nothing over three foot .I figured it was in between sets , so i suited up and paddled out anyway.
I surfed for a little while then and older gent paddled over . We got chatting and i said "i thought that the swell would be bigger." He responded with "Oh you're not from round here huh? Look here's what's happening . It's forecast to be bigger though... the swell is fading and we a surfing an outgoing tide, so that cuts the swell in half of pretty much of what they forecast"(he went on to explain the huge amount of water flowing back out around phillip island with the outgoing tide was pulling the energy and subsequent height out of the swell/ waves)..... ...... Well he was totally correct, surfed for a least another three quarters of an hour , then packed up and headed either somewhere else of home .
So every time i planned to surf phillip island . I would check the swell , wind and tides to see if it was going to be working from then on . old boy had it sussed out .!
This was almost twenty years ago and i would get the swell report from the paper! The vehicle ran on lpg and it was 19 cents a litre ....Great times, plenty of remote and uncrowded waves were found. Never did understand AFL though LOL!
Am with you Southey, but in essence this article is talking in time-frames of half days/days behind not hours as would be the case in your thinking.
All this article is pointing out if you're expecting the arrival of a long-range groundswell, don't head down the beach when the period on the buoy jumps significantly as you'll be too early. Tiny distinguishable lines may be there, but the bulk of the swell will be sometime behind this initial spike in period.
An interesting read, thanks!
Although I realise from the above article that a swells speed depends on its period is there a rough rule of thumb for determining how fast an average swell (eg 14-16 seconds) travels? In particular what is the approx time difference between a swell being recorded at Cape du Couedic and it reaching Victor?
Cheers
You can calcualte the travel time for a certain swell period using this equation:
Speed (km/h) = 2.81*period
So a 16s swell travels at about 45km/h and the distance between Cape du Couedic and Victor Harbor is about 200km so if it's a westerly swell it'll take just under 4.5 hours.
If the swell has more south in it, or is dead south you can't use this travel time as it will hit nearly Cape du Couedic and Victor at the same time.
Great thanks Craig
Here's one for you Craig & Ben .
It ties in a little with what i've previously said .
Apart from the obvious that one of the below examples has the swell travelling in the same direction as predominating winds and the other invariably tends to see Swell travel through opposing wind or light wind regimes.
But Generally speaking a solid period swell that passes the SW region of WA will usually reach NWA at about the same time it hits the Far Wets coast of VIC . On average esp. in a WSW swell that the travel time to both is about 24 hrs that the new swell outliers arrive there after first passing SWA .
I don't quite get how that could happen Southey, especially with a W/SW swell. Take for example the swell that hit WA this week on Monday and then travelled to Vicco arriving late in the arvo Tuesday.
This swell was standard SW in direction across WA, but W/SW for Vicco and you can see from the chart below that it would of hit NW WA a lot earlier than the West Coast of Vicco...
If however the swell has more south in its direction then the arrival times will become quite similar, especially from a polar low forming south-southwest of WA.
More the second part Craig .
But generally this is from an observed pattern not from watching the computer screen .
Now that you guys are doing written forecasting notes for Indo more often , your going to see the Computer fail on the swell arrival times more than once .
Also anything with any W component so SW or even SSW is pretty West compared to the majority of inshore swell they see up there which is usually SSW and further towards Sth . Ask the guys who surf Slaters Right irregularly .
You talk of the swell trainbeing like a group of slipstream bike riders , well their performance will change if they have a tail wind , compared to cross wind ?!
The other thing with the picture you've posted is that it looks like that swell is West of SW for SWA aswell . If you had it true SW of SWA and closer than 1000 kms from there as the swells source the numbers alone don't add up . The swell travel speed theory has some flaws . Its only when you use comparisons that it can be defined as imperical . Surface conditions will effect it along with currents .
All our feedback from Indo so far on swell arrival times and size have been that's it's very accurate. Would love a buoy up in the vicinity though or to get there personally to watch it for a few weeks.
And re the bike riding analogy for the way individual waves travel in groups through the ocean, this is just simply that.
Remember practically all the wave energy is stored below the ocean surface, so ocean wind on top of the swell won't accelerate or slow down the swells speed, it will only add additional small chops on the surface, or create a slight loss in swell energy due to white-capping.
Once a swell is established and moving through the ocean, the only factors that seem to be able to slow it down/speed it up is the ocean bathymetry and bottom friction, strong tidal or other currents, but not wind.
Craig , how come when a SOLID period swell has set in ( shown itself locally and inshore conditions are light , then we only see the long period Ground swell and not the ten other period swells that have been running through the same waters from as far away as Sth America ...
Over distance following swell energy must be combined into the dominating swell train energy / period . Even though the wind speeds are different when a Large period swell travels through the same direction winds of lesser strength for LONG a long fetch or multiple fetches then it must have an influence .
Being a SA boy i thought you'd see this , or was the Mid coast too unreliable breeding ground for life long obs , just like the MP in Vic where the majority of swells have refracted or felt the bottom for considerable time . When you guys started surfing the SE and NW of SA did it open you eyes ? I know it did here when i started travelling and then living at the 4 locations mentioned in the above discussion .
I don't have an answer to that sorry Southey, I think we need someone who's done serious research in wave dynamics and wave on wave interactions to try and explain.
I know that it's not even fully understood how waves fully interact in the open ocean, and that there are just very accurate theories and equations that have been made to try and explain it. It's quite a chaotic interaction, hence no one can even really predict where and when a freak wave will occur, only a statistical probability of when/how often it will occur.
I do see what you're getting at and you're right, at exposed spots across the country you don't usually see all that underlying energy in different periods from distant storms, just the more dominant swell.
I have however seen two strong and independent swells commonly interfere with each other across the East Coast and Southern Coast of Aus, wrecking waves by having the first swell right in front of the second when coming to break, making both waves nearly unsurfable.
If you've ever talked with a paddle skier they'll talk about linking together tiny windswells and waves that pop up and disappear right in front of them. This is the swell constantly moving and transforming through the ocean. They can't catch the longer groundswell energy as it's too fast to get onto, they only ride it for a short period.
You can see this happening with strong swells all the time if you stand above a headland. If you closely follow a set with your eyes you'll see the wave train fading and redeveloping right until it's about to break, fascinating to see.
Didn't really answer the question, but love chatting about it.
Just an observation from S WA kinda on topic, but i've noticed that the big swells, in the 16-18 second period, that come from a super broad frontal system tend to have larger variations in wave period than the swells that come from more acute, smaller frontal systems. For example, there will be 13, 14, and 15 second periods of smaller waves, from same/similar direction, but also still, there's the bigger drawn out bombs in the longer period range. Not sure if this translates to different wind speeds across such a broad fetch, hence a larger variation in resulting waves and wave energy reaching the coast, as opposed to a more specific wave period that comes off a narrower/more acute fetch. Might be barking up the wrong tree but that was my guess on it.
One more observation from down here is longer period swells tend to arrive ahead of forecast here by several hours where as i noticed on the East Coast, S Swells tended to usually fall behind forecast arrival.
Yep, nailed it. Those big, broad systems with multiple fetches of all different strengths have a bigger mix of different swells from mid-period to long-period. But if just a really intense fetch that dissipates quickly it'll be mostly just the long-period stuff.
And for East Coast and Indo, the swell source is usually further away, hence the delay in arrival.
Ahhh cool.
Thanks Craig.
And true to Murphys Law, I just bolted down to beach on dark and the swell that was meant to start showing this evening is running a little late. Ha! First time i swear!
Ha, yeah when the reporter mentioned a pulse in size this morning I thought it was coming early. Tomorrow will be different.
Buoys are ticking up now BD and Craig
lots of south in the swell
I'd love to see a breakdown at the molecular level of how swells are generated. All I can find in google image search though is that diagram with the circles decreasing in size to the sea floor. I'm too lazy to start pouring through textbooks :|
pouring? or pawing? or maybe poring???
boring Mitch ....
most wind / wave models only have basis in lakes or far less scale than groundswells . I spent a huge amount of time looking at this for feasability of large scale wave power theories . There's nothing out there , and if there is people are keeping close to their chest ... In all honesty , no -one out there has spent more time looking at swell behaviour than lifelong Surfers . It becomes part of our rhythyms , how we live our life and where / when , fishoes get it , but struggle to put it to use ... not many life long fishoes that have rarely surfed will snavel / survive a solid set at many locations during a large swell event ... some people unwittingly get to know the rhythyms of their oceanic surrounds and have it ingrained into their own understanding and timing of weather events / seasons .... not that they could convey that to others ... this is where i struggle .....
thanks Craig i appreciate the time you put into this unfortunately i'm wasting the time i need to earn ..... so until another time ....
PS i didn't mention wind swells only differing levels of ground swell ....
In point the swell i discussed with Ben a few years back had all day light winds and a BIG swell running , a new even BIGGER swell arrived , my issue was that with the arrival of this Massive swell , it's arrival heralded the complete end of any other waves , in between sets .... ( it sucked / drew all the other energy out of the in state swell ) . needless to say , i left it to the tow crew , and one or two extreme optimists .
Yeah cheers Southey, making the observations first is definitely more fun!
In s.a. Its common for a swell to arrive late but in w.a. It doesnt seem to happen much . Why?
Worth mention is when i say wa im talking about sw but in sa im talking ep where there are offshore islands & reefs
Camel, that's because WA is usually closer to the swell source and the long-period energy doesn't have the distance and time to really get too far ahead of the bulk of the swell.
You'll see it occurring for swells generated over near South Africa, but anything generated from Heard Island and closer, the lag will be minimal.
Say we have a swell 1000km off WA with 20s peak periods but the bulk of it is back in near 17s.
So the 20s swell travels at 56km/h and the 17s at 48km/h
The 20s fore-runners will hit WA in just under 18 hours and then the bulk of the swell in the 17s range will arrive 3 hours later, and in just under 21 hours.
But if this swell travels further to SA, it has to travel another 1,700 km and this is where you'll see a greater time difference between the forerunners and bulk.
The 20s fore-runners will hit SA in just over 48 hours and then the bulk of the swell in the 17s range will arrive 8 hours later, and in just over 56 hours.
Then if we look at Vicco, the time lag would be more again.
What's the equation? S period *2.8?
Yep, about that.
Thanks Craig, great article and great comments also.
But dont u think its anything to do with the size of continental shelf or offshore islands ?
No, the effects of the continental shelf on slowing the incoming swell would be much smaller than the difference in travel speed based purely on period.
Not sure the equations for the reduce in wave speed in shallow water, will have to try and hunt them down. But remember the period doesn't change as the swell pushes through shallower water. This is because a decrease in wavelength is seen as the swell slows down, preserving period.
A great example of this with the current W/SW groundswell filling in across SA and Victoria.
The forerunners arrived midday yesterday on the Cape du Couedic wave buoy (18s) but the bulk of the swell is 24 hours behind and filling in now.
Thats a good example Craig , hows the period lines merging closer together into a purified swell quality .
Yeah totally, with no windswell in the mix and pure groundswell the peak period and average period become closer to each other.
http://www.transport.wa.gov.au/imarine/mandurah-tide-and-wave.asp
http://www.transport.wa.gov.au/imarine/rottnest-island-tide-and-wave.asp
http://www.transport.wa.gov.au/imarine/cottesloe-tide-and-wave.asp
See directional data of all 3 buoys .
See how the swell direction comes into metro beaches from a nw direction . This is a perfect example here on the directional graph data.
http://www.transport.wa.gov.au/imarine/cottesloe-tide-and-wave.asp
( Copied from the other thread moved here to discuss ).
Hey Craig , ive been watching CB for 2 months and its wnw direction swell almost every day on the arrow . I dont believe current as much as the fact the waters too shallow in the sw direction so swell probably does come around the north of rotto .
FYi the Rottnest buoy averages a more west direction on average than any other buoy . The perth canyon , LC and bathymetry does this, according to my long term buoy obs .
The cott buoy shows pure wnw at times on the arrow but this week and today it has sw and nw direction combined . See graph )
I had never studied the cott buoy for obvious reasons before now . I plan to watch it for a year before making an official claim about it, as it could certainly (both cott and rott ) be current affected perhaps ?
http://www.transport.wa.gov.au/imarine/cottesloe-tide-and-wave.asp
Please post my picture will send it mms to you craig .
When the university did the study a few years ago . ( you still have the report craig , wouldn't mind it back if you can find it ) this was not mentioned , Ive never heard of anyone mention this before . Online version would be great thanks craig .
Clam .
I can't find the thread where it was originally brought up , but it was prior to the start of this thread .
" http://www.swellnet.com/news/swellnet-dispatch/2012/03/29/early-bells-fo... "
I brought up Rotto when speaking of current /tidal flows as evidence along with Bass Strait , SA mid Coast , Lombok etc . Not sure where it is , but may have been on one of those SA centric threads in 2010 or 2011 ?!?
The swell train may arrive late but the G Train comes right on time.