The cluster effect

Craig Brokensha
Swellnet Analysis

When comparing the current season to winters past, the years that come to mind are 2011, 2007 - when one of four succesive East Coast Lows grounded the MV Pasha Bulker at Newcastle - and 1974 when three Tasman Lows lashed the East Coast and caused enormous damage.

Different than a memorable storm, a memorable season usually involves the clustering of low pressure systems with resultant large and damaging swells, often from the east through to the north-east. You can throw the Black Nor'easter swell of 2016 into the mix, though this is an outlier as it was a lone storm and not a cluster.

So let's talk about storm clusters.

To start with, we know that the ENSO cycle - that being El Niño and La Niña - influences the East Coast wave climate more than other drivers such as the Indian Ocean Dipole (IOD) or Southern Annular Mode (SAM).

Numerous studies have looked at the wave climate off NSW, including peak wave heights and mean swell direction, during El Niño and La Niña years. Those studies found that, under La Niña conditions, on average, storm wave properties are of greater magnitude and the wave direction more easterly, while El Niño sees a reduction in storm wave properties and swells more from the south.

Wednesday July 15th - for many regions it was the best day of the year

For instance, a study by Davies 2016 which modelled coastal storm event sequences found the frequency of storms with direction north of 120° (i.e. easterly and north-easterly events) had a mean rate of 1.7/year in El Niño years and 3.9/year in La Niña years. This matched the historical data analysed from the wave buoy off Crowdy Head.

There was also a 15% increase in the mean annual number of storms in La Niña years compared with El Nño years off the Australian East Coast.

Further to this, it wasn't the mean storm wave height that was greater during La Niña but rather the storm wave duration.

That's because clustering of storms is more likely through La Niña events, and while we haven't yet ticked all the boxes yet for the coming La Niña, we've been feeling its influence on the East Coast since late autumn.

July 28th

July 30th, bringing south through south-east swell this weekend

While great for surfers who get to ride big wave boards on the reg, clustering is of great concern to coastal property holders, particularly during La Niña as storm events from the east to north-east cause the most destruction.

This is linked to the nature of sand flow. Littoral drift generally moves from south to north, so the northern ends of bays are loaded with sand, leaving southern ends with a thinner beach profile and less of a barrier to incoming storms. This means our beaches aren't setup for constant storm action and sand gouging from the east to north-east.

When back to back events hit, this creates a compounding issue and the results can currently be seen at Byron Bay, The Entrance, Wamberal, and Collaroy.

The solution to this problem is not clear, installing rock walls as defence will lead to erosion in the direct vicinity of this hard barrier, hence only moving the problem elsewhere. Instead returning the land back to council and the rate payers in the form of a natural dune system is likely the smartest approach and arguably the cheapest in the long term.

As things settle from the last two significant lows and swell events, another is starting to form in the Tasman Sea, but far enough away to not severely impact the already damaged areas.

Comments

freeride76's picture
freeride76's picture
freeride76 commented Thursday, 30 Jul 2020 at 3:01pm

"Littoral drift generally moves from north to south, so the northern ends of bays are loaded with sand, leaving southern ends with a thinner beach profile and less of a barrier to incoming storms. This means our beaches aren't setup for constant storm action and sand gouging from the east to north-east."

You mean from south to north?

Craig's picture
Craig's picture
Craig commented Thursday, 30 Jul 2020 at 3:04pm

Ah yes, fixed.

patricia.salter's picture
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patricia.salter commented Saturday, 1 Aug 2020 at 10:42am

Its interesting because I have lived on mid north coast for 4 years now and the cyclone swells affect MNC as well as the tail ends of the deep south lows-
Our locals especially pick up North easterly and easterly swells which seem to have greater damage factor-to dunes and the foreshore-especially when you factor in tidal heights-

freeride76's picture
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freeride76 commented Thursday, 30 Jul 2020 at 3:15pm

sorry for the nitpick, another great article.

Craig's picture
Craig's picture
Craig commented Thursday, 30 Jul 2020 at 3:18pm

Thanks, and not at all, keep me on my toes.

freeride76's picture
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freeride76 commented Thursday, 30 Jul 2020 at 4:14pm

can only seem to find archive charts back to '99 on the BOM site.

you know where to get earlier ones from Craig?

Craig's picture
Craig's picture
Craig commented Thursday, 30 Jul 2020 at 4:25pm

You can create your own.. https://psl.noaa.gov/data/composites/day/

Takes a while but select the variable (Sea Level Pressure), then go down to below all those boxes for creating composites and just after "Or" just select the year of the last day of range. I picked 1974, then picked May 27 to May 27 to get that day.

Then down below can put in lat/long boundaries (select custom) or click one of their pre made regions (Australia)..

This one below is.. Colour "Black and White", Shading Type "Contours(Black and White Only) and select Plot contour labels? "Yes"

Island Bay's picture
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Island Bay commented Thursday, 30 Jul 2020 at 5:56pm

Excellent link, Craig!

Westofthelake's picture
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Westofthelake commented Thursday, 30 Jul 2020 at 6:37pm

Cheers Craig. I had a crack and got what you did so feel I've passed the exam or something. When I tried widening the longitude it just defaulted back to 160E though.
A testament to your teaching skillz but.....at least better effort than this 'how to do' I found some time ago :)

uploadphotosjustaskcraig

Craig's picture
Craig's picture
Craig commented Friday, 31 Jul 2020 at 7:42am

Ha, so make sure you have "Custom" selected in the Region of Globe drop down.

Here's what I've done previous for a guide..

Westofthelake's picture
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Westofthelake commented Friday, 31 Jul 2020 at 10:34am

Thanks again. I got it to work.

JodyP's picture
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JodyP commented Wednesday, 5 Aug 2020 at 11:40am

Did anyone ever read Captain Ron Ware's account of that May 1974 storm in Tracks Magazine back then? An amazing read. Wave recorder buoys off sydney read 40 feet. Buoys in Botany Bay read 15 feet. He lived in a house on top of the hill at Bilgola, and had rocks thrown up over the top of the headland 100 feet above sea level. Waves breaking across the bridge at Narrabeen lakes. Etc. I have the BOM weather charts here for that storm (actually, 3650 daily weather charts spanning Sept 1972 to Sept 1982). The Double Island Point wave recorder buoy off the Qld Coast was even knocked out of commision and stopped recording. The difference with that low wasn't just the intensity and squeeze with the adjacent high to the west, it was that the high came through and was stalled dead by the low out at sea, then the low travelled WEST towards Sydney and accentuated the squeeze way more. A now unavailable CSIRO Research Report discusses the 'Anomalously Warm Sea Surface Temperatures In The Coral And Tasman Seas Between 1967 and 1977.

JP

thermalben's picture
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thermalben commented Wednesday, 5 Aug 2020 at 12:00pm

Fascinating Jody.

Did you mean this document?
http://www.cmar.csiro.au/e-print/open/CMReport_114.pdf

JodyP's picture
JodyP's picture
JodyP commented Wednesday, 5 Aug 2020 at 1:26pm

Yeah Ben. I did a websearch and a CSIRO data search quite a while ago and couldn't find it? I have the hard copy here. I just pulled it out and flicked through it. Noticed on page 9 on the discissussion about Eden coastal temperatures ... "the single exception could be the very warm temperature in 1974 arising out of the 1970-1973 warming event." I noticed that paragraph because in my variety of highlights and pencilled in comments from going through that document decades ago, I wrote "May 1974 storm. Low off Sydney. Worst ever recorded. Swell 40-60 feet". That would have been referencing Captain Ron Wades Tracks story where he in turn referenced the readings off the Sydney Wave Buoy Recordings and, I think from memory, the ships reports (read off depth sounders I assume)??

Just thinking Ben, I have friends who have worked on superyachts around the world for decades and say the Tasman Sea is the worst conditions they have ever encountered. A lot of people don't factor in the South running 'east equatorial current' running down the Australian coastline off Qld and NSW.

I notice the sea surface temp anomaly charts are showing adecent warming off the NSW and Tasmanian E coast at the moment?

JP

thermalben's picture
thermalben's picture
thermalben commented Wednesday, 5 Aug 2020 at 1:50pm

Yep, daily anomalies are up about 1-2 degrees (first image), weekly (second) and monthly (third) are up about 1 degree.


Pops's picture
Pops's picture
Pops commented Wednesday, 5 Aug 2020 at 12:09pm

An old mate's dad told me that his car got washed off the bridge at narrabeen in that storm...
Heard stories of waves breaking a long way up pittwater too!

He who hesitates is lost

shoredump's picture
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shoredump commented Wednesday, 5 Aug 2020 at 1:49pm

I was born in May 1974. I’ve always wondered what variety of 12 foot bombs I was born into. Anyway, thanks to Craig’s link, I can now see I was born in between systems. Doh :(
Great stuff anyway, cheers

freeride76's picture
freeride76's picture
freeride76 commented Thursday, 30 Jul 2020 at 4:30pm

Sick.

I was looking at some swell diaries from 88/89 and what struck me was the amount of swell during summers of those years.

wanted to confirm observations by checking mslp charts.

adsi's picture
adsi's picture
adsi commented Thursday, 30 Jul 2020 at 4:48pm

Currently studying coastal engineering so pretty interested in studying the erosion patterns...
I wonder how well artificial reefs in problem areas (ie clarkes beach, belongil) would fare. Will be good to see the review of the palmy reef to see if they've seen lower levels of sand transport.
Coastlines are constantly being reshaped though and i feel like it's probably a bad idea to build a house too close to a sand beach.

billie's picture
billie's picture
billie commented Thursday, 30 Jul 2020 at 6:00pm

win win win

wave quality, waves impacts on shoreline lessened, reefs for marine life

Billie

adsi's picture
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adsi commented Thursday, 30 Jul 2020 at 6:26pm

Yeh I would of assumed so but hard to know whether its disturbing the longshore drift and therefore starving somewhere further north of the sand.
Would be great to get some artificial reefs going along the coast, spread the shit ton of surfers out

the_b's picture
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the_b commented Friday, 31 Jul 2020 at 7:39am

I think some of these stretches of coast in the lee of the headlands (Byron, South West Rocks are in a natural process of shoreline recession anyway. The south-north movement of sand past the headland ends up in deep water or bypasses the bay on the northern side of the headland

groovie's picture
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groovie commented Friday, 31 Jul 2020 at 6:18am

Building houses on the dune system is a recipe for disaster for the tenants. Artificial reefs, if done properly may save some homes in danger but then impact others due to sand flows being disrupted. The only real option is retreat from the dune systems along the entire coastline. This will only become more common place due to sea level rise & the influence of climate change on the frequency of damaging storms( all of this driven by over population, check out Planet of the Humans on You Tube)! Cheers.

frog's picture
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frog commented Friday, 31 Jul 2020 at 8:45am

June 10 & 11 1994 - one of the biggest ever SW groundswells (lit up everywhere from Victoria and then up NSW coast) - checked the map generated for that date and it just looks like a long westerly fetch across the Bight. So the data is not that accurate or detailed enough for that type of system back then to be reproduced into a map that reflects the real situation.

Still it is an amazing resource.

Frogg

Craig's picture
Craig's picture
Craig commented Friday, 31 Jul 2020 at 8:47am

Ah, I'll have a look myself when I have a chance, but go back a week before the event to see the whole swell generating system to get the best idea.

thermalben's picture
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thermalben commented Friday, 31 Jul 2020 at 8:55am

Coupla things - hindcast data varies in resolution. Anything that far back will be much coarser than what we have available today (so, it won't pick up smaller features, for example a small region of storm-to-hurricane force winds within a broader system, which can often be the source of temporary though significant surf events). 

But also, it's a model reanalysis and given the scarcity of Southern Hemisphere data - including a complete absence of observational data in the Southern Ocean - I'd keep swell hindcast expectations low. 

freeride76's picture
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freeride76 commented Friday, 31 Jul 2020 at 9:20am

it must have done something in the Tasman if it generated something meaningful for NSW.

westerly fetches in the bight won't get the job done.

Craig's picture
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Craig commented Friday, 31 Jul 2020 at 9:28am

Yep.

frog's picture
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frog commented Friday, 31 Jul 2020 at 7:42pm

It was one of those deep lows that intensified as it moved below Vic and Tassie but had the angles right to push up with plenty of south in the swell so it aimed up the coast not just towards Fiji as is often the case. Sort of like the 1984 easter swell at Bells but went right up the main aussie coast. 1994 Queens birthday long weekend 3 day swell.

Frogg

freeride76's picture
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freeride76 commented Friday, 31 Jul 2020 at 7:47pm

yes! I remember that one.
was down the south coast.

got my arse kicked so hard trying to ride QLD banana boards.

Groper's picture
Groper's picture
Groper commented Saturday, 1 Aug 2020 at 8:30am

Maxing 8-12ft Central Coast reef, holding because of ~2.0m tide???? New moon. Waves of my life!!!

thermalben's picture
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thermalben commented Saturday, 1 Aug 2020 at 8:45am

FWIW, here's the printed synoptic charts in the days leading up to the 10th.

First chart is MSLP on June 8th, second is June 9th whilst the third is the 24 hour forecast chart for June 11th (issued June 10th) - reason for the last one being a forecast chart was because it was a weekend paper. Weekday papers back then seemed to print yesterday's analysis chart, but the weekend synoptic needed to provide enough info for Sat and Sun so they went with the forecast instead.

Looks like a Tasman Low to me and a nice run of S/SE thru' SE groundswell along the East Coast.

If Victoria had large surf at the same time then it would have originated from a weather system positioned out of view, off this chart (the early stages of the system that would become this Tasman Low may have generated a S'ly swell for Vicco, but it would have arrived a few days earlier than the 10th/11th).


freeride76's picture
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freeride76 commented Saturday, 1 Aug 2020 at 10:21am

Yep classic cut-off Tasman low. I remember that onion ring from the paper. Surfing 10 ft waves around the Dulla.

batfink's picture
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batfink commented Saturday, 1 Aug 2020 at 1:05pm

Ah yeah, takes me back Ben, pre-internet, when the newspaper was your best source for Synoptics, and you had to guess what was out there to the east and south where the map cuts off, and you would periodically get these mysto swells which you had to imagine the Synoptics for.

Mysto swells, the young folk will never know the pleasure of the mysto swell, or days of heading to the beach with no idea what was going to be there, and to come over that last hill and see glorious perfection.

thermalben's picture
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thermalben commented Saturday, 1 Aug 2020 at 1:09pm
batfink's picture
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batfink commented Saturday, 1 Aug 2020 at 1:49pm

:-) yes, I remember, now.

frog's picture
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frog commented Monday, 3 Aug 2020 at 7:54am

Thanks for the maps. The system had some power - long fetch and tight isobars.

Frogg

Yendor's picture
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Yendor commented Friday, 31 Jul 2020 at 1:16pm

Interesting read Craig,. Would you say the clustering of the systems occurs because the vacuum created by the onward movement of the last system allows a similar set of generation conditions for a new and similar system.

In La Niña scenarios without the westerly driver of El Niño is there more opportunity for this to happen?

I've often noticed a similar clustering in systems to the SE of NZ.

Of course strong El Niño seems to create endless clusters of flattened SW systems through the Southern ocean. It seems to me that La Niña allows for more weird and interesting weather events in our part of the world.

Island Bay's picture
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Island Bay commented Friday, 31 Jul 2020 at 1:23pm

Planetary troughs/ridges, Yendor.

But yeah, strong zonal flow during El Nino, with higher pressure over N Tasman/Coral Sea, and low SE of NZ. La Nina favours the opposite pattern.

stunet's picture
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stunet commented Friday, 31 Jul 2020 at 2:01pm

"But Risk Frontiers, a consultancy from Macquarie University, said the risks from the storms on assets such as coastal roads and property are being underestimated because they are typically treated as single events. Since east coast lows come in clusters, the impacts are magnified."

https://www.smh.com.au/environment/conservation/erosion-risks-underestimated-as-more-wild-weather-forecast-20200730-p55h43.html

Westofthelake's picture
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Westofthelake commented Friday, 31 Jul 2020 at 2:02pm

Looking like another ECL/Tasman Low for next weekend.

That would be 1 every fortnight for 6 weeks.

Clusteroclock.

MGB's picture
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MGB commented Friday, 31 Jul 2020 at 7:06pm

1990 & 1998 from memory were sick winters for swell as well

batfink's picture
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batfink commented Saturday, 1 Aug 2020 at 1:01pm

“Instead returning the land back to council and the rate payers in the form of a natural dune system is likely the smartest approach and arguably the cheapest in the long term.“

I agree with that idea, Craig. It was being looked at for our place over 20 years ago and I thought what a boon it would be for the local community, plus the people across the road would get a bonanza in terms of house price increases. But with the council owning the land (i.e. the ratepayers) council could terraform the land to both protect their roads and other properties as well as putting in protection measures for these big events.

Interestingly, at Norah head the beaches to the south that face south east were gouged out, and the beaches to the north that face north east we’re absolutely smashed. Especially to the north, it gouged out about 8 metres of dune that had been planted with what I think is the wrong type of grass, dunecare are helping the dunes and destroying the beach for surfers and swimmers. Banks there now are better than they have been for a long time.

Strangely, the beach at Cabbage Tree Bay, facing east and nor east but with various bombies around, has more sand on it than we have seen for years. I find myself lamenting that often when we think we are doing something good, the effects can be quite alarming down the track.

If I ever get off my arse I would like to bring this up with council. Interestingly one of the tracks to the beach further north, which has less sand build up and dune care activity had much less drop off and was in good nick. That area does protect some houses, but they are well back and not elevated in the way that Wamberal is. It’s a fascinating study, but from reading stuff on here and elsewhere I have come to the conclusion that dunecare that then leads to steep beach profiles is a disaster for users, and ends up more damaged during these cluster events.

Observations over a 25 year timespan.

redsands's picture
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redsands commented Saturday, 1 Aug 2020 at 3:56pm

Craig, I still think July 2011 will be hard to beat. Yes it wasn't 10ft, but with the LWT about as perfect as it could be for NSW, we ended up with two weeks of waves 3-6ft with westerly winds all day every day. I like the events that occur with west winds, as opposed to south winds where it's points n sheltered reefs only.

Pngy's picture
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Pngy commented Saturday, 1 Aug 2020 at 4:36pm

Love it Craig - another great piece! Any chance of producing a similar analysis centred around what impacts macro patterns in the southerns states of Vic, SA or WA?

bluediamond's picture
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bluediamond commented Sunday, 2 Aug 2020 at 5:10pm

Looks like there will be low pressure blanketing oz over the coming week. Wondering if this is La Nina related? Guessing there's gonna be some decent rain in some much needed places this coming week as well as some interesting little wave forming systems brewing sporadically.

big wave dave's picture
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big wave dave commented Monday, 3 Aug 2020 at 9:17pm

the clustering effect is a key phenomena with flooding as well. An initial flood is normally at least partly absorbed by a river valley, with pools and wetlands filling, and groundwater being recharged. A second flood in a cluster is likely to do much more damage to infrastructure. And often requiring a lesser rainfall event.it's very interesting. Thx Swellenet

Distracted's picture
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Distracted commented Friday, 7 Aug 2020 at 12:53pm

Wow, there’s the possible low off the NSW south coast next week Sunday/Monday (9th/10th Aug) but the long range models show yet another one the setting up the following weekend (15/16)!
Go the cluster!

Spuddups's picture
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Spuddups commented Friday, 7 Aug 2020 at 4:33pm

A lot of South swell due for the East Coast of NZ over the next week or so. At least three different systems one after the other. I think these are the systems that also send swell up to the Southcoast NSW. Would be interesting to see the state of the long wave trough at the moment.

Craig's picture
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Craig commented Friday, 7 Aug 2020 at 6:03pm

Yeah have a look at it drawing out and over you guys.. https://www.weatherzone.com.au/models/?lt=hemisphere&lc=sh

Spuddups's picture
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Spuddups commented Monday, 17 Aug 2020 at 6:11pm

Cheers man. As it turned out we’ve had eight days of solid long period SSW swell over here. These are the swells that march on up to Tahiti. They must have had some decent days up there. The long wave trough is a beauty when it lines up like that.

Craig's picture
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Craig commented Monday, 17 Aug 2020 at 6:37pm

How good!