Sydney's cold water episode explained
Up to the end of February southern NSW had bath-like water temperatures, however we've finally fallen under the effects of a cold water upwelling event.
If you've been surfing the last week or so you would of noticed the water temperature drop from a balmy 24-25 degrees to a crisp 20 degrees. While still relatively warm, it makes surfing in boardies a little uncomfortable, especially while sitting waiting for waves.
Every year this comes up, and people ask why have the water temperatures dropped, and it's worth a quick refresher (from 2006!).
For nearly two weeks we've been stuck in a diurnal cycle of light morning northerly winds ahead of fresher afternoon north-east sea breezes. While not especially strong these persistent north-east breezes have gradually shifted the warm surface water away from the coast under a process known as Ekman Transport.
Due to the Coriolis force, surface water is displaced to the left of the dominant wind direction in the Southern Hemisphere (to the right in the Northern Hemisphere). A north-easter down the East Coast will direct surface water to the south-east - i.e out to sea - which is then replaced/balanced by deeper cooler nutrient rich water rising to the surface.
Usually a string of days with strong north-east winds is needed to induce a cold water upwelling event, but as we've seen nearly two weeks of persistent weaker north-east winds each afternoon, the phenomena has been drawn out and will continue until we see a southerly wind change, pushing the warm water sitting offshore, back into the coast.
The image below left shows the current sea surface temperatures off the southern NSW coast and you can clearly see the cold upwelled water along the coastal fringe, especially off Batemans Bay and further south off Green Cape (Eden). Water temperatures are around 20 degrees, while further offshore the East Australian Current (EAC) is a balmy 25 degrees.
You probably would of also noticed the greener colour to the ocean water. This is because the cold water upwelled along the coast is full of nutrients which once brought to the surface and into sunlight causes phytoplankton to bloom. Phytoplankton contain chlorophyll in their cells and when they bloom in high densities this can be seen in the water and also from satellites in space.
The image to the right shows the chlorophyll concentration off the southern NSW coast. You can see it quite clearly along the coastal fringe where all the cold water upwelling has occurred.
Also worth noting is how the East Australian Current is nutrient poor - i.e. it's like an oceanic desert - and as a result, hardly supports any primary production.
Water temperatures will remain cool until we experience a pronounced southerly change up the southern NSW coast, pushing the water to the left and back into the coast. This currently looks to occur early next week, but until then take some extra rubber down into the surf.
Comments
Even tassie has been warm on the eastside thats quite interesting hey craig ?
Yes, Tassie is way above average off the East Coast, 3-4 degrees.
This is going to feed into a deepening surface trough off the East Coast next week, bringing lots of rain and swell.
I love these sort of articles but don't comment on them often.
Sometimes I remember sitting out the back in boardies when there's cold and warm patches around and you paddle into or through a cold patch and Brrrrr.... then out the other side into the warmth.
I kind of enjoy that in a weird way.
If it's crowded Zen the warm patches could be wee.
No complaints here. i have been hitting in boardshorts every afternoon...feels great!
Thicker skin than most.
Port Macquarie is the same. Even with a wetsuit and a rashy i was cold on monday. More around 16 degrees i think.
"Also worth noting is how the East Australian Current is nutrient poor - i.e. it's like an oceanic desert - and as a result doesn't hold many fish stocks at all."
Highly debatable Craig. It's the EAC that brings baitfish and pelagic fish concentrations. Plenty of papers around about that if the anecdotal evidence from fishing mags is insufficient.
I wouldn't say so. Lots of literature re this, and I've also dealt with marine biologists studdying the EAC. Chart shows it pretty clearly above as well.
Actually I should probably clarify and re-word that to primary production. That would be more applicable.
Yes, the primary production is related to the upwelling but the fish stocks, particularly large pelagics are often in the front water or warm cored eddies.
So, it's too simplistic to say the EAC is nutrient poor and doesn't hold fish stocks. It's a complicated marine ecosystem that can be highly productive.
Agree, but I stick with the nutrient poor statement, it's exhausted of nutrients once it starts pushing south.
Thanks, this is an interesting article. Can I ask why upwelling does not occur from the south? Or does it? Cheers
There's more info on that here:
https://www.swellnet.com/news/swellnet-analysis/2006/07/03/why-does-wate...
That makes perfect sense! Thanks
I've been driving up the NSW south coast from the start of Feb and have noticed that the water has been pretty warm. Just made it north of Sydney and the water is definitely colder now and I did notice the greener water around Treachery. I've been in boardies since I left Adelaide, the whole coast is like bath water!
Had to laugh at the 'Crisp 20º' comment Craig. I think in Melbourne we are around 21º at the moment and we think it is like a bath. Amazing what you get used to.
Most of the fisho's I know go looking for pelagics in the EAC? but you're saying the fish stocks are low in the warmer EAC....What am I missing here?
EAC brings a temp break and bait fish cannot push thru this bi lateral line fish tuna, Kingfish, marlin & Sharks are able. The temp break is like a brick wall to baitfish. So as I and many fisho's fish the EAC marlin will be found off Tasmania following the food train down from Cairns since birth.
Been lots of baitfish in the water off Sydney lately. And along with it bigger fish, penguins, birds and dolphins. Also something bigger seen chasing those fish, but I try not dwell on what it might be.
Oh yeah it's gotten fresh. Still boardies only after work but this mornings early I opted for my steamer which was a wise choice. It also pointed out the leaks in the fucker and reminds me I need a new steamer for when it's supposed to be steamer weather.
Why doesn't this happen with a SW wind in WA (which happens every day through summer)?
That's a great question!
SA sees upwelling with SE wind events and Indo(G-Land strong SE winds), but my guess would be because the depth of mixed layer (warm water) in the Leeuwin current is much more than the East Australian Current.
When the mixed layer is shallow, ie the warm mixed surface water not penetrating as deep, it takes less effort to upwell the cooler water, as there's less warm water to displace.
Will have to have a closer look into this.
Yeah, that makes sense. The Leeuwin is in the opposite direction from the EAC (ie against the Coriolis effect) so, combined with the shallow bathymetry, must restrict the upwelling.
Jezpo im sorry for not understand your question can u clarify what you are asking ? I did a little study on the leeuwin current so might have some info to help with this interesting stuff
?
It does happen , but as Stu has pointed out the bathymetry is to shallow .
The big difference I do see is that EAC is the dominate current , whilst over west , the cresswell , capes and Ningaloo currents are no where near as strong as the Lleuwin , which is weakened and occasionally stalled in summer . Infact only the Ningaloo current produces really noticeable effects on Temp , as it does have similar steep in shore bathymetry or short shelf . The major factor there though is as Ben pointed out the sea breeze up there is much closer to parallel to the coast , and as such the effect is stronger . The near 100 percent repetitive nature of near day long sustained SW breezes and strong in summer can infact reverse the flow up there .
But it's not unheard of down off Perth .
" http://oceancurrent.imos.org.au/news.htm "
Shows that Stu has been lucky on his occasions , some years I remember the double whammy of Strong SSE -SSW winds ontop of a XLarge swell running in excess of 4M . ( 2006 comes to mind , tough gig that day )
Anyone else up for " light " reading
" http://www.environment.gov.au/system/files/resources/5d5ddf48-0f87-4e76-... "
Pretty sure I was over there for 2006 Southey. I did it 4 or 5 times around the mid-noughties, one year a 5 hour crossing, another we got hit by a front mid-channel and it turned into 8 hours plus, and yet another year it was called off 'cos of the size (managed to score S Bay the next day!)
What time did you paddle out at S Bay Stu ?
Oh man...I don't know. Up at sparrows, rode the bike over around 6am, out at 6:30? From memory I had two sessions that day.
Did you notice a guy get out of the water as you and perhaps 2 others paddle out .
I had to get the first ferry to then catch a plane . ! You guys got rolled as there hadn't been a set in at least 20 mins .! ( will find the photo )
Got the photo Southey and it looks just how I remember it, however I can't recall a guy getting out of the water as we got in. One of us only lasted a short while and I can't remember if my mate and I got rolled though I do know it was pretty big.
Fuck you've got a good memory!
hahah . Yes Elephant memory .
But it was more a case that i was pissed that i could only surf it for an hour and a half after first light . I also stood on a duggite ( who had a go at me ) whilst getting out of my wetty which seemed to galvanise the day in my memory . Wasn't big though only had a few sets like that pre you guys arriving .
Reality is that place is best when the ferries can't run . :-))))) gets really good . It only stood out because i had to leave it this time . Like a breaking up with an Ex ....
I reckon your mate said hello / got washed over the inner shelf that sucks dry . nasty stuff
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For the Mandurah/Rockingham/Perth area I think it's due to how shallow the shelf is that extends off the coast. Even out in the Gage Roads shipping channels the water isn't very deep. I'm not sure of the depth but I've done the Rotto swim a few times and the bottom is visible much of the way.
Under a SW wind the top surface of warm water may move to the northwest, however the water that replaces it from below hasn't come from very deep and hence isn't as cold.
Only a theory...
Yep totally agree Stu!
Can someone explain what this question about the wa current is ?
So did you do the solo swim Stu or part of a team? I have a mate who swims it every year, he goes in a team but his son does it solo.
Only ever did it as a team BB. Rotating five minute hit outs for however long it takes, some years around 5 hours, others 8 hours. I've got a mate that's done it solo and he's a much better swim than I ever was, yet he was still a mess at the finish line.
Local bathymetry could be the issue here - it's relatively shallow inshore of the limestone reefs that extend north from Mandurah thru' Rottnest and beyond.
Also, SW winds aren't theoretically quite the right direction to induce upwelling, you'd ideally need a southerly flow (tending S/SE north of Perth to Gero). But again I suspect bathymetry is playing a strong role.
Yep, it's just too shallow
At bunbury the fishos have a saying, 1km = 1m
So 40km out and it's still only 40m
Basically it's one long shallow bank from Cape Nat to the abrolhos chain
Interesting topic
Ekman transport results in vertical upwelling (not horizontal).. it's pulling water from 'below' the surface, which is cooler (water temps generally decrease with increasing depth). However transport mechanisms in the ocean are very complex, and way outside my area of expertise.
No worries Mal, I finally worked it out, thanks. I think the Leeuwin current is a shallow current maybe due to the shallow bathymetry. Water temp variations on the lower west coast in my experience have been substantially less than those of the lower east coast.
The leeuwin isn't a shallow current . It runs quite deep especially in autumn . And it also doesn't run across the shallow bathymetry in close to shore . Waters at the west end if Rottnest can run up to 5-6 degrees warmer than inshore at Perth at the start to mid winter . !
Jamyardy the leeuwin isnt very strong lately too
And as I tried to point out ; offshore winds will keep it offshore but n-w type winds push it in close .thats why march-june will usually bring the current down to south coast wa & on to tassie at best . Due to regular nw type winds .
OK I'm getting the picture. We have had heaps of SE and SSE winds the last few months, however usually early mornings to 11am or so then the SW kicks in, so probably not consistent enough to produce an upwelling effect, however if it were to, would the water temp near shore change much anyway in WA, given that the Leeuwin current is some ways offshore, and the bathymetry near shore is shallow and/or unaffected by the Leeuwin. I have only noticed once this summer that the water temp changed cooler by a couple of degrees for a few days, and it wasn't much of a concern, noting significant. As for regular NW wind patterns the near surface water would move left so in a NE direction (toward land anyway), to be replaced by the deeper water, which as the Leeuwin is a deep water current kinda warm already. Would I be right to say a NE wind would be ideal for warmer waters on the immediate WA coast ?
Yes JY .
NE -ENE land breezes would help it . But it would need to last for a few days .
Cam , yes N- NW winds from Augusta to Port Macdonnel , help the Leeuwin reach the West Coast of Tassie .
Even west winds let it flow all the way
In my opinion a noreast wind thats strong for some days can remove the warm water & it can get cold if u are in the sw . But seeing the leeuwin current can be traced back to the Pacific Ocean theres other factors that might be contributing to the amount of warm / cool water aling the wa coast
Yes definitely . IOD would have the biggest impact on how much warm water was available to enter the start of the current SW of the Timor Sea .
Beat me to it Stu! We'll make a scientist out of you yet...
This here craig ↑
Yeah Stu and others have really answered that. Looks to be because the shelf is shallow and extends out further compared to the East Coast, hence there's no cool water to upwell.
Where the shelf comes closer like G-Land, Port MacDonnell, East Coast etc is where you see the strongest effects.
I think we can add that when an offshore blows all day or after the north Easters then water is colder yet again. It's again most likely up swelling but geez, the temp feels like 14 for Sydney area in mid January. I have only seen this a few times but it is so radically low that many swimmers suffer hypothermia.
So you have 40degrees outside and then jump into 14 - that is bloody cool.
Bathymetry is important as the closer the continental shelf the closer the deep cold water, which is why old mate at Port Macquarie is feeling the cold. Mid North coast nsw is about as close as the shelf comes in.
That chlorophyll image and its relationship with the cold ester is fascinating to see. On the north coast it is the red weed, polysiphonia, that moves in following the nor'easter and it is starting to get thick. Fingers crossed the southerly comes soon and sends it back out, it is crap to surf in.
Also the presence of prominent headlands, as the EAC pinches out during a NE there seems to be a vacuum on the south side of these headlands which initiates the upwelling. Usually seems to occur at a number of locations - Cape Byron, Smoky Cape/Hat Head, Seal Rocks, Wreck Bay
You can get similar effects in Indonesia with upwellings from the trench. I have had 17 degrees at G-Land and 19 at Uluwatu. With the high air temperature and a rapid rate of evaporation I was managing 40 minutes and that was with a wetsuit top!
So, cold current's hit ... how long until the shitty red "cornflake" weed is upon us then? Another summer phenomenon with it's own urban myth.
NOTE: sorry for being nerdy but I could help using that word ... phenomenon. It's on my gidgets spelling word list this week and we had to help her come up with a sentence in which it was used. Fuck me, took a while to be able to construct a sentence that made sense to her - fucking Yr 6 homework!
we're fine on the upwelling in this part of the world Wingy...the onshore flow has mostly been straight E so no ekman transport.
still warm as a bath here.
Why does everyone else understand the question jezpo asked ? Wheres the dunce hat & corner to sit in ?
Is this it ?; he asks why doesnt a sw wind make a cold upwelling in wa ?
Does he mean sw / nw area ?
Hey caml, my original question was why the WA coast, (bascially from cape leewin to Exmouth, but more specifically up to Geraldton) doesn't get upwelling (from my personal experience) with the prevailing wind, which, in Spring/summer is S'ly or SW'ly along the entire coast. i think everyone has agreed it's due to the shallow water and leewin current.
Cam, Craig answered it above. And I answered it before that.
"SW winds aren't theoretically quite the right direction to induce upwelling, you'd ideally need a southerly flow (tending S/SE north of Perth to Gero). But again I suspect bathymetry is playing a strong role."
I saw your answers but couldn't work out jezpos question cos it was very brief .thanks ben
Ok thanks jezpo, ben , I like to know about the wa currents & yeah it had piqued my interest but something wasn't clear . Now I see why ; as ben says a sw wind cant create an up welling . It might be cheeky to say but an onshore wind won't make upwelling, correct ben ? That can only be done by offshore winds , correct ? The leeuwin current flows strongest from feb to aug & the daily seabreezes only reverse that current a minor amount inshore close , but it can bring cooler water when the sou easters set in for long durations
depends on the coast angle Cam .
It needs to be as close to cross shore to get results . So that means direct Sth winds edging towards SSE up near Kalbarri . However as you are well aware Nth of Bluff SSW ( typical seabreeze is straight cross shore ) And that area has the most noticable temp. swings in Summer-Autumn . The lack of a wide continental Shelf between Ningaloo station and Exmouth exasperates this .
The biggest difference from West coasts to east coasts is that the Sth ward current on the east coast actually helps the NNE winds create the upwelling . Over west the dominant current opposes these winds . The winds are way stronger and more prevalent over west . So much so that they do create these micro counter ( Northward ) currents [ Capes and Ningaloo currents ] But the capes current speciifcially happens in shallow waters . Whilst the Ningaloo can actually redirect or push the Leeuwin out to sea at the NW cape on occasion .
Similar occurences can happen on the far Sth NSW coast with the EAC being driven out to sea in Eddies .
Reality is the Leeuwin always runs , its just that it isn't opposed as heavily in Autumn as the Morning land breezes last longer . Tides also help the ITF , and Timor Sea component , during these periods .
Sorry Ben beat me to it ... Shows how slow i am ..... 11 mins
Yeah I hear u southey taking ages to write a post then someone slips in before you !
It's sideshore winds that create upwelling. In general, southerly winds along west-facing coasts, and northerly winds along east facing coasts. Small perbutations in directional alignment (and also in wind speed, and duration) can have significant differences though.
Yeah thats right southey the leeuwin is always dominant , so ben are you saying that fresh offshore winds on wa west coast arent creating upwellings ? Thats a bit surprising .
Southey of course it depends on the coast angle ! A sou east wind from tassie to kalbarri for 3 weeks non stop im talking , that will push cool water to the souwest of wa
Caml , it's quite hard to explain ( which I won't try ) , but if you google Ekman spiral and then Ekman transport ( it's complex , but I'll let an expert explain it better than I )
Land Breezes ( direct offshores ) are what fuels the Leeuwin ( speeds it up ). You have to remember that due to the Coriolis effect the wind action on water doesn't cause the water currents to move in the same direction as the windflow it's to the left in the southern Hemisphere . Hence an Easterly ( offshore wind ) will push water in a Sth - SSW direction . Ekman spiral comes in the form that it causes the current to run deep ( as deep as the sea floor ) hence the leeuwin usually runs stronger and deeper a little way offshore . The distance offshore also gives the straight offshore a fetch ( for want of a better term ) , which gives a better influence of the wind over the sea surface . Very close to shore on beaches with little in the way of hills backing it . You may see more surface cooling due to shallow water to air temperature exchange in the coolest time of the year . But the bluff etc the water air temp differential can be quite extreme . Partly due to the coastal angle ( point ) projecting further out into the current /deeper waters . At Rottnest the current actually loops around the island counter clockwise . So strangely the south eastern side is warmer than the Northeast .
Too many big words for me southy ! I have noticed that sustained fresh offshore winds remove the top layer of warm water thus cold upwelling occurs . That was at margrets & other locations in oz . Isnt that correct ?
A browner fella smoking a cig, who is in such high position of yours Caml, only has to ask the sexatary about what those big words of Southey mean!
Its ridiculous , like what is he tryna say ? Wellingymon u get these scientists telling you some mumbo jumbo about spirals & how an upwelling isn't an upwelling , what do u make of it ? Exmouth is an incredible place to witness the current pouring around the cape
Will decipher later southey
North winds & earths eastward rotation , feb-aug lotsa n-w wind , cyclones do the most
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Cold comes from the deep...
And as expected sea surface temperatures are rising with the southerly wind (back to 25deg offshore).
Should fill in across the coast over the coming day or so..
And there we have it, the warm water is back!
Now that it's back I generally go on how many days of southerly it took to become warm vs how many days of noreast blows after it before it's cold again. Pythagoras would back this yeah?
How's this upwelling off the Lower Eyre Peninsula, from persistent strong S/SE winds.
It's down to 17deg!!
Check out the chlorophyll readings off the Hunter and lower MNC from all these north-east winds and upwelling!
Geez I reckon you'd be lucky if the MNC cracked much above 18 degree for much of summer in any given year. Steamers are the wetty of choice for most guys that aren't tourists.
https://www.mhl.nsw.gov.au/data/realtime/sst/
We tend not to get warm water till late Feb/March when the northerlies drop off and as you can see in the link above we sit well below every other section of coast for a good 2-3 months each year. I feel for the tourists trying to swim. If I was taking summer holidays with the hoards I'd be heading north an hour or so to Coffs with a six degree difference!
2018/12/30 03:00:
Byron Bay: 23.65
Coffs Harbour: 23
Crowdy Head: 17
Sydney: 21.25
Batemans Bay: 20.3
Eden: 19.2
Interesting