Re Swell direction

barley's picture
barley started the topic in Saturday, 19 Nov 2011 at 1:49pm

G'day Benny, a while ago I asked a question about South oz and why our year has been so shit for swell and the mid has been one of the best ever. You never really gave an answer just a forecast for swell to actually hit yorkes(finally). While spring and the last week has been fucken sick why have there been alot of wsw swells? and when they haven't been happening it has been ssw and hitting victor.It is really unusual for this side to be flat yet the mid 3ft. Apparently it is the worst winter for 20yrs! Is there a reason for this swell pattern

thermalben's picture
thermalben's picture
thermalben Sunday, 20 Nov 2011 at 8:35pm

I don't know of any specific reasons for this year's weather patterns to favour persistent W/SW swells in SA. It's been something I've been wanting to research for quite a while - however I suspect there'll be a lot of difficulty distinguishing between the swell directions in SA - it only takes a subtle difference to render a swell favourable (or unfavourable) for the Mid Coast.

Studies have been done to correlate Southern Ocean swell trends against the 'Southern Annular Mode' - but the SAM index covers the entire Southern Hemisphere (700mb height anomalies poleward of 20°S) so I'm dubious as to whether it'd provide a useful index for specific Australian coasts.

However, this area is of great interest to me and I'm hoping to allocate some research time to this over the coming years.

barley's picture
barley's picture
barley Monday, 21 Nov 2011 at 4:14am

Thanks for getting back to me mate. When the El Nino was present alot of High pressures dominated our state and pushed the low pressures and cold fronts down below us giving a lot of great surf and surface conditions. Last year with La Nina it was a really wet summer but since its dissipation for some reason the lows have been coming from a very westerly direction and not so much from the southerly direction. Would it be fair to say that as a result of a new La Nina presenting itself off the east coast we could be in for a repitition of 2011. Is there a defining weather pattern along South Africa or ocean/air temps that create these situations?

Craig's picture
Craig's picture
Craig Tuesday, 22 Nov 2011 at 7:40am


South Australian rain totals are so low that there would be near negligent flows reaching the open ocean from these falls. In fact the tidal movement every day through the gulfs would be factors of magnitude bigger than any run off outflow. And the tides are there all year every year.

Just on a side note, the natural cycle of the gulfs sees a slow clockwise flow of 'fresh' ocean water in the western side of each gulf before slowly dispersing throughout the gulf, while on the eastern side, at the ocean floor there is an outflow of high salinity bottom water. This hyper-saline water is created at the top of the gulfs where there is excess evaporation. The hyper-saline water is more dense than the ambient ocean water, sinks to the bottom and due to the Coriolis Force flows out the eastern side of each gulf. This outflow would be stronger than any freshwater outflow, and in saying that is so small compared to the tides it wouldn't come into play.

Oh and you can see why building a Desalination plant at the top of one of these gulfs where the water is very saline already is such a good idea for the environment.. Not!

Back to the west swells, I don't know if we can draw a link to La Nina for South Aus. I say yes there is a link for the East Coast with the propensity for more Tasman Lows or ECL'S due to the warm water/cool air interaction but how this translates west has to be studied more.

The reason though that there have been numerous and more W/SW swell events across SA this year is due to the Long Wave Trough being positioned multiple times over the south-west of WA or in that vicinity. This helps steer cold fronts up towards WA and the Great Australian Bight, generating swells with a lot more of a westerly component compared to a south-west.

This also translated to a poor winter on the Surf Coast in Vicco as the swells were always too west, with cold fronts being deflected on a south-east track as they approached Bass Strait, instead of coming from an ideal south-west direction if the Long Wave Trough were parked over Tassie.

There's obviously lots more room for research which Ben and I are very interested in delving into.

barley's picture
barley's picture
barley Tuesday, 22 Nov 2011 at 12:15pm

What has caused the long wave trough to be positioned over sw wa? we haven't had any high pressure systems over us to push the lows away. Infact we have had quite alot of nw/sw wind and good rainfall. Usually with good rainfall the cold fronts track vigorously up from the sw and sort of miss wa and the bight and sweep across us. At the moment we seem to have a weather pattern not yet setup for summer. ie no s/easterlies and alot of tropical/subtropical rain clouds and with La Nina expected to be reforming soething must be altering in this weather cycle.

Craig's picture
Craig's picture
Craig Tuesday, 22 Nov 2011 at 1:00pm

That's the big question Barley.

Was it just a coincidence that the LWT was parked up near WA more times than not this year, or was it linked to La Nina, the Southern Annular Mode or some other signal.

The current weather is related to the weak La Nina we are currently in, and also probably the higher than normal Sea Surface Temperatures off the WA coast. As cold fronts and other weather systems move in from the west maybe they pick up more moisture than normal and bring it across to SA, but I'm no expert on that stuff.

Also maybe the recent passing of the Madden-Julian Oscillation (MJO) across the north of the country during the 4th-8th of November helped along the way.

The MJO brings with it increased instability in the topics and moisture which can feed down from the north-west into South Australia and Victoria.

Also there's been warmer SST's to the south of Australia, and this would help to supress any strong (say 1040hPa) high pressure systems forming, possibly in turn decreasing the likely hood of a strong south-easterly ridge building over the state??

All very interesting stuff though!