General gripe...

So, we chose to immigrate specifically to Perth, a decision based largely on the strength of the BOM wind rose shown below, which implies that every 3rd day in summer gets 20-30 knot SW winds in the afternoons. Looked like consistent and paradise-like conditions for windsurfing. However, just as I got some equipment, the winds have been mostly offshore easterlies, with forecast for more of the same. Is this normal for here?

Yep what Gestie said.

Don't worry, it will come :-)

Thanks for the explanation Gestalt, makes sense. Looks like BOM has pretty advanced modelling the La Nina-related temperature changes, and their projections show gradual return to normal starting now, and finishing by the end of the year: www.bom.gov.au/climate/enso/archive/ensowrap_20120201.pdf

Good point, Pointman - I did assume it was in knots.

This is measured at the airport though, subject to all sort of drag / ground effects - presumably it's a bit more at the coast, especially in the case of onshore winds. 18-22k is a very nice wind range, glad to hear that.

I'll definitely take a drive to Geraldton one of these weekends, but might get lost around the Lancelin area :)

This is not unusual weather for this time of year but the troughing we have seen so much of this summer and last is NOT usual. Yes, la nina to blame.
Strong seabreezes 5 days out of 7 is more the norm.
Fingers crossed for next year. XXX

I reckon it made last winter a little better. General concensus?

hardie said...

Reasonable explanation Gesty, just to add a bit more, The Southern ocean is also a big determinant of the pressure systems that come across, and apparently, globabl warming means that the stronger southerly flows as they come across with both highs., and lows, are running further south of the continent, meaning we are not getting as many southerly flows, which bring strong seabreezes, so it may be a bit more than la nina and a bit more permanent

As for water temps and seabreezes? A personal observation was in DEc in Shark bay, water temps were about 25 to 27 degrees C, and the seabreezes were much stronger than perth which had 21 to 22 degrees C??? weather forecasting is very complex, chaos theory originated from it i think

I agree with you Hardie - it doesn't seem like temperature differential has much to do with the strength of the breeze otherwise logic suggests we would get stronger seabreezes on hotter days. As we all know, in fact the opposite is true.

On our strongest "seabreeze" days the wind is generally blowing from the south quarter pretty much all day and therefore it's cool.

Are our 25 knot+ seabreeze days actually thermal seabreezes then, or is the thermal effect just a kind of "turbo charger"?

Warmer water makes big fishies more active

i'm not convinced and not sure in the same breath.

i think la nina pushes the highs further south as it strengthens the sub tropical ridge.

a seabreeze though is a seabreeze. it needs a temperature gradient to form. the west coast trough is the result of the temperature gradient. it's conjecture on my part but it's possible that the trough forms further out to sea during a la nina event.

la nina could also provide too much cloud cover over inland australia which again reduces the temperature gradient.

with the basic setup of the trough the seabreeze is on the western side and the heatwave is on the eastern side. if the trough forms over land then you get seabreezes all day. if the trough forms over the ocean you get heat wave until the trough can move inland. it's the position and creation of the trough that controls your weather during summer.

the bom has some info saying the seabreeze in WA starts with E-NE winds early morning feeding into the trough. i suspect tho if those winds are too strong the trough won't be able to form. i think the magic figure is 20 knots.
then switches SW behind the trough and goes S as the day progresses due to the coriolis effect,

it could also be that with la nina the lows to the NE combined with the highs to the south create a pressure gradient strong enough to stop the trough from heading east. i don't know for sure.

what has me completely baffled is what moves the trough. i would have thought the trough could move under it's own force but articles i've read suggest it needs a cold front between two highs to move east.

anyways, the trough is forecast to return on monday by the bom. it's offshore.

be interesting to see what happens and if you get seabreezes or heatwaves.

Squid Lips said...

Mark _australia said...

AUS1111 said...

Winter 2009 was one of the best ever.
2010 was the worst ever, and 2011 was somewhere in between.

Have we had a summer worse than this one?

Yes, La Nina approx 2000 was just as bad or worse.

We will get 3 or 4 foooly sik summers after this.

I hope this is true! I've already sailed more in the last 4 months since moving than in the previous year on the east coast, so if this is a bad year, bring on the good

Yes, this is a bad year.

Mark _australia said...

AUS1111 said...

Winter 2009 was one of the best ever.
2010 was the worst ever, and 2011 was somewhere in between.

Have we had a summer worse than this one?

Yes, La Nina approx 2000 was just as bad or worse.

We will get 3 or 4 foooly sik summers after this.

I remember I had a Y2K job that meant I had nothing to do all day, and there wasnt any wind to take advantage of all that free time ! So it must have been the summer of 99 - 00

did some more reading and i think i understand it now. still it's only my best guess.
but seems hardie was onto it.

La Nina
during la nina periods the moonsonal trough is enhanced which pushes the subtropical ridge further south as low pressure system move further south over northern australia. the enhancing of the monson is driven by the MJO which cycles every 40-60 days and is defined by a pulse of hot moist NW winds across the top end of australia. it moves west to east and gets tracked by cyclone chasers as this same pulse usually means cyclones.

El Nino
the MJO also occurs during el nino cycles but the pulse/moonson is not as strong so the sub tropical ridge sits a little further north and allows blocking highs to sit in the bight bringing droughts to SA and heating up the western parts of WA while enhancing the west coast trough due to a higher temp gradient between the desert and the ocean. the enhanced west coast trough then gets pushed inland by the cold fronts along the subtropical ridge as the highs move west to east. As the subtropical ridge is further north during elnino periods or breaks in the monsoonal trough the cold fronts are in the right position to move the west coast trough inland.

so SA pain = WA gain.

currently there is a break in the monsoonal trough as the mjo moves away. i guess that may mean the return of the seabreeze until end of feb when the mjo returns.
doesn't really mean strong seabreezes though as it is still a la nina cycle so the temp gradients are not as high.

i guess you could look to the mjo when planning road trips etc.

the most dissapointing thing is that Mr sqid still sails more in WA during your bad season then we do on the east coast.

What gets me is quite a few times this summer we had the west north est trough which has just lingered for about a week at a time.

elmo said...

What gets me is quite a few times this summer we had the west north est trough which has just lingered for about a week at a time.

i found 2 papers that had specific studies of the trough. both indicated it is synoptic.

it forms where the temp gradient is. whether that's inland or over the ocean is just a result of where the temp grdient occurs. when you have a high in the bight pushing warm easterly winds and cool waters off the coast it forms. warmer easterly winds mean it deepens.

it won't move anywhere until a synoptic pattern occurs to push it. ie. a cold front comes along and pushes it eastward. what you would want is to be on the western side of the trough line as that's where the seabreeze is.

currently this year most of the cold fronts have been further south but as the monsoonal trough weakens these cold fronts move a little further north and then they are in a position to push the tough inland. it takes a few days and then the cycle starts over again usually but in la nina years it seems to just linger for weeks or not form at all because of lack of steering influences and or poor temp gradients

there's most likely another thousand things going on as well. the indian ocean dipole has an effect and the southern annular mode but it's all getting beyond my simple understanding. plus synoptic weather is 3 dimensional so upper level patterns will play a part.

d1 said...

General gripe...

So, we chose to immigrate specifically to Perth, a decision based largely on the strength of the BOM wind rose shown below, which implies that every 3rd day in summer gets 20-30 knot SW winds in the afternoons. Looked like consistent and paradise-like conditions for windsurfing. However, just as I got some equipment, the winds have been mostly offshore easterlies, with forecast for more of the same. Is this normal for here?

Can you give me a link to get a copy of one of these please? I want to see how the midnorth coast NSW looks. Pretty bad compared to this I'd say..