A look back on the December of surf

Craig Brokensha
Swellnet Analysis

The Mean Sea Level Pressure (MSLP) anomaly charts for the month of December have come in, and as you've likely read on the site and you'll surely read in the comments below, it was very poor across Victoria and South Australia, with Western Australia not faring much better.

The term 'anomaly' means the difference from the 30 year climate average, and when observing a high pressure anomaly it indicates that the pressure was higher across that region compared to the long-term average, vice versa for lower pressure.

Mean Sea Level Pressure anomaly (difference from long term average) chart for December 2020

Looking at the MSLP anomaly chart above, two main thing stand out. That being the strong high pressure anomaly south-west of Western Australia, spreading across southern Australia and into the Tasman Sea, and secondly the lower pressure across the north-west of the state and small dip across northern NSW.

With La Niña finally kicking in across the country we saw the subtropical high pressure belt shifting south, allowing the northern monsoon to drift south, feeding in moisture across the northern, eastern and central parts of the country.

December 2020 rainfall totals

This is to be expected under La Niña, as is the retraction of the westerly storm to the poles, known as a positive Southern Annular Mode (+ SAM) event. While this is great for bringing the rain and swell for the East Coast (discussed below) it's far from ideal for the southern states.

With the storm track pushed further away and an effective blocking pattern setting up south-west of Western Australia, we saw windy days across the southern half of the state (WA), intensified by a heat trough (the dip in the high sitting across the South West), with instability and moisture to the north.

The block resulted in little to no swell across Perth and Mandurah, with small to moderate events across the Margaret River region, broken late in the month by a large, long-period though inconsistent W/SW groundswell from over near South Africa.

The blocking setup was poor for South Australia, not because of the south-east wind regime (which we expect as this time of year), but because it prevented mid-latitudes fronts pushing in from Western Australia, bringing westerly swells for the Mid Coast.

Victoria deteriorated through December after offering plenty of fun options across the more exposed beaches and breaks through the start of the month. Onshore winds kick in with no quality swells of power and size.

With the polar frontal activity the South Arm in Tasmania faired relatively well with persistent small swells, though troughy weather brought funky periods of onshore winds.

The chart below shows the average winds through December and you can see the strong south-southeast winds off Western Australia, zonal westerlies aimed away from the continent and easterly trades in the Coral Sea.

Mean (average) winds for the month of December 2020

Moving to the east, with the subtropical high shifting south and the increase in instability across the Coral Sea owing to the Madden Julian Oscillation, a significant, supercharged trade-swell event developed, bringing large, stormy surf to the south-east Queensland and northern NSW regions through the middle of the month, followed up by an inconsistent E'ly groundswell event from Severe Tropical Cyclone Yasa.

This can be seen by the dip in the pressure anomaly chart sitting over northern NSW and south-east Queensland, with lighter winds and cleaner conditions topping off the end of the month with fun amounts of trade-swell.

Sydney and the southern NSW coast offered, cleaner, smaller surf and great waves with generally light, workable winds.

For eastern Tasmania there were plenty of fun surf days with surface troughs deepening into lows while moving off the mainland, initially generating swell from the south, then more beefy swells out of the east-northeast. There was also smaller swell energy emanating from the northern Tasman and Coral Seas.

Looking ahead, the La Niña signal is peaking in the equatorial Pacific Ocean, with it due to return to neutral by late autumn. With this the country will continue to fall under the influence of La Niña through summer and likely early autumn, with active swell generating systems for the East Coast, less so for the southern states owing to dominant though weakening high pressure.

Comments

James KC's picture
James KC's picture
James KC Thursday, 7 Jan 2021 at 4:26pm

Managed to miss that fun week and a bit of waves in Sydney because of COVID isolation... Bummer but especially because of the timing of it.

Roystein's picture
Roystein's picture
Roystein Thursday, 7 Jan 2021 at 7:39pm

Anyone know how the last chart (wind composite mean) translates to off season surf in the Maldives? Seems to look favourable?

nextswell's picture
nextswell's picture
nextswell Thursday, 7 Jan 2021 at 8:17pm

Definitely noticed the onset of earlier humidity/rain on MNC (north of Coffs). About 500mm for December. Some good waves though despite the brown water and seaweed/kelp. Shame that big swell of early December made a mess of so many banks around the area

yahabo's picture
yahabo's picture
yahabo Friday, 8 Jan 2021 at 7:08am

So even Autumn is going to suck in SA?

Craig's picture
Craig's picture
Craig Friday, 8 Jan 2021 at 7:32am

Here's the seasonal forecast for Jan/Feb/Mar and there looks to be that high pressure belt sitting across the Southern Ocean.

There is hope however with that lower pressure forecast through the Bight/WA we could see a couple of mid-latitude swell producers..

Let's hope so.

thermalben's picture
thermalben's picture
thermalben Friday, 8 Jan 2021 at 7:34am

Looks good for continuing East Coast swell potential, but of course, local winds always have the upper hand.

freeride76's picture
freeride76's picture
freeride76 Friday, 8 Jan 2021 at 8:57am

Nice wrap as always Craig but I do have to take issue with this:> " a significant, supercharged trade-swell event developed" in relation to the Dec 12-17 event.

That was a classic deep coastal trough that formed a coast hugging hybrid low.....a specific weather event and not at all a trade swell event.

appreciate the deep E'ly infeed into the trough/low was the fuel for the fire if you'll pardon the mixed metaphor.

we are still "suffering" the effects from that event. Dirty water, disturbed banks (mixed bag, some good and even improved).

Craig's picture
Craig's picture
Craig Friday, 8 Jan 2021 at 9:09am

Yeah I agree, though we had the developing trade-swell which then was super-charged by the tropical depression come coastal trough. The trade-swell then continued as the localised storm energy dissipated.

Two separate swell events yes, but related.

Ben Harding's picture
Ben Harding's picture
Ben Harding Friday, 8 Jan 2021 at 9:26pm

I see you've softened your stance on your "bank busting" comments pre, during and immediately post the swell event Dec 12-17th. These things are a lottery aren't they? Some spots deteriorate, whilst others may stay the same or improve, which happens throughout time anyway right.

freeride76's picture
freeride76's picture
freeride76 Saturday, 9 Jan 2021 at 7:00am

time heals all wounds.

generally though, net negative for this stretch.

hero sand and hero crowds has been the upshot, for the most part.

c'est la vie.

Ben Harding's picture
Ben Harding's picture
Ben Harding Saturday, 9 Jan 2021 at 8:54pm

I'm yet to experience what home looks like as Ive been away since the 17th of Dec. Mixed reports from mates around where I live on central Central Goldy. All in all, will probably just have to avoid low tide until they sort themselves back out if need be.

Such is life indeed.

freeride76's picture
freeride76's picture
freeride76 Friday, 8 Jan 2021 at 9:13am

copy that

velocityjohnno's picture
velocityjohnno's picture
velocityjohnno Saturday, 9 Jan 2021 at 12:45pm

Hey Craig, are you aware of the SSW (of the stratospheric variety) that the NHemi is copping at the moment?

We had one and it led to lots of westerlies here, cooler and swell. Wonder if it will do the same for Eurasia/N America?

Craig's picture
Craig's picture
Craig Saturday, 9 Jan 2021 at 12:47pm

Yep VJ, and will bring strong cold outbreaks etc to the Northern Hemi.

velocityjohnno's picture
velocityjohnno's picture
velocityjohnno Saturday, 9 Jan 2021 at 1:51pm

There have been a few records already this year:

https://www.seattletimes.com/nation-world/bomb-cyclone-breaks-records-of...

I'll try to find the China 400 year low temps and Mongolia record high pressure in an anticyclone... stay tuned

https://www.washingtonpost.com/weather/2020/12/29/mongolia-air-pressure-...

notice I'm using official links lol

for the China one I don't know all these sites, they range from official CCP to ?? to alt media:

https://www.sixthtone.com/news/1006649/china-ends-2020-shivering-in-cold...

https://electroverse.net/china-20-weather-stations-break-lowest-temp-rec...

https://www.globaltimes.cn/page/202012/1211414.shtml

given the spread of sources, it appears to be happening

velocityjohnno's picture
velocityjohnno's picture
velocityjohnno Saturday, 9 Jan 2021 at 7:13pm

All we need now is 3 big bomb cyclones to cover the NHemi, the fuel in the lines to freeze (saw that on the diesels at -10 Mt Hotham!), and we've got "The day after Tomorrow". Gulf stream is slowing too.

velocityjohnno's picture
velocityjohnno's picture
velocityjohnno Saturday, 9 Jan 2021 at 7:14pm

Spain, lowest temperature ever recorded:

https://www.euronews.com/2021/01/07/spain-records-coldest-ever-temperatu...

that'll be the SSW? How common are they up there?

Our one most recently was the first since 2002 I think.

AndyM's picture
AndyM's picture
AndyM Saturday, 9 Jan 2021 at 7:18pm

-35 in the Pyrenees!

Wow.

vbabin's picture
vbabin's picture
vbabin Monday, 11 Jan 2021 at 8:29am

I feel like december was real average surf overall in southern NSW (except for that TC YASA swell around the 20th dec)

Last two weeks been small-weak-windswell combined with persistent SE winds and horrendous crowd effect

greg-n.williams's picture
greg-n.williams's picture
greg-n.williams Monday, 11 Jan 2021 at 12:44pm

Large swell event was with howling on-shores & very little quality with small windows appearing when the wind backed off a bit(still onshore though). Cyclone Yasa produced a couple of days of fun waves but crowds were peaking around this tourist hotspot( as has been happening for months now). Jan has seen small trade-swells with on-shores getting up early most days with the crowd factor as above!