Looking Back: Winter 2021
While I've recently run a number of analysis articles: the lack of Roaring Forties, pumping surf on the East Coast, and then the southern states having their turn, this article looks at the winter season as a whole.
If we remember back to April, the Surf Coast broke its slumber after 2020 proved one of the worst winters in recent memory and continued into a poor La Niña summer. Conditions were great for a month but then things again subsided (though opening up good options on the exposed beaches), with the East Coast offering frequent good swells under favourable winds.
This trend continued into the start of winter proper, with the westerly storm track finally finding a bit of momentum through July, though not through the deep Souther Ocean. Quite the opposite, with relentless mid-latitude fronts pushing up and into Western Australia bringing relentless onshore winds and oversized stormy surf to that state, and less favourable westerly swells for Victoria.
However, on the positive side, these conditions opened up more options across the exposed beaches east of Melbourne (usually a no-go in winter) as the frontal systems were slipping south-east on approach to Victoria, clocking winds towards the north.
South Australia saw a wet, warmer-than-normal, winter owing to the frontal systems riding high, and the lack of cold polar air being advected north.
Once this pattern kicked in across the East Coast during mid-July, we saw the swell tap turned off for most locations, with small, background S'ly swell energy spreading sporadically north from fronts pushing though Bass Strait and deeper below Tasmania.
It wasn't until the end of August that we saw a more substantial and very slow moving low fire up in the Tasman Sea, bringing some larger surf to southern NSW, with a combined mix of easterly and smaller southerly swells across northern NSW and south-east Queensland.
All this can be conveyed in this Mean Sea Level Pressure anomaly chart for the three months of winter.
The anomaly is the difference in pressure from the climate average from 1981-2010 for the three month period and blue indicates lower than normal pressure, with yellow being higher.
What immediately stands out is the broad low pressure anomaly sitting across the southern parts of the continent and also its alignment. With it pushing so far north, we can see the influence the persistent mid-latitude storm and frontal activity had, bringing onshore winds to WA's South West, westerly winds across South Australia, and north to north-west winds across Victoria. This saw lots of westerly swell energy for South Australia and Victoria, and while the Surf Coast was mostly undersized (apart from a couple of outlying swells), it was clean and fun.
For interest's sake, if this low pressure anomaly was shifted further south and east (and weaker), it would have indicated a more standard South Australian and Victorian winter with strong cold outbreaks and large swells from the south-west.
Looking back to the anomaly chart, what is also evident is the peanut shaped high east of New Zealand which was squeezed by tropical depressions and mid-latitude lows on its northern flank. This provided periodic pulses of fun easterly swell to the northern half of the East Coast, mainly through early July and the end of August.
The other main feature to point out is the high sitting south-east of Madagascar, across Indonesia's prime south-western swell window. What this did was limit large, long-period swells developing in that region, with Indonesia relying on a mix of mid-period and groundswell energy from the south, pushing up past the Western Australian coastline. Not a classic Indonesian season at all. However, the ridge aimed persistent south-east winds towards the Maldives, creating a constant cycle of swell for that island chain.
This final chart shows the mean (average) wind for the three month period and this reinforces the above points.
Westerly winds across the southern parts of Western Australia, tending more west-northwest across South Australian and north to north-west in Victoria. The persistent westerly winds across the Eastern Seaboard also lead to a mostly dry, clear winter with temperatures pushing well above average. The persistent south-east winds aimed towards the Maldives are also evident.