A look back on the La Niña summer of 20/21
Rejoice can be heard from across the southern states as March 1st brings to a close one of the worst summers for surf in recent memory.
Of course this depends on which side of Port Phillip Bay you reside and who you talk to. Some claim that the beaches have been great fun, while others talk of the endless closeouts. One thing is true though, there's been nothing of note to the west. Since winter ended, I can't recall forecasting a large, clean run of solid groundswell for the Surf Coast reefs.
Luckily the drought was broken on the last day of summer with the arrival of a decent south-west groundswell arriving under light offshore winds, though it was only a one day wonder.
Below we have the last three months combined into two handy charts. The first shows the difference in Mean Sea Level Pressure for the summer months December, January, and February compared to the long term climate average (1981-2010).
Red indicates that the pressure was higher (positive) than the long term climate average during the summer period, while blue values show that the pressure was lower (negative).
What immediately stands out is the large high pressure anomaly stretching from under the Indian Ocean (south-west of Western Australia) across to Tasmania and then into the Tasman Sea.
While winds generally aren't favourable through summer for the southern states, and more often than not out of the south (onshore), what La Niña does is exacerbate this by shifting the sub tropical high pressure belt further south than normally seen through summer, putting a bigger block across the Southern Ocean engine room.
The position of the positive anomaly further south across the Roaring Forties shows that the highs were positioned further south than normal. With such a setup, polar fronts are steered closer to the ice shelf or are non-existent, producing a double whammy of minimal swell and onshore winds.
The reason for Western Australia's hot, windy weather can also be seen, that being a heat trough (low) running north-south along the coast, squeezing against the sub-tropical high to the south, pumping in heat from central Australia while also bringing strong coastal winds.
These winds at times met quality groundswells, more so through the end of January and February, but for the most part it was windy and small.
The flow on effects of such strong offshore winds was a less pronounced sea breeze season which also lead to less upwelling and a marine heat wave off WA's North West mid-late January.
The development of a tropical low off the North West early February converted this surface heat energy to torrential rain and flooding, while also reducing the ocean surface temperature.
Moving to the other side of the country, and the synoptic setup responsible for the great, consistent run of summer surf can be seen. That being lower pressure in the Coral Sea sitting atop the cradling high pressure belt stretching from the southern Tasman Sea to the south-east of New Zealand. This aimed fairly persistent easterly trade-winds towards the East Coast along with a couple of deeper intensifications, most notably a coastal trough in mid-December and Tropical Cyclone Lucas in early February. A final tropical low also pushed south through the Tasman Sea last week.
The chart that shows the swell generating engine best is the mean vector winds for the summer months (see chart below). These are the average overall winds throughout December, January, and February - not the anomaly (which is the difference from normal).
During this period, a broad swathe of easterly trades were positioned between New Zealand and New Caledonia. While looking across to Western Australia we can also see the strong, persistent south-east winds.
One noticeable positive from the setup across the East Coast was winds from the south-eastern quadrant which reduced upwelling, keeping inshore water temperatures toasty which in turn inhibited the development of afternoon sea breezes. This provided plenty of great afternoon and evening sessions, especially across southern NSW.
If we had upwelling, the temperature differential from the heating land during the day and cooler waters would have brought in stronger sea breezes. The extra cloud cover also associated with La Niña also prevented land temperatures reaching any great heights, with Australia recording it's coolest summer in nine years.
Tasmania faired really well through the summer, with the East Coast seeing plenty of localised north-easterly swells mixed in with stronger, less consistent east-northeast swells from the tropical activity.
The South Arm managed to pick up persistent small waves from whatever polar frontal activity there was though with plenty of winds from the north-east direction to keep conditions clean.
Looking at the coming autumn, La Niña is weakening and we're due to see a run of southerly swell for the East Coast over the coming week, with the tropical wave of activity (MJO) not forecast to be back in our region until at least the second half of March, if that.
So it looks like we're in limbo for now, though keep an eye on the regional Forecaster Notes for more detailed and up to date rundown.