Explained: Western Australia's return to winter and East Coast flat spell
During the past month:
- Indonesia experienced back-to-back XXL groundswells
- The Western Australian coast has seen relentless onshore winds, large stormy surf, and a return to winter conditions
- A prolonged flat spell is driving East Coast surfers around the twist
Are all these events related? And if so, what's the cause?
Those keeping an eye on the Forecaster Notes across southern Australia would be aware of the current negative Southern Annular Mode [SAM]. The SAM is an index representing the north/south position of the westerly storm track.
When negative - as we're currently seeing - the storm track is positioned further north and closer to the equator, bringing cold fronts up to southern Australia. Conversely, when the SAM is positive we see the storm track positioned further south, resulting in stable high pressure across southern Australia.
As you can see in the graph below, SAM has been negative since late July. However, this is just one piece of the puzzle.
As well as a strong negative SAM index, we've seen the Long Wave Trough [LWT] focussed just off Western Australia over the past month and this has exacerbated the situation. The LWT has been steering polar storms up and across the south-west of the country, namely Western and Southern Australia.
This explains the prolonged period of large surf across Indonesia and relentless onshore stormy surf across Western Australia, but what about Victoria and the East Coast?
The flow on effects of the storm track sitting further north is that Victoria has been receiving endless westerly swells which generally don't bend around Cape Otway into the Surf Coast anywhere near as efficiently as south-west swells.
This has also meant persistent north-west offshore winds have generally gone to waste, while there also hasn't been enough north in the wind to clean up the Mornington Peninsula beaches.
Further down the line, the East Coast has been the biggest loser, with nearly a month of small surf across southern NSW.
The reason for which is mainly due to the negative SAM event but also the position of the LWT to the west.
Generally under a zonal (west-east) weather pattern, the storm track will still be positioned far enough south to generate diffracted southerly swells either from eastern Bass Strait or from under Tasmania.
But with the storm track shunted north and also dipping east-southeast across New South Wales, the frontal systems pass straight across the country with no chance for any southerly swell to be generated at all for the East Coast. This also prevents any other possible swell generating system developing in the Tasman Sea.
Instead we see endless clear sunny skies, clean smooth seas and no swell.
This can be easily visualised by the latest Mean Sea Level Pressure anomalies for July (top diagram, below). The anomaly is the difference in surface pressure from the long term climatological average and as expected, we can see that across the south-west of the country, surface pressure was around 3hPa lower than normal, even more through the Southern Ocean.
If we compare this to autumn last year when we ran an article explaining the excellent run of offshores and good weather across Western Australia and South Australia, which was linked to strong positive SAM events and higher than normal pressure across most of the country (bottom diagram).
When will this setup breakdown?
The SAM is tricky to predict, but forecasts have it remaining negative over the coming week or two which will keep the storm track positioned further north than usual.
One positive for the East Coast is that we'll see the Long Wave Trough shifting further east and sitting across the Tasman Sea.
This will result in the storm track shifting more south-west to north-east across the south-east of the country, though intersected by Victoria. This is likely to slice in half some of the frontal activity, but we'll still see a much more active southerly swell regime developing from this weekend and more so next week.
Keep an eye on the regional Forecaster Notes for a run down of your region.