Autumn Review: Back In Black
Well the verdict is in: After three years of poor swells and winds thanks to a triple dip La Niña, the Southern Ocean is back in black.
The Victorian Surf Coast has seen a stellar run of swell and offshore winds, climaxing last Sunday with one of the best days of surfing in years. A large mix of groundswells provided pumping 10ft surf under perfect offshore winds, glassing off into the evening providing a huge silky canvas for those with the ability and heart to take it on.
The change in fortune is linked to the transition in sea surface temperatures throughout the equatorial Pacific Ocean. That being from a cool signal to a warming signal, La Niña to El Niño.
The curious amongst you may wonder how water temps in the Pacific Ocean can influence surf in the Southern Ocean. It's a good question.
During La Niña, the belt of sub-tropical high pressure shifts to the south, which is great for the East Coast as it increases the easterly swell energy. However, much as the sub-tropical high pressure belt shifts south, so too does the westerly storm track that runs below Australia, and this suppresses storms in the Southern Ocean.
Now that we're transitioning into a warming equatorial Pacific Ocean and El Niño phase, we've seen a return to normal Southern Ocean frontal activity which has been focussed to the south-southwest of Tasmania, Victoria's prime swell window.
This is clearly evident looking at the Mean Sea Level Pressure (MSLP) anomaly charts for the autumn just gone. The term anomaly means the difference from the long-term climate mean, and what immediately catches the eye is the big low pressure anomaly south of Tasmania. This indicates lower than normal pressure in that region through March, April, and May, meaning there was an increase in swell-creating frontal systems.
This is in stark contrast to last year (see image at left), where a large swathe of higher than normal pressure dominated the Southern Ocean as lower pressure enveloped northern locations.
It wasn't only Victoria's Surf Coast that benefitted from the setup seen above. South Australia, Tasmania, and regional Victoria all saw periods of favourable winds and swell aplenty.
Looking at the East Coast and while the regime has recently shifted to episodic southerly pulses spreading up through the Tasman Sea, there was still plenty of quality easterly swell through April and May thanks to the lingering effects of the weakening La Niña.
This can be seen as the weaker (compared to last year) but still persistent low pressure anomaly through the Tasman and Coral Seas.
After a slow March, the following months of April and May seemed to provide non-stop swell energy out of the east, becoming large at times and with favourable winds. It hardly dropped below 2ft and there were plenty of pumping sessions to chase across the East Coast's varying coastline from the Sunshine Coast to Eden.
The Tasmanian East Coast was also fairly consistent but on the smaller end of the scale as the easterly swells lost a bit of their punch once they pushed south of Bass Strait.
Over to the west, and the autumn was dominated by favourable winds and moderate to large groundswell events.
With the more distant sources, Perth and Mandurah weren't overly consistent, but the amount of clean, solid days in the South West were numerous. This is clear when looking at the mean (average) wind speed and direction for the past three months, with a south-east direction blowing across most of the West Coast.
Under the country the Roaring Forties roared, while the Coral Sea continued to provide persistent easterly winds.
The reason for the slow season in Indonesia can be seen in the first MSLP chart, represented by the large (blocking) high pressure anomaly sitting across their prime swell window, south-west of Western Australia.
With the El Niño signal continuing to strengthen throughout the Pacific Ocean we can expect the southerly regime to continue across the East Coast with plenty of activity for the southern states. How Indonesia and Western Australia will fare is unknown, though it looks like we'll see the first proper swell of the season hitting tropical waters during next week.
More on this over the coming weeks.