October In Review
Steve Shearer suggested that we could be seeing, not just a shift, but a reversal in seasons. The last three Octobers have provided excellent surf for his neck of the woods, while further south across the Mid North Coast and Southern NSW, the October just gone provided non-stop surf from the east-northeast, often accompanied by favourable offshore winds.
The main culprit is, of course, the persistent La Niña signal throughout the Pacific Ocean these last three years, with this third event peaking in spring (as opposed to a summer climax which is more typical of La Niña ).
This brought a summer-esque synoptic pattern to the Australian region through October. An area of semi-stationary high pressure sat under the country and spread east across New Zealand, squeezed on its northern flank by instability and lower pressure in the Coral and Tasman Seas.
Instead of the strong northerly winds and junky, low-period windswell that's usually associated with October, we saw endless lines of groomed east-northeast energy that at stages provided some of the best waves of the year at marquee breaks.
This favourable wind and surf pattern (low squeezing high pressure) is evident on the Mean Sea Level Pressure (MSLP) anomaly charts for the month of October. That being the difference in average sea level pressure for the month compared to the long-term climate mean for October (1991-2020).
For a clearer idea on the drivers behind the consistent swell, we only have to look at the wind anomaly for the month. The Tasman Sea was flush with stronger than normal E/NE winds, while along the coast, troughy weather provided generally lighter and more favourable winds apart from the far South Coast.
Victoria and South Australia also benefited from the predominatly north-east flow, as it blew offshore into swells from the west and south, providing fun waves for exposed coasts along with the the gulf waters closer to Adelaide.
The easterly wind anomaly through Victoria's prime swell indicates the lack of cold fronts, south-westerly winds and in turn, swell in that region.
Western Australia wasn't overly special but into the end of the month, a large SW groundswell was met with offshore winds, creating excellent conditions across most of the state. The Margaret River region was a standout.
We can also see the influence of the negative Indian Ocean Dipole on the local winds across Sumatra and eastern Indonesia. That being north-west bursts for the former and weaker than normal trades across the latter.
On a final note, this last image shows the precipitable water anomaly throughout the atmosphere for October. Precipitable water is the amount of water potentially available for rainfall (not the rain that did fall) and what's stark is the influence of the negative Indian Ocean Dipole event and La Niña in relation to the water content in the atmosphere across the country during October.
The warmer than normal water surrounding Indonesia associated with the negative IOD saturated the atmosphere in that region. During spring we see the upper level jet stream dragging this moisture down across the country (evident as north-west cloudbands) bringing the flooding rains to central and south-eastern Australia.
The warm water signal through the Coral Sea and south-western Pacific Ocean, thanks to La Niña, also provided increased precipitable water to our east - a double whammy of sorts.
Looking at the months ahead, and a weakening La Niña signal (article to come) will still influence our weather and synoptics over the coming months, with lower than normal pressure expected throughout the Coral Sea and south-western Pacific, continuing to drive easterly swell energy to the East Coast, while higher than normal pressure is forecast to stretch across the Southern Ocean, south of the country.