Double-dipping into La Niña
If you've been reading Swellnet over the past few years, particularly our weather analysis articles, you may appreciate how certain global climate indices influence the seasonal surf in Australia.
The biggest player in this space is the El Niño-Southern Oscillation (ENSO) which develops along the equator throughout the Pacific Ocean. The ENSO signal starts to form during spring and influences our weather and surf into summer and autumn.
El Niño (Spanish for 'little boy' as Peruvian fishermen noticed it occured around Christmas) develops under a slackening and reversal of the easterly trade winds across the Pacific, which results in warm water building up in the central to eastern Pacific Ocean, with a corresponding cool water signal developing further west, closer to Australia.
The opposite is La Niña (the 'little girl') which sees stronger than normal easterly trade winds (blowing from east to west) cause an upwelling of cool water along the equator in the eastern to central Pacific Ocean, while warm water pools towards the west - meaning the north-east coast of Australia and around Indonesia.
Neutral events are the most common state that the equatorial Pacific Ocean reverts to, with steady easterly trade winds and convection in the western Pacific Ocean.
Under La Niña setups, the cooler than normal water throughout the equatorial Pacific Ocean suppresses convection (storms/rainfall) in that region while to the west (i.e closer to the Australia) we see increased convection and instability due to the warmer than normal water. This brings increased rainfall to the continent, mainly focussed to the east and north.
El Niños see the opposite occur with a suppression of convection and rainfall in the Australian region (on average - we can still see wet El Ninos), with it enhanced further east towards Central and South America.
Now, not all La Niñas are equal and the same goes for El Niños, they come in different flavours and strengths.
As the the Niña/Niño signal develops across the equatorial Pacific Ocean during our spring, historically the cool/warm water anomaly (measuring the difference from normal) has been positioned towards the east. But over the past few decades we've seen the focus of the temperature anomalies developing further west towards the Date Line (180°).
What this does is shift the general circulation pattern further west, and with La Niña's this means the convection and rainfall is also shifted further west, away from the coastal locations along the East Coast, more inland towards the Murray-Darling Basin and the north-west of the continent.
This variation of the traditional La Niña/El Niño is given the name Modoki (Japanese for 'similar but different').
Following last year's La Niña, and after forecasting the probability of a double-dip La Niña (back to back) event this coming summer, we're currently on the cusp of it coming to fruition.
The difference this year is that we're set to see a bit of a Modoki flavour to the developing La Niña, with the cool sea surface temperature anomalies being strongest further west, towards the Date Line, compared to last year where they were focussed more towards 140-120W. You can see this shift in the two charts below, the first from 2020, the second from 2021. You can also notice the sea surface temperaures in both the Coral and Tasman Sea are a bit cooler than last year.
This means that the coming summer and autumn should see moisture focussed more towards central and north-west Australia, however what does it mean for surf potential?
During La Niña, the increased convection and lower pressure across the north of the country shifts the sub-tropical high pressure belt further south during summer. This results in an increase in easterly swell energy across the East Coast, while suppressing the westerly storm track to the polar shelf, further south than normally seen during summer. Meanwhile, the southern states usually experience smaller, weaker swells, and unfavourable winds from the south-eastern quadrant, and that's just what happened across most locations through last summer.
So how does Modoki effect this outlook?
Despite the moisture being focussed more inland, surf wise it looks like we'll see a similar season to the last on the East Coast, with plentiful swell from the east along with more favourable winds (i.e not the usual northerly pattern).
Below is the sesonal forecast (provided by the UK Met Office) for the coming summer months (December, January, February). It shows the probability of lower/higher pressure during the coming three month period.
Victoria looks to fair a little better, with the position of the blocking high being more favourable than last summer, which was to the south-west of Western Australia. Current seasonal forecasts have it positioned more under Tasmania and across New Zealand which hopefully points to a little more swell action.
Also, with lower pressure through central parts of Australia it'll hopefully tip winds towards a more east to north-easterly bias, with this hopefully also impacting South Australia.
Western Australia looks to be dry and hot with that low pressure anomaly indicating a semi-persistent heat-trough which will bring gusty offshore winds across the surfable regions, while swell wise they should do a little better than last summer with high pressure anomalies forming further west and east of its immediate swell window. This looks to bring energy more from the south-southwest though.
In summary, when looking at the broad scale synoptics it's looking like an active summer ahead . As always, we'll continue to provide updates on the coming summer season in the comments below.