Niña Won't Budge
Two too much
While it's been fascinating to predict then watch the double-dip La Niña event the last two years, what's even more intriguing is watching the Pacific Ocean try, but fail to shake the current oceanic and atmospheric setup.
Whether it be El Niño or La Niña, the signal in the Pacific Ocean usually breaks down (thanks to a rebounding Kelvin wave - more on this later) through our summer and autumn, returning to some form of neutrality during winter. Alas, this year the current La Niña, the second in two years doesn't want to budge.
After warming temporarily through February the Pacific Ocean is starting too cool again and this will extend La Niña's hold on our surf and weather into the middle of the year at least.
Just a quick refresher, La Niña events develop when we get stronger than normal easterly trade winds blowing along the equatorial Pacific Ocean, upwelling cool water in the central and eastern Pacific while piling up warmer water to the west. This warm water fills in and around northern Australia as well as Indonesia, increasing convection and thus rainfall, as well as increasing the chance of tropical cyclones.
El Niño is the opposite, as the easterly trade-winds slacken off and sometimes even reverse, upwelling cool water in the western Pacific while piling up warm water through the central and eastern Pacific Ocean. With the cooler water surrounding Australia there's less convection and hence generally less rainfall.
Neutral years fall in between with steady (neither strong or weak) easterly trade winds blowing across the Pacific and no major heating or cooling sea surface temperatures.
Coming back to La Niña, and with the warm water, increased convection and rising air to our north we see lower pressure develop across the tropics, and this shifts the sub-tropical high pressure belt further south, towards the South Pole during our summer. As a result we see the East Coast opened up to moist, onshore easterly winds, creating more swell activity, but the high pressure belt puts a block across the Southern Ocean, effectively cutting off the southern state's swell sources. It also brings unfavourable east to south-east winds, as many could attest to regarding the last two summers.
So what are the drivers that neutralise a La Niña event and settle things back to the neutral or El Niño status?
In summer, as the warm water builds up across the western Pacific Ocean, we see this water downwelled, producing a warmer than normal sub-surface layer.
This warm water spreads back east under the ocean, known as a Kelvin wave, and shuts off the source of cooler sub-surface water that the stronger than normal trade winds upwell. This occurred during January and February with the Kelvin wave tracking east and then surfacing in the eastern Pacific Ocean (near Peru) as shown in the images below.
Meteorologists sensed the current La Niña was coming to a close, predicting a March exit for 'the little girl'. However, since then waters have started to cool again at and below the surface in the central Pacific Ocean.
This, plus a forecast of stronger than normal easterly trade winds will upwell cooler water again, tipping us below the threshold for La Niña.
And what that means is La Niña will keep on chugging along.
Our weather and surf will continue to be influenced by La Niña all through autumn and possibly even winter. Another factor effecting our climate into winter is the Indian Ocean Dipole, with a negative event seeing warmer than normal water building up around Indonesia, and cooler towards Africa. Negative events usually coincide with La Niña's and this is the forecast for the coming months, with the warm water build up north-west of the country bringing a wetter than normal winter and spring to the country.
So what does this all mean for autumn in your region?
In Western Australia, you only need to look back at what happened last year. As per last autumn and winter, fronts and lows will spawn at the mid-latitudes (meaning further north than usual), which will create more westerly swells. In general this means wet, onshore surf for the Margaret River region with higher than normal rain forecast, but further north at Perth and Mandurah will cope better with lighter winds and plenty of swell.
West swells will favour the South Australian Mid Coast, where they can run the gauntlet into Gulf Saint Vincent, while surfers on Victoria's Surf Coast, who've endured a restless summer, unfortunately won't experience a snapback to normality. A westerly swell regime will see the Surf Coast miss a lot of size, though with winds more from the west than south, at least it'll be clean - gotta look for the positives!
This pattern will remain until winter when the pattern is forecast to slowly break down (images below). Keep in mind the Rip Curl Pro at Bells starts April 10th, meaning the early indications aren't ideal.
For the East Coast it looks like we've got another active few months ahead with the pattern of mid-latitude activity likely to favour southern NSW, while hopefully also providing plenty of energy further north.
Keep an eye on the Forecaster Notes and further articles on the coming autumn, winter and beyond.