Welcome La Niña ... again
It’s not news to Swellnet readers, but La Niña has been officially called by the Australian Bureau of Meteorology for the coming summer. It's the return event we’ve been talking about since early autumn.
Following any first year La Niña, statistically the odds of a second La Niña - called a 'double-dip' La Niña - sit at 50%, and earlier this year the Pacific Ocean showed signs that a back to back event was likely. The chances of a trifecta are remote. The last time that happened was in the years 1973/74/75.
As the rain comes down across the eastern seaboard, anyone who’s travelled inland over the past month would have noticed how lush the landscape is. These inland falls have been associated with the negative Indian Ocean Dipole (IOD) through winter and spring. The IOD is a signal similar to El Niño/La Niña though in the Indian Ocean where warm water piles up towards the east, under Indonesia and north-west of Australia.
The increased sea surface temperatures provide mositure to the atmosphere which is then drawn down, south-east across the country.
Negative IOD events generally coincide with La Niña, so as we transition from spring to summer, Australia will fall under the influence of the Pacific Ocean rather than the Indian Ocean.
The mechanics behind El Niño/La Niña events are the easterly trade winds across the equatorial Pacific Ocean. If these winds become stronger than normal during winter and spring, they upwell cool water in the eastern Pacific while warmer water piles up to the west, around northern Australia.
This is known as La Niña while the counter to this El Niño, where the trades weaken and even reverse to the west which sees warm water piling up to the east, with cooler water around northern Australia. Neutral years fall between these two scenarios. In our last article I mentioned the possiblity that this season's La Niña will have a Modoki twist, but this looks less likely with the cold water anomalies now being strongest further east.
With warmer water surrounding Australia under the current La Niña scenario, we're seeing more convection and instability, resulting in increased rainfall across the north of the country. Also, the corresponding lower pressure is pushing the sub-tropical high pressure belt further south than normal, opening up the East Coast to easterly trade-winds and a stream of moist, onshore air.
Besides the wetter weather it means more swell for the East Coast and with generally favourable winds owing to troughy, unstable weather.
There’s also an increased risk of Tropical Cyclones (TC) owing to the warmer water which they feed off. Last month, the Bureau of Meteorology released their Tropical Cyclone outlook for the coming season and the Eastern Region - which extends from the Queensland coast to roughly New Caledonia - is forecast to see four TCs with a 59% chance of more, while further east towards Fiji - a region that can still generate swell for SE QLD and N NSW - six are forecast.
For the north-western and western regions there are five and seven cyclones forecast respectively with a 61-62% chance of more forming.
Keeping in mind that cyclones aren’t always the best swell producers due to their small, focussed nature. Regardless, the persistent easterly trade-wind setup should provide plenty of surf out of the east for the coming summer and early autumn on the East Coast.
The downside of the sub-tropical ridge drifting further south is that it generates a blocking pattern across the southern states resulting in smaller swells out of the south and winds out of the east. One positive this year compared to last is that winds look to be more favourable due to instability inland, providing a few more options on the beaches.
For Western Australia, a large blocking pattern to the south-west looks to also keep a lid on any incoming swell activity while coastal troughs bring hotter, offshore winds.
We’ll provide a more detailed seasonal outlook for the summer ahead once the next update from the seasonal forecast models comes through.
The Bureau of Meteorology have created this great clip explaining the IOD and La Niña.