La Niña: A Trio Of Dips
La Niña three-peat.
Someone's got to be taking the piss? Surely.
While the La Niña signal has started to weaken throughout the equatorial Pacific Ocean thanks to the eastward spread and upwelling of warm sub-surface water, it's not over yet. We've got an interesting and dynamic outlook for the coming spring and summer.
We may be staring down a three-peat La Niña event, a trifecta only seen twice in the last fifty years.
The last time we saw three consecutive La Niña's was during the period 1998-2001 and then before that in the 1970's (specifically 1973 – 1976).
That 70's period saw a succession of destructive storms claiming houses and damaging properties across the greater Hunter and Sydney regions. All the regulars were involved, from Manly to Collaroy and Narrabeen, Bilgola, Wamberal and way up inside Port Jackson. It claimed the Norwegian bulk carrier MV Sygna which washed ashore at Stockton Bight and caused wide spread coastal erosion over the three year period.
The late 90's and early 2000's wasn't remembered to be as destructive coastally but did produce the most costly natural disaster seen to date for the country. That being a significant supercell thunderstorm and resultant large hail which impacted Sydney's eastern suburbs on the afternoon of April 14th, 1999. The storm pushed into the central business district causing $5.57B of damage as it punctured roofs, flooded homes and dented cars.
The flooding seen earlier this year across the eastern seaboard has now reached a cost of $4.8b, the third most costly event in the country's history, falling just behind Tropical Cyclone Tracy in 1974 at just over $5B.
The context of a three-peat La Niña has already been realised with us seeing the three most costly natural disasters for the country correlating with each of these periods over the past 50 years.
Looking ahead to the coming months and most seasonal forecast models now have the sea surface temperature forecast through the all important Nino 3.4 region falling back towards La Niña thresholds during spring.
The catalyst for this will be a strengthening wave of tropical activity (known as the Madden Julian Oscillation) shifting in from the Indian Ocean, to the north of the country during July. The MJO signal brings an increase in monsoonal activity (rainfall and convection) which is fed by strengthening easterly trade-winds on its eastern flank.
This is what's forecast over the coming weeks, that being significant bursts of stronger than normal easterly trade-winds in the western and central Pacific Ocean. The image below shows the forecast wind anomalies (difference from normal) along the equator for the coming week. Blue/purple represents stronger than normal easterly winds, with red/orange representing westerly winds. The equatorial Pacific Ocean falls between Australia and South America.
This will induce westward flowing surface currents, upwelling cooler water sitting above the warm sub-surface layer flowing east while piling up warmer water to our north and north-east again.
There is a chance that the warm sub-surface water will prevent us reaching La Niña thresholds over the coming months, though either way it looks like the coming spring and summer will have a La Niña twist to it.
Below are the forecast probabilites for La Niña, El Niño and Neutral conditions for the coming months, with a weakening La Niña signal for the rest of winter, ahead of an increased 58-59% chance for late spring and early summer.
As per the last couple years, the tell tale sign of the lingering La Niña influence on the local synoptic weather and patterns across the country is the persistent mid-latitude frontal and storm activity. We're seeing this setup currently right across the country and this looks to persist over the coming months with higher than normal pressure across the Roaring Forties. Not ideal for a typical end to the winter across the southern states while boding well for the East Coast.
We'll continue to provide updates as the coming weeks and months unfold.