Are we heading for a double-dip La Niña?
- El Niño events generally return to neutral the following year, unlike La Niña which has a 50% of returning back to La Niña
- This is called a double-dip La Niña
- A recent burst of west-flowing surface currents may be the catalyst for a double-dip La Niña
- The prediction is bolstered by a developing negative Indian Ocean Dipole event - which usually coincides with La Niña
- La Niña brings moisture to Australia, plus increased swell for the East Coast but a decrease in the southern states
The chances of back to back La Niña events (know as a double-dip La Niña) into the 2021/22 summer have just increased slightly.
This is great news for East Coast surfers though not what Victorian and South Australian surfers want to hear, especially with the last event being so fresh in their memories.
Before we get stuck in, it's worth noting that El Niño and La Niña don't swing back and forth like a pendulum, and that there's even a neutral phase between the two.
Historically, once a La Niña event has run its course the chances of back to back La Niña events is 50%, yet the same isn't true for El Niño. The reasoning behind this is explained in more detail below, but as we move through winter we're starting to see a few signals which are favouring a double-dip Niña event.
El Niño and La Niña events aren't symmetrical. Meaning they don't mirror each other from one side of the Pacific Ocean to the other.
We know that during La Niña events, warm water piles up in the western Pacific Ocean, bringing increased convection and moisture to the Australian region, with cooler waters upwelled to the east. El Niño is the opposite with warm water to the east, and cooler towards us in the western Pacific Ocean, leading to subdued convection and less moisture for our region.
Where the peak of this cool/warm (La Niña/El Niño) water anomaly sits across the Pacific varies between the two phases, and this is where the symmetry falls apart. La Niña's see the cold pool developing further west than where the warmer El Niño pool sits, which is further east (shown below). If all were equal we'd see the same region of ocean cycling between cold/warm events but they're slightly offset.
Also El Niño's are generally stronger regarding the shift in surface temperature from normal. That being the warm water anomaly for the ten strongest El Niño's is larger than the cool water anomaly.
So if the shift in sea surface temperature is larger for El Niño, then why don't we see back to back El Niño events compared to the relatively weaker La Niña signal?
The answer is in the strength of the feedback loop. During warmer water El Niño events, heat is released into the upper atmosphere and flows back to the poles. This feedback system is much greater than La Niña's helping it reset to neutral more quickly. La Niña's instead see a slower return to neutral and this in turn slightly primes the ocean and atmosphere for another La Niña event to kick back in the following year.
This phenomena is shown in the graph below which plots the change in sea surface temperatures across the Niño 3.4 sector (central) equatorial Pacific Ocean.
Every first year El Niño/La Niña is plotted from 1950 (left), with the following year's developments to the right.
What these statistics show us is that during the year following an El Niño, conditions, on average, return to neutral and sea surface temperatures back to normal.
However, following first year La Niña events, conditions have a tendency to stay on the La Niña side, though on average not as intense as the previous year's event. This is known as a double-dip La Niña and statistically it's a 50% chance following the first year of La Niña.
Knowing this, we've been keenly observing events in the central Pacific Ocean regarding any sign of a double-dip La Niña, and in the past fortnight we've seen a few indications that increase the chance slightly above 50%.
Firstly, Kathleen Dohan from Earth and Space Research has done amazing work correlating surface current changes across the equatorial Pacific to the evolution of a La Niño/Niña event.
After filtering out seasonal differences, a clear signal appears which shows that following a westward surface current anomaly (flowing to the west and shown when the blue line dips negative) we see a shift in the Sea Surface Temperature (SST) anomaly the opposite way shortly after. This shift is positive (red line going positive). La Niña events are shown when the red SST line pushes above 1 standard deviation and El Niño events when it dips below -1.
This correlation plays out beautifully and you can clearly see last year's La Niña develop after the surface current dipped towards 2 standard deviations and the SST accordingly rose above 1 standard deviation.
Looking at the past two decades, the strong 1997/98 El Niño clearly stands out, as well as 2010-12 La Niña and 2015/16 El Niño.
This analysis gives us a slight heads up on a possible developing El Niño/La Niña event by monitoring the surface current changes from normal.
So coming back to recent observations, we can see SST (red line) is still returning slowly to neutral following last summers La Niña but with the recent westward surface current anomaly (negative dip in blue line) we can expect those surface temperatures to head back north over the coming weeks, towards La Niña thresholds.
This appears to further increase the chances for a double-dip La Niña. We also have a developing negative Indian Ocean Dipole event in the Indian Ocean (warm water off Indonesia, cooler towards Africa) and this usually goes hand in hand with La Niña.
As for swell, another La Niña would see a summer and autumn similar to the one just gone. It's a plus for those on the East Coast with an increase in swell energy from the east along with winds from the south-east to east, while a shift in the storm track further south brings less swell to the southern states and unfavourable south to south-east winds.
NOAA's Climate Prediction Centre has the chance of La Niña reforming at just over 50% this November/December/January in its latest update so we'll continue to keep a close eye on the developments throughout the Pacific Ocean over the coming months.