Analysis : La Nina watch
While there've been local indicators, such as heavy, persistent rain events, we're now officially on La Nina watch for this coming summer.
The Bureau of Meteorology made the switch earlier this week which puts the chance of La Nina developing over the coming months at 50% (double the the average likelihood). NOAA's Climate Prediction Centre are also on board with 40-50% chance of La Nina forming.
If we take a look at the sea surface temperature indicators across the equatorial Pacific (see image below), a clear cooling has taken place in the eastern and central equatorial Pacific Ocean, while there's been some warming in the western Pacific. This setup usually drives a feedback loop, with strengthening easterly trade-winds across the Pacific Ocean, promoting further upwelling and cooling off the South American coast.
Easterly trade-wind anomalies (difference from normal) are slightly stronger and the sub-surface temperature profile is also pointing towards a La Nina event. That being cool water extending deep below the sea surface to the east, with warm water throughout the water column to the west.
Whether this continues and develops into a La Nina event is still uncertain, though as mentioned, most climate models are trending towards either a neutral or La Nina event.
If we do see La Nina developing it will mean an increase in rainfall across central and eastern Australia. The last significant event being 2010/11 when we saw wide-spread flooding across many locations. That was the summer of the Brisbane floods, when the rising waters caused havoc in the Lockyer Valley, 2010 was also the wettest calender year for the Murray-Darling Basin.
With the current forecasts only just reaching La Nina thresholds, a repeat of 2010/11 is unlikely, if it occurs at all.
Here on Swellnet we've frequently discussed the Indian Ocean Dipole (IOD. The difference between this and El Nino/La Nina is that the IOD, which develops during winter, effects rainfall across central and inland Australia through our spring and early-mid summer. While El Nino/La Nina effect rainfall throughout the entire year though focussing on the central and eastern half of the country.
So what does this mean for surf potential?
In 2015 we penned an article looking at the effects on mean significant wave height during El Nino/La Nina. That article used data sourced from a 2011 study by Mark Hemer, “The wind-wave climate of the Pacific Ocean”.
During La Nina years the mean significant wave height increases through the Coral Sea and northern Tasman Sea , largely owing to more frequent easterly trade-swell. However, there's a reduction in swell size for Tasmania and Victoria that's linked to blocking highs setting up across Victorian latitudes.
While having more swell is all well and good, it's the local winds that will make or break a good surf season, and that's yet to be seen.
So sit tight over the coming months as we monitor the developments through the equatorial Pacific Ocean.