Forecaster Blog: Climate Models Tipping Towards a Stronger La Niña
It’s official. We’re now in the early stages of a proper La Niña event (as of 29 September, according to the Bureau of Meteorology): one that’s projected to endure throughout the remainder of 2020 and at least through to the first quarter of 2021. On top of that, we’ve seen a notable shift in climate model guidance surrounding the ENSO outlook. Roughly half the models are now forecasting a strong La Niña: a little more reminiscent of the last big event that unfolded throughout 2010 and 2011.
A quick look back: The first half of October pumped.
In line with the general perception that La Niña events equate to regularly pumping surfing conditions across the East Coast, you might have noticed that it did just that throughout the first half of October. The first notable event arrived out of the SSE at long peak intervals of 16 to 17 seconds, arising from a remote source that set up some 2,000 plus nautical miles away. That culminated in powerful 4 to 8ft conditions across exposed NSW breaks on Thursday 1st October, with a couple of good days following as the swell eased throughout Friday 2nd and Saturday 3rd.
That was backed up by an excellent and unseasonal round of mid-period easterly tradewind swell, generated by an early season tropical depression that developed south of Fiji, supporting a broad easterly fetch in conjunction with a ridge to the south. That lit up the entire East Coast throughout the week of Monday 5th, culminating in good to excellent, 3 to 5ft condition up north on Wednesday 7th and across Sydney, Newcastle and the South Coast on Thursday 8th.
The key tropical Pacific Ocean indicators are continuing to tip towards a moderate to strong La Niña event. Those indicators include:
- Below average sea surface temperatures (SSTs) and sub-surface temperatures across the central and eastern tropical Pacific: Over the past week, cool SST anomalies have expanded westward and strengthened over the central Pacific.
- Warmer than average SSTs are persisting, both over the western tropical Pacific Ocean and the Tasman Sea.
- Negative SST anomalies over the key, Nino 3.4 region, are at -0.8C: Persistent values cooler than -0.8 are indicative of La Niña.
- The Southern Ocean Index (SOI) was +10.5 as at 30 September. Positive values are indicative of a La Niña.
- Stronger than average tradewinds and low-level easterly wind anomalies are persisting across the tropical Pacific Ocean.
So what’s in store for the rest of 2020?
Once again, we’re taking a look back at the 2007/08 and 2010/11 La Niña to get an insight into what we might expect see in the way of surf across the East Coast over the months of November and December 2020. If you’re still scratching your head at the mention of ‘La Niña’ and ‘ENSO’, then have a quick read of the broader explanation of these terms below.
El Niño Southern Oscillation (ENSO) and the Walker Circulation
ENSO is a natural atmospheric cycle encompassing the tropical Pacific Ocean. It describes a process known as Walker Circulation, whereby tropical heat is moved via easterly trade-winds, transporting warm water and air from east to west. That warm, moist air eventually rises over northern Australia and the Coral Sea, condensing to form clouds, precipitation and general storminess during the Southern Hemisphere summer. Now drier and cooler, the air is then transported back east at high altitudes, where it sinks back down over the eastern Pacific Ocean.
La Niña: The supercharged version of the ENSO cycle. La Niña occurs when easterly tradewinds are stronger than average. This has the dual effects of causing warm, tropical water to pool over the western Pacific Ocean (including the Coral Sea) while causing cooler than average SSTs in the central and eastern tropical Pacific. These mechanisms create a feedback loop, effectively sending the Walker Circulation into overdrive.
The above average SSTs over the western Pacific Ocean can result in an earlier onset of the monsoon and a higher incidence of tropical lows, troughs, and tropical cyclones in the Australian region throughout the season. That can also spill over to above average easterly swell activity over the Tasman Sea.
ENSO is only One of the Big Drivers of Australian Climate
Once again, keep in mind that La Niña (and ENSO in general) is just one driver of Australia’s weather and hence surf-potential. There’s a host of atmospheric mechanisms that influence Australian seasons. The other big ones that shape our weather patterns also include the Indian Ocean Dipole (IOD) and the the Southern Annular Mode.
Think of the IOD as the Indian Ocean’s version of ENSO. Positive and negative phases of the IOD also fluctuate with shifts in the SST profile across the tropical Indian Ocean. A majority of climate models predict a weakly negative IOD throughout October, before it returns to neutral in November.
Having said all that, two seasons are never quite the same. Each year we see a unique set of atmospheric drivers producing a unique surf season. That doesn’t preclude looking back at previous La Niña events to get an idea of what we might see in the way of swell over the coming months. The last strong La Niña event was a decade ago, when the SOI fell well below zero, peaking at -1.7 during the Spring/ Summer of 2010. Prior to that, there was a moderate event back in 2007/08. You might recall that the transition towards La Niña during the 2007 winter saw a cluster of ECLs, producing a good to epic run of large surf, drawing parallels with the spectacular procession of Tasman lows that left us reeling over the 2020 winter.
So let's sample some data from the previous notable La Niña's, including the moderate event of 2007/08 and the strong event straddling 2010/11 and 2011/12.
November and December 2007: Both months saw a pretty typical, decent run of small to mid-sized surf across the entire East Coast, arriving almost exclusively from the easterly quadrant. That was comprised of fluctuating surf, occasionally down to 1 to 2ft, but mostly up in the fun-sized, 2 to 4ft range. That all changed towards the end of December.
That all changed with the development of a slow-moving tropical low off the southern Queensland coast towards the end of the month, resulting in large, stormy 8-10ft swell peaking on 30th December. That energy then spread down the coast to produce a clean 4-6ft NE swell across Sydney and locations south on New Year’s Eve.
In contrast, November 2010 saw a consistent run of small to mid-sized easterly swell, with the vast majority or days falling somewhere in the 2 to 3ft range, with a few outlying days falling a foot or two outside this range.
Conditions throughout November 2010 epitomised what you'd expect to see during a moderate to strong La Nina: a consistent run of small to mid-sized swell, arriving almost exclusively arriving from the eastern Quadrant for the entire month.
Compared to the previous month, December 2010 was pretty lacklustre. Both up and down the coast the Tasman Sea didn’t produce a great deal of action, for the most part featuring small east and SSE swells in the 1 to 2ft range, with only a handful of days reaching the 2 to 3ft range, with records showing only one 3 to 4ft day across Sydney and surrounds on the 1st of the month.
Stay tuned for the next update in the coming weeks, looking at La Nina and its implications for Tropical Cyclone activity this season.
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