Forecaster Blog: The 2020 La Nina, East Coast Tropical Cyclone Seasonal Outlook
Following close on the heels of one of the best winters in recent memory, the East Coast is continuing to benefit from the effects of the 2020 La Nina. The month of October has seen one trough/ low combo after another migrating eastward across the mainland, before eventually moving out into the Tasman Sea to produce a range of solid, short to mid-period swells, exhibiting an array of sizes and directions.
So what about this summer? Based on what you’ll read below, the La Nina wave-party is, at least in theory, set to continue unabated. As in any La Nina year, we’re seeing elevated chances of an above average number of TCs forming inside our swell window this season. But before you go out and custom order a quiver of guns and buy a floatation vest, it’s worth considering a few salient points when it comes to tropical cyclones and surf.
A named tropical cyclone will always capture the public’s imagination – surfers or otherwise, but the reality is TCs are often fickle swell producers. In many cases they’ll spend the best part of their short-lived existence tucked away inside the swell-shadow of a distant tropical island nation, before accelerating rapidly away to the south or southeast, without hanging around long enough to send any swell our way.
Even TCs that are favourably positioned inside our swell window are pretty needy, requiring a host of synoptic elements to come together to produce powerful surf. The primary one is a stable high pressure ridge to the south, allowing a large swell-producing fetch to develop. TCs also need the support of weak upper-level steering winds (low wind-shear) and high sea surface temperatures (SSTs) above 26.5C to remain in existence as they move south.
At the end of the day, each season is unique and we’ll just have to wait and see what transpires. The official start to the 2020 Cyclone Season is November 1st – and going on recent model runs there’s already a small tropical depression forecast to form over Southwest Pacific. Let’s hope that’s a sign of things to come.
In summary: over the 2020/21 Summer and autumn cyclone season:
- Chances for tropical cyclone genesis are elevated for the Coral Sea, northern Tasman Sea, extending as far west as New Caledonia.
- There’s a reduced risk of TC development east of the dateline.
- A 67% chance exists of a greater than the average number (four) of tropical cyclones developing over the Eastern Region, according to the BOM.
- Typically, the monsoon sets in across northern Australia a couple of weeks earlier than normal.
- The first Tropical Cyclone of the season often develops earlier in the season.
- Heightened TC activity is more likely during the mid to late part of the season, from January onwards.
La Nina and TC genesis: As discussed in the recent blog, convection and evaporation associated with the Walker Circulation is not only stronger than normal during a La Nina year, but is also displaced westward over the Maritime Continent and Coral Sea. It follows that TC activity is similarly enhanced by the higher than normal sea surface temperatures (SSTs) and resulting, additional tropical moisture in the atmosphere.
Above average number of Tropical Cyclones Forecast
It follows that a higher than average number of TCs are forecast over the Coral Sea and northern Tasman Sea this season, specifically encompassing the region bounded by the Queensland coast and New Caledonia. This is clearly illustrated by the image below, depicting the average number of TCs over various parts of the Coral Sea and broader Southwest Pacific basin, using five La Nina analogue seasons as a reference.
NIWA’s Southwest Pacific Tropical Cyclone Outlook zeros in on New Caledonia as the focal point for TC genesis this season. While that doesn’t bode well for residents of the tropical nation, isn’t a bad thing for surfers residing on Australia’s East Coast. Indeed, you could say a TC developing over the southern Coral Sea near New Caledonia places it in the ‘goldilocks zone’: not so close that it brings associated gale force winds, wild weather and stormy conditions to the coast – but not so far away that it dilutes the size and power of the swell.
In fact, the entire Coral Sea, extending from the Queensland coast to New Caledonia and as far east as Fiji are forecast to see enhanced TC genesis. That’s predicated on an average sum of tropical cyclone activity for a La Nina season, using five analogue seasons: 1970/71, 1995/96, 2005/06, 2007/08 and 2017/18.
The great un-named: Tropical lows, the sub-tropical ridge and Extratropical transition
The focus on tropical cyclone genesis during the tropical swell-season belies a plain fact: the vast bulk of easterly swell arises from unnamed sources; originating from tropical lows or an active monsoon trough - not strong enough to be named as a TC - but supported by a strong ridge the south. These kind of systems are Queensland’s bread-and butter during the summer months: delivering long-enduring runs of small to mid-sized easterly swell, often lasting anywhere from a week to several weeks at a stretch. Here's a good example from November 2007.
Extratropical Transition: There are other, less obvious effects that may come into play in a La Nina pattern. One is a logical consequence of enhanced tropical cyclone activity over northern Australia, the Coral Sea and the broader Southwest Pacific west of the dateline.
That presents higher chances for secondary swell-events, triggered by the re-birth of an ex-tropical cyclone tracking south across the Tasman Sea and undergoing extratropical transition over the mid-latitudes. Such extratropical transitions of ex TCs can produce larger storms of similar strength to a TC, albeit with much larger wind-fields than their tropical predecessors.
As you’ll see below, there are plenty of examples of this phenomena occurring throughout Australian longitudes over the years. More often than it’s the extratropical reincarnation of a TC that produces the largest and most powerful groundswells for the NSW coast.
Extratropical transition is a global phenomenon and over the last few days we’ve witnessed an impressive example in effect over the North Atlantic.
Stronger extratropical lows a potential by-product of La Nina
A La Nina pattern can have couple of other, less obvious effects on cyclone behaviour and development. The first is a weird one: reduced sinuosity. The absence of a strong equatorial steering winds can result in straight-line trajectories, taking TCs more directly from a given point A to point B. That’s in contrast to a pattern featuring pronounced upper-level winds that can often result in squiggly, difficult to forecast tracks meandering over the tropics.
A southward displacement of steering winds can also allow TC’s feed off their energy source that being warm SSTs above 26.5C) for longer than normal. That can result in tropical cyclones retaining greater intensity as they move south and commence extratropical transition over the northern Tasman Sea – in theory producing stronger hybrid lows and hence bigger easterly swell-events for the East Coast.
Madden Julian Oscillation (MJO)
The MJO is a tropical circulation pattern that brings enhanced tropical activity west to east around the equatorial region. When the MJO passes through the Maritime Continent and out across the Coral Sea and broader Southwest Pacific, it's usually the catalyst for TC genesis - more often than not triggering a cluster of tropical lows and cyclones as it goes.
Analogue Seasons and historical cases
To try and get an inkling of what might occur this season, we’ve taken a look at how some past cyclone seasons played out during analogue years, specifically seasons that hold similarities with this year’s early La Nina signature.
The 2010-2011 La Nina was one of the strongest on record, but contrary to high forecast-expectations there wasn’t a whole lot of tropical cyclone-swell action. As depicted below, most the cyclones that formed over the Southwest Pacific basin tracked away to the southeast, out past New Zealand, thereby robbing the East Coast of associated swell potential.
One of the few to set up off the Queensland coast was TC Anthony. The storm meandered eastward across the Coral Sea in late January, before re-curving and moving westward, back towards – and eventually across the Queensland coast near Bowen as a category 2 system on Saturday 30 January. The storm contributed to a decent 3 to 5ft peak in easterly swell across southern Queensland over the weekend of Saturday 30th.
That was closely followed by one the strongest Coral Sea cyclones on record: TC Yasi. Tropical Cyclone Yasi commenced its lifecycle as a tropical depression north of Fiji on Friday 28 January and steadily intensified to cyclone strength northeast of Vanuatu by Sunday night. TC Yasi intensified at a greater than forecast rate as it tracked steadily westward across the Coral Sea on Monday and Tuesday 1st February.
By Wednesday 2nd the cyclone had intensified to a phenomenal central pressure of 922 hPa: a category five system located 300 nautical miles east of Cairns. TC Yasi eventually made landfall as a category five system between Innisfail and Cardwell on the north Queensland coast on Wednesday night, but the resulting swell was a bit of a fizzer. TC Yasi’s far north-western proximity to the southern Queensland coast kept the strongest wind fetch confined to latitudes north of the 22S parallel: predominantly affecting locations north of Frazer Island.
Ex TC Vania and Zelia: The only notable TC related swell event followed the extratropical transition of ex-TCs Vania and Zelia, south of NZ in January 2011. In its tropical phase, TC Zelia was an underwhelming swell-producer, tracking south too swiftly to generate any notable easterly swell. It wasn’t until it intensified over the far south-eastern Tasman Sea that it set up a notable pulse of SE groundswell for the NSW coast, picking up into the 3-5ft range on Friday 21st January.
The low was comprised of the remnants of TC Vania and TC Zelia, moving across NZ’s North Island. Extratropical transition of the low occurred as it collided with a weakening frontal low traversing beneath the South Island, triggering rapid intensification as the low’s central pressure bombed to 975 hPa southeast of the South Island on Wednesday morning.
The 2007/08 season provides a good analogue for this season, with respect to the tropical Pacific SST profile and other La Nina indicators like the Southern Oscillation Index and ENSO 3.4. This time around there were only two notable TCs that had an influence on the East Coast in early 2008:
The first and easily the most significant was TC Funa. The system formed northwest of New Caledonia on 15 January 2008 and didn’t have a notable influence over our swell window until it moved poleward in the days following, eventually going extratropical just above NZ’s North Island and moving slowly east into the northeast Tasman Sea from between 19 and 21 January.
The second was TC Gene. The system formed on 26 January 2008 and remained active through to 9 Februay. Overall, however, the 2007/08 was a major let-down, falling well short of the above average number of TCs that were forecast during the season.
The 2005-2006 season saw a total of four TCs active over the Coral Sea and northern Tasman Sea: TC Jim formed on 25 January 2006 and took a slow eastward track across the Coral Sea, before curving around the northern side of New Caledonia and dropping south: eventually going extratropical just north of New Zealand.
TC Larry formed on 14 March 2006 and was a moderate swell-producer: moving westward across the central Coral Sea and contributing to an upswing in ENE swell across southern Queensland coasts before it made landfall near Innisfail on Monday 20 March.
The best of the season arrived on TC Larry’s coattails. TC Wati set up shop over our swell window from 15 to 27 March and was a major swell-producer, initially setting up days of large easterly swell for Queensland as it intensified over the Coral Sea. The storm then turned its attention to the NSW coast as it moved poleward and went extratropical late in the month, cradled by a strong high pressure system to the south all the while:
Finally, there was late season TC Monica, that formed off far northern Queensland on 16 April 2006, but remained positioned too far northwest off the far northern Queensland coast to generate notable swell.
The 2017/18 season saw plenty of activity.
TC Gita stood head and shoulders above the rest for surf potential. The system formed out near Fiji on 9 February, moving east before curving south, and then west with intensification between the 11th and 19th, setting up the kind of tropical cyclone swell we spend most seasons dreaming about.
In conclusion, it’s fair to say there’s a weight of La Nina historical precedents tipping the scales towards a good to epic cyclone season. It lends weight to higher than average chances for a season featuring consistent easterly tradewind swell, punctuated by at least one, if not several large TC related swell-events. Having said that, the 2007/08 season is a stark reminder that even the best laid seasonal forecasts don’t pan out as anticipated. Stay tuned for updates to the tropical-swell season outlook over the coming months.
Forecaster Blog: Dual Tropical Cyclones Lining up to Deliver Days of Pumping Easterly Groundswell to the East Coast.
Dual cyclones active over the tropical Pacific Ocean hold definitively large east-swell potential for the East Coast.
A distant Southwest Pacific cyclone holds latent east swell potential in the lead-up to Christmas.
A resurgent La Nina is about to rock the East Coast this December.
The Story Behind Tom Curren's Offbeat New Search Vid
The Story Behind Tom Curren's Offbeat New Search Vid
Australia, All of Australia, May Soon Be a CT Bubble
It's called Free Scrubber
Pipeline comes to Australia's East Coast, featuring Paul Morgan, Laurie Towner, Matt Dunsmore and more
Almost the day of dreams. Almost.
Offshore barrels and dreamy setups
A stoked out portrait of Australian Junior Surfing in the year 2018.
Golden hour at perfect Padang Padang - it's been a good week to be in Bali... Photo: Childs
Well, almost all. But definitely a solid weekend ahead.