Forecaster Blog: The 2020 La Nina, East Coast Tropical Cyclone Seasonal Outlook

30 Oct 2020 4 Share

Ben Macartney

Chief Surf Forecaster

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.


Cyclone season officially kicks off on November 1st, but it's usually not until the new year that this kind of action kicks into gear. Photo: Otwaydundee.

Cyclone season officially kicks off on November 1st, but it's usually not until the new year that this kind of action kicks into gear. Photo: Otwaydundee.

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.

The National Institute of Water and Atmospheric Research (NIWA) have assigned elevated risk of TC impacts to New Caledonia and the North Island this season.

The National Institute of Water and Atmospheric Research (NIWA) have assigned elevated risk of TC impacts to New Caledonia and the North Island this season.

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.

The Walker Circulation, during a La Nina year. Source: NOAA.

The Walker Circulation, during a La Nina year. Source: NOAA.

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.

Keep in mind that the average number of four TCs over the Eastern Region doesn’t include potential TC genesis east of 160E. Source: BOM.

Keep in mind that the average number of four TCs over the Eastern Region doesn’t include potential TC genesis east of 160E. Source: BOM.

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.

Based on NIWA's analysis, New Caledonia will be the hot-spot for TC genesis this season. Source: NIWA.

Based on NIWA's analysis, New Caledonia will be the hot-spot for TC genesis this season. Source: NIWA.

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.


This image depicts average TC activity spread over five La Nina analogue years. Source: NIWA.

This image depicts average TC activity spread over five La Nina analogue years. Source: NIWA.

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.

Based on a study by Philip Malsale back in 2011, TC tracks are straighter during La Nina years. This image depicting TC tracks for the 2010/11 season lends some support to this trend. Source: BOM.

Based on a study by Philip Malsale back in 2011, TC tracks are straighter during La Nina years. This image depicting TC tracks for the 2010/11 season lends some support to this trend. Source: BOM.

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.

TC Anthony was one of four that developed over the Coral Sea during the 2010/11 season. Source: BOM.

TC Anthony was one of four that developed over the Coral Sea during the 2010/11 season. Source: BOM.

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.

TC Anthony's surf influence was mostly confined to southern Queensland coasts in late January 2011. Source BOM.

TC Anthony's surf influence was mostly confined to southern Queensland coasts in late January 2011. Source BOM.

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.

TC Yasi's track eventually turned west, towards and then over the QLD coast. Source: BOM.

TC Yasi's track eventually turned west, towards and then over the QLD coast. Source: BOM.

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.

TC Yasi was one of the strongest cyclones on record over the Coral Sea, but its far northern location rendered it a minor source of surf for the Queensland coast. Source: BOM.

TC Yasi was one of the strongest cyclones on record over the Coral Sea, but its far northern location rendered it a minor source of surf for the Queensland coast. Source: BOM.

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.

Here's a good example of extra-tropical transition coming to fruition inside our swell window. Source: BOM.

Here's a good example of extra-tropical transition coming to fruition inside our swell window. Source: BOM.

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.

TC Funa delivered a powerful round of easterly swell to the East Coast following sub-tropical transition near NZ. Source: BOM.

TC Funa delivered a powerful round of easterly swell to the East Coast following sub-tropical transition near NZ. Source: BOM.

Tropical cyclone tracks for 2007/08 reveal a fizzer of a season, featuring just two cyclones moved through our swell window during the season. Source: BOM.

Tropical cyclone tracks for 2007/08 reveal a fizzer of a season, featuring just two cyclones moved through our swell window during the season. Source: BOM.

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.

TC Gene was the source of a powerful easterly groundswell in early Feb 2008. Source: BOM.

TC Gene was the source of a powerful easterly groundswell in early Feb 2008. Source: BOM.

Our surf report archives show easterly groundswell from TC Gene peaked in the first week of February 2008.

Our surf report archives show easterly groundswell from TC Gene peaked in the first week of February 2008.

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 Jim as another major swell producer for the entire East Coast back in early Feb 2006. Source: BOM.

TC Jim as another major swell producer for the entire East Coast back in early Feb 2006. Source: BOM.

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.


TC Larry's swell influence was mostly confined to southern Queensland back in March 2006. Source: BOM.

TC Larry's swell influence was mostly confined to southern Queensland back in March 2006. Source: BOM.

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:

TC Wati first set up powerful swell for SE QLD late in March, 2006. Source: BOM.

TC Wati first set up powerful swell for SE QLD late in March, 2006. Source: BOM.

TC Wati then went extratropical, resulting in a monster easterly swell for the entire East Coast. Source: BOM.

TC Wati then went extratropical, resulting in a monster easterly swell for the entire East Coast. Source: BOM.

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.

Easterly groundswell arising from TC Gita peaked around the 8ft mark on 18 Feb, 2018.

Easterly groundswell arising from TC Gita peaked around the 8ft mark on 18 Feb, 2018.

Gita's location and movement was ideal for swell generation for the East Coast. Source: BOM.

Gita's location and movement was ideal for swell generation for the East Coast. Source: BOM.

Other notable swell-producers included TC Linda, TC Fehi, TC Hola and TC Iris.

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.  


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