Forecaster Blog: Tropical Cyclone 2017/ 2018 Seasonal Outlook

12 Dec 2017 2 Share

Ben Macartney

Chief Surf Forecaster

Issued Tuesday, 12 December 2017

Remember last year’s cyclone season and all those pumping east and northeast swells of late 2016 and early 2017? No? Me neither. That’s because last season was virtually devoid of notable tropical cyclones, let alone cyclone swells. Only one system formed over the Coral Sea and that was TC Debbie. The storm turned out to be a relatively minor and indirect swell producer; initially crossing the northern Queensland coast late in March before moving back offshore and out into the Tasman Sea early in April. It was in this ex-TC guise that it contributed to a minor rise in NE windswell along the NSW coast, preceding the onset of a more substantial SSE swell as it tracked away towards NZ.

With any luck the excellent run of ENE swell through early December is just the beginning of an excellent tropical-swell season along the East Coast. Photo: Tarron Bell.

With any luck the excellent run of ENE swell through early December is just the beginning of an excellent tropical-swell season along the East Coast. Photo: Tarron Bell.

So going on probability alone, it’s fair to say this season is likely to be better than the last. The Eastern Region usually sees an average of four tropical cyclones – and with any luck we’ll see these numbers stacking in our favour. Like last season, we have are now officially in the midst of a weak La Niña pattern that gradually became established throughout the tropical Pacific Ocean during spring. While the La Nina pattern hints at slightly above average TC numbers over the western Pacific Ocean (ie inside our short to mid-range east and northeast swell windows), the absence of any major coupling between the sea-surface and atmosphere lends more weight to near-average TC numbers.

Key Points

  • Weak La Nina conditions indicates slightly above average number of tropical cyclones will occur inside Australian longitudes between 1 November 2017 and 30 April 2018.
  • Over the eastern region (i.e. the Coral Sea) near average numbers of tropical cyclones are forecast, with a 54% chance of above average numbers (down from 58% this time last year) and a 46% chance of below average numbers.
  • The long-term average indicates 4 tropical cyclones are likely to form within Eastern region this season.
  • According to the National Institute of Water and Atmospheric Research (NIWA), forecast numbers of tropical cyclones for Papua New Guinea and New Caledonia are slightly elevated.
  • Across the broader Southwest Pacific, east of the dateline, all regions are expected to see below average or non-existent tropical cyclone activity, with 8 to 10 named cyclones expected to form within the Southwest Pacific Basin as a whole.
  • La Niña years are typically associated with above-average tropical cyclone numbers over the western Pacific, west of the dateline (ie over the Coral Sea), and an earlier than normal start of the season occurring sometime in December.
  • The existence of a tropical cyclone does not guarantee surf

The BOM's TC forecast shows slightly elevated chances for above average numbers of TC's across Australia this season. Source: BOM.

The BOM's TC forecast shows slightly elevated chances for above average numbers of TC's across Australia this season. Source: BOM.

Weak La Nina persists through late 2017 and early 2018
So what, exactly are the implications of a La Nina event? The primary indicator of such a pattern is warm sea surface temperature (SST) anomalies throughout the western Pacific Ocean, while cool anomalies take hold throughout the tropical central and eastern Pacific Ocean. TC formation requires SST’s of 26.5C or higher, so prima facie this is good news for TC related swell-potential throughout our short and mid-range swell windows. The warm SST anomalies over the western Pacific draw the focus of convective activity to the Coral Sea and broader tropical Pacific Ocean about as far east as the dateline. Conversely, the cooler SST’s prevalent throughout the broader South Pacific inhibit TC formation, so it’s far less likely we’ll see any notable long-range swells emanating from TC activity east of Fiji (such as Tropical Cyclone Winston, which has to be rated as one of the greatest tropical surf-producers of the last decade).

This chart depicts average sea surface temperature anomalies from November to December. Note the warm anomolies over the western Pacific and Coral Sea and the cool anomalies over the central and eastern Pacific; the signature of a La Nina event. Source: NOAA.

This chart depicts average sea surface temperature anomalies from November to December. Note the warm anomolies over the western Pacific and Coral Sea and the cool anomalies over the central and eastern Pacific; the signature of a La Nina event. Source: NOAA.

Like last season, the Coral Sea immediately north and east of Australia is 1 to 2 degree warmer than average and this is the reason for heightened TC formation potential inside Australian longitudes. Along with the Coral Sea, the Tasman is also exhibiting high SST anomalies – and this is good news for ex-TC sources that undergo extratropical transition over the mid-latitudes. These reborn tropical lows are known for delivering powerful groundswells to the entire Eastern Seaboard long after the original TC has disappeared from the charts.

Tropical Cyclone Ita's extratropical transition over the south-eastern Tasman Sea in mid April, 2014 is a good example of the latent swell-potential ex-TC's hold as they move through the mid-latitudes. Source: BOM.

Tropical Cyclone Ita's extratropical transition over the south-eastern Tasman Sea in mid April, 2014 is a good example of the latent swell-potential ex-TC's hold as they move through the mid-latitudes. Source: BOM.

NIWA
New Zealand’s National Institute of Water and Atmospheric Research Centre’s (NIWA’s) analysis encompassing the broader Southwest Pacific also points to slightly higher than average TC numbers occurring west of the dateline. Their analysis assigns elevated risks to Papua New Guinea and the Solomon Islands and further afield, to Tonga and Niue. Their forecast draws on five analogue seasons as precedents of what to expect this season. These are 1970/71; 1978/79; 1995/96; 2005/06 and 2007/08. 

NIWA's TC risk map clearly shows the westerly bias in TC activity forecast across the Southwest Pacific this season. Source: NIWA.

NIWA's TC risk map clearly shows the westerly bias in TC activity forecast across the Southwest Pacific this season. Source: NIWA.

This image depicts the average number of TC's to form over the Southwest Pacific during the analogue years. Source: NIWA.

This image depicts the average number of TC's to form over the Southwest Pacific during the analogue years. Source: NIWA.

The last two last seasons in the series (2005/06 and 2007/08) are notable, both for the tropical cyclones that developed over the Coral Sea – and for the string of East Coast Lows that subsequently formed over the Tasman Sea during the winter of 2007. Once of the most significant TC’s to form during this period was severe TC Wati.

Tropical Cyclone Wati's extratropical transition over the Tasman Sea produced a giant round of E swell for the entire East Coast late in March 2006. Source: BOM.

Tropical Cyclone Wati's extratropical transition over the Tasman Sea produced a giant round of E swell for the entire East Coast late in March 2006. Source: BOM.

What is a Tropical Cyclone and how do they generate surf?
A tropical cyclone (TC) is a low pressure systems that forms over tropical waters, drawing energy from latent heat present in sea surface temperatures of 25 degrees or higher. Storms are only named TCs once they begin to support core wind-speeds of 34 knots or higher. TC strength is based on core wind-speeds and is catagorised from 1 to 5, with 5 (the strongest) denoted by winds of 151 knots or more. Their formation is contingent on sea surface temperatures of 26.5 degrees or higher and they are characterised by a circular eye at the centre of phenomenal wind vortices. Their erratic movement is usually difficult to forecast beyond two or three days in advance and hence it’s usually difficult to accurately forecast surf potential in mid to long-term outlooks.

Although their phenomenal core-wind speeds and personification give them a high profile in the media and an imposing appearance on weather charts, a tropical cyclone is generally regarded as a tenuous source of surf. In isolation tropical cyclones can be fickle swell producers; generating clockwise winds over confined areas of the sea surface. Tropical cyclones are usually compact storm systems exhibiting a short radius and well-defined, clockwise winds of gale to hurricane force strengths (34 to 150 knots plus). While these phenomenal wind fields can generate large and powerful surf when positioned close to a coastline, their fetches are often confined to an area just a few hundred nautical miles in diameter, so that the resulting swell rapidly dissipates once it departs the source.

Often an absence of fetch length and breadth, coupled with other elements like blocking landmasses and an unfavourable track can act as effective constraints on wave potential. On the other hand, when tropical cyclones develop adjacent to a significant high pressure system, the resulting, gale or near gale force winds can set up over vast areas of open ocean for days at a time, producing large and/or long-enduring swell events. TC Winston, which formed over the Southwest Pacific during the 2015/16 season, is probably one of most prominent examples in recent history of how good a TC swell can be. 

At the end of the day, forecasting when and where a tropical cyclone might form, and whether or not it will generate swell is still a short-term proposition. The lack of predictability associated with TC formation and development often means the resulting surf-potential only becomes clear in the 24 to 72 hour before the event. Here’s hoping this season is a good one.

Tags: Forecaster , Blog , tropical , cyclone , outlook , Ben , Macartney , topnews (create Alert from these tags)

blog comments powered by Disqus
Features
The O'RIGINALS, Beau Cram – Chippie and Surfer

The O'RIGINALS, Beau Cram – Chippie and Surfer

His J-Bay lines are serious fun.

3 17 Sep 2019
9 Surf Trips for your Honeymoon

9 Surf Trips for your Honeymoon

Dreamy getaways for loved-up surfers

15 Sep 2019
Recent
The O'RIGINALS, Beau Cram – Chippie and Surfer

The O'RIGINALS, Beau Cram – Chippie and Surfer

3 17 Sep 2019
Perfect Point Barrels and Punchy Beachies – Creed and Crew Shred Fun Northern NSW

Perfect Point Barrels and Punchy Beachies – Creed and Crew Shred Fun Northern NSW

12 Sep 2019
Nick Carroll: A Shot in the Arm for the QS

Nick Carroll: A Shot in the Arm for the QS

3 10 Sep 2019
Nias: "It Is a Complete Disaster"

Nias: "It Is a Complete Disaster"

16 22 Aug 2019
Latest News

The O'RIGINALS, Beau Cram – Chippie and Surfer

His J-Bay lines are serious fun.

Nick Carroll: A Shot in the Arm for the QS

Everyone’s a winner with the Challenger Series!!

Nias: "It Is a Complete Disaster"

Construction continues at Lagundri Bay; local surfers up in arms

Popular This Week

Mini-Margo Ripping to Tool! the New QS, & Longboarders Take on the Kelly Pool

This Week in Surfing: Ten Things from Surfing & the Internet on the Week That Was September 13, 2019

9 Surf Trips for your Honeymoon

Dreamy getaways for loved-up surfers

Weekend Surf Forecast 13 - 15 September, 2019

The weekend is just around the corner.

Go to Top