Forecaster Blog: Tropical Cyclone 2017/ 2018 Seasonal Outlook
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.
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.
- 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
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).
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.
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.
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.
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.
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