2016/17 Tropical Cyclone Outlook - What Does It Mean For Swell?
Tropical Cyclone 2016/ 2017 Seasonal Outlook
By Chief Swell Forecaster Ben Macartney
Monday, November 14, 2016
Expectations are likely to run high in the wake of last year’s epic cyclone season. The big El Nino of 2015/16 delivered several major swell events of tropical origins; culminating in a long-enduring run of powerful E groundswell originating from Tropical Cyclone Winston.
This season there are also favourable indications for a heightened tropical swell-season; albeit in a very different guise from last season’s El Nino dominated Pacific Ocean. A steady weakening of the El Nino gave way to a neutral ENSO pattern, verging on weak La Nina levels during the first half of 2016 – and this ENSO state is expected to persist into the summer of 2016/17, making it the primary driver of the seasonal forecast.
IN THIS OUTLOOK YOU WILL FIND:
- 2016/17 Outlook Summary
- How Tropical Cyclones Generate Surf
- The Eastern Region Outlook
- Historical TC Breakdowns
- TC Zelia 2011
- TC Ului 2010
- TC Betsy 1992
- TC Grace 1984
- TC Harvey 1984
(Video below by Craig Halstead - Montage of SE QLD & NSW far north coast cyclone swells 2010/11. Featuring surfers; Mick Fanning, Joel Parkinson, Occy. Bede Durbidge, Josh Kerr, Dean Morrison, Asher Pacey, Noah Lane, Clint Kimmins, Dale Richards & Nick Heath.)
- Neutral to weak La Nina conditions indicates slightly above average number of tropical cyclones will occur inside Australian longitudes between November 2016 and April 2017.
- Over the Coral Sea (eastern region) expect near average numbers of tropical cyclones, with a 58% chance of above average and 42% chance of below average numbers.
- Chances for above average numbers of tropical cyclones for both Papua New Guinea and the Solomon Islands are elevated.
- Across the broader Southwest Pacific, most regions are expected to see a normal cyclone season, with 8 to 10 named cyclones expected to form within the Southwest Pacific Basin.
- Within Eastern Australian waters, a long term average indicates 4 tropical cyclones are likely to form, with a 58% chance of more.
- A minimum of 5 to 6 severe tropical TCs (category 3 or higher) are projected to develop anywhere across the Southwest Pacific.
- La Niña years are typically associated with above-average tropical cyclone numbers, and an earlier than normal date for the first cyclone to impact Australia.
- The existence of a tropical cyclone does not guarantee surf.
Neutral / weak La Nina for the 2016/17 Season
Whereas the big El Nino of last season was characterised by abnormally warm sea-surface temperatures affecting the central and eastern tropical Pacific Ocean, the current near-La Nina pattern features warm anomalies much closer to home. The Coral Sea immediately north and east of Australia is 1 to 2 degree warmer than average – and this implies heightened TC formation inside Australian longitudes this season.
The weak La Nina trend also suggests the focus of cyclogenesis will shift west of the dateline, lending the focus of TC activity to the Coral Sea and the western Pacific Ocean; specifically encompassing the region bounded by Papua New Guinea in the west and Tonga in the east. In past seasons exhibiting a similar atmospheric state (analogue years), the strongest TC anomalies were focused in the region between Papua New Guinea and the Solomon Islands – a trend associated with a south-westward displacement of the South Pacific Convergence Zone (SPCZ) during La Niña events.
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 accurately forecasting surf potential is often difficult in longer three to seven day time frames.
Although their phenomenal core wind speeds and personification give them a rock-star presence on weather charts, a tropical cyclone isn’t always a guarantee of swell. 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 powerful swell in the right circumstances, their fetches are often confined to an area just a few hundred nautical miles in diameter. Usually, the absence of any length of fetch, coupled with other elements like blocking landmasses and an unfavourable track can act as effective constraints on wave potential.
On the flipside, when tropical cyclones form adjacent to a significant area of high pressure, the resulting, gale-force winds can set up over vast areas of open ocean to produce spectacular swell-results. As mentioned above, TC Winston is probably one of most prominent examples of how good a TC swell can be the last decade.
The Eastern Region
The BOM’s sphere of influence only extends so far out across the Pacific; being bounded by the 160E meridian in the east. That means any tropical cyclone originating further afield (for example near Fiji, the Solomon Islands or Vanuatu) falls outside their area of influence and is thus not covered in their forecasts. Hence, the BOM’s analysis is really only relevant to our close-range swell windows encompassing the Coral Sea and the Tasman Sea.
The BOM’s analysis says the Eastern Region will experience average or slightly above average tropical cyclones numbers this season; assigning a 58% chance of more than 4 forming over the region. However, just to make things a little more complicated, if you include cyclones originating outside of, but traveling through Australian waters, a total of seven TCs are expected to be active over the region.
New Zealand’s National Institute of Water and Atmospheric Research Centre (NIWA)
NIWA’s analysis encompassing the broader Southwest Pacific also points to 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 uses a sophisticated methodology to derive six analogue TC seasons. The diagram below shows average TC numbers for the sum of these analogue years (ie neutral to weak La Nina years): 1969/70, 1983/84, 1992/93, 1995/96, 2005/06 and 2010/11. The diagram shows average TCs that formed between November and April each year. This effectively provides further confirmation we’ll see a concentration of TC formation occurring over the Coral Sea - and prima facie this looks pretty good for surf-potential:
While poring over analogue seasons doesn’t provide any concrete forecast on how the impending TC season will pan out, it can give us an inkling into what could occur. The 1983/84 season in particular, was notable for a record number of 19 TCs that that formed within the Australian region – a statistic that’s only matched by the 1973/74 season. One of the notable trends – particularly with some earlier examples from the 1980’s and 90’s, is the trend for TCs originating over the Coral Sea to track south-eastward; invariably bringing them within range of the East Coast during the later stages of their lifecycles. Whether or not this trend holds true this time around is yet to be seen.
Fiji: RSMC Nadi
Fiji’s Regional Specialised Meteorological Centre (RSMC Nadi) have also issued a tropical cyclone outlook. Although their analogue years are completely different, their overall outlook is in broad agreement with NIWA’s assessment; forecasting an elevated risk of tropical cyclone formation throughout the Southwest Pacific. They’re anticipating 2 to 3ft tropical cyclones effecting Fijian waters, with one forecast to reach category 3 or above. RSMC Nadi has also assigned an elevated TC risk for Fiji and Tonga and an elevated risk of severe TCs for the Solomon Islands. Fiji’s local meteorological agency, RSMC Nadi, show a breakdown of past tropical cyclone numbers for Fijian waters during their listed analogue years:
Analogue Years: Tropical Cyclones
Trawling back through the Bureau of Meteorology’s archives reveals some notable TCs that developed during the analogue years. However, Coastalwatch’s records don’t contain archived wave-data relating to the 1980’s and 90’s – and hence ascertaining the surf-related impact of these storm’s is no simple task. Still, the names of the TCs listed below are selected for their favourable tracks into southern Queensland’s swell window – and some of them are bound to ring a bell with the mature age surfers among us. The TCs listed below are in chronological order.
Tropical Cyclone Zelia: January, 2011
Severe Tropical Cyclone Zelia initially developed over the northern Coral Sea approximately 750 kilometres northeast of Cairns on the 14th January 2011. Zelia began to move in a southeasterly direction under the influence of a mid-level ridge situated across the western Coral Sea. Zelia rapidly intensified to a category 3 system on the 15th January and then further to a category 4 on the 16th while accelerating to the west of New Caledonia. Severe Tropical Cyclone Zelia began weakening on the 17th January as the system approached Norfolk Island due to a combination of cooler sea surface temperatures and increased wind shear. Zelia was downgraded to a low-pressure system at 4am on the 18th January. Zelia was the second tropical cyclone in the Queensland area of responsibility during the 2010/11 season. Zelia was also the second tropical cyclone to affect Norfolk Island during the 2010/11 season following tropical cyclone Vania only the week before.
Maximum Category: 4
Maximum sustained wind speed: 185 km/hr
Maximum wind gust: 270 km/hr
Lowest central pressure: 943 hPa
Not all cyclones do their best work up north – and Zelia was a case in point. Every once in a while the remnants of a tropical cyclone moving south over cooler waters merge with a Southern Ocean front or trough and transform from a heat-dependent system into a cold-core, extratropical cyclone – a process known as extratropical transition. Surfers along the NSW coast benefitted from this back in 2011. Tropical Cyclone Zelia tracked swiftly southeastward to become a weakening tropical depression near Norfolk Island on 18 January. This is where things got interesting, as the combined remnants of Vania and Zelia merged with a low pressure system over the southern Tasman Sea. The resulting deep, extratropical low off New Zealand’s South Island aimed a gale force SE fetch at the NSW coast during January 19 and 20. Check out the archived feature here:
Severe Tropical Cyclone Ului was initially analysed as a tropical low situated in the Pacific Ocean to the north of Vanuatu on March 8th. The low then traversed across the northern islands of Vanuatu and into the far northeast Coral Sea before it developed into a tropical cyclone late in the day on March 12th. Ului then rapidly intensified over the next couple of days into a severe tropical cyclone as it moved in a westerly direction to the south of the Solomon Islands. From the 15th to 17th March, Ului became a slow moving system as it gradually adopted a south to southwesterly track. Ului underwent a weakening phase as the system moved across the Coral Sea due to a combination of increased wind shear from an upper trough crossing the Tasman Sea and cool sea surface temperatures.
As Ului fell under the influence of an upper ridge on 18th March the system accelerated towards the Queensland east coast, and reintensified into a severe tropical cyclone. Severe Tropical Cyclone Ului made landfall near Airlie Beach on the Whitsunday Coast at approximately 1.30am on March 21st.
Maximum category: 5
Maximum sustained wind speed: 215 km/h
Maximum wind gust: 290 km/h
Lowest central pressure: 930hPa
Tropical Cyclone Betsy: 6 - 13 January 1992
Tropical cyclone Betsy was the second cyclone to affect Vanuatu during 1991-92 and was considerably more damaging than its predecessor, tropical cyclone Tia . Unusually for the time of year, Betsy formed as a southern hemisphere twin to typhoon Axel in the northern hemisphere. ( Axel tracked westward and was subsequently associated with the spin up of tropical cyclone Mark in the Gulf of Carpentaria.)
Between 3 and 5 January, strong to gale-force westerly winds developed on the northern side of the southern hemisphere monsoon trough. Enhanced convection associated with the westerly burst gradually became more organised within the trough west of Tuvalu, and by 1800 5 January, a deep tropical depression was evident. The depression continued intensifying as it drifted slowly west and at 1900 UTC 6 January was named tropical cyclone Betsy. During the next 12 hours, Betsy steadily deepened and tracked south-southeast at around 5 m/s. Mean winds associated with the cyclone reached storm force at 2100 UTC 7 January and the system turned to the southwest in response to increasing deep-layer mean easterlies. This brought the cyclone on a direct course for the central Vanuatu Islands.
By 1800 UTC 8 January, mean winds associated with tropical cyclone Betsy had reached hurricane force. Six hours later, the destructive core of the cyclone struck the islands of Ambrym and Malakula in central Vanuatu. On Malakula, sustained winds of 33 m/s were recorded between 0400 and 0500 UTC 9 January and the MSLP dropped to 959 hPa. At this point, Betsy possessed a large ragged eye with a diameter of over 110 km. Betsy continued to intensify after leaving Vanuatu, and reached maximum intensity (mean winds estimated to 45 m/s) at around 0000 UTC 10 January whilst 200 km to the north of New Caledonia.
By 0600 UTC 11 January, satellite imagery indicated that Betsy was losing organisation, with the large eye (diameter of 165 km) rapidly shrinking and becoming elongated and ragged. Around 1200 UTC 11 January, Betsy recurved sharply to the south under the influence of an approaching upper trough. Environmental shearing increased over the next few days, and by 13 January Betsy had dissipated as a tropical cyclone. The extratropical remnants continued drifting southeast and then east, passing close to the northern coast of New Zealand on 17 January.
Severe Tropical Cyclone Grace: 11 - 20 January 1984
A small low developed approximately 200 km east-northeast of Townsville on 11 January and then moved north northeast and slowly deepened during the next two days. The low reached its most northern position approximately 250 km north northeast of Willis Island at 0000 UTC 13 January and then moved southwards with cyclone status being reached at 0300 UTC 14 January. Grace continued moving south and passed 95 km east of Willis Island, 125 km to the west of Lihou Reef (AWS) and then turned towards the southeast and passed 50 km to the southwest of Marion Reef (AWS). Deepening continued with Grace becoming a severe cyclone when it reached its lowest estimated pressure of 970 hPa around 0600 UTC 16 January. Continuing to move southeast, Grace approached to within 30 km of Frederick Reef (AWS) and 25 km to the northeast of Cato Island (AWS).
At 1600 UTC 16 January, Frederick Reef (AWS) reported the strongest wind speed of 130 km/h and a pressure of 991.5 hPa when Grace was located 60 km to the west.
Visual Satellite imagery on 15 and 16 January indicated a ragged eye. Mackay radar identified about one-half of a hemisphere of the eye wall and allowed for positive fixes of the eye for some hours when at a range of almost 350 km. Upper-level shearing became evident on 17 January as the low-level circulation became exposed. Weakening continued as the cyclone moved into the Nadi region on the evening of 18 January and lost cyclonic intensity on 20 January. The remnant low decayed to the southwest of Noumea on 21 January. Grace 's influence on Queensland's weather was confined to the central coast where moderate to fresh easterly winds and moderate to heavy rainfalls occurred on 16 January. No damage reports were received and only minor sand losses were reported from the Gold Coast and Sunshine Coast beaches.
Tropical Cyclone Harvey: 3 - 7 February, 1984
The tropical low that was to become Harvey developed in the monsoon trough on 3 February to the northeast of Willis Island. The monsoon trough was lying across far northern parts of Australia to tropical cyclone Beti which had developed in Nadi's area of responsibility. Beti was centred near 18°S 164°E at 0000 UTC 3 February and was moving to the southeast. The low began to deepen and move southeast on 4 February as the low-level cloud-banding became more organised and cyclone status was reached about 0000 UTC 5 February.
During the next twenty-four hours Harvey moved southeast to south and deepened to a pressure of about 985 hPa. Harvey then took a more easterly track and continued to intensity until 0000 UTC 7 February when it reached its lowest estimated central pressure of 980 hPa. At this time Harvey was located 30 km to the west-northwest of Chesterfield Reef (AWS) which then had winds of 111 km/h (the strongest winds reported) and a pressure of 990.5 hPa. GMS-1 imagery indicated a clearly defined circulation centre though no eye was visible.
Harvey commenced to weaken as it moved in a general easterly direction until passing into the Nadi region just after 1800 UTC 7 February. The track then became southeast with Harvey losing cyclone status around 1800 UTC 8 February. The remnant low lost its identity on 10 February to the west of Noumea. No damage was reported.
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