Forecaster Blog: Tropical Cyclone 2015/ 2016 Seasonal Outlook

11 Nov 2015 3

Ben Macartney

Chief Surf Forecaster

By Chief Swell Forecaster Ben Macartney and Swell Forecaster Matt McKay, Wednesday 11 November 2015

  • El Nino conditions point to below average numbers of tropical cyclones throughout the Australian region between November 2015 and April 2016.
  • Above average tropical cyclone numbers are expected across the broader Southwest Pacific this season, specifically for nations such as Vanuatu, Fiji and the Solomon Islands.
  • Within Eastern Australian waters, 4 tropical cyclones are likely to form, with only a 27% chance of more.
  • Including the broader Southwest Pacific Ocean east of 165E, anywhere from 15 to 18 tropical cyclones are likely to form.
  • The existence of a tropical cyclone does not guarantee surf.

SEE ALSO: Cyclone Vania Produces World Class Swell On Gold Coast, January 2011

When a tropical cyclone passes within close range of southern Queensland the results can be spectacular. This was the by-product of tropical cyclone Fina back in 2011. Photo: ODs Surf Photography.

When a tropical cyclone passes within close range of southern Queensland the results can be spectacular. This was the by-product of tropical cyclone Fina back in 2011. Photo: ODs Surf Photography.

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Tropical Cyclones and Surf Potential

Before delving into the seasonal outlook itself, I’ve taken a step back to look at what a tropical cyclone (TC) is, how they generate swell and why they often result in more froth than surf. Tropical cyclones are a low-pressure systems that form over tropical waters and draw their energy from latent heat present in sea surface temperatures of 25 degrees or higher. TC’s are only classified as such once they begin to support core wind-speeds of 34 knots or more. TC strength is based on core wind-speeds and is categorised from 1 to 5, with 5 (the strongest) denoted by winds of 151 knots or more. Their formation is contingent on sea surface temperatures being 26.5 degrees or higher and they are characterised by a circular eye at the centre and their often erratic movement is difficult to forecast beyond two or three days in advance.

So what implications do tropical cyclones have for surf? If there’s one thing we know, the development of a tropical cyclone is no guarantee of swell. In isolation tropical cyclones can be fickle swell producers; generating clockwise winds over a confined area, often bounded by tropical islands that block any swell potential for more distant shores. Hence, unless they exist within close range of a coastline their swell-impact can be minimal. Indeed, it’s not unusual for TC’s developing south of Fiji or New Caledonia to track swiftly away to the southeast without sending any notable swell our way. Similarly, tropical cyclones forming over the Coral Sea often remain too far north to play a part in swell production.

SEE ALSO: 2015/16 Long Range Summer Forecast & Board Review 

What is it about TC’s that capture our collective imaginations? Tropical cyclones are usually very compact storms exhibiting a short radius and a well defined, clockwise wind circulation of gale to hurricane force strengths (34 to 150 knots plus). While these phenomenal wind fields can generate powerful swell in the right circumstances, these clockwise winds 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 unfavourable movement 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 span vast areas of open ocean to produce spectacular swell-results. 

Tropical cyclone Vania combined with strong high pressure over the southern Tasman Sea to set up a large ENE swell event for the entire Eastern Seaboard during January 2011. Source: BOM.

Tropical cyclone Vania combined with strong high pressure over the southern Tasman Sea to set up a large ENE swell event for the entire Eastern Seaboard during January 2011. Source: BOM.

El Nino and the 2015/16 Season

Unless you’ve been hiding under a rock, somewhere far removed from mainstream media, you’d be well aware we are in the midst of the biggest El Nino event since 1997/98. But really, what does that mean? And how does it relate to the coming cyclone season? To briefly address the former question, El Nino is a climate extreme describing persistent negative values of the El Nino-Southern Oscillation (ENSO) index.

ENSO is derived from a confluence of atmospheric indicator that describe conditions over the tropical Pacific Ocean, averaged out over time. These include sea surface (and sub-sea surface) temperatures (SST’s), atmospheric pressure, cloudiness and tradewind anomalies throughout the Southwest Pacific. The ENSO index is the oscillation between the extremes – the ends of which are the El Niño and the La Niña phases. An El Nino state is characterised by extensive warming of the central and eastern tropical Pacific Ocean – and this is the source of major shifts in weather patterns for Pacific nations such as Australia, New Zealand and the Americas. 

SEE ALSO: The Best Surfing Fails At The Superbank

This graph depicts the current state of the Southern Oscillation Index, showing strongly negative values indicative of a big El Nino event. Source: BOM.

This graph depicts the current state of the Southern Oscillation Index, showing strongly negative values indicative of a big El Nino event. Source: BOM.

What the heck does this have to do with tropical cyclone formation? For Australia and the island nations throughout the Southwest Pacific, the extensive warming of the central and eastern equatorial Pacific Ocean tends to shift the focus of tropical cyclogenesis further east – specifically near and east of the dateline

On the one hand that means more distant island nations such as Samoa, the Cook Islands and even French Polynesia are likely to see above average tropical cyclone numbers – and that doesn’t necessarily bode well for Australia’s East Coast. However, as discussed in more detail below there’s no shortage of cyclone-swell potential for the region this season, with warm SST pools over various parts of the Southwest Pacific pointing to elevated chances of cyclone activity inside the Eastern Seaboard’s swell window.

This diagram depicts basic El Nino characteristics and some climatic effects. Note the pooling of warm sea-surface waters over the central and eastern Pacific, Source BOM.

This diagram depicts basic El Nino characteristics and some climatic effects. Note the pooling of warm sea-surface waters over the central and eastern Pacific, Source BOM.

This chart depicts sea surface temperature anomolies throughout the Pacific Ocean during October this year. Source: NOAA.

This chart depicts sea surface temperature anomolies throughout the Pacific Ocean during October this year. Source: NOAA.

Australian Bureau of Meteorology (BOM)

The Australian Bureau of Meteorology (BOM) use a statistical relationship between cyclone numbers and previous El Nino years to derive an average number of tropical cyclones likely to form this season. Further, they divide Australia into three distinct forecast regions – and only two of these are relevant to swell-potential. What’s been made clear by the BOM is an overall lower probability of cyclone formation throughout Australian longitudes this season. This is based purely on statistical relationships between past tropical cyclone numbers and the state of ENSO – and for this season that means comparing average TC formation to other El Nino years – aka analogue years.

This diagram summarises the BOM's Tropical Cyclone Outlook; pointing to an average of 4 systems over the Coral Sea in 2015/16. Source: BOM

This diagram summarises the BOM's Tropical Cyclone Outlook; pointing to an average of 4 systems over the Coral Sea in 2015/16. Source: BOM

The Eastern Region

As the above image shows 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 a little further afield (for example near Fiji, Solomon Islands or Vanuatu) falls outside their area of influence and is thus not factored into their forecast. 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 probably experience four tropical cyclones, with a 27% chance of more and a 73% chance of less. They also add roughly a quarter of all tropical cyclones that form region make landfall, but given a lack of reliability on statistical analysis among the various analogue years they assign low accuracy to the outlook, so it’s definitely not worth taking these probabilities as gospel.

NIWA

New Zealand’s National Institute of Water and Atmospheric Research Centre’s (NIWA’s) analysis encompassing the broader Southwest Pacific is more encouraging. They anticipate higher than average TC numbers for various island nations throughout the Southwest Pacific and specifically assign elevated risks to some regions ideally placed inside out tropical swell window. These include the Solomon Islands, Vanuatu, Fiji – not to mention an array of other islands to the north and east.

An elevated risk of TC formation over much of the Southwest Pacific lends weight to a better than average tropical cyclone swell-season for the East Coast. Source: NIWA.

An elevated risk of TC formation over much of the Southwest Pacific lends weight to a better than average tropical cyclone swell-season for the East Coast. Source: NIWA.

What’s even more encouraging is NIWA’s analysis of ex-tropical cyclones coming within close range of New Zealand. NIWA says the majority of analogue seasons (1972/73; 1982/83; 1987/88; 1991/92; 1997/98 ) sees multiple ex-tropical cyclones coming within close range of New Zealand (within 550 km) during the season – and this is clearly good news for any surfer residing along Australia’s East Coast. Any ex-TC coming within cooee of New Zealand is predisposed to redevelopment as an extratropical low over the mid-latitudes – and such hybrid low’s are often ideally placed to aim powerful E swell our way as long as they remain outside NZ’s swell-shadow (ie east coast).

Overall, NIWA hints at a higher than average number of TCs forming inside Australia's eastern swell window this season. Source: NIWA.

Overall, NIWA hints at a higher than average number of TCs forming inside Australia's eastern swell window this season. Source: NIWA.

Fiji: RSMC Nadi

Fiji’s Regional Specialised Meteorological Centre (RSMC Nadi) have also issued a tropical cyclone outlook– and overall it’s 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 the focus of storm formation centred north and east of Fiji. RSMC Nadi has also assigned a highly elevated TC risk to the Solomon Islands, while allocating an elevated risk to nearby Vanuatu. While this is definitely bad news for the local inhabitants, any TC forming within these regions are situated within plumb swell-striking distance of Australia’s Eastern Seaboard.

Analogue Season: 1997/98

While it intuitively makes sense to compare the last big El Nino event (1997/98) with the current one, it’s worth noting that each event has unique characteristics – not only with respect to the various indicators (eg the distribution of warm SST anomalies throughout the tropical Pacific Ocean) – but also the broader state of the global atmosphere. Never the less, all meteorological agencies lean heavily on ENSO data to construct a tropical cyclone seasonal outlook – primarily because it has proved a statistically reliable way to forecast seasonal weather patterns.

A look back through the BOM’s records suggests tropical cyclone formation was less frequent over the Coral Sea through most analogue years. During the very strong El Nino of 1982/83 only two tropical cyclones named over the Coral Sea (TC Des and TC Elinor). Similarly, the moderate to strong El Nino of 1987/88 also only saw two tropical cyclones over the same region (TC Charlie and TC Agi).

The moderate to strong El Nino of 1991/92 saw three TCs active – and two were major swell producers: TC Daman and TC Betsy. However, of key interest is the last big El Nino of 1997/98, during which three tropical cyclones active over the Coral Sea: TC Katrina, TC Nute and the infamous TC Yali.

Tropical Cyclone Yali

Although this isn’t intended to imply we’re likely to see any monstrous swell events of tropical origins linked to the current El Nino, it’s always interesting to look back at significant swell events that occurred during analogue years.

Tropical cyclone Yali formed over the Southwest Pacific at the tail end of the last big El Nino Event in March 1998. The system commenced its lifecycle near Vanuatu on March 17 before curving it’s way slowly southeast with steady intensification in the days following. TC Yali achieved peak strength (category 3) on March 22 before beginning to weaken as recurved south-west, towards Australia’s East Coast.

TC Yali then evolved into extratropical storm as it began to interact with an upper-level low south of New Caledonia on March 22 and this hybrid system re-intensified as it tracked south across the Tasman Sea between 24 and 27 March. An associated, gale/ strong gale force fetch produced a huge E swell across the entire Eastern Seaboard in the days following.

Tropical Cyclone Yali delivered the kind of E swell event associated with heavy coastal erosion and tow-surfing across the East Coast at the tail end of the last big El Nino event in March 1998. Source: NOAA

Tropical Cyclone Yali delivered the kind of E swell event associated with heavy coastal erosion and tow-surfing across the East Coast at the tail end of the last big El Nino event in March 1998. Source: NOAA

TC Yali's meandering track across the Southwest Pacific Ocean and into the Tasman Sea was ideal with respect to swell-production for the Eastern Seaboard. Source: BOM.

TC Yali's meandering track across the Southwest Pacific Ocean and into the Tasman Sea was ideal with respect to swell-production for the Eastern Seaboard. Source: BOM.

RSMC Nadi have provided a breakdown of past tropical cyclone numbers (above) within their area of influence during other analogue (ie El Nino) years:

Season

TC Occurrence

Severe TC’s (Cat 3-5)

1972/73

8

2

1982/83

14

8

1986/87

12

6

1997/98

16

7

Average

12.5

5.8

If one were to draw any conclusions from this, it’s that tropical cyclone numbers are higher during strong El Ninos (ie 1982/83 and 1997/98). With any luck that bodes well for the coming season, but again it’s no guarantee. More often than not, it’s the presence of an intense high pressure system over New Zealand and an active monsoon trough featuring embedded tropical depressions that tend to be the reliable E/NE swell producers for the Eastern Seaboard.

Although these synoptic patterns aren’t much to look at on your daily MSLP and usually exhibit lower, sub-gale force wind, they are more likely to exhibit the prerequisites for a robust, mid-period E swell: Significant length of fetch supported over the same region for several days at constant wind speeds. 

The West Coast

With Australia’s north western coastline being the most cyclone prone location in Australia, we as surfers take great interest in the conditions needed to generate one those epic NW swells.

It has been well documented that the Pacific Ocean has generated cyclone after cyclone (north of the equator) over the past 3 months, but what about the Indian Ocean? Well, last week, the Western Indian Ocean produced Tropical Cyclone Chapala formed over warm waters and managed to make its way into the Arabian Sea (very rare considering the amount of dry air in the region).

In an unprecedented fashion, another cyclone named TC Megh formed just days later and followed a similar path. However this sudden increase in cyclone activity isn’t entirely random, it is strongly correlated to the location of the Madden-Julian Oscillation (MJO). The MJO is associated with weekly to monthly variations in wind, cloudiness, and rainfall across the tropical atmosphere. It’s best characterised as an eastward moving 'pulse' of cloud and rainfall near the equator that typically recurs every 30 to 60 days, but isn’t always discernible. It consists of broad scaled patterns in convective activity and rainfall that migrate slowly eastward along the equator. It acts as a teleconnection for the atmosphere and ocean, effectively enhancing tropical and extratropical weather – and this includes monsoonal activity and tropical cyclone formation. The strength of the MJO varies annually and is linked to the El-Niño Southern Oscillation (ENSO).

During El-Niño events, the MJO is often weak or absent, whilst erratic MJO activity is evident during the La-Niña phase. The MJO can also effectively alter the low-level trade flow around tropical regions, causing changes in SST and thermocline depth. Thus, indicating that it may influence the intensity and development of phases within the ENSO cycle indirectly through changes in oceanic conditions.

SEE ALSO: Asher Pacey Surfs Deserts Alone

Since the formation of TC Megh, the MJO has moved eastward over the Indian Ocean. Unfortunately, for surfers along Australia’s west coast it is expected to completely dissipate before it enters Australian waters; thus limiting tropical cyclone potential for the region. Another kick in the gut for surfers awaiting that rare north-westerly swell is the Bureau of Meteorology’s (BOM) latest tropical cyclone outlook.

* In their latest release, the BOM states that the Western region will most probably experience fewer tropical cyclones than average this season; showing a 75% chance of receiving below average TC activity.

Remember Tropical Cyclone Bianca? The system delivered a rare NW swell to the West Coast back in January 2011. Source: BOM.

Remember Tropical Cyclone Bianca? The system delivered a rare NW swell to the West Coast back in January 2011. Source: BOM.

The tropical cyclones off the Western Australian coast are often easier to track than their Coral Sea counterparts. The classical movement of a TC evolving off the NW coast involves an initial WSW movement, which tends more southerly as time goes by.

If atmospheric and oceanic conditions are favourable, then they may move past 22S and then curve towards the SSE. Luckily for us, this is just the ‘general’ case. Ideally, we would like to see the TC remain offshore for some time in order to generate a decent fetch, conducive of swell production.

The clockwise rotation of these systems will shift our focus toward the north to north-eastern flanks of most of these storms. This is illustrated in TC Bianca example, where the north-westerly fetch is directed at the southern W.A coastline. Typical winds within tropical cyclones range greatly from <125 km/h (Category 1) through to 280 +km/h (category 5), and so we can expect huge variations in wave heights. 

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