Cyclone season in the Pacific has started earlier than ever before, and with a blast.
Vanuatu’s Tropical Cyclone Lola reached category 5 on 24 October – the strongest, earliest cyclone ever recorded in the region, according to Radio New Zealand. It blew up and dissipated quickly, leaving a trail of destruction across the islands.
Lola was battling for media attention with Hurricane Otis, causing mayhem in Acapulco in Mexico, and Cyclone Tej in Oman and Yemen.
Meanwhile in Australia the weather bureau is anticipating a later start and fewer cyclones this year than usual.
But the early arrival of Tropical Cyclone Lola in the Pacific this week is a reminder of the variability and randomness in cyclones, and the need – for those in affected regions – to be prepared.
When is a storm a cyclone?
According to the Bureau of Meteorology the technical definition of a cyclone involves a “warm-cored, non-frontal low pressure system” with wind speeds faster than 63 kilometres per hour, lasting at least 6 hours.
These severe storms usually form over warm tropical waters when the surface temperature is above 26.5 degrees Celsius. As moist air rises, the cyclone begins to spin, pulling in more warm, moist air, gathering momentum.
Greg Browning, a tropical climatologist at the Bureau of Meteorology, says: “a tropical cyclone needs several factors to develop, including warm oceans, a particular wind profile that extends through the lower atmosphere and vorticity – this is basically spin, due to winds from different directions meeting and being focussed into the centre of the tropical cyclone.”
And what makes a cyclone stop? Browning says: “A tropical cyclone usually dissipates when it moves over cold waters or over land – in both cases this cuts off the ‘fuel’ supply for the tropical cyclone – warm, energetic water. If a tropical cyclone moves into an environment with an unfavourable wind profile this can also weaken or breakdown a tropical tyclone.”
What to expect in Australia this cyclone season?
In Australia, most cyclones occur in the tropical north of the country in Western Australia, the Northern Territory and Queensland, usually between 1 November and 30 April.
Dr Andrew Dowdy, a Principal Research Scientist at the University of Melbourne’s School of Geography, Earth and Atmospheric Sciences says, while you’d expect fewer cyclones on average during an El Niño season, there can be “a lot of variability and randomness in tropical cyclones”.
In Australia an average season involves about 11 tropical cyclones of which an average of 4 cross the Australian coast (based on data from 1969 – today).
According to Geoscience Australia, the greatest number of cyclones in any one year was 20 in 1983. While the lowest was 3 in 2015.
How is climate change affecting cyclones?
He says, “it’s really the weaker categories that tend to be reducing in frequency. We don’t see such a large change in the severe tropical cyclones.”
In addition to strong winds, cyclones bring rain and flooding. Dowdy says sea level rise and more intense rainfall may exacerbate some of these impacts.
“A warmer air with climate change can hold more water vapour […] That means that there’s more water that can turn into rainfall,” he says.
Severe hurricanes and cyclones have also been arriving 3 to 4 days earlier each decade since the 1980s according to research published in Nature.
Dr Simon Bradshaw is Research Director at the Climate Council of Australia.
Bradshaw says “while we may not see more cyclones in the future, and may even see slightly fewer. Those that do develop, have the potential to become stronger and more destructive, with higher maximum wind speeds, and a greater amount of rainfall.”
In recent times, he say, cyclones appear to be lingering and retaining their strength for longer periods after making landfall.
He adds that climate change appears to be increasing the chances of rapid intensification – “cyclones becoming stronger very quickly. Obviously, that creates extra hazards because there’s less warning, less time to prepare,” he says.
He says intensifying cyclones are: “another way that climate change is really compounding the pressures on less developed countries.”
“Especially in countries in the tropics, and Pacific Island countries who have contributed almost nothing to the causes of climate change, but are now dealing with the fallout through rising seas, more disruptive tropical cyclones,” Bradshaw says.
That’s another reason why it’s important for countries like Australia to reduce its fossil fuel use and support for climate change adaptation and resilience in the region, including the Pacific, Bradshaw says.
What do the 5 categories mean?
Cyclones are classed according to their severity based mainly on maximum wind speed. This ranges from 1 (weakest) to 5 (strongest). Cyclones of any category can be hazardous.
- Category 1 – average wind speeds reaching 63 – 88 km/h and gusts up to 125 km/h
- Category 2 – average wind speeds reaching 89 – 117 km/h and gusts 125 – 164 km/h
- Category 3 – average wind speeds reaching 118 – 159 km/h and gusts 165 – 224 km/h
- Category 4 – average wind speeds reaching 160 – 199 km/h and gusts 225 – 279 km/h
- Category 5 – average wind speeds above 200 km/h and gusts greater than 279 km/h.
Dowdy says cyclone wind observations are made from some weather stations and satellites. “You can also get a measure of cyclone intensity from the pressure gradients. So, in the middle of the cyclone, if it’s very low pressure, then that’s associated with a more severe cyclone in general.”
Where does a cyclone do the most damage?
Tropical cyclones are hazardous due to extreme winds, heavy rainfall and storm surges.
They can have a range of impacts including injury and loss of life, and can cause damage to critical infrastructure (water, food, fuel supplies; energy, communication and transport systems), says the Australian Climate Service.
From the eye of the storm, to the edge
The calm ‘eye’ in the centre of the cyclone is where cool air sinks. The eye can range from 10km to 100km wide.
The strongest winds occur at the eye wall. The highest cyclone wind speed ever recorded was 408 km/h in Tropical Cyclone Olivia in 1996 off the Pilbara Coast in Western Australia.
Tropical Cyclone Tracy in 1974 is the smallest observed cyclone with a radius of 50 km, according to Geoscience Australia. Typhoon Tip in 1979 is the widest, with a radius of 1100 km.
Errol, Iggy and Zane – the naming of cyclones
Cyclone names help with communication, and distinguishing between cyclones when there are more than one.
The closest country to where the cyclone forms names it, and that name is retained even if moves into a neighbouring region. The name is approved by the World Meteorological Organisation.
The BoM lists historical cyclones dating back to 1970. In Australia, the first cyclones with official names were Audrey and Bessie in 1964.
If a cyclone has particularly devastating impact, its name is permanently retired. ‘Tracy’ was retired after the 1974 cyclone which all but levelled Darwin on Christmas Eve.
I say cyclone, you say hurricane
In Australia (and the South Pacific and Indian Oceans) large-scale intense tropical storms are called tropical cyclones.
In the US – in the northern Atlantic Ocean, central and eastern North Pacific Ocean – similar events are called hurricanes.
Meanwhile in Asia, and the Northwestern Pacific Ocean, the systems are called typhoons.
Tropical cyclones spin clockwise in the Southern Hemisphere and anticlockwise in the Northern Hemisphere.
In the US, hurricane season runs from 1 June to 30 November in the Atlantic and 15 May to 30 November in the Eastern Pacific, meanwhile typhoons generally occur from May to October,although they can be variable.
A ‘bomb cyclone’ is a similar system that forms outside the tropics, usually intensifying quickly.
Spinning storms – what about tornadoes?
Dowdy says: “Outside of the tropics, the more damaging winds phenomena are often caused by thunderstorms, including from severe wind gusts that can come out of thunderstorms. In extreme cases, thunderstorms can sometimes generate tornadoes.”
While tornadoes and twisters are also storms involve rotating air, they are different to cyclones. These events are usually much smaller with shorter duration.
However, tornadoes can also be very damaging.
In 2016 at least 7 tornadoes with wind speeds in the range 190-260 km/h damaged transmission lines in South Australia (Cyclone Tracy wind speeds were measured at 217 km/h). This led to a state-wide black system event on 28 September 2016 in which 850,000 households and businesses lost electricity supply, according to the final report on the incident by the Australian Energy Market Operator.
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