Tropical Cyclones – we have tropical depressions, tropical storms and hurricanes. There are typhoons and super typhoons. Cyclonic storms, extremely severe cyclonic storms and super cyclones all with different naming lists allocated around the world. We’ll look at the areas of responsibility, the seasons and various intensity scales.
“A Cyclone: An atmospheric closed circulation rotating counter-clockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. “
Tropical cyclones are deep low pressures that have a certain set of characteristics. They usually form in the tropics, but rules are there to be broken. They are known around the world by different names such as Hurricanes, Typhoons or just a named Cyclone. Examples include Hurricane Katrina, Typhoon Haiyan or Cyclone Fani. The first part relates to the area it formed in and the second (often a person’s name) comes from pre-determined lists overseen by the WMO (World Meteorological Organisation) which are often alphabetic and sometimes used in rotation.
Hurricanes are found in the Atlantic and eastern or central north Pacific. Typhoons in the western Pacific and Cyclones, the rest of the world.
There are various levels of development for tropical systems. Tropical Depression TD – given a number, Tropical Storm TS – given a name, then Hurricane. Typhoon and in some regions Super Typhoon. Cyclones: areas of interest are given a number and letter suffix for that region, e.g. 03A near Oman in the Arabian Sea which was then was named Tropical Cyclone Hikaa as it strengthens. Various categories up to Super Cyclonic Storm or Severe Tropical Cyclone.
These categories follow wind scales, but tropical cyclones bring not just wind damage but heavy rain with flooding and the deadliest factor of storm surge.
There is also the Dvorak technique (see later under JTWC) Real observations from hurricane hunter planes or land sensors (which manage to survive the wind onslaught) give direct in-situ measurements. Obviously, they often aren’t possible. Satellite imagery can provide data for locating and estimating the intensity of tropical cyclones.
“The Dvorak technique is a method of evaluating satellite signatures of tropical systems to quantitatively estimate their intensity.” The interpretation of satellite imagery by watching development to estimate the current and future intensity. By looking at cloud features as indications of development or weakening, an intensity analysis and forecast can be made. It is worth remembering that this is a forecast, not a true observation. The scale is 0 to 8 in 0.5 intervals for a T number. T4 and T4.5 would be equivalent to a Cat 1. T7 and above is a Cat 5.
Tornadoes- A tornado is a different type of vortex, still a spinning column of potentially destructive winds but not a tropical cyclone. It forms from the bottom of a thunder cloud, the energetic cumuliform clouds becoming a tornado once it has spiralled down to touch the ground. They are not as large but can have stronger winds, they last for part of an hour rather than the days of a hurricane/typhoon. There can be hundreds in a year and although warning areas are highlighted, the exact location is still very difficult to pinpoint with only 10 to 30 minutes warning for people on the ground. Not a tropical cyclone.
The seasons vary around the world. “The portion of the year having a relatively high incidence of cyclones”.
The hurricane season in the Atlantic, Caribbean, and Gulf of Mexico runs from June 1 to November 30. However, hurricane Alex formed in January. ‘The season’ is just a guide.
The hurricane season in the Eastern Pacific basin runs from May 15 to November 30 and in the Central Pacific basin from June 1 to November 30.
South Pacific and Australia, the tropical cyclone season runs from 1 November to 30 April. North Indian Ocean, including the Bay of Bengal and also the Arabian Sea, sees two peaks, May then again October until mid-December for the most intense storms.
SW Indian Ocean peak is mid-November to mid-April.
Typhoons in the NW Pacific do occur all year round but the main season is July to November.
There are six tropical cyclone Regional Specialized Meteorological Centres (RSMCs) together with six Tropical Cyclone Warning Centres (TCWCs) which have responsibility for cyclone advisories and bulletins and oversee the naming of the storms.
Cyclones obviously aren’t interested in the boundaries so if a cyclone forms in one region (such as Indonesia or Fiji) it will be named there and will keep that name if it moves into the Australian region. A cyclone from the South China Sea which crosses Thailand and emerges into the Bay of Bengal will not change its name either. Hurricanes that manage to move from the Atlantic into the Pacific, say crossing Mexico, do not now change their name. They did use to. A cyclone moving from SE Indian Ocean to SW Indian ocean will also keep its original name.
1. RSMC Miami - Hurricanes - Caribbean Sea, Gulf of Mexico, North Atlantic and eastern North Pacific Oceans, the western hemisphere. The NHC, National Hurricane Centre (USA) and National Weather Service NWS.
“Tropical Cyclone: NHC A warm-core non-frontal synoptic-scale cyclone, originating over tropical or subtropical waters, with organized deep convection and a closed surface wind circulation about a well-defined centre. Once formed, a tropical cyclone is maintained by the extraction of heat energy from the ocean at high temperature and heat export at the low temperatures of the upper troposphere. In this, they differ from extratropical cyclones, which derive their energy from horizontal temperature contrasts in the atmosphere (baroclinic effects).”
Naming - East North Pacific has six lists which are alphabetic and used annually starting with A. Caribbean Sea, the Gulf of Mexico and the North Atlantic Names also has six lists, used annually, starting with A each season. Central North Pacific has four name lists and are all used, in order.
Hurricane Lowell, eastern Pacific August 2014
2. RSMC Honolulu - Hurricanes - Central North Pacific Ocean. Hawaii Central Pacific Hurricane Centre CPHC
For the huge area of the mid-Pacific, there is very little land. The Hawaiian Islands are affected by high surf from large storms but often hurricanes curve away or stay away from the tropical islands.
January 2016 saw Hurricane Pali, in the central Pacific. Pali was the earliest central North Pacific hurricane to form in a calendar year on record. Pali peaked at a Cat. 2 and large surf reached Hawaii. It was unusually close to the equator and so wandered about slowly with little steering flow.
"A tropical cyclone in which the maximum sustained surface wind (using the U.S. 1-minute average) is 64 kt (74 mph or 119 km/hr) or more. The term hurricane is used for Northern Hemisphere tropical cyclones east of the International Dateline to the Greenwich Meridian. The term typhoon is used for Pacific tropical cyclones north of the Equator west of the International Dateline". NHC and CPHC.
3. RSMC Tokyo -Typhoons- Western North Pacific Ocean and the South China Sea. The Typhoon Centre JMA Japan Meteorological Agency.
Naming - Countries such as China, Japan, the Philippines, Thailand and the US contribute to the six lists of typhoon names. Typhoon names are used sequentially and for tropical cyclones of tropical storm strength or above.
Typhoon Hagibis shot to worldwide fame when it interrupted the Rugby World Cup. This was one of the most powerful typhoons to hit Japan in decades. Previously an equivalent to a Cat 5, it hit as a Cat 1, remaining intact as it crossed Tokyo and brought torrential rain and lethal flooding. The capital itself saw over 8” of rain and rivers burst their bands adding to the flooding problems. The typhoon was very well forecast but over 70 people still died and if a cyclone is going to hit a highly-populated area even with evacuations the damage can still be devastating, even if structures survive the floodwater issues last for months.
A slight anomaly in this region is the Philippines. Although warnings systems have improved greatly, the Philippines are located in an area with a lot of cyclone activity and is likely to get a direct hit five or six times a year, often by devastating cyclones. They use their own naming convention. You may have seen two typhoon names alongside major events in this region. PAGASA (the Philippine Atmospheric Administration) currently has four sets of names that are used in rotation every four years.
Typhoon Haiyan affected the Philippines in November 2013. It was one of the strongest tropical cyclones ever recorded with sustained winds of 194mph, equivalent Cat. 5. In some areas, 281.9 mm of rainfall was recorded, much of which fell in under 12 hours. Waves of up to 7 m in height battered the coast. More than 7,000 people died and nearly 2 million were left homeless. Locally Haiyan was known as Super Typhoon Yolanda.
The JMA do not use “Super Typhoon” you won’t see this on their graphics.
4. RSMC New Delhi - Cyclones - North Indian Ocean (Bay of Bengal and the Arabian Sea). IMD. Indian Met. Dept
“Tropical cyclone: Generic term for a non-frontal synoptic-scale cyclone originating over tropical or subtropical waters with organized convection and definite cyclonic surface wind circulation.”
Northern Indian Ocean names are listed alphabetically by the country, not the cyclone name. They are sequential order through 13 columns. The names are not repeated. New names are used which should not already be on any other RSMC lists around the world. Bangladesh India Iran Maldives Myanmar Oman Pakistan Qatar Saudi Arabia Sri Lanka Thailand UAE and Yemen all contribute.
Bangladesh, (formerly East Pakistan) and NE India have seen the most deadly cyclones worldwide with 100s of thousands of people dying as a cyclone heads northwards over the Bay of Bengal and pushes a huge storm surge up against the low lying delta area. Modern warning systems and mass evacuations have managed to save many lives in recent times although the flooding leaves devastation for a long time.
5. RSMC La Réunion - Cyclones - South-West Indian Ocean including the Mozambique Channel. Météo-France
The RSMC La Réunion is responsible for the monitoring of all the tropical cyclones from the equator to 40°S and between African coastlines to 90°E. It provides guidance information (analysis, forecasts) to the 15 member states of the region: Botswana, Comoros, France, Kenya, Lesotho, Madagascar, Malawi, Mauritius, Mozambique, Namibia, Seychelles, South Africa, Swaziland, Tanzania and Zimbabwe.
There are three lists of names to be used in the SouthWest Indian Ocean. They start with 'A' each season following alphabetic order. A new list is created replacing used names with never before used ones but keeping the unused later ones.
March and April 2019 were devasting for SE Africa as Cyclones Idai and Kenneth hit. Catastrophic flooding from the heavy rains affected close to 2.2 million people in Mozambique, Zimbabwe, and Malawi. Even a year later thousands of people were still living in basic resettlement sites.
6. RSMC Nadi - Cyclones- South-West Pacific Ocean. Fiji Met. Service
Officially designated by WMO in 1995, it covers Tonga, Samoa, Polynesia and many other Pacific islands in Oceania, Australia and New Zealand. The naming conventions - four lists A, B, C, D with the full list used. Standby list E.
Around this area are TCWCs Tropical Cyclone Warning Centres. The TCWCs complete the tropical systems monitoring and forecasting coverage over smaller areas of responsibility.
Australia has three Brisbane, Perth and Darwin. New Zealand has one – Wellington. Indonesia has the Jakarta TCWC covering the sea area around the equator and there is the Port Moresby, Papua New Guinea one covering the Solomon Sea and Gulf of Papua
Indonesia isn’t an area that sees cyclones as it is on the equator but its area has been extended recently further east. The Port Moresby area doesn’t get many cyclones and if used, the name is then retired. Cyclone Guba affected Papua New Guinea in November 2007. There was heavy rain, extensive flooding and massive damage with the loss of at least 200 lives.
Bureau of Met Australia BOM “Tropical cyclones are low pressure systems that form over warm tropical waters. They typically form when the sea-surface temperature is above 26.5°C. Tropical cyclones can continue for many days, even weeks, and may follow quite erratic paths. A cyclone will dissipate once it moves over land or over cooler oceans.”
Australia BOM - Brisbane looks after the Coral Sea, Perth – southeastern Indian Ocean and Darwin - the Arafura Sea and the Gulf of Carpentaria.
Australia has alphabetic lists but slightly different to Atlantic hurricanes as they use PQ, UV and WXYZ and just in sequence, no starting at A for a season.
Northern Queensland was hit by Severe Cyclone Yasi in early 1911. It had reached the maximum category 5 just offshore and brought a large storm surge, becoming one of Australia’s most powerful cyclones. This region was where Cyclone Mahina struck in 1899, Australia’s most deadly cyclone again with a lethal storm surge.
The cyclones will move from one area to the next. Cyclone Fehi in Feb 2018 was a challenge as the system had a double handover for warning responsibility. Fiji handed TC warning responsibility to New Zealand, but as the system was immediately reclassified to an ex-TC, warning responsibility was then transferred to Australia (warnings will come from New Zealand for a TC, but from Australia for a non-TC). New Zealand itself can experience ex-cyclones, post-tropical lows just as the UK can witness ex-hurricanes.
Other sources of cyclone information
JTWC – Joint Typhoon Warning Centre. A US govt website for other parts of the world, often where the US may have a military presence. Northwest Pacific/North Indian Ocean, Central/Eastern Pacific and Southern Hemisphere. From the JTWC FAQs
Q: How are JTWC forecasts different from forecasts issued by tropical cyclone warning centres (TCWCs) of other countries?
A: JTWC and RSMC/TCWC tropical cyclone warnings may differ for several reasons. Measurement of maximum sustained surface winds. JTWC reports the maximum sustained surface winds in tropical disturbances and cyclones in terms of 1-minute mean wind speed, like NHC. Other nations, however, report maximum sustained surface winds averaged over a different time interval, which in many cases is 10-minutes. The difference generally means that JTWC will report higher maximum sustained surface wind speeds than non-U.S. tropical cyclone forecasting centres for the same cyclone. Another difference is that JTWC will issue forecasts out to 120 hours. Finally, JTWC does not apply the same tropical cyclone numbering scheme used by the regional centres. Hence, the cyclone number assigned by JTWC may not match the numerical designation assigned to the same cyclone by the responsible RSMC/TCWC.
Q: How is tropical cyclone intensity determined?
A: JTWC uses several tools and techniques to estimate tropical cyclone intensity, including subjective Dvorak estimates, objective fix data, and observations. Over most of the JTWC area, the Dvorak technique is the primary means to estimate tropical cyclone intensity. The Dvorak technique is based on the analysis of cloud patterns in visible and infrared imagery from geostationary and polar-orbiting satellites. The Dvorak technique results in a decimal number, called a T-number, which in turn corresponds to an intensity estimate.
Typhoon Haiyan (Super Typhoon Yolanda) November 2013
The Canadian Hurricane Centre CHC opened in 1987 following a need for warnings as east coast hurricanes or tropical storms made their way further and further north. In late September 1985, Hurricane Gloria affected the eastern Maritime provinces. Canada had to mostly rely on U.S. forecasts which didn’t highlight the impacts for Canada from this intense storm. After considerable confusion for the media and public, the CHC was created. In 2019, warnings were issued as post-tropical Dorian left its devastation of the Bahamas and reached eastern Canada.
Will the UK or Europe need a hurricane centre one day, to help protect people and property from the impacts of tropical cyclones, whether on land or at sea?
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