Tropical Cyclone Monitoring
The World Meteorological Organization (WMO) Tropical Cyclone Programme has been established to minimise the loss of life and damage caused by tropical cyclones.
- Who is watching for Tropical Cyclones?
- Why do we need to know about Tropical Cyclones?
- Important facts about Tropical Cyclones and New Zealand
- Where to get the latest Tropical Cyclone information
- What is a Tropical Cyclone?
- What are the different categories of Tropical Cyclone?
- When do we get Tropical Cyclones?
Internationally there are six Regional Specialised Meteorological Centres (RSMCs), together with six Tropical Cyclone Warning Centres (TCWCs), which have regional responsibility to provide tropical cyclone advisories and bulletins.
Map of WMO Tropical Cyclone monitoring centres, courtesy of the Australian Bureau of Meteorology.
MetService is the Wellington TCWC, so all advisories and bulletins about Tropical Cyclones that could affect New Zealand originate from us.
Tropical Cyclones are one of the most dangerous weather hazards for humans. Every year they do massive damage to property and can cause considerable loss of life. They generate strong wind, heavy rain and large sea surges, which can affect New Zealanders on sea or land across the Pacific Region. Warning people before any major weather event can minimise loss of life and property damage.
- On average, about 10 tropical cyclones form in the South Pacific tropics between November and April each year, and about one of those will affect New Zealand as an ex-tropical cyclone (most commonly in February or March).
- The characteristics and structure of any tropical cyclone will change dramatically by the time it reaches New Zealand, and it will almost certainly be re-classified as an ex-tropical cyclone. This is due to it moving over colder waters and encountering strong upper level winds as it moves south out of the tropics. See our blog http://blog.metservice.com/TC-extra-tropical-transition for more information about extra-tropical transition. Although these storms will no longer be classified as tropical cyclones, we will often still refer to them by their given name e.g. ‘Cyclone Winston' for communication purposes.
- Re-classification as an ex-tropical cyclone does not necessarily mean the system has weakened or been downgraded, but rather that it has transformed into a completely different type of weather system. Ex-tropical cyclones may still have considerable potential for severe weather, and under the right meteorological conditions they can intensify and acquire lower pressures than they had before being re-classified. Many of New Zealand’s most severe storms have been ex-tropical cyclones.
- In the tropics, the strongest winds and most intense rain associated with a tropical cyclone usually occur just outside the ‘eye’ (cyclone centre). However, after the cyclone has undergone extra-tropical transition, it loses its symmetric cloud pattern and the strongest winds and heaviest rain can be hundreds of kilometres from the cyclone's centre, usually in a large area south of the centre. This means that the position of the cyclone centre is no longer a good indicator of where the most severe weather will be. During Cyclone Bola in 1988, the heaviest rain and strongest winds over New Zealand occurred well away from the centre of the cyclone. It is wrong to assume that because the cyclone centre is missing New Zealand, the severe weather will also miss New Zealand!
For New Zealand:
- MetService produces a Tropical Cyclone Potential Bulletin which appears as a 'Tropical Cyclone Activity' link in the Warnings banner and on the Warnings Home page if there is any activity in the tropics that could potentially turn into a Tropical Cyclone, or has been named as a Tropical Cyclone. Details of the latest updates, positions and expected movements can be found on the Tropical Cyclone Activity page.
- MetService will often produce blogs and/or news releases on significant TC activity - watch the Home page of the website for these, or visit the blog here.
- NOTE that any significant weather effects on New Zealand caused by Tropical Cyclone activity are covered by Severe Weather Watches and Warnings.
For our region:
- You can see all current Tropical Cyclone Warnings from RSMC Nadi in Fiji here: http://www.met.gov.fj/current_warnings.php
- You can see all current Tropical Cyclone Warnings from Australia's TCWCs in Perth, Darwin and Brisbane here: http://www.bom.gov.au/cyclone/
- Other national weather services may issue warnings for the impact of tropical cyclones on their national territory, local areas and coastal waters. Use these links to visit the websites of other South Pacific weather services: Cook Islands - Fiji - Kiribati - Niue - Samoa - Solomon Islands - Tonga - Tuvalu - Vanuatu
- WMO's Severe Weather Information Centre http://severe.worldweather.wmo.int/ provides a global snapshot of all tropical cyclones and depressions, typhoons and hurricanes currently being monitored by WMO monitoring centres.
A tropical cyclone is a low pressure system which forms over warm waters in the Tropics and has gale force winds (63km/h or more) at low levels near the centre (winds turning clockwise in the southern hemisphere), with organised convection (i.e. thunderstorm activity).
The gale force winds can spread out hundreds of kilometers from the centre of the cyclone, and if the winds reach 118km/h then the system is call a Severe Tropical Cyclone. See the table below for what severe tropical cyclones are called in other parts of the world.
This table outlines how Tropical Cyclones are categorised under the Australian and South Pacific Category System used by the Fiji Meteorological Service and the Australian Bureau of Meteorology (any tropical cyclones that impact New Zealand originate from these regions, and are no longer 'tropical' when they reach us).
In the southern hemisphere, the tropical cyclone season usually lasts from November to April. In the northern hemisphere, most tropical cyclones occur between June and November with a peak in September. However, in the north-west Pacific it is not unusual to have the occasional tropical cyclone outside of this period. Tropical cyclones are occasionally observed in the South Atlantic, but this is a very rare occurrence.
Image courtesy of COMET, NASA