Tropical cyclone season 2014

For the South West Pacific, tropical cyclone season is said to begin in November and continue right through to April the following year. However, the weather doesn’t follow a rigid calendar and tropical cyclones have been known to form as early as October and as late as the month of June.

Around the globe, the monitoring and forecasting of tropical systems is looked after by a Regional Specialized Meteorological Centre (RSMC) or a Tropical Cyclone Warning Centre (TCWC), depending on the location of the cyclone. TCWC Wellington, based at MetService, looks after the area that extends from 160E to 120W and between 25S and 40S. Although it is very rare for any tropical cyclones to form in TCWC Wellington’s area of responsibility, fully-fledged tropical cyclones do arrive from the Brisbane or Nadi areas and they may retain their cyclone status until 30S. Sometimes an ex-tropical cyclone will approach New Zealand and Severe Weather Watches and Warnings need to be issued. Even if land areas are not affected, warnings are issued for vessels over the open sea.

RSMC locations and areas of responsibility.

RSMC locations and areas of responsibility.

Our neighbouring TCWCs are Brisbane, run by the Australian Bureau of Meteorology, and RSMC Nadi, run by the Fiji Meteorological Service.

Cyclone categories

Cyclones are classified by a Category (Cat) system, numbering Cat 1 to Cat 5 depending on the strength of the winds near the centre of the system:

Cyclone Category

Average Wind Speed (km/h) Typical Strongest gusts(km/h) Central pressure(hPa)

1

Gale 63-88 Less than 125 Greater than 985

2

Storm 89-117 125-164 985-970

3

Hurricane 118-159 165-224 970-955

4

Major Hurricane 160-199 225-279 955-930

5

Major Hurricane Greater than 200 Greater than 279 Less than 930

Forecast for this season

Every year MetService works alongside NIWA as well as national meteorological services from other Pacific nations to produce a Tropical Cyclone Outlook.

Although this Outlook cannot say exactly when or where tropical cyclones will form, it is used as a guide to the expected activity over the South West Pacific. The outlook for 2013-2014 is for a near-average number of tropical cyclones (around nine) to form in the area from the Coral Sea to French Polynesia. Of these, four could potentially reach category 3 or 4. Under the forecast conditions for this season, a category 5 cyclone is unlikely but cannot be discounted. As with all forecasts, the best advice to stay up-to-date with the latest information.

The Australian Bureau of Meteorology (BOM) also produces forecasts for the areas to the north and west of Australia. You can find their latest forecast here.

How does this compare to normal?

On average there are nine tropical cyclones in a season but the actual number can vary greatly. The most active season in the SW Pacific since 1969 was the 1997-98 season when 17 tropical cyclones were recorded:

Number and intensity of Cyclones in the South Pacific

Number and intensity of Cyclones in the South Pacific

Tropical cyclones: extra-tropical transition

On average, about nine tropical cyclones form in the South Pacific tropics between November and April. Three or four of these leave the tropics and nearly all of them undergo a marked transformation to a mid-latitude depression – a completely different weather system – before they reach New Zealand.

For a while after this extra-tropical transition, the system may be referred to as “low formerly cyclone so-and-so”. At this stage of its life, the system may still have considerable potential for severe weather, despite its name change – for example, the low that was formerly tropical cyclone Bola, in March 1988.

So, how does this extra-tropical transition take place?
When a well-developed tropical cyclone reaches its peak in the heart of the tropics, it has an eye. The eye is often fairly cloud-free, nearly circular, and surrounded by a ring of very active thunderstorms. In the early and middle parts of their lives, tropical cyclones stand up quite vertically in the atmosphere, like large columns.

Infra-red satellite image for 10pm 02 February 2011 (New Zealand Daylight Time); this image displays colour-coded cloud top temperature. Tropical cyclone Yasi is just about to make landfall on the Cassowary Coast of northern Queensland.

Besides encountering cooler seas, tropical cyclones heading towards New Zealand eventually come under the influence of the westerlies. The westerlies of the mid-latitudes increase in strength with height, a phenomenon known as vertical wind shear. This shear almost literally chops off the upper part of the tropical cyclone and sweeps it away, not unlike a woodcutter chopping off the upper part of a coconut tree to leave a section just above the ground (except it’s a much more gradual and subtle process). Along with the lower sea temperatures of the mid-latitudes, this destroys the positive feedback processes within the cyclone. What remains is the former cyclone’s low-level circulation, which may get carried off in the westerlies or become the focus of further development if conditions are right. Either way, tropical cyclones approaching the New Zealand area undergo drastic changes of structure and appearance as they undergo this extra-tropical transition.

Let’s take a look at the extra-tropical transition of Wilma, which affected New Zealand in late January.

While in the tropics, Wilma had a similar look to Yasi, but was a less intense tropical cyclone. In the above infra-red satellite image (for 2pm Wednesday 26 January 2011 New Zealand Daylight Time), we can see a well-developed eye and the ring of thunderstorms surrounding it.

24 hours later (2pm Wednesday 26 January 2011 New Zealand Daylight Time), at the margin of the tropics, in response to increasing wind shear, the eye of Wilma became obscured amongst a broader mass of colder topped clouds.

And 24 hours further on, Wilma’s cloud structure bore little resemblance to that of 26 January. The eye and eye wall are no longer identifiable.


Finally, we take a look at the visible image for the same time as the last infra-red image above. This shows the more active, colder cloud tops (bright white) are now east of Wilma’s low-level centre. Therefore, Wilma is on the verge of losing its tropical cyclone status and becoming classified as a depression. Like all other cyclones before Wilma, Wilma lost its tropical cyclone status before reaching New Zealand.

Tropical Cyclone YASI

Tropical Cyclone Yasi formed north of Fiji on Sunday 30 January 2011 and was named by the Tropical Cyclone Warning Centre of the Fiji Meteorological Service.

Yasi is a girl’s name and also the Fijian word for sandalwood.

Click to animate satellite loop showing 'Yasi' from 5pm Wed 2 Feb to 10am Thu 3 Feb 2011 (NZDT)

As Yasi moved westwards  and absorbed energy from the warmer-than-normal waters of the Coral Sea, its diameter expanded to 800 km: much larger than the 200-300km typical of tropical cyclones in this part of the world.  It intensified to Category 5 (on the Australian category scale) with gales out to 250km from its centre.

This was the largest category 5 tropical cyclone to cross the Queensland coast since 1918.

A circle of gales 500km wide is large enough to cover most of the North or South Island.  Even so, Yasi was not the largest cyclonic circulation on the planet at the time: over the last few days, a depression over the North Atlantic brought a major winter storm to the United States.

The conditions required for tropical cyclones to form are:

  • Warm seas. The flow of heat and moisture from the sea into the atmosphere powers the deep thunderstorms that form the building blocks of a tropical cyclone and enables the subsequent release of latent heat.
  • Converging winds. These draw the thunderstorms together and organize them.
  • Spin. The thunderstorms need to be drawn together and organized in such a way that they rotate cyclonically (clockwise, in the Southern Hemisphere). This creates a positive feedback, strengthening the winds and uptake of moisture, generating even more thunderstorms. To initiate spin, the developing circulation needs to be not too close to the Equator. (For a brief mention of the Coriolis Force, see the comments at the end of Chris Webster’s post titled “Year 12 Maths”).
  • Low wind shear. If the winds aloft are not too different from the winds near the Earth’s surface, we say that there is little vertical wind shear. Low wind shears allow the thunderstorms to grow unhindered; high wind shears stunt their growth.

Eye and eyewall: Click on this image to read more about the structure of low pressure systems.

Just outside the eye of a tropical cyclone is a region called the eye wall, where rain and wind are at their most intense. The eye of Yasi expanded to a diameter of 100 km at one stage, which is very large. Within Yasi’s eye wall, wind gusts of  up to 290 kph and rain amounts of 300mm were reported.

The track that a tropical cyclone takes is partly due to imbalances in its own symmetry and partly due to the environment around it. Yasi was embedded in a reasonably steady easterly flow aloft and this carried it along a fairly straight path towards northern Queensland.

Apart from wind and rain, the other destructive component of Yasi has been its storm surge. This in itself has several components. Firstly, lower than normal atmospheric pressure near the cyclone’s centre raises the sea level. Secondly, strong winds blowing from the sea to the land generate large waves and push the sea onto the land. These latter effects are most pronounced to the left of the tropical cyclone’s track (in the Southern Hemisphere), where the movement of the cyclone adds to the wind strength. If all these coincide with high tides, the effect of storm surge is increased.

Yasi weakened quickly as it moved inland. This is mostly because it became cut off from its energy source, the warm sea. But also, the low-level air in the circulation encounters more friction over land than over sea, and this slows everything down.

Summer in the Australia – New Zealand region continues in a remarkable way under the influence of La Nina.

Tropical Cyclone Names

February and March are the peak months for tropical cyclones in the South Pacific.

The first storm in our area this season was named TASHA on Christmas Day, it very quickly made landfall near Cairns.  TASHA played a small part in the annual wet season that has proved so extreme over northern Australia during December and January.

Cyclone VANIA was the second of the season, named near Vanuatu on 12 January. It was followed by VINCE, named on the same day off NW Australia, then by ZELIA which was named on 14 January in the Coral Sea.  On 22 January,  WILMA was named near Samoa and the next day ANTHONY was named in the Coral Sea.

Tropical Cyclone WILMA , 7am Fri 28 Jan 2011

The History of naming storms

Christopher Columbus was the first recorded writer about tropical cyclones.  For several hundreds of years storms were named arbitrarily: an Atlantic storm that ripped the mast off a boat named Antje became known as Antje’s hurricane.  In the Spanish-speaking islands of the Caribbean, storms were named after the Saint’s day on which they arrived.

Near the end of the nineteenth century, Clement Wragge, an Australasian meteorologist, named tropical cyclones (and droughts) after political figures.  It is recorded that on occasion he described the tropical cyclone in uncomplimentary terms, such as causing distress, displaying erratic behaviour, wandering aimlessly, or frequently changing its mind.

During World War II when American bomber crews flew missions from Micronesian to Japan, they informally used the names of girlfriends and wives for the tropical cyclones they encountered.  In the early 1950s, American meteorologists used their phonetic alphabet (ABLE, BAKER, CHARLIE, etc.,) to identify tropical cyclones in the Atlantic. Starting in 1953, female names in alphabetical order were used by the US National Weather Service.  In the late 1970s, the lists of names were extended to include alternating male and female names.

Who names the names?

When winds rotating around the core of a cyclone in the tropics build to gale force, the cyclone is given its own name to uniquely identify it.  This is done by the appropriate Tropical Cyclone Warning Centre (TCWC).  The current list of available names is prepared well in advance by the Tropical Cyclone Committee of the World Meteorological Organisation (WMO) through its World Weather Watch and Tropical Cyclone Programmes.

Traditionally, the tropical cyclone formation areas are divided into seven basins (see map).  To help monitor cyclone activity in these basins the WMO Tropical Cyclone Programme has established Tropical Cyclone Warning Centres (TCWCs) for each basin. Their areas of responsibility extend across the regional boundaries of the Regional Specialized Meteorological Centres (RSMCs) of the WMO, which are outlined in red on the map.  The RSMCs have regional responsibility to provide weather advisories, bulletins and warnings on tropical cyclones, while the TCWCs supplement this work by observing, naming and forecasting tropical cyclones.

In our region, MetService operates the Wellington RSMC, working closely with the Nadi TCWC and the Nadi RSMC (operated by the Fiji Meteorological Service) to coordinate on the various bulletins and warnings for our respective areas of responsibility. MetService also serves as a backup for the Nadi TCWC in the event that Nadi experiences an outage. The Australian Bureau of Meteorology runs a TCWC at Brisbane. The naming boundary between the Brisbane and Nadi centres is along longitude 160 degrees East, which explains why the names of cyclones heading for New Zealand may seem to jump around the alphabet.

The Tropical Cyclone Committee provides each TCWC its own list of names.  These are chosen to be short, familiar to users, and easy to remember so that their use helps communicate warnings, especially when multiple cyclones have formed.  The names are used in alphabetical order, alternating male and female, but occasionally letters such as Q are skipped.  A name may be skipped if it is deemed inappropriate for some reason – e.g., a similarly-named cyclone is already active in the area, or it is the name of a public figure currently in the news.  Cyclones retain the same name if they cross a regional boundary (except in the Indian Ocean) or if they decay and then regenerate.

For more information on cyclone names click here, or here (NOAA), or here (WMO).

In regions such as the NW Pacific and the Atlantic each season starts with A on the list.  In our part of the world the lists are used sequentially, so the first name used each season is the one immediately following the last one used.

In the AUSTRALIAN list, this season started with Tasha, followed by Vince, then  Zelia.  Their next alphabet is: Anthony, Bianca, Carlos, Dianne, Errol, Fina, Grant, Heidi, Iggy, Jasmine, Koji, Lua, Mitchell, Narelle, Oswald, Peta, Rusty, Sandra, Tim, Victoria, and Zane.

In the FIJI list, this season started with Vania, followed by Wilma, Yasi, and Zaka.  Their next alphabet is: Atu, Bune, Cyril, Daphne, Evan, Freda, Garry, Heley, Ian, June, Kofi, Lusi, Mike, Nute, Odile, Pam, Reuben, Solo, Tuni, Ula, Victor, Winston, Yalo, and then Zena.

CATEGORIES

Mariners may have heard of the intensity of a tropical cyclone given as a category between one and five.  Worldly mariners will have noted that the Category Scale used in the South Pacific is DIFFERENT from the Saffir-Simpson Category scale that is used around America.  For more details on this point, click here .

Here is a copy of the category scale that applies to the Australia/New Zealand area.

On average, there are 86 tropical cyclones each year with 47 reaching hurricane strength.  These systems act like safety valves, helping to spread out any build-up of heat in the tropical oceans.  In the South Pacific the annual average is between eight and ten, of which one is expected to reach New Zealand in nine years out of ten.  The South Pacific cyclone season is officially from the start of November to the end of April.

So, there may be a few more tropical cyclones forming in our area, especially during February and March.  Keep an eye out to the north at http://bit.ly/7daywx and at our wide-scale satellite image.