Natural hazards and emergencies

Tropical cyclones

Tropical cyclones typically form close to the equator in the southwest Pacific Ocean. It is from the warm equatorial waters that cyclones derive their energy.  Most cyclones play out their whole life in the tropics. Sometimes though, a few cyclones migrate out of the tropics into the mid-latitudes, close to New Zealand. As they move south into cooler waters generally, their central core cools causing the cyclone to weaken. Some cyclones can re-intensify to become mid-latitude 'ex-tropical cyclones'.

Southwest Pacific cyclones tend to form between December and April with at least one ex-tropical cyclone passing within 500km of New Zealand most years. The severity of each storm depends on many factors including where the cyclone forms and the strength of the dominating La Niña or El Niño cycle.

 


Cyclone classifications

Cyclone classifications differ between the northern and southern hemispheres.  In the Southwest Pacific, whether a storm is large enough to be classified as a cyclone generally depends on the strength of the storms winds. 

The table below shows tropical storm classification in the southwest Pacific and some of the likely effects.

Storm type

Sustained wind (km/h)

Likely effects

Tropical depression

<63

N/A

Tropical cyclone category 1

63-125
(gales)
  • minimal house damage 
  • damage to some crops, trees and caravans.

Tropical cyclone category 2

125-164
(destructive)
  • minor house damage 
  • significant damage to signs, trees and caravans 
  • heavy damage to some crops 
  • risk of power failure 
  • small boats may break moorings.

Tropical cyclone category 3

165-224
(very destructive)
  • some roof and structural damage 
  • some caravans destroyed 
  • power failure likely.

Tropical cyclone category 4

225-279
(very destructive)
  • significant roofing and structural damage 
  • many caravans destroyed and blown away
  • dangerous airborne debris 
  • widespread power failure.

Tropical cyclone category 5

>280
(extremely destructive)
  • extremely dangerous with widespread destruction.

As tropical cyclones move into the mid-latitudes, where New Zealand is situated, they transform into ex-tropical cyclones as they move into cooler seas.  Generally Auckland is affected by such storms.

On average, about 10 or 11 tropical cyclones form in the southwest Pacific per year, although in any one season the number can range from about two to 16.  La Niña conditions are more favourable for cyclones to form in the Coral Sea instead of further east, which increases the chance of ex-tropical cyclones tracking towards Auckland.

Some of Auckland’s highest wind gusts have occurred from ex-tropical cyclones. Winds such as those resulting from cyclones Bola (March 1988) and Giselle (April 1968) are examples of tropical cyclones that transformed into mid-latitude systems, causing damage from extreme winds. Cyclone Bola produced gust speeds of 107km/h in Auckland.  In the last few decades, the worst storms affecting Auckland were Cyclone Fergus in December 1996 and Cyclone Drena in January 1997.

View footage of Cyclone Drena in Auckland on YouTube.  

 


Impacts of cyclones

Tropical and ex-tropical cyclones are generally associated with heavy rainfall, high winds, and storm surge. Ex-tropical cyclones are often the cause of Auckland’s most extreme weather leading to extensive flooding and wind damage. In addition, particularly during high tides, flooding of low-lying coastal areas is common because of storm surges and waves.

General impacts on property and people in Auckland include:

  • uprooted trees damaging buildings and utility infrastructure such as power lines
  • injuries to people from airborne debris
  • boats blown onto shorelines
  • disruption to air travel
  • land movement in the form of landslides, rockfalls and coastal erosion
  • flood inundation of properties in floodplains and low-lying coastal areas.


More information

For more information on the Tropical cyclone season 2016, visit the MetService Blog.

Visit the NIWA website for more information on cyclones in the southwest Pacific.

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