Natural hazards

Cyclones

Five tropical cyclones have passed within 220km of Auckland City between 1970 and 2001.

The main hazards associated with tropical cyclones are wind gusts and heavy rainfall, but they can also generate significant storm surge and coastal erosion.

Cyclone consequences

A severe cyclone event, of a magnitude expected on average once every 100 years (winds up to 170 km/hr, rainfall up to 85mm/hr), could have the following consequences:

• 5-20% of steeper slopes in coastal areas and in the Whitford, Hunua and Waitakere Districts may fail
• coastal flooding and erosion
• a 1:100 year cyclone is expected to disrupt land transport. Road transport disrupted by debris and flooding for several days, or up to a week or more for routes affected by landslips. Rail could take 2-7 days to recover, up to a several months if widespread slope failure occurs
• power outages for several hours
• disruption of water supply, wastewater and stormwater networks
• short term economic losses to home-owners, utility companies and territorial authorities for cleanup, repair and reconstruction.

(Source: Auckland Engineering Lifelines Project, Stage One report, 1997. (Publication # 116) Order here.)

Tropical cyclone formation

Tropical cyclones typically form close to the equator in the West Pacific. It is from the warm equatorial waters that cyclones derive their energy. Most cyclones play out their whole life cycle in the tropics. Sometimes though, a few cyclones migrate out of the topics into the mid-latitudes, where New Zealand is situated. During this process they exchange their characteristic tropical core of warm air for one of cold air. This exchange of warm for cold air often occurs when cyclones interact with troughs of cold air moving through in westerly winds and cause re-development of the storm.