What creates a tropical cyclone?

The cyclone will build momentum throughout today [Monday] and continue through until the early hours of Tuesday morning, when the rain will begin to ease but winds will remain at gale force. –Thames-Coromandel, Civil Defence, 13 February 2023

By Roger Childs

Tropical cyclone is our term for what the Japanese call a typhoon and the Americans call a hurricane. Tropical is the key word as these massive low pressure systems originate within a few degrees of latitude north and south of the Equator.

High sea temperatures are the source of tropical cyclones, and in the late summer they reach a heat ideal for spawning the storms. 

Cyclone Gabrielle originated over the Coral Sea off the coast of Queensland and is now causing havoc in the north and east of the North Island. The paths of cyclones are difficult to predict and they can change rapidly, however the formation process follows a set pattern.

The recipe for cyclone formation

TAKE 1 A large warm ocean area.

ADD 2 Add storms developing on the edge of the Inter Tropical Convergence Zone where trade winds from the two hemispheres interact.

ADD 3 Have air sucked up into the upper atmosphere sometimes aided by the jet stream.

THEN 4 Stir with the Earth’s rotation – the Coriolis Force (this give the tropical cyclone / typhoon / hurricane its characteristic swirling shape)

THEN 5 Add heat given off by condensation of water in the air.

Typhoons / hurricanes / tropical cyclones need an on-going supply of energy from warm seas to keep them going. In Japan’s case the warm Kuroshio Current provides this fuel. It flows north-east from the South China Sea and passes south of the Japanese islands. Off the coast of the United States the Gulf Stream serves the same function. 

Approaching New Zealand a cyclone will lose some of its strength when it passes across colder waters and over land, however as Gabrielle is showing, it will continue to pack a punch for some time after hitting the coast.

Tropical cyclones only travel at about 25-35 km/h, but the winds that swirl around the calm eye of the storm can reach over 200 km/h. These intense low pressure systems rotates anti-clockwise in the Northern Hemisphere and clockwise in the Southern.

Devastating outcomes

Cyclones are massive hazards as they bring a cocktail of appalling weather which adversely  impacts on the land. Some of its features include:

• very strong winds which cease for a short time as the eyes passes over
• high seas and large waves driven by the winds
• torrential rain which invariably causes flooding and landslides
• storm surges which result from a temporary rise in sea level where the ocean waters are literally pulled up by the rapidly rising air in the low pressure system.

The results of these factors, as exhibited by Gabrielle, are widespread damage to property; flooding of settlements, and farmland; road closures and power outages; landslides, slips and erosion; damage to crops, trees and stock. Not surprisingly, areas close to or on the coast are most vulnerable to the severe impact of cyclones.

For the areas affected there is the inevitable subsequent massive job of cleaning up, clearing slips and repairing the damage to buildings once the cyclone passes.