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correspondence

Is New Zealand outside South Pacific cyclone zone? To the editor: Every year hunBelow, Livia Gilstrap’s 35-foot Wauquiez Pretorien in the South Pacific. Right, a chart showing the number of cyclones by quadrant in a 10-year period, showing that New Zealand is not wholly cyclone free.

dreds of boats transit the South Pacific, leaving from ports in North or Central America and arriving in tropical paradise. By the end of the cruising season, after visiting a handful of Pacific Island nations, each boat has to decide where to weather out the South Pacific cyclone season. By far, the most common strategy is to reach New Zealand in early November, with plans to return to the tropics at the end of the cyclone season in April or May. One reason New Zealand is the default option is because it is commonly considered to be outside of the cyclone belt...but is it? Yes: By definition, a cyclone ceases to be a cyclone the moment that it leaves the tropics. This exit is called an extratropical transition and the storm is now considered by experts to be not a cyclone, but a storm of tropical origin. In addition to the cyclone exiting the tropi-

cal arena and reaching colder waters, it often (but not always) loses other physical characteristics, such as the eye, that are necessary to be considered a named cyclone. No: Even though this storm of tropical origin is no longer correctly called a cyclone, mariners and news outlets usually continue to use the official cyclone name as a former cyclone approaches New Zealand because, semantics aside, the strength of the wind in the storm of tropical origin can be as extreme as any tropical-based cyclone. Over a 27-year period (1970-1997), Northland averaged one storm of tropical origin with cyclone-strength winds per year. In April 1968, Tropical Cyclone Gisele re-intensified near Wellington, producing winds gusting to 145 knots and blowing the roofs off of houses as far south as Christchurch on South Island. As recently as March 2014, Cyclone Lusi

brought cyclone-strength winds to North Island. It depends: These former cyclones, as they reach New Zealand, are rapidly weakening and for that reason rarely reach very far down the North Island. While an average of one extratropical cyclone per year hit the area from Auckland north, the area between Auckland and the top of South Island averaged half as many. Still, extratropical storms of considerable strength, such as both Cyclones Lusi and Gisele described above, have reached all of the way down to South Island. Although El Nino is generally associated with increased cyclonic activity, the strongest extratropical storms affect New Zealand when the ENSO cycle is neutral, not El Nino. To sum up, while the risk is still relatively low, New Zealand is not accurately described as being unaffected by cyclonestrength storms. New Zealand

Livia Gilstrap

Ocean Navigator January/February 2015  

Ocean Navigator January/February 2015

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