Blog by Philip Duncan

It’s a question I’ve been asked plenty of times lately – has a main centre in New Zealand ever recorded winds equal to a category 5 cyclone?

The answer… is yes – although it wasn’t because of a tropical cyclone.  But it did have connections with the tropics.

In 1968 a former tropical cyclone called Giselle was tracking across the North Island.  At the same time a polar storm was racing out of the Southern Ocean.  The two collided over Wellington creating what is known as the “perfect storm”.

It was this storm that sunk the Wahine ferry in Wellington harbour as it blasted the capital with hurricane force winds.  NIWA records show winds gusted to 275km/h – which is equal to a category 5 cyclone.  Around 100 homes lost their roofs. (See new FOOTNOTE added below story)

But the winds were very different to a cat 5 cyclone.  With a cyclone, the strong winds are generated around the eye of the storm over open water.  In this case it was the merger of the two systems and Wellington’s localised topography that created the incredible winds – and they only existed as this strength in one part of Wellington.

It was the first and only time winds of that speed were recorded in New Zealand.

Another question I’ve been asked lately – Could New Zealand ever receive a Category 5 tropical cyclone?

The answer… is no. 

Image : Current sea surface temps / WeatherMap.co.nz

The simple answer lies in New Zealand’s location – we’re just too far south.  Tropical Cyclones need warm water to fuel them.  In northern New Zealand, where our sea waters are the warmest, the average temperature at this time of year is roughly around 22 degrees.  Compare this with the tropics where it’s up to 34 degrees at the moment.

Even a drop of a few degrees can dramatically decrease the strength of a tropical storm as we saw with Zelia which was a category 3 cyclone just a few hundred kilometres north of New Zealand was barely even a low by the time it reached our shores several hours later.

---

 

April 2018 Footnote:  From a NIWA employee: The 275 km/h is a reference to a 3-s gust speed from a Munro anemometer located at Oteranga Bay during the storm. I’ve always understood that this reading was highly questionable due to an issue with the anemometer at that site at the time of the storm. I’ve double-checked with Steve Reid (retired employee of NIWA and before 1992 MetService) who was the wind-expert at both institutions for several decades. He had in the past checked the instrument file for Oteranga Bay and noted that the next technical visit to the site after the Wahine storm had remarks that a “substitution resistor was missing from the installation†and this would result in speeds 25% too high.  he resistor was used in installations where no dial was in the circuit. For this reason, the observation is highly questionable at best, and should not really be accepted as a record.  – NIWA

---

– Philip Duncan, WeatherWatch.co.nz head weather analyst