Gabrielle re-energised as it reached NZ, with tragic results

A109 Light Utility Helicopter flight with mayor Gisborne City from the air in November 2023.

Opinion

Early analysis of Cyclone Gabrielle indicates it was a stronger cyclone system than Bola in 1988 or Giselle in 1968, with historically low pressure levels. Gabrielle was also re-energised as it moved over the northeast of the North Island last Monday night, ingesting a vorticity or area of spin in the upper atmosphere that increased its power, slowed it and yanked it closer to land — “with tragic results for the East Coast,” science writer Jamie Morton in today's NZ Herald.

Meteorologists have compared our most destructive ex-tropical cyclones in recent history to gain a clearer picture of Gabrielle's power, finding that its pressure reached as low as 963hPa, against Bola's 982hPa and Giselle's 967hPa.

“The lower the pressure, the faster winds are going to be blowing in towards the centre of the storm,” explained Niwa meteorologist Ben Noll, who calculated the values.

“As this happens, the winds come together and ascend upward through the atmosphere, which then lowers the mass at the centre of the storm. This creates a hole . . . in the atmosphere, and in order to fill that hole, the winds need to spin faster.”

Gabrielle's widespread, gale-force winds — which dramatically stripped forests in the central North Island — included gusts of over 130kmh.

Bola hit the East Coast on March 7, 1988. Three days of torrential rain over Gisborne and Hawke's Bay caused widespread flooding and landslides. Three people drowned at Mangatuna near Tolaga Bay.

Giselle hit Wellington on April 10, 1968 bringing heavy rain and severe winds. One extreme gust reached 235kmh. The inter-island ferry Wahine sank with the loss of 51 lives. Three other people were killed.

Gabrielle dropped nearly 560mm on the ranges above Gisborne in the space of a day and a half: values comparably intense as the 900mm that Bola delivered to the East Coast over 72 hours in March 1988.

Gabrielle likely had more fuel for heavy rain, as total atmospheric moisture is 5.3 percent higher than in the 1960s and 3.2 percent higher than in the 1980s. Three years of La Nina have also contributed to abnormal warmth in the tropical waters where Gabrielle formed and quickly reached category 3 strength.

“It's important to note that, by the time it reached New Zealand, Gabrielle wasn't the very intense tropical cyclone it once was,” Noll said. “It needed to be assisted by this other vorticity feature to deepen and become more intense once again — and of course, it did that right on New Zealand's doorstep, and at the worst possible time.”