It has been a summer to remember, but not in a good way, says a NIWA spokesperson.

February 2023 will go down as the month that New Zealand experienced one of its worst weather disasters.

On February 13 -14, Cyclone Gabrielle caused historic extreme rainfall and river flooding, catastrophic wind damage, and substantial storm surges across the North Island. Unfortunately, this culminated in widespread destruction and loss-of-life, with a long and costly recovery ahead.

Rainfall

February rainfall was nothing short of exceptional. Parts of the North Island received at least 400 per cent of their normal February rainfall:

Napier had its third wettest month since records began, receiving over 600 per cent its normal (and 45 per cent of its annual normal)

The highest one-day rainfall was 316 mm, recorded at Tūtira (Hawke's Bay) on February 13

Despite this, in parts of the South Island, rainfall was below (50-79 per cent) or even well below normal (

Summer rainfall was also noteworthy, as it was the second wettest summer on record for the North Island.

The Auckland region received over 5.5 times its normal summer rainfall and 63 per cent of the entire annual normal, and it was the wettest summer on record for several major centres, including Napier, Auckland, Whangārei, Gisborne, and Tauranga/

For the South Island however it was the fifth driest summer on record.

A meteorological drought developed in Otago during February, with many areas recording less than half of their normal summer rainfall

Pressure

Extreme low-pressure readings speak to Cyclone Gabrielle's power. February 14 saw air pressure dip to 968 hPa at Whitianga in the Coromandel Peninsula - the second lowest daily minimum value observed in the North Island since 1960

Soil moisture

This was a tale of two halves. At the end of February, soil moisture levels were well above normal across most of the North Island and in parts of Marlborough and Canterbury

Soil moisture levels in the northern West Coast, western Tasman, the Mackenzie Basin, and Southland were below normal.

Wind

Cyclone Gabrielle brought destructive winds. There were 18 locations that observed a record or near-record high summer wind gust

The highest wind gust was 150 km per hour, observed at Mokohunau Islands on February 12.

Temperatures

It was not only wet, but warm. In February:

- The nationwide average temperature in February 2023 was 18.5°C, 1.1°C above the 1991-2020 February average

- Temperatures in the West Coast were particularly noteworthy, being the warmest February on record for Greymouth, Westport, and Arapito

- Greymouth recorded seven days with a daily maximum temperature above 25C (between 1972-2000, Greymouth only had six total February days when the temperature exceeded 25C!)

- The highest temperature was 35.6°C, observed at Middlemarch (Otago) on February 4.

Summer:

- It was the third warmest summer on record

- 65 locations experienced record or near-record warm minimum temperatures

Marine heatwave

New Zealand's waters are also part of the picture. Summer sea surface temperatures were record breaking (since at least 1982) in the west of the South Island.

In the north and east of the South Island, summer sea surface temperatures were second highest on record.

Sunshine

The sunniest four locations so far in 2023 are:

- Central Otago (562.6 hours)

- West Coast (548.6 hours)

- Mackenzie Basin (545.4 hours)

- Queenstown Lakes District (526.7 hours)

- Sunny in the west! The sunshine hours in Hokitika were twice as much as those in Dannevirke and Dargaville

The ‘why' behind the weather

Typical of La Niña summers, higher-than-normal air pressure was observed to the east and south of New Zealand, with lower-than-normal air pressure to the north and west

This resulted in more easterly and northeasterly winds than usual, drawing in warm and humid air from the tropics and sub-tropics.

This partly explains why persistently wet and cloudy weather was experienced in northern and eastern parts of both Islands, with sunnier and drier conditions in the west and south of both islands.

Exacerbating the warmth, humidity, and moisture availability to passing low pressure systems was a protracted marine heatwave (MHW) that peaked during January and rivalled the MHWs of 2017-2018 and 2021-2022

Climate change: a warmer atmosphere can hold more water vapour, which leads to heavier rainfall totals - NIWA scientists are contributing to an attribution study on the influence that climate change had on Gabrielle. More details will likely become available in the next month or two.