A SUMMER OF FLOODS AND DROUGHTS, AND VERY WARM

Summer

2022–23 in Aotearoa New Zealand

was marked by striking contrasts across the two islands.

February 2023 will go into the record books as a month during which Aotearoa New Zealand experienced one of its worst weather disasters in modern history, by way of an ex-tropical cyclone. Two ex-tropical cyclones, Hale and Gabrielle, impacted the North Island during the summer months. Hale resulted in considerable damage and Gabrielle caused widespread devastation, including unprecedented flooding, severe coastal erosion, and loss of life. A national state of emergency was declared, for only the third time in New Zealand’s history.

The North Island experienced exceptionally wet conditions across much of summer, with Auckland, Northland, Bay of Plenty, and Hawke’s Bay all experiencing their wettest summers on record – marred by several bouts of extreme rainfall and devastating flooding, including an event where Auckland recorded over 280 percent of its normal January rainfall in under six hours.

In contrast, the South Island saw prolonged periods of hot, dry and sunny weather, leading to some areas experiencing meteorological drought, as measured by NIWA’s New Zealand Drought Index (NZDI).

Persistent La Niña – for the third year running –played a big part

Cyclone Gabrielle occurred amidst a unique set of climate drivers – specifically, a La Niña ‘triple dip’ (third consecutive), which although its strength was waning, continued to have a meaningful influence on New Zealand’s weather patterns.

As is typical of La Niña summers, higher-than-normal air pressure was observed to the east and south of

New Zealand, while lower-than-normal air pressure was recorded 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. Other climate drivers also contributed to warm temperatures Adding to the warmth, humidity and moisture availability for passing low-pressure systems, was a protracted marine heatwave that peaked in January. The sea surface temperatures for February were the warmest on record in the west and east of the South Island, and second-warmest on record in the north of the South Island.

The Southern Annular Mode (SAM) was mostly positive throughout the summer, but dipped into negative in February when ex-tropical Cyclone Gabrielle arrived. A positive SAM during summer is usually associated with belts of high pressure near the South Island, which was indeed observed during summer 2022–23.

Temperatures during the summer were generally above average, with some areas experiencing well above average temperatures. All three months were in the top ten warmest on record.

The nationwide average temperature for summer 2022–23 was 17.9°C, which was 1.1°C above the 1991–2020 summer average from NIWA’s seven station temperature series, which begins in 1909.

Looking ahead to autumn

New Zealand is expected to experience a transition from La Niña to El Niño Southern Oscillation (ENSO)-neutral conditions during March, resulting in more variable air flow, temperature and rainfall patterns during autumn.

Niwa Seasonal Climate Outlooks

Predictions

Low pressure is expected to occur more frequently over the Tasman Sea and the South Island, leading to spells of westerly winds – a distinct change from summer. These westerly winds, while not dominant, will bring some more typical cold fronts.

Rainfall is equally likely to be near normal or above normal in the east of the North Island, and most likely to be near normal in all other regions. In the North Island, this represents a drier signal compared to previous outlooks.

Low pressure systems to the western side of both islands are expected to produce rainfall that will gradually ease long-term rainfall deficits in the west and south of the South Island. However, the remnant effect of La Niña could still produce some sub-tropical and/or tropical low-pressure systems, occasionally increasing the risk for heavy rain in the North Island.

Seasonal temperatures are most likely to be above average in the west of the South Island and about equally likely to be near average or above average in all other regions. Cold spells will become more common as autumn progresses, and this transition will be accompanied by an increasing risk of frosts.

NIWA’s scientists are monitoring the chances for a transition to El Niño conditions during winter. El Niño tends to be associated with drier conditions in the North Island and eastern South Island during winter. The likelihood of a transition to El Niño will be updated each month in NIWA’s seasonal outlooks available on the NIWA website.