‘Exacerbated by climate change’ – NZ experiences warmest May on record

By RNZ

New Zealand has experienced the warmest May on record, with temperatures at least 2 degrees Celsius higher than average in parts of every region, except Northland.

The National Institute of Water and Atmospheric Research (Niwa) said the nationwide average temperature was 13.1C.

Temperatures were exacerbated by climate change, it added. Of the five warmest Mays on record, all have occurred since 2011.

The Crown research institute said lower sea level air pressure (MSLP) over the Tasman Sea, and higher than normal pressure east of New Zealand resulted in more northerly and northeasterly airflows than usual over much of the country.

“The first eight days of the month were exceptionally warm for the time of year, and dozens of locations observed record or near-record high daily maximum and minimum temperatures during this period.

“The warmth persisted for the month overall, and monthly temperatures were above average (0.51-1.20°C above average) or well above average (>1.20°C above average) in every region of New Zealand.”

Niwa said May sea surface temperatures near the South Island were the second warmest on record, as a marine heatwave delayed the seasonal transition to colder temperatures.

“The widespread nature of May’s exceptional warmth, both on land and in the sea, has been exacerbated by climate change,” Niwa said.

Rainfall was also well above normal average and 149 per cent of normal for Northland, Auckland, Waikato, Bay of Plenty, Taranaki, parts of Manawatū-Whanganui, Wellington, Tasman, Nelson, northern Marlborough, West Coast, inland Otago and western Southland.

At the end of May, soils were wetter than normal for parts of Gisborne, Manawatū Whanganui, northern Marlborough, eastern Canterbury near Christchurch, and inland Otago.

Drier than normal soils were recorded in coastal parts of North Otago and South Canterbury.

Other May statistics:

Overall, the nationwide average temperature for autumn was 14.5C, well above average.

“Autumn 2023 was characterised by higher than normal mean sea level pressure (MSLP) east of Aotearoa New Zealand and lower than normal MSLP over New Zealand and the Tasman Sea. This generally resulted in more northeasterly winds than normal, bringing moist air from the tropics and sub-tropics across New Zealand, resulting in periods of wet and warm weather,” Niwa said.

“The season saw a transition from a long-term La Niña pattern to an ENSO-neutral state, allowing for some variability in the monthly air flow patterns. While March saw more westerly winds than normal, both April and May returned to northerly-quarter air flow anomalies, more typical of a La Niña regime.”

An ongoing marine heatwave event contributed to the unusual warmth.

“Overall, the most impactful event during the season was an atmospheric river originating in the tropics that brought heavy rain to large swathes of the country during approximately the first 10 days of May. The atmospheric river varied in strength and location during this time, but nevertheless delivered periods of heavy rainfall and thunderstorms to much of the North Island and the western and northern South Island.”

Despite that, there were hints of colder months to come.

They included March 21 when snow fell to about 800 metres above sea level across parts of the lower South Island, Niwa said.

Another incident was on May 11, when a powerful cold front brought the coldest day since October 2022 for large parts of the South Island and snow down to around 400 metres for parts of Southland, Queenstown-Lakes District, and the Mackenzie Basin, Niwa said.