“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:
- The highest temperature was 27C, observed at Waiau on May 3
- The lowest temperature was -5.2C, observed at Middlemarch on May 16
- The highest 1-day rainfall was 241 mm, recorded at Milford Sound on May 2
- The highest wind gust was 169km/h, observed at Secretary Island on May 10, and South West Cape on May 26
- Of the six main centres in May 2023, Auckland was the warmest, Tauranga was the wettest and sunniest, Christchurch was the coolest and driest, and Wellington was the least sunny
- Of the available, regularly reporting sunshine observation sites, the sunniest four locations in 2023 so far are Taranaki (1101 hours), Central Otago (1100 hours), Mackenzie Basin (1096 hours) and West Coast (1060 hours)
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.
