Niwa climate scientist Gregor Macara said the mean sea level air pressure was higher than normal over much of New Zealand and the Tasman Sea, and lower than normal to the east of the country during May.
"This resulted in more southwesterly airflows than usual over the country, and reduced mean wind speeds for many parts.
"These southwesterlies contributed to relatively cold temperatures for the month, with New Zealand observing its coldest May since 2009.
"The nationwide average temperature in May 2024 was 9.8°C. This is 1.3°C below the 1991-2020 May average from Niwa’s seven-station temperature series which began in 1909."
He said it was also a dry month for many parts of the country.
Rainfall was below normal or well below normal for the majority of the South Island, and near normal in southern Southland.
"The lack of rainfall was accompanied by clear skies for many parts of the country, resulting in relatively high sunshine hours for the time of year.
"Tauranga and Mt Cook Village each observed their sunniest May on record; and Whangārei, Paraparaumu, Franz Josef and Queenstown observed their second-highest total sunshine hours for May, respectively."
Niwa meteorologist Seth Carrier said the weather conditions for the autumn season as a whole, were similar.
He said the past three months were also characterised by higher-than-normal mean sea level pressure west of New Zealand and lower than normal south and east of the country.
Weakening El Nino weather conditions drove the southwesterly air flow anomaly from the Southern Ocean, and New Zealand coastal water temperatures were slightly below average during autumn, which contributed to cooler air temperatures and drier-than-normal conditions for much of the country.
"Overall, the nationwide average temperature for autumn 2024 was 12.8°C, making autumn 2024 the coolest autumn since 2012, and the 4th-coolest autumn since 2000."
No autumn or May weather records were broken in Otago or Southland.
