New Zealand has just recorded its warmest June and July on record - and with one month to go, 2021 is on track to have the second successive warmest winter since records began more than a century ago.
In a statement released on Thursday, the National Institute of Water and Atmospheric Research (NIWA) noted the temperature for the past two months was an incredibly high 1.53C above the long-term average, according to its data.
NIWA meteorologist Ben Noll says the previous record was 1.15C above average for June and July five years ago in 2016.
Noll says several factors have contributed to the warmer temperatures, including higher-than-normal air pressure in the east during the winter months.
"[This] has brought more northeasterly winds to New Zealand, bringing warmer air from the subtropics," he said.
Sea temperatures around New Zealand's coastline have also been warmer than average, mitigating the colder air masses tracking towards the country.
Noll added the Southern Hemisphere polar vortex above Antarctica has also been stronger than normal.
"This helps keep filaments of frigid air tucked away deep in the Southern Ocean," he said.
Atmospheric patterns have also leaned towards a La Niña-like direction, which tends to produce warmer conditions in the Southwest Pacific.
In short, La Niña is a weather pattern that occurs in the Pacific Ocean. In a normal year in this pattern, strong winds along the equator blow warm water at the surface of the ocean westward from South America to Indonesia. As the warm water moves west, cold water from the deep rises up to the surface, which ends up on the coast of South America, according to the National Oceanic and Atmospheric Administration.
La Niña events have different impacts on New Zealand's climate. Typically northeasterly winds tend to bring moist, rainy conditions to the north-east of the North Island, and reduced rainfall to the south and south-west of the South Island, according to NIWA.
Some areas, such as central Otago and South Canterbury, can experience drought during La Niña.
Warmer-than-normal temperatures typically occur over much of the country during La Niña, although there are regional and seasonal exceptions.
Another climate driver in the Indian Ocean, called the Indian Ocean Dipole, developed during July, Noll continued, which brought extra warmth and humidity to New Zealand. July in particular recorded above or well-above-normal rainfall in the inner and western South Island, with Mt Cook recording 146 percent of its typical monthly rainfall, and Arthur's Pass on 181 percent.
Monthly temperatures were 1.7C and 1.9C above average respectively.
Noll said with the June and July temperatures tracking so far above average, this winter has a firm lead on last year's figures.
"This is consistent with our expectations. In the background, the long-term tailwind of climate change continues to churn. Of the seven warmest June and Julys, six have occurred since 2000," he said.
The year to the end of July 2021 ranks as the sixth warmest on record.
