Kiwis are in store for a dry start to summer, but "moist, subtropical disturbances" could bring heavy rainfall and flooding to New Zealand as it goes on, NIWA's latest seasonal weather outlook warns.
The forecaster released its latest outlook for the country on Monday, a day before summer officially begins, and it confirms La Niña is looking likely to bring unsettled weather conditions over the next three months.
NIWA predicts air temperatures will be above normal throughout the country, alongside elevated humidity levels in the North Island. It says average local sea temperatures, subtropical winds and strong climate model agreement supports this outlook.
This will be flanked by a marine heatwave that's been building in northern parts of the North Island and the Tasman Sea during November, and could result in "unusually warm seas" in other coastal regions between now and February.
This could further result in even warmer air temperatures, NIWA says.
"Abnormally warm ocean temperatures can have a profound impact on air temperatures over land, and provide fuel to cyclones approaching from the north," its outlook reads.
"The marine and fisheries sectors should keep an eye on this evolving situation, as it could be similar to what occurred in late 2017 to early 2018."
The forecaster says extended dry spells are expected, but are likely to be interspersed with unsettled periods brought on by moist, subtropical disturbances - particularly in the top half of the country.
The outlook also reveals an "elevated" risk of significant rainfall, damaging winds and coastal inundation during the tropical cyclone season, which takes place between November 2020 and April 2021.
"Rainfall is about equally likely to be near normal or below normal in the east and west of the South Island, and about equally likely to be near normal or above normal in all remaining regions of the country," the outlook reads.
La Niña, which is currently sweeping the globe, is behind the unsettled conditions. The complex weather system has recently moved to moderate strength, NIWA says, which means New Zealand is more likely to get a summer "that has a mixture of drier westerlies and wetter northerlies".
La Niña is caused by strong wind blowing warm water at the ocean's surface from South America and across the Pacific Ocean, and is expected to peak in the first few months of next year before petering out.
"With La Niña’s peak expected in early 2021, tropical trade winds are expected to continue to be stronger than average over the next several months," it reads.
"While La Niña is expected to be the primary planetary climate driver over the next three to six months, an eastward advancement of the West Pacific warm pool was noted at depth in the equatorial Pacific during November.
"Should the advancement of this warm pool continue, it could cause a transition away from La Niña during mid-2021."
