Cooler weather is on the horizon, with La Niña — which brought heavy rains to the northeast of the North Island and dry conditions down south — expected to transition to ENSO-neutral during March.
El Niño and the Southern Oscillation (ENSO) refers to variations in the temperature across the surface of the equatorial Pacific Ocean, and influences rainfall, temperature and wind patterns around the world, according to NIWA.
The transition to a neutral state means the country can expect to see more variable air flow, temperature and rainfall patterns during autumn, NIWA said in its seasonal outlook.
It will also see low pressure occur more frequently over the Tasman Sea and the South Island in March, resulting in spells of westerly winds and, in turn, more cold fronts typical of the cooler autumn months.
Low pressure systems to the west of both islands are also expected to lead to rainfall, gradually easing the rainfall deficits in the parched west and south of the South Island.
From March to May, seasonal temperatures are most likely to be above average in the west of the South Island and about equally likely to be near average or above average elsewhere, cold spells will become more common, such as during early to mid-March. It's also expected to be accompanied by an increasing risk for frosts.
While it signals drier conditions compared to previous outlooks, the "remnant effect of La Niña" could see some sub-tropical and/or tropical low pressure systems, occasionally increasing the risk for heavy rain in the North Island.
Rainfall, meanwhile, is "equally likely to be near normal or above normal" in the east of the North Island, and "most likely to be near normal" elsewhere in the country.
Regional outlook
According to NIWA's regional predictions from March to May 2023, temperatures in Northland, Auckland, Waikato, Bay of Plenty, the central North Island, Taranaki, Whanganui, Manawatū and Wellington are equally likely to be near average (45% chance) or above average (45% chance). Rainfall totals are most likely to be near normal (45% chance).
In Gisborne, Hawke's Bay and Wairarapa, temperatures are about equally likely to be near average (40% chance) or above average (45% chance). Rainfall totals are equally likely to be near normal (40% chance) or above average (45% chance).
Tasman, Nelson, Marlborough and Buller are about equally likely to be near average (40% chance) or above average (45% chance). Spells of cooler air are likely in early to mid-March. Rainfall totals are most likely to be near normal (45% chance). However, the remnant effect of La Niña could see a sub-tropical and/or tropical moisture plume reach the region at some point during the season, particularly in Marlborough.
The West Coast, alps and foothills, inland Otago and Southland are most likely to be above average (55% chance). Marine heatwave conditions continued in February, increasing the odds for above average autumn air temperatures. However, spells of cooler air are likely in early to mid-March. Rainfall totals are most likely to be near normal (45% chance). During March, an increase in westerly winds will likely bring more frequent fronts and low pressure systems to the west and south of the South Island, easing some of the current dry conditions.
Coastal Canterbury and east Otago are about equally likely to be near average (40% chance) or above average (45% chance each). Spells of cooler air are likely in early to mid-March. Rainfall totals are most likely to be near normal (45% chance). An increase in westerly winds will see some dry spells develop during the season, but the remnant effect of La Niña will still result in onshore wind flows and rain at times.
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