Niwa’s autumn weather outlook shows conditions in Otago and Southland will continue to be warmer and drier than average between March and May, but it warns there will be intermingling spells of cooler and wetter weather.
Niwa National Climate Centre forecasting principal scientist Chris Brandolino said the La Nina pattern that had been affecting New Zealand’s weather for the past three years was now expected to transition to ENSO-neutral during March.
Trade winds were stronger than normal in the equatorial Pacific during February, but those are now expected to be significantly reduced and a possible westerly wind burst is predicted for March, he said.
"This is linked to a pulse of the Madden-Julian Oscillation crossing the Pacific.
"This radical change in equatorial circulation represents the first real sign that the ocean-atmosphere system has a chance of moving toward El Nino during 2023.
"First and foremost, however, will be a likely transition to ENSO-neutral conditions during March."
The transition was expected to result in more variation for air flow, rainfall patterns and temperature, Mr Brandolino said.
"Low pressure will occur more frequently over the Tasman Sea and the South Island during March, leading to spells of westerly winds — a distinctive change from summer."
On the West Coast, the Alps and foothills, Otago, and Southland, temperatures were likely to be above average for the next three months, he said.
"Marine heatwave conditions continued in February, increasing the odds for above average autumn air temperatures.
"However, spells of cooler air are likely in mid-March and will become more common.
"The seasonal transition to autumn will also be accompanied by an increasing risk for frosts."
Rainfall totals were most likely to be near normal.
In coastal Otago, an increase in westerly winds would result in some dry spells developing during the season, but the remnant effect of La Nina would still result in onshore wind flows and rain at times.
For the rest of the southern region, the increase in westerly winds during March would likely bring more frequent fronts and low pressure systems to the west and south of the South Island, easing some of the recent rainfall deficits, he said.
However, soil moisture levels and river flows were still most likely to be below normal inland, and near normal or below normal along coastal Otago.
Niwa autumn weather outlook
• La Nina is expected to transition to ENSO-neutral during March.
• Low pressure will occur more frequently over the Tasman Sea and the South Island during March, leading to spells of westerly winds that will bring more cold fronts.
• Rainfall is most likely to be near normal in the South, easing some of the recent rainfall deficits, but there may still be dry spells in some areas.
• While temperatures are forecast to be generally above average, cold spells will become more common.
