“This one is one of only a very few in the southern hemisphere that have occurred in the last 20 to 25 years.”
This event is competing with the regular climate events that typically affect New Zealand, including the shifts in the El Nino–Southern Oscillation - a naturally occurring global climate cycle every few years in the Pacific Ocean which influences trade winds and ocean currents, in turn shifting global weather patterns.
The cycle oscillates between El Nino and La Nina phases which may last up to one to two years each, with neutral conditions in between. Conditions in the Pacific have been neutral in recent months but are gradually shifting towards La Nina.
A La Nina watch is in effect for the Tropical Pacific. Earth Sciences New Zealand found an 85% chance of La Nina conditions becoming established between October and December.
During a La Nina event, New Zealand typically has an increase in northeasterly winds, causing wetter, rainier conditions in the northeast of the North Island and drier conditions in the lower and western South Island.
Cooler-than-average surface-level temperatures were recorded in the eastern Pacific in September, which is in line with the shift towards a La Nina event.
In the short term, Carrier said the westerly winds brought by the SSW might continue throughout October, causing increased rainfall and unsettled weather conditions.
In the longer term, the shift towards La Nina wind patterns could create drier conditions in November and December.
“As we progress through the next three months, these westerlies that we have been experiencing, and probably will experience over the next few weeks, are likely to shift and change into more easterly or northeasterly winds,” Carrier said.
He said as the events collided over the next few weeks and brought competing low and high pressure systems, the strength and consistency of winds would likely be higher in the northern parts of the South Island and lower North Island.
“We have high pressure basically north of New Zealand and then low pressure south of New Zealand,” Carrier said.
“When you get high pressure and low pressure physically close to each other, which is what we have, you tend to get a lot of wind between the high pressure and low pressure.”
For the Whanganui-Manawatū region, there is a 45% chance of above-average temperatures and 40% of near-average temperatures from October to December.
Moisture levels in soil and river flows are likely to be average with a 45% chance of near-normal levels.
Rainfall for the region is projected to be typical for this time of year with a 40% chance of near-normal rainfall.
“Without doubt, the most unusual thing we have going on right now is the effects of that southern stratospheric warming,” Carrier said.
“That’s not something that happens on a regular basis by any means. It’s definitely the most unusual thing we’re dealing with at the moment.”
