Why summer's about to get warmer - and maybe wilder

New Zealand's summer is set to heat up again, after a post-New Year cool spell. Photo / Jason Oxenham

New Zealand's summer is set to heat up again after a post-New Year cool spell.

And forecasters are also pointing to the possibility of cyclone-driven storms later in the season.

Niwa meteorologist Ben Noll said a large high-pressure system was expected to move toward the country around the middle of the month.

"Over the next week, it will begin to slide over the east of New Zealand – and that may allow some warm air to spill on to the west and north of the country as we go toward January 17, 18, 19 and 20," he said.

"So we're going to see some warmer weather coming our way in the next 10 to 15 days, we just have to give it a little bit of time."

There was also a chance that sea surface temperatures around New Zealand could climb higher.

"As we go through the rest of the season, we're likely to see westerly winds die out a bit, and that gives an opportunity for seas to warm up a bit as north-easterly winds can drag warmer air toward the country."

Looking further into the season, Noll said forecasters would be closely watching the tropics to the north of New Zealand – particularly around New Caledonia, Vanuatu and Fiji – where conditions were expected to become more active.

"If you have plans to travel up there, you may want to keep a close eye on forecasts."

Niwa has forecast the usual one ex-tropical cyclone coming within 550km of New Zealand, potentially bringing heavy wind and downpours.

"But, because we are on the verge of a pattern where it's potentially going to become more active, I'd definitely impress upon people to not let their guard down, because as we saw two years ago, we had three tropical cyclones come near our coastlines."

He added that, for some parts of the upper North Island that had dried nearly to the point of being in a meteorological drought, any extra moisture might not be such a bad thing.

Over coming weeks, forecasters also expected to see a shift in an important climate indicator called the Southern Annular Mode, or SAM.

Effectively a ring of climate variability encircling the South Pole, the SAM involved alternating windiness and storm activity between the mid-latitudes, where New Zealand was located, and higher latitudes, such as over the Southern Ocean and Antarctica.

In its positive phase, the SAM was associated with higher than normal air pressure in the New Zealand region, bringing relatively light winds and tranquil weather conditions.

But in its negative phase – as it had been now for much of the past several months – the SAM typically resulted in lower than normal air pressure, with a higher chance of westerlies and unsettled weather.

"The expectation is that negative pattern is probably going to ease, and we're headed in the direction of getting more days with positive SAM," Noll said.

"So if you don't mind the cooler weather, then enjoy it, because this is something that will become less and less frequent. Soon enough, we'll be having a heatwave and people will be complaining they can't wait for things to get cold again."