Why New Zealand Could Have Another Unusually Hot Summer

New Zealand could be heading for another unusually hot summer, with a leading climate scientist already calling the arrival of a “colossal” La Niña system, along with likely temperate seas.

Professor Jim Salinger said it was possible that a “marine heat wave” for 2020-21 could even approach the scale of a bizarre event three years ago, while fueling high temperatures on land.

A Niwa meteorologist also said today that the chances of a La Niña weather system messing with our weather in the coming months are now very likely, while the potential for a marine heat wave “certainly has our attention.”

An advisory published late last week by the US-based Center for Climate Prediction and other major agencies reported that La Niña conditions were present and 75 percent likely to continue through the hemisphere’s winter. north.

Niwa’s most recent climate outlook, released earlier this month, puts the probability of a La Niña developing in the Pacific before the end of November at 57 percent, with most of its criteria for alertness being met. .

The last time a strong La Niña formed was in 2010-11, with weaker systems in 2016-17 and 2017-18, and a moderate one in 2011-12.

During a La Niña event, ocean water from the coast of South America to the central tropical Pacific cooled below average, as a result of stronger-than-normal eastern trade winds, churning out cooler seawater. and deep to the surface of the ocean.

This unusually cold water in the eastern Pacific suppressed clouds, rain, and thunderstorms as sea temperatures at the western end of the ocean warmed to above-average temperatures.

Here in New Zealand, winds further from the northeast generally brought rainy conditions to the northeast of the North Island and drier conditions to the south and southeast of the South Island, the reverse image of El Niño.

Thanks to the northeast winds, warmer temperatures also tended to manifest themselves in much of the country, although there were always regional and seasonal exceptions.

The weak La Niña of 2017-18 helped establish an unusually intense marine heat wave around New Zealand during what turned out to be the hottest summer in the country, along with another ingredient that, according to Salinger, also seemed to be making a comeback on stage.

That was the southern annular mode, or SAM, a constantly changing storm indicator at latitudes around and deep in New Zealand, simply shifting to a positive phase.

A “positive” SAM had meant that there were weaker than normal westerlies over the South Island with higher pressures and fewer cold fronts crossing New Zealand with gusts of cold air.

In a positive phase, it also blocked the highs in the east of the country and at times rose back over New Zealand, with gentle air currents from the north through New Zealand.

Salinger said that since the SAM had swung positive, the seas would begin to calm down, helping prepare the oceans for heat wave conditions.

In the short term, he expected the northwesterly winds to continue this month and part of the next, before the northeasterly winds and “great southern highs” became more frequent weather systems.

“During the summer holidays, it will be great to go west and south of the South Island; areas like the southern lakes will be the places to be.”

More broadly, warmer coastal waters could drive what he described as a “New Zealand regional heat wave,” with days particularly hot across much of the country during the summer.

Niwa meteorologist Chris Brandolino said weather conditions continued to trend toward an emerging La Niña, although the Pacific Ocean and atmosphere had to “couple” for an extended period before one could formally declare.

“Most of the dynamic models we use, which help us understand not only the weather, but also the climate, indicate that we will be in La Niña as spring unfolds and we move into summer,” he said.

“Certainly more likely than not.”

Brandolino said that La Niña in New Zealand had historically been linked to warm sea temperatures, and that waters across the country had already been near or above average.

Some models indicated a “significant sign” that ocean temperatures would become “unusually abnormal.”

“That’s important because basically if our ocean temperatures are above average, there is a pretty high probability that we have above average air temperatures,” he said.

“The fact that there are signs for this to continue, and some of them are quite strong, the potential for a marine heat wave certainly has our attention.”

But Brandolino thought it was too early to say whether a marine heat wave would develop or reach extremes of the event during 2017-18, when sea surface temperatures rose to 1.5 ° C above average and even 6 ° C above normal in some places. off the west coast.

But Salinger said it was possible that 2020-21 could be of “similar magnitude.”

If one were to develop again, it would be the third in just four years.

The impact of back-to-back events between 2017 and 2019 was perhaps seen nowhere more starkly than in our glaciers, which contracted amid one of the largest melts ever observed in the Southern Alps.

Marine heat waves also have dramatic consequences for ocean ecosystems, sometimes even driving tropical fish to colder climates, and are expected to grow stronger, longer, and more frequent under climate change.