Hot and Wild: Three Ways La Niña Will Affect Our Summer

Kiwifruit can expect a two-halved summer, one dry in the south, wet in the north, but generally warm throughout, if an anticipated La Niña weather system brings its usual flavor to the table.

Forecasters have also pointed to the potential for warm sea temperatures and possibly increased tropical cyclone activity.

A week after kiwi climate scientist Professor Jim Salinger told the Herald that a La Niña system had developed in the Pacific Ocean, the Australian Bureau of Meteorology issued an official statement yesterday.

Niwa is delaying doing the same, but could probably go from its current La Niña “alert” state to also confirming an event in the coming weeks.

Salinger said he expected the next La Niña to be a moderate event, or similar in scale to one that unfolded in 2011-12.

He predicted that it would peak around December and January, with a prevailing northwest-to-northeast wind shift around the end of spring.

Meanwhile, Niwa’s newly released seasonal outlook has forecast above-average temperatures for the remainder of 2020, along with a high probability of extreme high temperatures, particularly on days with a strong northwest wind.

Air pressure was also forecast to be higher than normal in the southeast and lower than normal in the north of New Zealand.

This was expected to be associated with the development of La Niña-like northeast airflow anomalies, although a potentially strong western flow anomaly was favored to continue through much of October.

So what kind of weather could we go the farthest when La Niña settles right?

A divided summer

In a La Niña event, the ocean water that stretches from the coast of South America to the central tropical Pacific cools below average.

This is due to stronger-than-normal easterly trade winds, which churn colder and deeper seawater to the ocean’s surface.

While the water in the eastern Pacific runs unusually cool, suppressing clouds, rain and thunderstorms, sea temperatures at the western end of the ocean warm to above-average temperatures.

Here in New Zealand, more winds come from the northeast, bringing rainy weather to the northeast of the North Island, but drier conditions to the south and southeast of the South Island, or the other way around El Niño.

Niwa meteorologist Ben Noll said the shift from northwest to northeast may mean a wet summer season for exposed regions like Northland, Coromandel and the East Cape.

“At the same time, areas like Taranaki, Manawatu and much of the South Island, especially the interior or the west, are protected [from] those same northeast winds, “he said.

“As that northeast wind travels across the North Island and then onto the South Island, the air is drying up as it moves south.

“And La Niña often comes with large areas of high pressure that are near the South Island and sometimes over the South Island.”

We will likely start to see signs of this general pattern in late October, and then in November and December, he said.

“Sometimes those large areas of high pressure can also cradle the low pressure as it descends from the north, and it can get stuck near the top of the North Island,” he said.

“Historically, that’s when we can have a lot of rain in a short period.

“We’re probably still a month or two away from that becoming a bigger risk, and it doesn’t seem like it’s in the cards right now.

“But as we move into the next three-month period, it’s a gradual thing that will play out in our climate.”

Auckland residents especially can also have a more humid climate during the summer, as flows from the north draw warm air and moisture from the tropics.

Noll said many would recall Auckland’s excessively sultry heat during the record hot summer of 2018, which came after a weak La Niña formed in previous months.

Warmer oceans

That summer also saw a strange “marine heat wave”, when sea surface temperatures rose to 1.5 ° C above average and up to 6 ° C above normal in parts of the west coast. .

It was also the result of skewed La Niña conditions, coupled with blocking of anticyclones centered over the Tasman Sea, fewer low-pressure systems, and a strongly positive Southern Annular Mode that calmed ocean waters, all in context. of climate change.

At their most severe, marine heat waves can have dramatic effects on land, raising air temperatures, melting glaciers and bringing growing seasons forward weeks ahead of schedule.

The 2017-18 big event also saw cascading losses of beds of mussels and other seaweed along the coasts of Southland and Otago, while New Zealand’s beaches were crowded weeks earlier than usual.

Meteorologists have been tipping warmer seas this summer, and coastal waters across the country have already been unusually warm in recent months.

Noll said that historical records establish a clear link between La Niña and above-average temperatures in the Tasman Sea.

“And then there are what we call analog years, or years in the past that have climatic similarities to the present,” he said.

“Many of the springs and summers of those years have come with an increase in ocean temperature, especially in the Tasman Sea.”

Weather systems that have spread in recent weeks have churned the seas and caused a moderate drop in coastal water temperatures, but they are still “a little” above average.

“But as we move into the next three months, high pressure is expected to become an increasingly important factor in our weather pattern.

“That means more sun, less wind and drafts from the northeast, which can help bring some warmer air down from the subtropics and tropics to our north, and this would help warm ocean temperatures here in the Tasman Sea. and around New Zealand. “

More cyclones?

Meteorologists consider a “severe tropical cyclone” one that explodes with a force of up to 118 km / h, and each season from November to April, about 10 of them form in the southwest Pacific basin.

Only a few of them achieve category 4 strength, where the average wind speeds are over 159 km / h, or more.

Vanuatu and New Caledonia typically experience the most activity, with an average of two to three named cyclones passing close to land each year.

At least one reaches 550 km from New Zealand each season, usually around February and March.

Niwa is still weeks away from issuing its 2020/21 tropical cyclone forecast, but Noll said La Niña could also have an influence on the outlook.

“What can happen is that the Southwest Pacific Convergence Zone, which is the area of ​​converging winds that is near the Pacific islands to our north, may be more active than normal during La Niña events.

“Ocean temperatures from Fiji to Vanuatu and New Caledonia to the Coral Sea can also be warmer than average.

“It is one of the reasons that Queensland often sees a high risk of extropical cyclones during La Niña events.

“And those systems that form to our north, depending on the pressure pattern further south, will basically determine how likely they are to continue south.”

“So if there are more cyclones in that part of the world, there is a higher overall probability that New Zealand will feel the impacts of one of those systems at some point during the cyclone season.”

Noll noted that the 2017-2018 season saw three cyclones, Gita, Hola and Fehi, and the damage from Gita alone generated more than 4,000 insurance claims, costing insurers just over $ 28 million.

“So even though we averaged one a year, La Niña tips the scales a little bit toward that higher end of the scale.”