NZ's seas are running hot: Is a marine heatwave coming?

Meteorologists have been keeping a close watch on warming sea surface temperatures - and some pockets have been reaching 3C above average. Image / Niwa

New Zealand's coastal waters are running hotter now than they were at this point three years ago - when the country was enveloped by a freak marine heatwave that fuelled our warmest summer.

Meteorologists have been keeping a close watch on warming sea surface temperatures (SSTs) - some pockets have been reaching 3C above average - as an arriving La Nina climate system begins to influence our weather.

Already 15 spots around New Zealand are on track to register record or near-record dryness for October.

Niwa forecaster Ben Noll said local SSTs had warmed "quite notably" since September, with waters north of the North Island sitting at just over 1C above average.

"So that's pretty impressive. The most anomalous waters are sitting off the west coast of Auckland where, over the past seven days, SSTs have been as warm as 2C above average," he said.

"And looking further west, as you go more into the Tasman Sea, there are actually pockets of water that have been as much as 3C above average."

Source / Niwa

Sea temperatures around the South Island had also risen to between 0.5C and 0.8C above average.

"So all areas are at least a half degree above the the long-term average for the month of October," he said.

"And unfortunately, things are tracking in the way we kind of expected, with really warm temperatures arriving, as well as northeasterly wind flows, which are a La Nina-like signature here late in October."

Forecasters have been picking a "moderate" strength La Nina, comparable to one nearly a decade ago.

The ocean-driven phenomenon - the opposite of El Nino - is expected to deliver a summer of two halves, bringing dryness to the south, moisture to the north, but above-average warmth everywhere.

Over the record summer of 2017-18, weak La Nina conditions, combined with other factors, precipitated tthe largest marine heatwave ever observed around New Zealand.

New Zealand's coastal waters experienced an unprecedented marine heatwave in November-December 2017 🌊 📈

3 years later and conditions are warmer at this point in 2020 than they were in 2017.

We are watching closely.

2017 2020 pic.twitter.com/S4KjbCU3n4

— NIWA Weather (@NiwaWeather) October 28, 2020

It pushed SSTs to 1.5C above average - and as high as 6C above normal in some spots off the West Coast - as beaches became crowded weeks earlier than usual, and mussel beds and seaweeds around Southland and Otago suffered cascading losses.

Interestingly, Noll said, our waters were running warmer now then at this point in 2017.

Along with a La Nina, another similarity with three years ago was a positive phase of a key indicator called the Southern Annular Mode (SAM).

A positive SAM had meant weaker westerly winds than normal over the South Island with higher pressures - and fewer cold fronts crossing New Zealand with bursts of cold air.

It also blocked highs to the east of the country, while less churn of local ocean waters allowed the sea surface to warm.

"For people who work within the fisheries and marine sectors, I'd say keep a very close eye on the situation - they might even be able draw on their experiences from late 2017 and early 2018 to get a guide of this potential."

Another marine heatwave would mark the third in just four years - and these events are expected to become longer, stronger and more frequent under climate change.

Warmer seas also correlated to warmer air on land - driving hot spells and the further melt of our alpine glaciers.

"It means that, when it comes to things like high temperature extremes, the dice are probably loaded as we work our way through the end of spring and into summer."

As it stood, Niwa was forecasting above average temperatures around the country for the next three months - and a slightly more active tropical cyclone season.

Noll added October had been unusually dry for many places.

Rainfall data from the past week showed Waiouru, Whakatane and Auckland - at Motat - were on track to record their lowest levels of rainfall on the books.

Other locations including Hawera, Levin, Hicks Bay, Martinborough, Masterton, Te Puke and other parts of Auckland have similarly seen one of their driest Octobers on record.