NZ study aims to predict the fate of our oceans

Kiwi scientists have begun creating a state-of-the-art model, allowing us to forecast, decades into the future, what will become of New Zealand's oceans. Photo / File

The La Nina-driven "marine heatwave" that's engulfed the Tasman Sea this summer has delighted beach-goers with warmer water, but likely also had some serious effects on sea life.

Now Kiwi scientists have begun creating a state-of-the-art model, allowing them to forecast, decades into the future, how frequent such events might become as the world warms.

At the core of the Niwa-led project, supported by a $300,000 Marsden Fund grant, is what's known as the subtropical front, or STF.

The front forms the boundary of the warm, salty and nutrient-poor tropical ocean to New Zealand's north, and the cold, fresh, nutrient-rich sub-polar ocean to the south.

The position of the front is highly influenced by currents deep beneath the surface, especially along the Macquarie Ridge off the bottom of the South Island, where water has to pass through narrow channels.

Future predictions suggest that the westerly wind in the Southern Ocean will become stronger and shift south, ultimately driving a change to the STF.

"How and how much it is going to change is unknown and can only be tried to be answered by ocean models," said Dr Erik Behrens, a Niwa ocean modeller leading the study.

But those used now couldn't tell scientists how the small channels of
the Macquarie Ridge might affect the picture, and ultimately weren't detailed enough to show how the STF would respond to future changes in wind and sea levels.

Given oceans were a major natural resource - the value of New Zealand's seafood industry has been placed at more than $4 billion - better information was crucial.

"If the STF shifts further south, more nutrient-depleted water will surround New Zealand, and thus impact the entire ecosystem by limiting chlorophyll production," Behrens said.

"Correct ocean predictions are necessary to estimate how much this water mass boundary will change in the future, and thus influence the ecosystem."

The Subtropical Front is shown here, running along the bottom of the South Island. Others shown are the East Cape Current (ECC), Tropical Front (TF), Antarctic Circumpolar Current (ACC), Subantarctic Front (SAF), East and West Auckland Currents (EAUC and WAUC), D'Urville Current (DC), Westland Current (WC) and Southland Current (SC). Image / Niwa

Aided by New Zealand's new NeSI high performance computing facility and satellite data, the team will build a cutting-edge ocean model for the New Zealand region.

It would be used to compare simulated present-day conditions and potential future scenarios.

"The largest challenge of this project is to simulate the ocean around New Zealand as realistically as possible," he said.

"Various model parameters need to be adjusted, especially around New Zealand, where the ocean bathymetry is so exceptionally complex."

But what he and his team ultimately produced would hold huge benefits, allowing us to answer colossal questions over how much warmer our oceans may become over the next few decades, whether ocean heat waves will become more frequent, and how marine ecosystems may respond.