Marine heatwaves & oceanic changes

New Zealand is surrounded by ocean, and both our climate and our climate extremes (such as droughts, heatwaves, floods and tropical storms) are highly impacted by ocean temperatures. The ocean is also changing rapidly – it’s absorbing more heat, ocean currents are shifting, and we’re experiencing longer and more intense marine heatwaves. The more we understand the pace and scale of these changes, the better we can plan to adapt.

In the summer of 2021-22, we saw one of our most intense marine heatwaves, and our work indicates that this will start to become “normal”. Marine heatwaves kill off corals, disturb ecosystems, pose risks to fishing and aquaculture, and contribute to heatwaves on land and other climate extremes across the country. We are also seeing changes to our oceanic system, with fish and marine species seen further south than ever before.

This suite of research is examining how the ocean system around New Zealand is changing. Using results from the New Zealand Earth System Model, we are currently trying to understand how the highly fertile environment of the sub-tropical front might change. Future changes will determine the locations of fishing hotspots, influence species’ distribution and provide pathways for invasive species. Improved modelling of the ocean system through this research will feed back into global modelling efforts. We are also engaging with other research programmes to make sure climate projections for Aotearoa take into account all current knowledge.

You can find detailed information about the research underway in the drop-downs below.

Key findings:

Modelling findings:

By 2100, the 40-odd marine heatwave days we currently see in a normal year will increase to between 80 days (low emissions, best-case scenario) and 170 days (high emissions, worst-case scenario).
For some regions, such as southern tip of the South Island, there is a high chance that marine heatwaves start to last more than a year.
In regards to the intensity of future marine heatwaves (or just how warm they will be), for coastal waters, average marine heatwave intensities will increase by 20% (best case) to 100% (double, worst case) by the end of the century.
For the North Island, this means an average marine heatwave could be between 0.5°C to 2°C more intense than they are today.
The datasets available through this project range from the complete model variable output (ocean temperature at different depths, horizontal velocities, salinity, etc.) and metrics related to marine heatwaves relevant to research purposes, to locally-specific temperature and heatwave projections most useful for industry members and practitioners. An infosheet describing these datasets and how to access them will be available shortly. In the meantime, please email our Knowledge Broker from more information.

Observational findings:

Salinity variability is more important to the Tasman Box than was previously understood, which impacts how the ocean sequesters heat and carbon dioxide and influences primary productivity.

How you can use this research:

Fisheries managers and aquaculture companies can measure future risk (e.g. spread of invasive species or nutrient supply), and adjust business strategies (e.g. relocation of fish farms)
Policy makers can consider these projections for the design of fisheries and ocean policy, quotas, and managing marine protected areas
Researchers can use this research in impacts studies on marine ecosystems, for example, looking at the future health and distributions of particular marine species
Policy makers can consider the severity of these findings when setting emissions reductions targets

Who we are engaging with:

Fishing and aquaculture industry groups and members
Coastal hapū and iwi researchers investigating local and taonga species
Fisheries NZ and MPI
DoC (for marine protected areas)
Other research programmes such as the Moana Project and NZSeaRise

In the media:

NIWA gives dire warning about ‘severe’ marine heatwaves, 1News, TVNZ
Marine heatwaves forecast to devastate, Otago Daily Times
Marine heatwaves expected to get longer, hotter and more severe, Stuff
Why scientists are expecting another ‘crappy’ year for glacier melt, NZ Herald
Marine heatwaves: Why do they make our seas scorch?, NZ Herald
Major climate modelling effort captures New Zealand’s exceptional oceanic conditions, leading to better climate simulations, Deep South Challenge
Narrowing in on Southern Ocean eddies, Deep South Challenge

Both industry and recreational fishers are already noticing changes in the kinds of species that are caught and where. It’s hard to see current fisheries being resilient enough to withstand increases between 80% and 100% of median marine heave wave intensities by the end of the century.
Tony Craig, partner with marine consultancy Terra Moana

Stages of research:

Future oceanic changes around New Zealand: 2022-2024

Our changing sub-tropical front and ocean currents

Budget: $200,000

This stage of the project is assessing how boundary currents around New Zealand, in particular their volume and heat transport, are projected to change. We’re comparing climate simulations from the New Zealand Earth System Model with other climate models and observations (including from mooring arrays around the coast, hydrographic sections and satellite data). The model mesh of New Zealand’s Earth System Model is fine enough to capture boundary currents and their variability. Previous assessments, using coarse ocean model data, have been unable to resolve boundary currents.

We are also examining if, how and why the highly fertile environment of the sub-tropical front might change. Our own NZESM results suggest the location of the sub-tropical front, as it skirts around New Zealand, is changing, due to an expanding sub-tropical gyre, and changing to the western boundary currents in the Southern Hemisphere. We are porting technical advancements of the New Zealand Earth System Model back to its parent model (the UK Earth System Model) to support global modelling efforts and grow the user group.

Marine heatwaves and the link with climate extremes: 2019-2022

Ocean heat content changes around New Zealand: a link to climate extremes

Budget: $500,000

The state of our ocean is controlled by the interplay of heat and moisture between the ocean and the atmosphere over the Tasman Sea. Here, oceanic heat travels from the subtropics through the East Australian Current (EAC), raising the heat in the Tasman Sea. When the heat content in the Tasman Sea is elevated, or increases rapidly, then the ocean “catches a fever,” which can result in heatwaves and climate extremes over New Zealand.

This project improved the simulation of this heat content and heat transport in the New Zealand Earth System Model (NZESM), and, in particular, significantly refined the modelling of ocean currents in the Tasman Sea (e.g. EAC, EAC-Extension and Tasman Front). This project also analysed the NZESM projections to understand future intensity, frequency and seasonality of marine heatwaves around New Zealand in both deep and coastal waters. Datasets from this project stage are now supporting stakeholders in planning and policy.

Warming in the Tasman Sea: 2019-2020

The Tasman Box: filling and warming up

Budget: $170,000

The temperature of the Tasman Sea has a strong influence on New Zealand’s climate system. Because of our prevailing winds, the heat content of the Tasman Sea in particular is a key player in New Zealand’s climate experience. Heat in seawater is easily shifted, even by slow ocean currents and subtle changes in its vertical composition. When the amount of heat stored in the ocean changes, so do our climate and our climate extremes (such as tropical storms).

This project collected temperature, salinity and ocean current data to calculate heat fluxes within the “Tasman Box” – an area of ocean that lies between New Zealand, Australia and Fiji.. These observations improved the accuracy of the New Zealand Earth System Model (NZESM), which in turn improved our understanding of what’s causing changes in the Tasman Sea’s temperature, and how these changes might impact our weather, marine heat waves and sea levels.

Salinity influences water density and, therefore, how the ocean mixes vertically, but changes to salinity in the Tasman Box hadn’t been studied before due to a lack of a long-term data record. Using new Argo float data since 2004, the team found that inter-annual salinity variability in the Tasman Box is large, with an effect on water density as large as that of temperature variability

Simulating New Zealand’s changing climate: 2015-2019

These ocean modelling projects grew out of our large, foundational, earth system modelling and capability project. Read more about that project here.

The Southern Ocean in a warming world: 2015-2019

These ocean modelling projects built on research into the Southern Ocean in a warming world and its influence on New Zealand’s climate. Read more about that project here.