Marine heatwaves and the link with climate extremes

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

New Zealanders have experienced a series of marine heat waves. Temperatures in some regions have exceeded 6°C above average. While some of us enjoy the warmer water temperatures for water sports and holiday activities, most marine organisms experience heat stress, and warmer water drives further changes in our weather and climate.

New Zealand is surrounded by ocean, and both our climate and our climate extremes (such as droughts, floods and tropical storms) are highly impacted by ocean temperatures. 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. Recent research has shown that 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.

Marine heatwaves also affect the fishing and aquaculture industries (e.g. mussels and salmon farms). Exceptional southward migrations for some fish species looking for cooler waters has been reported during these heat waves. These species rely on a constant supply of cold water year-round to thrive. But many marine organisms can’t swim, which puts them under severe pressure and risk of death.

At the time of this research project, the current version of the New Zealand Earth System Model (NZESM) did not simulate this heat transport as precisely as it could, and modelled ocean currents in the Tasman Sea (e.g. EAC, EAC-Extension and Tasman Front) required significant refinements. Improved modelling of these ocean currents was to lead to a better representation in the NZESM of ocean heat content and heat transportation, enabling our models to more accurately predict changes in future change, including to temperatures in the ocean and on land and related climate extremes.

Better knowledge of future climate extremes is vital for effective decision-making on how to respond, adapt and thrive in a changing climate.