Coastal marine heatwaves in New Zealand are becoming stronger, longer, and more frequent

Worldwide, marine heatwaves (MHWs) have increased in intensity, duration, and frequency – oftentimes with severe impacts on important coastal marine ecosystems.

Now, new research on seasonal trends in marine heatwaves has confirmed this is happening in New Zealand’s coastal regions, and that these trends have been accelerating over the last decade.

The results of a study have been published in the New Zealand Journal of Marine and Freshwater Research.

Since 1982, the number of marine heatwave days has risen by 5.75 times, the average and maximum intensity has increased by 0.1°C and 0.45 °C, respectively, and the cumulative intensity of MHWs is 7 times higher.

Different regions have experienced more marine heatwaves in different seasons; for instance, the greatest increases in frequency occurred in north-eastern New Zealand in summer and autumn, Central New Zealand in autumn, the Chatham islands in winter, and Bounty and Antipodes Islands in spring. 

According to the researchers “understanding seasonality of MHW trends is critical since biological processes like survival, growth, reproduction, and species interactions are seasonally conditioned, and species-specific thermal tolerance often vary seasonally and with [development].”

They analysed changes to MHWs across seasons and ecoregions within 12 nautical miles of New Zealand’s coastline using sea surface temperature data collected from satellites, ships, buoys, and Argo floats (a fleet of robotic instruments that drift with the ocean currents) between 1982 and 2021.

Since this is an area with high biodiversity and endemism – meaning species are found nowhere else in the world – the researchers are calling for better monitoring of these habitats.

“Our analyses demonstrate that coastal communities in New Zealand, over the last few decades, have experienced increasingly stronger, longer and more frequent MHWs that should have implications for conservation and provide an incentive to initiate effective, adequate, and standardised monitoring of coastal habitats (currently almost non-existing),” they write.