‘Extreme’ marine heatwave developing West Coast and Fiordland

While the North Island is recovering from the latest storm, ocean temperatures along the South Island’s south-western coast are reaching new records. According to MetService oceanographer Dr Joao de Souza, sea surface temperatures may reach an incredible 6 degrees higher than normal – an ‘extreme’ marine heatwave – and the elevated temperatures will last for at least a week.  

Dr Souza has been tracking marine heatwaves as part of the Moana Project for years and explains that he has never seen temperatures reach 6 degrees above normal before. “By comparing the current or forecasted temperature to the average of the last 25 years for each location and day of the year, we get a picture of how much warmer than normal the oceans currently are. And what we are finding is disturbing. A six-degree anomaly is a whole new level compared to what we have seen before.  

“Our research shows that for the last couple of years, marine heatwaves have persistently impacted New Zealand coastal waters. The ocean has broken all records - the longest marine heatwave, which lasted just over a year in the Bay of Plenty, and now the biggest departure from normal, with 23-degree warm surface waters off Haast. This is similar temperatures to waters off Northland, and normally never happens at these latitudes.” 

Dr Robert Smith, Moana Project oceanographer from the University of Otago, explains that the warm water is likely caused by a mixture of climate change and the ongoing La Nina conditions: “A blocking high pressure system with light winds in the area is leading to reduced vertical ocean mixing and reduced heat loss from the ocean, triggering the marine heatwave. This is exacerbated by warming in the eastern Tasman Sea, which is currently more than 2 degrees warmer than normal at depths of 100-400 m. These warm underlying conditions makes it easier for the surface ocean to tip over into marine heatwave conditions.” 

While swimmers may enjoy the warm water, it can cause problems for marine life. “Ocean temperatures don’t vary like air temperatures,” continues Dr Souza. “The ocean takes a long time to heat up and cool down. Observations collected in partnership with the fishing industry show waters in Fiordland have been a lot warmer than usual for the last few days, even at depth. This trend will worsen over the coming week. For marine life experiencing 23-degree water off the South Island’s west coast is very unusual. Recently, marine heatwaves off the West Coast have been linked to kelp die-off.” 

Craig Jones, a commercial fishing operator off the West Coast who works with the Moana Project has noticed the effects of warmer waters over the past few years. "The patterns in fish are changing, the fish are still around but it’s not the same. This summer in particular we are seeing snapper and kingfish down our way more than we traditionally have. They seem to be trending south.” 

Lisa Murray, MetService Head of Weather Communication says, “The South Island has experienced the best of the settled summer weather over the Christmas and New Year holiday season. The ridge of high pressure intensifies over the West Coast over the next few days, continuing the fine weather and light winds.” 

The 7-day forecasted sea surface temperatures and how they differ from the 25-year average (called the temperature anomaly) for New Zealand’s ocean can be explored on SwellMap, the free MetService marine forecasting website. Users can zoom in to areas of interest and click on the map to get the exact temperature or difference from normal. 

Sea surface temperatures forecast for 16 January 2023, showing the ocean off the southern West Coast and Fiordland due to reach 23 degrees – a temperature similar to that of waters off Northland. Image from https://www.moanaproject.org/marine-heatwave-forecast.

Difference between forecasted sea surface temperatures and the long-term average for 16 January. Red shading indicates warmer than normal ocean temperatures, blue shading colder than normal temperatures. The dark red shading shows an extreme marine heatwave off the southern part of the West Coast and Fiordland, with water temperatures reaching 6 degrees warmer than expected for this time of year. Image from https://www.moanaproject.org/marine-heatwave-forecast