Marine heatwaves of unprecedented strength forecast
Flooding and heavy rainfall in the North Island have been matched by another climate-related extreme event in the South Island – marine heatwaves.
Water temperatures above 18C have been forecast by MetService near Stewart Island and Otago for this weekend.
In a press release, MetService oceanographer Dr Joa˜ o de Souza said a marine heatwave of unprecedented strength will affect the ocean off the southern South Island.
‘‘While you might expect a small blip in sea surface temperatures lasting a day, the heatwave that is now surrounding Stewart Island is concerning because 4-6 degree warming is forecasted to last three or four days,’’ he said.
De Souza said this was a warning sign, as two heatwaves have already hit the South Island this year.
In January, marine heatwaves caused extensive bleaching of sponges which then resulted in a loss of marine sponges’ population of up to 10%.
The warming of the water could bring more exotic fish to the Southern Ocean, which can threaten local species like Hector dolphins – as well as trigger a higher number of salmon deaths in fish farms.
De Souza, who has been working on the MBIE-funded Moana Project to create the first ‘‘first nation-wide ocean modelling system to explain oceanic changes’’, said tracking the scale of marine heatwaves has become more difficult.
Since MetService started tracking marine heatwaves a scale with different colours has been used to denote the warming of the water, but those colours might not be sufficient to denote the extreme marine heatwaves Aotearoa New Zealand has experienced in recent times.
‘‘Our graphics only go up to 5 degrees C above normal, but now that we’re increasingly seeing even higher temperatures it may be time to change the scale. The warming is literally off the scale,’’ de Souza said.
The warming of the water turned into an extreme event when the temperatures were ‘‘higher than 90% of what we’ve seen in the past,’’ where the comparison was between ‘‘the current or forecasted temperature to the average of the last 25 years for each location and day of the year’’.
‘‘Extreme marine heatwaves are when waters are warmer than four times the difference between normal conditions and the marine heatwave threshold,’’ he said.
Stewart Island will experience water temperatures above 18C this weekend.
‘‘Extreme marine heatwaves are just that – extreme. It’s the highest category in existence for marine heatwaves, reflecting how unusual this level of warming is, both in New Zealand and internationally.’’