A moderate to strong marine heatwave around New Zealand subsided before summer started. But there could well be more marine heatwaves through the summer months.
University of Otago physical oceanographer Robert Smith said despite a spring heatwave that lasted for five weeks, ocean temperatures at the sea surface had dropped below the threshold for a heatwave over the past few weeks.
Although marine heatwaves had become more frequent, more intense, and longer lasting over the past century — and were projected to continue to trend up with climate change — it was ‘‘good to be cautious’’ and not extrapolate too far, he said.
Dr Smith, who is also a principal investigator for the Moana Project, said the ability to forecast heatwaves was an area of active research.
‘‘There are likely to be more marine heatwaves this summer — but when they kick off, and quite how long they last, and how intense they get is a bit of a question,’’ he said.
Globally, marine heatwaves could lead to huge impacts on marine ecosystems.
‘‘But right at the moment we are at a little bit of a respite,’’ he said.
As summer began, Wise Response climate change spokesman Jim Salinger warned New Zealand could face intense environmental impacts in a summer heatwave.
Dr Salinger said if another marine heatwave were to develop similar to the one in the summer of 2017-18, which began near the start of summer and lasted for three months, environmental impacts again would be significant.
October was the fifth-warmest on record, and November temperatures were 0.9degC above average, he said.
And below the sea surface the water was up to 3degC warmer than normal off the coast of the South Island in the spring.
The impact on marine life could be intense if ocean temperatures rose significantly over summer, Dr Salinger said.
Coastal kelp forests along Otago’s coast reportedly struggled in the warm sea and there had been impacts on some fish and seabird species in 2017-18, he said.
Wise response chairwoman Liz Slooten said she was worried high marine temperatures could cause issues for species’ survival or reproduction.
Prof Slooten, a University of Otago marine biologist, said high marine temperatures could cause reduced breeding in the nationally vulnerable Hector's dolphins, or increased deaths of very young calves.
The number of Hector's dolphin calves born last year was the lowest she had seen in her 35-year study.
A further reduction would be ‘‘the straw that broke the camel's back’’.
University of Otago geography Associate Prof Nicolas Cullen co-authored a paper with Dr Salinger that documented the marine heatwave in 2017-18.
The warm conditions that summer caused massive ice loss in South Island glaciers, estimated to be the largest annual loss of glacier ice in nearly 60 years of records for the Southern Alps.
Associate Prof Cullen said the ice loss from the Southern Alps was likely to be significant if marine heatwave conditions returned to the country again this summer.
Niwa is predicting a La Nina weather pattern, which is warmer and drier than average for Otago, for the summer.
