Southern seas warming faster than northern New Zealand

08 November 2017
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Two-spot demoiselles, taken at Poor Knights Islands by Paul Caiger.

Three of the longest-running sea temperature records in the Southern Hemisphere have been analysed by scientists and show that while coastal waters near Auckland are not getting any warmer, southern New Zealand and Tasmania are.

Scientists have long been concerned about ‘tropicalisation’ of oceans due to sea temperature rise from climate change. Predicted changes in marine ecosystems include tropical species moving into areas that were previously too cool and vice versa.

Dr Nick Shears from the University of Auckland’s Institute of Marine Science and Dr Melissa Bowen from the School of Environment say there is as yet no firm evidence this is happening near Auckland but their study shows ocean warming is highly variable between regions.

Their research analysed 50 years of daily sea temperature records taken at Leigh Marine Laboratory north of Auckland; 64 years of records from Otago University’s Portobello Marine Laboratory near Dunedin; and 71 years of data from Maria Island, eastern Tasmania. Satellite and other meteorological data, including major ocean currents and wind patterns, which both have a significant influence on sea temperature, were also included in the analysis.

The research found strong warming off the coast of Tasmania, moderate warming in southern New Zealand and little evidence of long-term warming in coastal waters around the North Island.

However some seasonal variation in warming trends was detected. At Leigh there was evidence of long-term warming in May and June and, conversely, a drop in temperature in November and December.

Down south at Portobello, there was an increase in autumn-winter sea temperature of 1.3 deg C but no long-term changes in summertime temperatures.

“Seasonal patterns may be influenced by a number of different factors including ocean currents and meteorological conditions,” Dr Bowen says. “This is one aspect of the research we think deserves further investigation.”

The variation in warming trends observed indicate that tropicalisation is unlikely to be uniform but instead will vary between different ocean regions, Dr Shears says.

“For example, we would not expect tropicalisation of northern New Zealand to occur to the same degree as what is currently being observed in eastern Australia. Conversely, we are likely to see an increase in the arrival of warmer water species from northern New Zealand into southern waters.”

Dr Shears says warming in southern New Zealand is of particular concern for unique systems and ecosystems found there, such as yellow-eyed penguins and giant kelp forests.

“These results clearly demonstrate the importance of long-term records in understanding changes around our coasts, predicting how it will change in the future, and predicting how species and ecosystems will change as the oceans warm.”

The study is published in Scientific Reports.

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