NZ seas show signs of new heatwave
New Zealand is on the cusp of a second consecutive marine heatwave, with temperatures along parts of the east coast now about 3 degrees Celsius warmer than summer averages.
Hot patches of water were already lying off the Hawke’s Bay, Kaiko¯ura and Canterbury coasts during the first week of this year, according to National Institute of Water and Atmospheric Research (Niwa) maps of sea-surface temperatures. Much of the surface of the Tasman Sea is also between
1C and 2C warmer than normal, and there are more hot spots close to the shore of Australia.
Another map shows the sea surface from East Cape down to Otago during the first week of this year was at least 2C warmer than it was in the first week of
2018. The marine heatwave this time last year was a major contributing factor to the country’s hottest summer on record.
However, Niwa meteorologist Ben Noll said Kiwis should not get too excited thinking the warming sea meant a heatwave was also inevitable on land this summer. ‘‘It doesn’t necessarily imply that this JanuaryFebruary-March will have the extreme warm temperature anomalies that the same period last year did. But, since we are an island nation, as the seas go we also tend to go.
‘‘Warmer than average temperatures, particularly during January, are favoured.’’
Scientists will leave it another couple of weeks before determining if the current seasurface warming definitely counts as a marine heatwave.
Two marine heatwave summers in a row would be very unusual, Noll said.
Niwa principal climate scientist Dr Brett Mullan said there were different thresholds for defining what constituted a marine heatwave. But if summer seasurface temperature anomalies were used, the last New Zealand marine heatwave was in the 2001-02 summer.
‘‘Curiously, though, this peak does not occur in the NZ sevenstation land temperature record: the summer minimum temperature was high but not the maximum, so maybe it was cloudy and humid or wet.’’
It was more than 80 years since a marine heatwave that ‘‘reached anything like the same level of severity’’ as that of the
2017-18 summer, he said.
Noll said what happened last summer might still be having an impact on current sea-surface temperatures.
Generally, higher air pressures east of the country and settled weather were also allowing the sea to warm.
‘‘Overall, if you look under the skin of the sea, you’ll find there has been a more than usual persistence of warmth at depth over
2018. That may have added to this. ‘‘Also, we haven’t had a lot of southerlies lately. That is a player,’’ Noll said.