0814 GMT April 25, 2019
Hot patches of water were already lying off the Hawke's Bay, Kaikoura 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, stuff.co.nz reported.
Much of the surface of the Tasman Sea is also between 1°C and 2°C 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 2°C 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 January-February-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 favored."
Scientists will leave it another couple of weeks before determining if the current sea-surface warming definitely counted as a marine heatwave.
Two marine heatwave summers in a row would 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 sea-surface 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 seven-station 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."
In January last year, RNZ reported Niwa's Dr Dennis Gordon as saying the warmer water brought different marine species, including jellyfish and pseudo-jellyfish, close to New Zealand beaches.
Noll said a recent paper in the journal Nature discussed how marine heatwaves would increase with climate change.
There had been a doubling in the number of marine heatwave days between 1982 and 2016.
"This number is projected to further increase on average by a factor of 16 for global warming of 1.5°C relative to pre-industrial levels and by a factor of 23 for global warming of 2.0°C," the paper said.