Warm seas around New Zealand could take edge off cold snap
Friday, 3 November 2017
Sea surface temperatures up to three degrees Celsius warmer than usual around parts of New Zealand are likely to help make the coming cold snap easier to handle.
The east and south coasts of the South Island are the warmest areas, a chart published by NIWA on Thursday shows.
That patch of unusually warm water stretches from near Marlborough to south of Stewart Island and includes the Chatham Islands.
Sea surface temperatures around NZ are currently above or much above average, in some cases (South Island) 3 degrees warmer than average 🌡️ pic.twitter.com/XJUbKhJWue
— NIWA Weather (@NiwaWeather) November 3, 2017
Most of the rest of the water around the country is warmer than normal, too.
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The only place where it is slightly colder than average is in the eastern Bay of Plenty.
NIWA meteorologist and forecaster Ben Noll says the warmer waters have been driven by the heatwave in the south.
'Ocean temperatures can bring about warmer than average air temperatures and likewise warmer than average air temperatures can increase ocean temperatures,' he says. 'It's a bit of a positive feedback, if you will.'
Looking at how sea temps have changed over the last week, the effect of the South Island heatwave is clear: 0.5 to 1.5°C worth of warming. pic.twitter.com/LJoY5gzJIp
— NIWA Weather (@NiwaWeather) November 3, 2017
Warm off-shore winds on both sides of the South Island have contributed to the water temperatures off both coasts.
Those temperatures are handy for NIWA's seasonal forecasting, too.
'When water temperatures are that much warmer than average it makes it a much lower probability to then have colder than average air temperatures, so it makes our forecast a little bit easier when oceans are warm.
'Since New Zealand is an island nation, as sea surface temperatures go, often the land goes.'
Along the South Island's East Coast, temperatures are between 13-15C, with patches up to 16 or 17C off South Canterbury. Off the West Coast, they are about 13-14C in the south and 15-16C further north.
While anyone getting in the water is likely to still feel cold at those temperatures, they're likely to make next week's cold snap a bit easier to handle.
'[They] reduce the sting a little bit as these air masses move over the warmer water and they are modified, so the air is not quite as cool as it would be if sea temperatures were closer to average or below average,' Noll said.
Warmer sea surface temperatures were expected to stick around for the next three months, but not necessarily at 3C above usual for the whole time, he said.
The sea surface temperature map included the first 10 metres of the water column.