It’s hot in the city
A searing nationwide heatwave caused scorching temperatures across New Zealand but proved a good day at the beach for many Wellingtonians.
The capital’s temperature is forecast to reach 28 degrees Celsius today, with the heatwave set to continue for several days.
Temperatures were in the midtwenties yesterday, with many residents flocking to beach hotspots such as Oriental Bay.
Masterton reached 33C about 3pm – the hottest temperature the Wairarapa centre had posted this year. The mercury went even higher in Blenheim and Hawke’s Bay, both recording a blistering temperature of 35C.
The heat was something of a first for Napier resident Phil O’Donnell. ‘‘Coming from Wellington, I’ve become used to hot summers in the last few years, but this is something else . . . I wouldn’t want it like this every day.’’
MetService meteorologist Tui McInnes said evenings across the week were also forecast to be muggy with overnight temperatures of between 18C and 23C.
Otago University environmental health senior lecturer, Dr Alex Macmillan urged people to ‘‘take care".
‘‘Even short duration heatwaves can increase deaths and hospital admissions from heat stroke, heart and lung disease.’’
Analysis: Take several helpings of hot air from Australia and the sub-tropics, some long and sunny days, warm ground, New Zealand’s mountains, and a nearby anticyclone accompanied by light winds, and you have the recipe for this week’s heatwave.
Usually, a very warm air mass from the Australian outback would be cooled significantly by contact with the Tasman Sea as it moves across to New Zealand.
This summer, however, with another marine heatwave under way and sea-surface temperatures in parts of the Tasman up to 4 degrees Celsius warmer than average, that modifying influence on the lowest layers of heated air has been significantly reduced.
How much this particular hot spell can be sheeted home to climate change is difficult to quantify. But the marine heatwave – for the second consecutive summer – is perhaps the clearest indicator of such change.
Meteorologist Ben Noll, from the National Institute for Water & Atmospheric Research (Niwa), in a recent paper in the journal Nature discussed how marine heatwaves would increase with climate change.
The number of marine heatwave days has doubled between 1982 and 2016.
Feeding the heatwave this week are two sources of hot air.
Not only is the air coming from Queensland and New South Wales but also, because of a highpressure system north of the country, steamy sub-tropical air is being added to the mix.
That more humid air will feed the mugginess and for many particularly in the top half of the North Island, it will get worse before it gets better towards the end of the week.
The end of January and the first half of February is peak summer, with the warmest ground and atmospheric temperatures occurring regardless of any heatwave conditions. So this torridity comes on top of our hottest days.
The jetstream – a core of strong winds about 10km up which steers the surface fronts and depressions – has also retreated well to the south of
New Zealand, allowing the hot air and high pressure to stage a sit-in.
If the wind is from the west or northwest this week, and you live on the eastern side of either island, particularly if you are more than 10km from the coast, the effect of the mountains will add another few degrees to the heat. Residents of Gisborne, Hawke’s Bay, Wairarapa, Marlborough and Canterbury are used to these conditions. For them, this week’s heatwave is probably more about its length, rather than the maximum temperatures.
While it appears we are all in the furnace, a closer look reveals the impact of microclimates, especially local winds. In coastal places, seabreezes, and associated cloud, can cut 5 or more degrees from the maximums. But in inland valleys and basins, there is little wind to bring relief. The heat can build, day after day, under largely clear skies, until a major change in the weather arrives.
It is here that this week’s true ‘‘heatwave’’ exists.
The mercury is on the move again into the 30s. With peak temperatures at or above this level likely through to the weekend, where do the forecasters think the hottest places will be?
MetService’s Tui McInnes picks Blenheim, towns in the central North Island and parts of Central Otago.
WeatherWatch’s Philip Duncan suggests Blenheim, Alexandra and Hamilton.
New Zealand’s hottest temperature remains at 42.4C at Rangiora on February 7, 1973. It seems unlikely that it will be threatened this week.