New Zealand heatwave - the science behind why it's so hot

Monday, 28 January 2019

Temperatures are set to hit 30 degrees Celsius in many parts of the country as a heatwave hits New Zealand.

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.

**READ MORE:

Signs of another hot day, with the back edge of the Canterbury nor'west arch flying above Christchurch.

* Australian heatwave will push New Zealand temperatures into the 30s

* Christchurch temperature hits record December high of 36.1C

Manny Robert with Lachlan Johnstone-Robert, 7 months, and Nadya Johnstone-Robert, 7, cool off in the paddling pool in the Christchurch Botanic Gardens.

* NZ's hottest ever day: Melted roads, striking workers and 26,000 dead chickens

* Old diary sheds light on weather trends

Belle Anderson of Rollickin Gelato serves customers at the New Regent St, Christchurch, store.

* Adelaide swelters through hottest day on record

* Australian businesses urged to cut power to avoid blackouts

* Timaru had hottest day at 41.3C**

Niwa meteorologist Ben Noll in 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.

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 the position of the high-pressure system north of the country, steamy sub-tropical air is being scooped up and added to the mix.

That more humid air will keep the mugginess going and for many North Islanders, particularly those in the top half of the 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 are traditionally peak summer in New Zealand, with the warmest ground and atmospheric temperatures occurring regardless of any heatwave conditions. So this torridity comes on top of our of hottest days.

The jetstream - a core of very 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 across the country.

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 baking in these conditions. For them this week, the heatwave is probably more about the length of the hot spell rather than the maximum temperatures.

It is also a matter of scale. Look broadly at New Zealand's temperatures this week and it certainly appears like we are all in the furnace.

Look closer, though, and you can see the impact of microclimates, especially local winds. In coastal places like Christchurch, seabreezes, and associated cloud, are knocking off five or more degrees from the maximums.

But in inland valleys and basins, there is little or no wind to stir up the air and bring in some refreshing relief from these dog days. 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 42.4C at Rangiora on February 7, 1973. It seems unlikely this week will threaten that.