Computer weather forecasts may struggle with warming Antarctic skies

Monday, 26 August 2019

MetService National Forecast for August 26.

Weather forecasting models may find it difficult to cope with the kind of rapid warming some say is now happening above the Antarctic.

Blue Skies Weather and the National Institute of Water and Atmospheric Research (Niwa) believe the exceptionally rare sudden stratospheric warming (SSW) event predicted last week is under way and could last into next month.

The impact of that on New Zealand's weather is impossible to predict at this stage. Any potential effect could still be several weeks or longer away and it is hard to determine what kind of weather it might bring where, and how severe it may be.

Niwa and Blue Skies Weather say an extremely rare sudden stratospheric warming (SSW) is under way above the Antarctic and may cause headaches for forecasters several weeks down the track.

Both organisations, however, say such an extremely unusual event may affect the accuracy of forecasting models they use.

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That would be a concern if it meant stormy weather was more difficult to predict.

The aurora australis above Antarctica. These occur in the thermosphere, two layers of the atmosphere higher than where the sudden stratospheric warming is believed to be happening, in the stratosphere.

The stratosphere is the second layer of the Earth's atmosphere, in which temperatures generally increase with height due to the concentration of ozone. It extends to about 50km above the surface, and starts at altitudes of between 7 and 10km over the poles and about 17km above the equator.

Below it lies the troposphere, the layer which contains most of the weather.

Blue Skies forecaster Tony Trewinnard said the magnitude of the SSW by the end of the week – with a forecast temperature difference of nearly 90 degrees Celsius from one side of Antarctica to the other at an altitude of 20 kilometres – might be 'beyond the physics of the models'.

The phenomenon – which has only been recorded twice before, in 2002 and 2010 – was unlikely to have much effect on southern hemisphere weather for several weeks, he said.

'It could release cold air into the weather systems of the southern ocean and affect New Zealand. Or it could give us a week of really strong winds, or a week of intense anticyclones with sunny weather and frosts, or snowy outbreaks, or stormy easterlies with heavy rain.'

MetService meteorologist Georgina Griffiths said it was important to note that over the next week the weather would actually be settling down, after a very stormy month.

A decent anticyclone would cross the country at the weekend, followed by milder northerlies early next week.

National Institute of Water and Atmospheric Research (Niwa) meteorologist Ben Noll said there was now 'high confidence' the SSW would peak on Friday, with strong agreement from several models on its location and severity. It could now be a protracted event, he said, continuing into September.

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