The stratosphere above the South Pole is rapidly becoming ‘hot news’ as meteorological interest is focused on a sudden stratospheric warming (SSW) event that has raised the temperature by 40-50 degrees C, promising to disrupt weather patterns as far away as southern Australia and Patagonia, in South America.
In the northern hemisphere, these SSW events are relatively frequent and such an event led to the ‘Beast from the East’ last winter and early spring. This ultimately brought a stream of intensely cold air to the UK and north-western Europe from as far as Arctic Siberia.
Now, the Antarctic continent is undergoing a similar sequence of events to what happened in the northern hemisphere last year, involving a complete breakdown of the southern stratospheric polar vortex, followed by a massive rise in temperature high up in the stratosphere.
Long-range forecasts showed that an SSW event was likely during late September.
How often do we have these events?
SSW events are a natural feature of the atmosphere but it is difficult to estimate how often such a rare event like this occurs. Our computer models have raised the prospect of its potential occurrence in the past when we have occasionally seen SSW events in computer model simulations of the polar vortex in the southern hemisphere. However, it was only in 2002 when the only other event on record occurred in the real world that we realised that the computer models were alerting us to a real possibility. Professor Adam Scaife said: “These events are rare in the southern hemisphere but we recently analysed thousands of computer simulations to calculate the chance of a second sudden stratospheric warming in the southern hemisphere and we now know that it is about four per cent per year, so it is likely to occur on average about once every 25 years.”
What does this mean for the weather?
The change in the stratospheric winds burrows down with time and on reaching the tropopause – the layer separating the higher stratosphere from the lower troposphere – it is followed by a change in the southern hemisphere jet stream which is expected to shift towards the equator.
Harry Hendon is a senior principal research scientist at the Bureau of Meteorology in Melbourne. He said: “We’re not really anticipating impacts until October and maybe extending through to January. So typically what happens is the westerly jet stream shifts towards the equator. So in those locations that pretty much sit right underneath the westerly jet stream – like the southern part of the South Island of New Zealand or Patagonia in South America – they can expect to see enhanced storminess and more clouds and more rainfall.”
Harry added: “Our modelling capability here – where we’ve been anticipating what’s going to happen both in the short term and long term – really depends on our partnership between the Bureau of Meteorology and the UK Met Office. We share the same modelling capability and it’s a really good example of the benefit of this long-running partnership that the Bureau of Meteorology has had with the Met Office.”
Although there is likely to be weather disruption in parts of the southern hemisphere, there is some good news in that the ozone hole is likely to temporarily shrink this year with the benefit that there will be less ultraviolet radiation reaching the Earth’s surface.