Sudden Stratospheric Warming and a summer of swell?
Last month I began making regular posts in the Swellnet forums about changes in the weather occurring in the atmosphere above the South Pole. The chain of events appears to be leading towards what's called a Sudden Stratospheric Warming, though it hasn't quite reached that classification just yet.
The mainstream press has also caught onto the unfolding story so you may have read about Sudden Stratospheric Warming there.
Irrespective of whether it offically becomes a Sudden Stratospheric Warming, the event will have an impact on Australia's weather this coming spring and summer.
The current event originated in the lower atmosphere (known as the troposphere) from a phenomena known as mountain torque. In simple terms, mountain torque is a change in wind flow caused by mountain ranges. In this instance, it's caused by the Andes Mountains of South America interfering with the southern polar vortex. The polar vortex is a stationary low pressure system positioned at each pole, circled by strong westerly winds.
The Andes, which rise 6-7km above sea level, sometimes disrupt the westerly winds, setting off mixed waves through the lower atmosphere (troposphere) which can propagate to the upper atmosphere (stratosphere).
Think of a steady flow of water hitting a barrier and then becoming turbulent downstream.
Regular disturbances aren't powerful enough to break through to the stratosphere, but on rare occasions it can happen with the turbulence altering the layers of the atmosphere: cold air will sink through the stratosphere, with the reponse to this being warm air lifted towards space. This is a 'warming' event.
If the warming event is strong enough it can break down the polar vortex and even reverse the flow of the westerlies. This would be a Sudden Stratospheric Warming event (SSW), though they're extremely rare in the Southern Hemisphere with only one of them being identified since records began back in 1979. That lone example was in September 2002.
SSWs are more frequent in the Northern Hemisphere owing to the presence of large land masses and mountain systems that disturb the polar vortex flow. Except for the Andes, Antarctica is surrounded by mostly ocean and the flow is more stable.
When the sudden warming event occurred last month, temperatures in the upper stratosphere jumped from -80 to -5 degrees - a warming of 75 degrees (see graph below)
This is the earliest warming event measured across the Southern Hemisphere, with the 2002 event occurring on September 24th and 25th. However, it's just falling short of thresholds to be classified as a SSW event at the moment. There's been a wind reversal at 2hPa and this is forecast to occur at 5hPa in the coming days, but it has to occur at 10hPa to be classified as a true SSW event.
Even so, in the last 39 years there have only been six reversals of the westerlies at 2hPa and three times at 5hPa indicating that it's still quite a significant event.
Over the past fortnight there's been numerous deep low pressure systems tracking the Southern Ocean below Australia, however these are only an indirect influence of the warm stratosphere.
The warming event is linked to the Southern Annular Mode (SAM) which describes the changing latitude of the westerly wind belt - positive SAM means the westerlies are more polar, negative SAM and they track closer to Australia.
At present there's a strong positive SAM which has not only moved the westerlies towards the poles, but also strengthened them, resulting in the strong storm activity under the country.
It's believed the warming event - sudden stratospheric or not - will filter down to the troposphere around the start of October. We won't see any significant impact on surface patterns until then.
We spoke to Professor Ian Goodwin and he's expecting a similar spring and summer to that of 2002/2003 - the occasion of the last Sudden Stratospheric Warming - where continuous southerly swell events were guided up the Tasman Sea. The swells encouraged northwards longshore drift, flooding all the East Coast pointbreaks with sand. Goodwin called it "one of the more peculiar summers" he's experienced. Peculiar in the good sense. Surprising.
The drivers for this particular warming event have occurred above the Indian Ocean with the downstream regions - i.e Australia, New Zealand, and the islands of the western Pacific - feeling the most profound effects. In regards to the latter, the incredible seasons that Fiji, Samoa, and Tahiti have experienced are set to continue, while New Zealand may be cooler owing to a persistent south-west flow.
Lastly, it should be noted that this isn't linked to climate change per se and is a naturally occuring phenomena.
We'll continue to provide updates on this event over the coming weeks and months.