Σάββατο, 23 Ιουνίου 2012

No more ‘horror winters’ for Europe – not for several years ahead – and indeed also not this year

Extremely cold winters for Europe persisting for several months are very rare. The last one to fit the definition occurred in 1963, almost half a century ago. Still many weather forecasters and media seem inclined (as again happened this year) to report on pending ‘horror winters’ – often without presenting much more than an animated air pressure chart to show how it would appear, and little or no published background information as to why it would happen.

As such winters are extremely rare, but theoretically possible; one could argue there is a fair chance for such predictions to become reality. But, thanks to recent advances in climate science, no more.

New insights* have led to improved predictability of a few key factors. Judging by these critical indicators at least the European winters of 2011-2012, 2012-2013, 2013-2014, 2014-2015 will have some normal climatic variation, but will not present persistent temperatures significantly below average – winters that keep the deep freeze on for several months.

Therefore it seems it would be a good thing to help end the horror winter hype – for now – and probably for some time longer as well.

[*) Here our key references are to peer-reviewed scientific literature from 2011 and 2012 in the journals Nature Geoscience, Science and Nature Climate Change - with additional academic references as side notes.]

Why no horror winters: (1) the not-stopping Gulf Stream, (2) the non-existent solar minimum, (3) the clearly observed rising temperature trend

All factors that forecasters could – theoretically – base extreme cold winter scenarios on in fact point to the opposite direction, rather favouring (within normal variability range) mild and wet Atlantic winters – like the current European winter.

Instead of a solar minimum [or even a proclaimed ‘new’ Maunder Minimum] ongoing NASA observations show we’ve already entered a normal new solar maximum that is forecast to stay until somewhere around 2017. This decreases the chance of high pressure blockades and the inflow of cold air from Arctic regions and Siberia.

As can be read in our European winter outlook of November 10 2011 [which clearly turns out] the increased number of sunspots (as indicator of increased solar radiation) influences air patterns in the higher atmosphere. As a consequence [correlation shown for instance in Nature Geosciencepublication by the UK Met Office Hadley Centre, Nov 2011] the Arctic Oscillation (AO) is (somewhat) more inclined towards a positive phase [which is why we forecast that during the 2011-2012 winter the US would experience fewer snowstorm outbreaks than in years before - as is indeed clearly the case] and (a stronger correlation) the North Atlantic Oscillation (NAO) also tends towards a positive phase.

Europe therefore has a smaller chance for the formation of high pressure blockades over Scotland or Scandinavia – scenarios with respectively a direct influx of cold Arctic air and cold continental air, from the Russian Arctic or Siberia.

Instead depressions persist around Iceland and follow easterly trajectories across northern Europe. There is a semi-permanent west circulation with mild Atlantic air and episodes of high precipitation in the form of rain.

What is a real horror winter: the Little Ice Age – or the Younger Dryas?

Although a repetition of the Maunder Minimum, reminding of the considerably colder European winters during the Little Ice Age does theoretically seem a good candidate* to enable ‘horror winters’ – one other local climatic driver is more potent.

[*) Not all scientists agree a strong solar minimum caused the Little Ice Age. A 2011 publication in Geophysical Research Letters argues total solar irradiance during a persistent minimum does not sufficiently decline to explain significant cooling.]

Graph showing Maunder Minimum of Little Ice Age and current episodes of solar minima and solar maxima. As we reported in November the new solar maximum is already clearly developed and forecast to be of incluence until around 2017. [Image courtesy: Berkeley climatologist Robert A. Rohde.]

That other candidate is the Gulf Stream, the northward ocean current that carries tropical warm waters across the surface of the ocean all the way to the North Atlantic and thereby also influences sea surface temperatures in European coastal waters.

In the North Atlantic the water cools and sinks to the ocean floor as a driver to the planetary thermohaline circulation that connects the 4 major oceans (excluding the Arctic Ocean). This process of warm water transport to the north, sinking and cool water transport back south over the ocean floor is also referred to as the Atlantic Meridional Overturning Circulation (AMOC) – which is a better physical description.

In a theorised scenario in which the AMOC would halt or slow down considerably the Gulf Stream would no longer transport heat towards Western Europe. As a consequence especially the winters would become much colder – ice age cold. That is what paleoclimatologists tell us, because that is what they think happened –suddenly– at the onset of the Younger Dryas – a rebound of ice age conditions some 12,000 years ago. [Again however research from 2011 undermines something of the hypothesis – as we may have lost our explanation as to why the Gulf Stream would have stopped.]

Whether or not we can find evidence of a stopped AMOC in the paleoclimatic record is in fact irrelevant. Climate models show Europe would indeed be much colder without the Gulf Stream. We can therefore focus on whether it will stop – and the answer is no.

First of all there is some research from 2011 indicating greater AMOC stability than previously thought, as for instance a new overturning site has been discovered – and there are not yet any monitored problems with salt content of boundary waters.

But if you want to us to be more precise: it will at least not happen before the year 2015. This insight we owe to climate researchers of the German Max Planck Institute for Meteorology who proudly announced in their November 2011 publication in Science that they have devised an AMOC ocean circulation model with great reliability (it neatly passes hindcast – or ‘retrospective forecast’ – skill tests for a 4-year period) with a Gulf Stream reference point at 26.5°N.

This is not only good news for seasonal forecasters as they now have an extra tool in their boxes trying to predict Atlantic hurricane seasons, Sahel droughts, or European weather patterns, but also shows a nice and quiet forecast period ahead. In fact, the German model shows over the next couple of years the AMOC will be somewhat more stable than it has been over the last couple of years – and these went without major dips in Gulf Stream velocity – and as you can remember (although December months have been cold thanks to the then solar minimum) without horror winters.

A European horror winter in 2008 would have had to try 1 degree Celsius harder than one in 1990

If we look beyond the 4 to 6 year predictability range for AMOC and for solar activity there is yet another important climatic indicator that points away from European horror winter scenarios – that is becoming of increasing and indeed measurable influence: the warming temperature trend.

Just three days ago researchers of France’s National Centre for Scientific Research (CNRS) stated in a Nature Climate Change publication that the European average warming between 1990 and 2008 has been as high as 1 degree Celsius – which means the European warming is happening 2-3 times as fast as the world average land temperature rise.

And when you look at season long temperature anomalies a one degree rise can mean the difference between an expectionally cold winter – and a winter like that of 2009-2010 – some snow, some ice, some thaw, nothing special – nothing like 1963 

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Άρης Τσιλιφώνης(Aris_tsili)