Winter 2009 / 2010 – The Explanation

Winter 2009-1

Image from NASA

The current cold spell affecting most of the northern hemisphere is causing quite a stir with widespread scientific and media attention and confusion.

The proponents of human caused climate change seem to be in a panic and most sceptics carry a smirk.

However it all fits in very nicely with the forecast that I issued last August concerning the coming winter which was as follows:

“I hate trying to predict because everything could change tomorrow but on the basis of the present set up here goes:

The jets are still well south of us and if that persists into autumn and winter then of course there will be greater influence from cold high pressure over Europe and Greenland.

The matter of precipitation amounts depends on where the main battleground is between cold and warm air. No two years are the same and last winter was unusual. The cold spells were very immobile. Large chunks of cold air were dumped on us for weeks at a time with little movement and relatively little snow despite a couple of notable falls.

For the coming winter I think that, instead, we will get more occasions when the battleground is over southern UK or northern France and Germany with more snow over wider areas than we have had for many years. However so much depends on day to day variability of the precise positions of all the weather systems that I realise I am a bit out on a limb there. Nevertheless I think it a higher likelihood than for many years past.

I’m also unsure what the balance will be between northerly and easterly flows which give very different weather types. The chance of a return to prolonged south westerlies is low but it could happen depending on the synoptic situation over USA.

Last winter also showed a change from the previous two winters when cold plunges over the USA distorted the jets and gave us persistent warm wet south westerlies so we did not then share in the general slow cooling trend.

I think that the overall global trend is still moving towards a colder regime but only slowly. The position of the jets in both hemispheres still indicates overall cooling. We seem to be getting increasing cold weather reports in winter in both hemispheres despite the current absence of a strong La Nina so the effect of the previous one seems to be persisting.

Meanwhile ENSO is less negative than it was but the anticipated EL Nino seems to be stuttering. I would say that overall the rate of cooling in the air will slow down a bit thanks to the extra energy flow into the air from the less negative ENSO but remember that if the sun is weak it will not fully replace the energy lost from ocean to air via the warmer SSTs so there remains a general background loss of energy for the system as a whole.

So, (gulp!) UK coming winter cooler than recently and likely to further reduce the warming trend of the 1975 to 2000 period. Not necessarily back down to the longer term average but well on the way with an outside chance of a memorable winter.

More snow than we have been used to but generally drier than average in the north and wetter than average in the south.

A lot could happen to change my opinion but that’s where it stands at the moment”

And that turned out to be an accurate assessment.

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Published by Stephen Wilde January 11, 2010

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