Snow Madness and the North-West European Anti-Monsoon

In India, apparently, legions of weather-forecasters, from eccentrics to supercomputer-operating state-funded agencies, are devoted to attempting to forecast the characteristics of the annual monsoon.

A collective snow madness grips the UK during cold winters. Legions of weather-forecasters, from eccentrics to supercomputer-operating state-funded agencies attempt to explain events, nowadays with particular reference to global warming.

Now that the snow here is melting and low grey cloud has parked on top of London, the weather is simply downright miserable here.

So it seems a good day to stay indoors and join in the speculative fun.

Monsoon conditions occur when the land heats faster than the ocean during the summer, causing air masses to rise. The rising air – or low pressure – relative to the ocean, draws moist air inland, creating rainfall.

Given the cultural significance of cold winters in the UK – I have a deep desire to roast an ox on the Thames, and fond memories of 1684 are starting to fade – I suggest we should characterise the weather-pattern that brings cold weather to north-west Europe as an anti-monsoon (or perhaps “antisoon”).

The point is that in late December or early January there is often a plunge of cold air towards Western Europe. In many years this fails to reach the UK, but occasionally it does. The breakdown in the normal westerly airflow is caused by the reverse of monsoon conditions. Rapidly cooling air over land (high pressure) tends to flow out over the warmer ocean (low pressure).

I’ve noticed the meteorological profession making bold – in my view even reckless – predictions about future British winters. Rob (or “Reg” as he was introduced by a BBC anchorman!) Varley insists that our cold winters will be less cold in the future. We’re even told that future generations won’t know what snow is!

Actually this is what I thought until recently. Whenever it snows I rush out with my camera, thinking I might never see the white stuff again.

But now I’ve thought about the matter a little more deeply. In fact, I’m ready to spout some speculative scientific ideas.

I now doubt we’ll never see snow again, for the simple reason that the temperature difference between average and cold UK winter weather (the “anomaly”, perhaps) is of the order of 5-10C. Global average temperatures have risen by an order of magnitude less.

Winter weather in Britain depends not on the global average temperature or even the temperature of the North Atlantic. Rather, it simply depends on which direction the wind is blowing in. And it so happens that high pressure over Scandinavia and/or Greenland causes easterly winds over the UK.

When in the future can we expect such high pressure systems to develop? Will they become less frequent? Or perhaps the real question is why they have been rare over the past 30 years or so, leading to a succession of mild winters in the UK?

The meteorologists have an alarming tendency to attribute causal significance to what are in fact the phenomena in need of explanation. The jet stream is a case in point.

During cold UK winters the jet stream moves further south – over Gibraltar, perhaps, rather than the Channel. Depressions therefore track south of rather than across the British Isles.

But the jet stream must be caused by something. In fact, it can only result from pressure differentials in the atmosphere – rising air (to its north) and falling air (to its south) if you like.

This brings us to another meteorological concept: the North Atlantic Oscillation (or NAO). This isn’t really an oscillation at all, rather a measure of the pressure difference between the Azores (usually “high”) and Iceland (“low”). The NAO measure fluctuates rather than oscillates (unlike the El Nino Southern Oscillation, ENSO, which is driven by movements of water). But the NAO concept does bring us a little closer to an explanation of the root causes of UK winter weather. Here’s a graph of the NAO over the last century or so:

I’d say there appears to be some correlation between the NAO and cold and mild winters. 1947, 1963 and 1979 were all years of low (negative) NAO, whereas the recent run of mild winters occurred while the NAO was high.

But what determines the NAO?

Here’s my hypothesis. We need to look at not how warm the planet is, but at whether or not it is warming.

The point is that there is a lag between atmospheric warming and warming of the ocean (we’re concerned here with the ocean surface, not the deep ocean, where the warming lag is centuries rather than just years). When the planet is warming, therefore, the land (affected directly by the atmospheric temperature) will be warmer relative to the ocean than is usual. The reverse will be the case when the planet is cooling.

When the planet is cooling, therefore, the North-West European Anti-Monsoon will tend to be stronger, bringing the possibility of cold winters to the UK.

Here’s another graph:

Compare this graph of temperatures with the NAO graph above. Someone can so some maths (differentiate the temperature graph!), but I’d say the NAO correlates with the rate of change of temperature. When the planet is warming rapidly, the NAO tends to be positive; when the underlying warming trend slows or reverses, the NAO tends to be negative.

And when the NAO is negative we get those proper UK winters that somehow satisfy the soul.

This set me thinking a little more.

A sudden cooling trend can result from volcanic eruptions. The cold decade of the 1810s is explained this way. Remember, the average global temperature declined by “only” 0.5C below normal back then, but the UK experienced White Christmases more often than not!

The current levelling off in warming cannot be explained by volcanic activity. Neither can 1963, for instance, although the eruption of Mt Agung occurred during the cold snap!

But the evident cooling in the 1960s can be explained by “global dimming“, resulting from particulates emitted by fossil-fuel burning.

Could the same thing be happening now? We keep hearing how many coal-fired power-stations are being opened in China. They must be having some effect, surely?

It might be worth pointing out that reflection of sunlight by particulates in the atmosphere would be expected to be more significant in winter than in summer, because the angle of incidence of sunlight would mean it having to pass through more air to reach the surface (and random scattering would be more likely to deflect the light out of the atmosphere altogether).

Or maybe the sceptic-fuelling hesitation in the rise in average temperatures over the noughties is just unexplainable “natural variation”, as climatologists suggest.

The warmest year on record, 1998, was the start of an el Nino, when warm water spreads across the surface of the Pacific. In 2010 we are seeing the start of another el Nino. I can believe the start of an el Nino would disrupt normal weather patterns and help cause a cold UK winter (cooling and a negative NAO is a necessary, but not a significant causal factor). Here’s my wager, though: next year (2010-11) will not be a cold winter. The el Nino warmth will by then have had time to spread through the entire atmosphere. We’ll have a positive NAO and a weak antisoon. I should say, in case anyone really does want to put money on the prediction, that a perusal of the historic record does suggest that while cold winters can, and often do, occur in an el Nino year, they don’t tend to occur the year afterwards.

We might be able to blame China for this cold winter, but a longer-term problem with warming the planet may be that at some point we have to stop. And maybe even let it cool down, bringing on some strong antisoons. With a bit of luck, though, it won’t be so warm by then that we won’t be able to put that ox on a spit in front of Parliament!

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PS I do realise that embanking the Thames during the 19th century and the demolition of the old London Bridge (which obstructed the flow of the river) make it less likely it will freeze over. I still dream of the day, though!