This website uses cookies. Read about how we use cookies. OK

Extra - July 21, 2012
Worst first half of summer ever
Why it's the fault of the jetstream

By Philip Eden

The last of the hosepipe bans in the southeastern corner of England were lifted a couple of weeks ago, and not before time. The water year begins in October, when the reservoirs and aquifers - the water-bearing rocks beneath our feet - begin to fill up again after the summer. Between October 2010 and March 2012 the Southeast of England recorded 240mm less rain than normal. Since then we have had an excess of 220mm, the reservoirs are full, and so are the aquifers.

It is all the fault of the mid-latitude jet stream, that powerful conveyor belt of winds in the upper atmosphere which, if it's in the right place, brings aircraft returning from America back to the UK hours ahead of time.

Let me try to explain why the jet stream is always there. It marks the boundary between cold polar air and warm tropical air, and it is located in the zone of biggest temperature contrast. These huge temperature contrasts provide the energy which drives this narrow stream of powerful winds.

There is another feature of the jet stream which is crucial to understanding how it works. It controls the behaviour of the depressions that we see on our TV weather charts, and these depressions routinely bring widespread rain, and often strong winds too.

The jet stream blows all the way around the northern hemisphere - and the southern hemisphere too - but its character changes from time to time. In the northern hemisphere the Rocky Mountains provide a major obstruction and therefore disrupt the flow. The result of this disruption is that the jet stream follows a series of sinusoidal waves downwind of the Rockies. When the flow is strongest, these waves are elongated and flat, and the winds in the upper atmosphere blow from the west, give or take a couple of points of the compass, all the way around the planet. But when the jet stream is relatively weak, rather like a sluggish river, the flow meanders widely, first into polar regions, then deep into the tropics, then poleward again.

If we are stuck under a ridge, when the jet stream heads polewards, the weather becomes dry and sunny. But if we are under a trough, when the flow aims towards the tropics, we end up with weeks or months of cloud and rain.

Throughout the 1990s and the early-2000s the jet stream blew powerfully from the west, and the distribution of weather across the UK was relatively normal, with just a few exceptional droughts and wet periods. Since 2003, however, the flow has been sluggish and meandering, and as a result we have experienced long periods when the weather has appeared to get stuck in a rut. So we had heat-wave summers in 2003 and 2006, appalling floods in 2007 and now in 2012, and an extended drought in 2010 and 2011.

This poses the question why the jet stream has been so weak over such a long period. Such periods have happened before, notably in the 1950s and '60s, so it may not be necessary to blame anything other than "natural variability". But in terms of the degree of sluggishness of the jet stream, the last ten years have certainly been pushing hard at the boundaries of previous experience. It is therefore wise to develop theories about why this may be happening, while also retaining the "natural variability" theory.

The first likely culprit to be identified was solar activity. The numbers of sunspots visible on the surface of the sun is a measure of that activity, and it follows an approximately 11-year cycle. We also know that there is a measurable statistical link - admittedly not a very strong one - between solar activity and the strength of the jet stream. The last "solar minimum", when the numbers of sunspots are at their lowest, was appreciably longer than usual, lasting from late-2003 to early-2011, but it has now ended. That may appear to scupper this particular theory, but the next "solar maximum", when the sunspots are most numerous, due in 2014, is predicted to be 50 per cent lower than the last one.

The latest flavour of the month is a theory which depends on the fact that the Arctic is warming more rapidly than the tropics. This results in a reduced temperature contrast between the two regions which in turn provides less energy to drive the jet stream. End result: sluggishness. We know that the ice extent at the late-summer minimum is now 35 to 40 per cent lower than it was 33 years ago, but so much of the multi-year ice has now melted that the ice volume at the late-summer minimum is estimated to be 75 per cent lower than it was in 1979. Of course the extent of ice recovers sharply during the winter and early spring as it always has done, but even in winter the average thickness of Arctic ice is now very much less than it was three decades ago.

Those who have taken an early holiday at home have been suffering terribly, with campsites and caravan parks in many parts of the country under water. Root crops are rotting in the fields, the fruit harvest is predicted to be very poor, while vine growers are praying for a change in the weather. Some retailers are, however, doing quite well, with DIY stores reporting a surge in activity, while umbrella manufacturers must be coining it!

But I may, at last, have some good news. It seems that the six-week long period of thick cloud and heavy rain may be drawing to a close. After further heavy showers in the southern half of Britain on Friday this week, it does appear that the weather will take a turn towards something rather drier, sunnier and warmer from the beginning of next week onwards. Don't expect it to lurch to the other extreme - to a sweltering heat-wave - and you won't be disappointed.

Actually, having said that, the last time the Olympics came to London, in 1948, we had a dreadful summer, apart from two weeks at the end of July. King George VI opened the Olympic Games on the hottest day of the year with 35C (95F) approached in London and the Home Counties. But sadly that year the rains returned in August.

By Philip Eden

transparent picture
transparent picture