Wettest summer in 100 years

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  • Strawberry crops suffered during the wet summer. Photo: EMPICS Sport/EMPICS Sport

    Cloud covering much of the UK on 6 July this year, during one of summer's very wet spells.

    A chart showing sea surface temperature (SST) anomalies on 29 August 2012. Note the extreme warmth in the North Atlantic, 5 deg C or more warmer than average, and the "horseshoe" of anomalously cold water in the eastern Pacific - the signature of the current strongly negative Pacific Decadal Oscillation (PDO).

    A farmer and two sons during the 1930s Dust Bowl episode in Oklahoma, USA.

  • Wettest summer in 100 years
    31.08.2012 11:54

     

    If you thought it was a wet summer then you were not wrong. The statistics show it.

    Both the UK and the England and Wales rainfall series show that 2012 has been the wettest summer for 100 years. Moreover, the England and Wales data go back to 1727, and with one day of summer to go it looks like being the fourth of fifth wettest summer since then.

    This will not be news to anyone who was trying to enjoy a holiday at home, unless their break happened to coincide with one of the periodic but short-lived dry and warm spells, nor to farmers who have seen a huge impact on, for example, soft fruits such as strawberries.

    Crops have had a hard time as well, initially from the drought that extended from winter into spring and then from the influx of excessive rain at just the wrong time.

    It is hard to point the finger at any one cause for such a disappointing summer, or indeed for the fact that this has been pretty much the sixth in a row, with 2006 the last with any lengthy periods of dry and hot weather.

    We can say that the atmospheric circulation around the northern hemisphere has been rather weak for much of the time during those years, and this lends itself to what meteorologists call “blocking”, whereby a pattern of low and high pressure around the hemisphere becomes locked in for long periods as the jet stream that shapes and holds them loops lazily and extravagantly around the globe.

    It is unfortunate for those wanting to break out the flip-flops and shorts that the British Isles have found themselves under stagnant low pressure rather more often than high pressure.

    Looking east and west of these isles we can note that eastern Europe has been hot and dry for much of the summer, as of course has the eastern half of the USA. Even Greenland and Iceland had a long stretch with temperatures well above average. Crops such as corn and soya have been suffering in the USA and Ukraine, to name just two countries, for rather different reasons – prolonged dryness and heat with, at times, record breaking temperatures. Farther east much of Russia has had a hot, dry summer, and Siberia is close to having its hottest on record.

    But why is the circulation weak? If we knew that for certain then meteorologists might have a better handle on forecasting for seasons ahead, which at the moment is a nascent science.

    There are plenty of atmospheric connections on broad scales that might be partly responsible. The general prevalence of La Niña episodes during the past few years might have caused a tendency for weak circulation. And this could also be tied in with unusual sea surface temperatures in both the Atlantic and Pacific might have an influence.

    A pattern known as the Pacific Decadal Oscillation has been strongly negative, meaning that there has been an abnormally cold horseshoe of cold water from the north Pacific down past the west of North America and back west into the sub-tropical Pacific. It is hard for La Niña’s opposite El Niño to become established under such circumstances and induce stronger atmospheric circulations.

    At the same time, especially this year, the north Atlantic has been abnormally warm. In fact, the contrast in these sea surface temperatures between the Pacific and the Atlantic in July is almost unprecedented. 1955 had a greater difference, while 1952 and 1933 were fairly close.

    Although we should not jump too quickly to conclusions on such relatively scarce evidence, it is at least interesting to note that these years also featured lengthy summer drought in the USA – and 1933 was one of the Dust Bowl years.

    By: Stephen Davenport