Weather all over the world

  • Weather all over the world
    06.01.2011 17:17


    The UK just had its coldest December for 120 years while Greenland was unusually warm. Abnormally torrential rain has fallen in California, USA, and appalling floods have afflicted Queensland, Australia. So what it going on with the weather? Are these events linked?

    Extreme events occur periodically around the world and not too much should be read into what might seem to be increasing occurrences. The speed of communication in the 21st century and the ease of reporting events through both traditional and social media mean that we are all more likely to become aware of these phenomena. Moreover, the rapid growth of the world’s population means more people moving into areas heretofore uninhabited and plagued by severe weather about which they might have been unaware.

    To get down to specifics, though, the UK has suffered such frigidity in December because, like at the end of the 2009/2010 winter, Atlantic weather systems were suppressed, and they normally bring unsettled and relatively mild conditions with occasional cold interludes. That is the sort of winter we were getting used to for many years before 2009.

    Instead of southwest or westerly winds we had a greater frequency of northerlies and easterlies, and that was because the atmosphere became “blocked”. That is to say, high pressure had a tendency to stick at high latitudes, stopping low pressure systems moving eastwards and encouraging prolonged Arctic outbreaks.

    The other side of that coin was a surge northwards of unusually mild air across northeast Canada and Greenland as a counterbalance. This in itself helped to propagate high pressure over Greenland and Iceland which further enhanced the northerly flows on our side of the Atlantic.

    Queensland’s deluge has been one of the worst known but similar floods swamped eastern Australia in 1974, and those were described as the worst in 200 years. The early to mid-1970s were also notable for record-breaking downpours.

    When it rains like this it is known by Australians as the Big Wet, and it is typical of the sort of weather that can be expected during a La Niña episode (the cyclical cooling of the waters of the tropical Pacific), the latest of which has only just started to weaken. The eastern half of Australia becomes much wetter in the southern hemisphere’s summer, and during the winter more rain falls everywhere than usual except the west and the extreme south.

    On the eastern side of the Pacific La Niña also has well documented effects. One of these is usually to make winters in California very dry – which is why the persistent heavy rain, mountain snow and floods in December were so extraordinary.

    The atmosphere is a highly complex system, however, and connections are rarely straightforward. La Niña seems to have had a hiccup this winter because of the Pacific Decadal Oscillation, which is a slow cycle of cooling and warming of the northern Pacific.

    The is going through a slow cooling there at the moment and it is possible that this has caused the jet stream and associated weather systems to track farther south than might ordinarily be expected because of, as with the UK, a degree of blocking at higher latitudes. This has given California so far its third wettest winter on record, and Los Angeles its wettest December since 1889, victim to a conveyor of moisture originating near Hawaii, dubbed 'The Pineapple Express'.

    So, to answer the opening question, in a way these events are all indeed connected. No simplistic connections or parallels can be drawn but the atmosphere must be viewed as a fluid whole. Nothing happens in isolation.