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Archive for gener de 2011

Severe weather in 2010 — a global perspective

Posted by Costa M. a 20 gener 2011

It certainly seems to have been a year of headline making weather. In the summer of 2010 many meteorologists were asking what could be the causes behind the Russian heatwave and the floods in Pakistan and China. At the end of the year it was the UK’s turn with many areas experiencing heavy snowfalls and very low temperatures; while on the other side of the world eastern Australia saw flooding of “biblical proportions”.

But as with the summer’s extreme weather conditions, the early winter’s snow and ice in the UK and the extreme flooding in Australia are just as much a part of the natural variability of our climate. In the UK, although this winter’s weather is very unusual – especially after a run of mild winters in the last 20 years or so – it is not unprecedented.

Last summer, the jet stream – the current of air in the upper troposphere that circles the northern hemisphere and guides our weather systems – was very disturbed with large amplitude troughs and ridges moving warm air northwards and cold air south. This winter we have seen similar large disturbances in the position of the jet stream and some very large temperature anomalies as a result. There have been persistent signals of substantially above normal temperatures over Greenland and Chukot and of below normal temperatures over Canada, into Florida, and over central Russia and Western Europe.

So what is driving the formation of these large amplitude troughs and ridges? Since the autumn, the global circulation has been in the grip of arguably the strongest La Niña event for over a century. La Niña, the cold counterpart of El Niño, brings colder water to the surface of the tropical east and central Pacific and confines the warmest ocean waters to the West Pacific. The direct consequences for tropical rainfall patterns have been profound.

We also know there is a link between La Niña and colder winters over Western Europe (including for us, here in the UK) as it can produce a negative North Atlantic Oscillation (NAO) pressure pattern. The effect of this is to shift the Atlantic jet stream southwards with higher pressure building over Greenland. This means that in La Niña years our weather is more likely, although not always, to come from the colder and drier north and east as it has in 2010, rather than the milder and wetter south and west. However, the intensity and persistence of the blocking weather pattern experienced in late 2010 was quite exceptional, so it is very likely that other factors will have contributed.

One such additional influence could be the strength of the negative phase of the NAO [or the pressure difference between the Azores and the Norwegian Sea]. We know that there is considerable variability in the NAO and that on longer timescales of up to a decade or more variations in ocean temperatures influence the weather to either positive or negative NAO patterns. At the moment, ocean temperature anomalies show a ‘warm-cold-warm’ sea surface temperature distribution from north to south across the Atlantic Ocean. This pattern has been in place for some time and it is one that encourages a negative phase of the NAO, and a blocked pattern in our weather.

Other factors that may be influencing our weather include the declining extent and thickness of Arctic sea-ice and variations in the radiation from the Sun. These factors are very much at the cutting edge of our research, but there is increasing evidence that both may have contributed to the negative North Atlantic Oscillation pattern that we have experienced this winter and last winter.

What is clear from our analysis is that the cold early winter we have just experienced and the other recent extreme events around the world such as the Queensland and Sri Lankan flooding can be explained by the natural variations in our climate, especially La Niña. Our climate always has varied and will continue to vary naturally. Furthermore the range of that natural variability is much greater than the longer-term warming we have seen over the last 150 years or so. So we should not be surprised to experience cold winters; nor do a few cold winters signal the end of global warming.

2010 has just been confirmed as the warmest years on record, and even though our planet is warming, lots of other factors come into play to determine what our weather will be like at a regional and local level from one year to the next. Broad scale influences on our winter weather include atmospheric pressure systems that define where our weather comes from, Arctic sea-ice conditions, Atlantic Ocean sea surface temperatures, El Niño and La Niña in the tropical Pacific, and variations in the sun’s output.

Our understanding of some of these aspects is still immature and some are not well represented in contemporary climate models. This means that on decadal timescales it is difficult to predict the likelihood of severe winter weather conditions going forward. That being said the accuracy of our weather forecasts has never been better. As our understanding grows of the range of possible drivers for our climate and we develop improved climate models so that we can predict further ahead, so we hope to provide confident, robust and actionable advice to the government and the public in the future at longer time scales.

Source: Met Office – Professor Julia Slingo

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