The image displays 2011 mean precipitation (in mm) as departure from normal across the US. There are two key things to note: firstly, the extremely high rates of precipitation in the north-east US, and secondly, the extremely low-rates of precipitation in the south (500mm above and 500mm below respectively). I have touched on precipitation extremes in previous posts, but Philadelphia is now the wettest it's been since 1867 and there are 20 other cities who have broken long-term records or are close to. The increase in precipitation in 2011 has largely been down to the tornado outbreak in April/May combined with hurricane Irene and other heavy summer thunderstorm rains.
Now the heavy rains are only half the picture, sections of 16 states have precipitation rates 500mm below normal this year. It has been a significant year of contrast with many places being washed away with heavy rain and floods or whilst other areas have faced and suffered from significant dry conditions.
I have yet to really touch upon droughts in any of my posts, and now would probably be a good time to do so. Arnell (who's based at the Walker Institute for Climate System Research in Reading) in 2004 published this paper on climate change and drought, which has been recently built upon in the SREX report. Essentially droughts are likely to become more frequent and intensify with the intensification in the hydrological system. Precipitation in large parts of southern Europe, north Africa, central Asia and southern Africa (which are often subject to frequent dry conditions), are likely to experience even less rainfall, increasing drought risk. Moreover, the climate change risk is super-imposed on top of things such as land cover changes which could exacerbate the problem even further. It is well agreed upon that there will be an increased (physical) drought risk, but there is still considerable uncertainty over the magnitude of this change.
The variability in climate change and precipitation is also displayed in a study released last month in which regional climate models were developed for Cyprus in order to project weather extremes. Despite, increases in temperature across the entire country (of 1-2 degrees Celsius in both summer and winter), some areas are expected to experience less frequent precipitation events of lower magnitude (inland places such as Nicosia) whilst other areas such as Saittas and Limassol may experience more precipitation events, with increased rainfall.
Climate change science predicts that if the Earth continues to warm as expected, wet areas will tend to get wetter, and dry areas will tend to get drier. Therefore you would expect this year's side-by-side extremes of very wet and very dry conditions in the US should become increasingly common in the coming decades.
