http://www.wunderground.com/blog/JeffMasters/comment.html?entrynum=1824Nature's fury reached new extremes in the U.S. during the spring of 2011, as a punishing series of billion-dollar disasters brought the greatest flood in recorded history to the Lower Mississippi River, an astonishingly deadly tornado season, the worst drought in Texas history, and the worst fire season in recorded history. There's never been a spring this extreme for combined wet and dry extremes in the U.S. since record keeping began over a century ago, statistics released last week by the National Climatic Data Center (NCDC) reveal. Their Climate Extremes Index (CEI) looks at the percentage area of the contiguous U.S. experiencing top 10% or bottom 10% monthly maximum and minimum temperatures, monthly drought, and daily precipitation. During the spring period of March, April, and May 2011, 46% of the nation had abnormally (top 10%) wet or dry conditions--the greatest such area during the 102-year period of record. On average, just 21% of the country has exceptionally wet conditions or exceptionally dry conditions during spring. In addition, heavy 1-day precipitation events--the kind that cause the worst flooding--were also at an all-time high in the spring of 2011. However, temperatures during spring 2011 were not as extreme as in several previous springs over the past 102 years, so spring 2011 ranked as the 5th most extreme spring in the past 102 years when factoring in both temperature and precipitation.
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Was climate change involved?Whenever an unprecedented series of extreme weather events hit, it is natural to ask how climate change may be affecting the odds of these events, since our climate is undergoing unprecedented changes. This spring's unusual precipitation pattern--wet in the Northern U.S., and dry in the South--does fit what we'd expect from a natural but unusually long-lived winter La Niña pattern (Figure 2). However, it also fits the type of precipitation pattern climate models expect to occur over the U.S. by the end of the century due to human-caused warming of the climate (though shifted a few hundred miles to the south, Figure 6.) This drying of the Southern U.S. and increased precipitation in the Northern U.S. is expected to occur because of a fundamental shift in the large scale circulation of the atmosphere. The jet stream will retreat poleward, and rain-bearing storms that travel along the jet will have more moisture to precipitate out, since more water vapor can evaporate into a warmer atmosphere. The desert regions will expand towards the poles, and the Southern U.S. will experience a climate more like the desert regions of Mexico have now, with sinking air that discourages precipitation. A hotter climate will dry out the soil more, making record intensity droughts like this year's in Texas more probable. So, is it possible that the record extremes of drought and wetness this spring in the U.S. were due to a combination of La Niña and climate change. It is difficult disentangle the two effects without doing detailed modelling studies, which typically take years complete and publish. One weakness in the climate change influence argument is that climate models predict the jet stream should retreat northwards and weaken due to climate change. Indeed, globally the jet stream retreated 270 miles poleward and weakened during the period 1970 - 2001, in line with climate model expectations. Thus, a more southerly position of the jet stream over the U.S. during the spring is something we should expect to see less and less of during coming decades.
Figure 6. The future: simulated change in precipitation during winter and spring for the years 2089 - 2099 as predicted by fifteen climate models, assuming we continue high emissions of heat-trapping carbon dioxide. Confidence is highest in the hatched areas. Compare with Figure 7, the observed change in precipitation over the past 50 years. Image credit: United States Global Change Research Program.
Figure 7. U.S. annual average precipitation has increased by about 5% over the past 50 years, but there has been pronounced drying over the Southeast and Southwest U.S. Even in these dryer regions, though, heavy precipitation events have increased (see Figure 4.) Thus, rainfall tends to fall in a few very heavy events, and the light and moderate events decrease in number. Image credit: United States Global Change Research Program. Data plotted from
http://www.ncdc.noaa.gov/oa/climate/research/ushc n/.
Keep in mind, though, that climate models are best at describing the future global average conditions, and not at predicting how climate change might affect individual continents--or at predicting how rare extreme events might change. Major continent-scale changes in atmospheric circulation are likely to result over the coming few decades due to climate change, and I expect the jet stream will shift farther to the south in certain preferred regions during some combination of seasons and of the natural atmospheric patterns like La Niña, El Niño, and the Arctic Oscillation. For example, there has been research published linking recent record Arctic sea ice loss to atmospheric circulation changes in the Arctic Oscillation that encourage a southwards dip of the jet stream over Eastern North America and Western Europe during late fall and winter. Until we have many more years of data and more research results, we won't be able to say if climate change is likely to bring more springs with a circulation pattern like this year's.
One thing we can say is that since global ocean temperatures have warmed about 0.6°C (1°F) over the past 40 years, there is more moisture in the air to generate record flooding rains. The near-record warm Gulf of Mexico SSTs this April that led to record Ohio Valley rainfalls and the 100-year $5 billion+ flood on the Mississippi River would have been much harder to realize without global warming.
http://www.wunderground.com/blog/JeffMasters/comment.html?entrynum=1824
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