Blocking patterns: How global warming might have worsened US drought

As the summer of 2012 winds down, with drought and searing temperatures its hallmark for much of the continental United States, researchers are trying to get a better handle on the factors that contribute to the persistence of weather patterns responsible for the extremes.

The immediate culprit: patterns of atmospheric flow that steer storms along a given path for weeks, heating and depriving some areas of needed rain while drenching others. Such blocking patterns are a global phenomena, a normal component of Earth's weather systems.

But some researchers suggest that global warming's influence on the Arctic and on the tropics can change circulation patterns in ways that keep blocking patterns in place longer than they otherwise might.

For the continental US, blocking has been a byword for much of the year. The first eight months of 2012 have gone into the books as the warmest January-August period on record for the continental US, according to the National Oceanic and Atmospheric Administration's National Climatic Data Center in Asheville, N.C. The 12-month span ending in August 2012 was the warmest 12 months on record. The summer itself ranks third among the warmest summers on record.

At the end of August, 62.9 percent of the continental US was experiencing moderate to exceptional drought.

"I can tell you that 2011 and 2012 are shaping up to be ripe for study. It's really been a sustained period of climate extremes up to this point," says John Fasullo, a researcher at the National Center for Atmospheric Research (NCAR) in Boulder, Colo.

As researchers try to untangle the atmospheric influences behind persistent weather patterns, the Arctic and the tropics have come under increasing scrutiny.

The decline in summer sea ice, which this summer has reached a record low, deprived the Arctic Ocean of much of its reflective coat of white. This leaves more open ocean from which heat escapes back into the atmosphere in fall and winter. This tends to slow the jet stream's wind speeds and stretch its meanders north and south, according to researchers from Rutgers University and the University of Wisconsin at Madison.

Both effects tend to slow the movement of these meanders – the boundary between warm air from the south and cold Arctic air, and the track storms follow – as they work their way from west to east across the continent in the fall and winter.

In addition, the researchers show how earlier snow melt in the spring and a later onset of snow in the fall dries out the region, contributing to warmer land temperatures in spring and summer – and with a similar, if somewhat less-pronounced, effect on the jet stream then.

Another study, by scientists at NCAR, looked at the reasons behind a prolonged heat wave and drought in Russia, drought in Brazil, and heavy rains and flooding in Pakistan, Colombia, and northeastern Australia in 2010.

It identified four expanses of tropical oceans – the northern Indian Ocean, the Gulf of Mexico as well as Caribbean Sea, and the tropical Pacific – where sea-surface temperatures either broke or approached record levels at various times between May and September. In the meantime, the shift from El Niño to La Nina – the seesaw cycle in ocean temperatures and wind patterns in the tropical Pacific – shifted the center of thunderstorm action in the tropical oceans to these unusually-warm regions.

An atmosphere already warmed by climate change and capable of holding more moisture built vigorous storms that provided the heavy rains. The release to the atmosphere of heat as the water vapor feeding these storms cooled and condensed triggered a chain of changes in circulation patterns.

Over the tropical Atlantic, it led to the establishment of a stubbornly persistent high-pressure system over southern Europe and the western part of the Russia, bringing with it the heat wave, which set the stage for wildfires the country experienced. The blocking pattern also kept steering amped-up monsoon storms to flood Pakistan.

Meanwhile, intensified convection over the Caribbean also brought heavy rains to Colombia, where flooding displaced an estimated 1.5 million people.

The study, led by NCAR's Kevin Trenberth, was published earlier this month in the Journal of Geophysical Research.

The direct effect of global warming on features such as Russia's heat wave and drought is small, acknowledges Dr. Fasullo, who teamed up with Dr. Trenberth on the study. But that small contribution appears to have triggered processes that amplified the warming, he adds.

Over time, as climates change plant species change and soils lose moisture. In the US Midwest this summer, scant winter snowfall left soils with little moisture on hand to evaporate and serve as an air conditioner when spring and summer arrived. These conditions can help reinforce the blocking pattern that keeps at bay storms that otherwise might reach the area.

The same thing happened in Russia, according to the study. The warming it has undergone has been greater than warming implied by the 1-degree Fahrenheit increase in the global average temperature since the 1950s, Fasullo says. This set the stage for feedbacks that amplified the warming the country experienced when the heatwave arrived.