研究：「超頑強高壓」擾亂 致加州異常大旱

The ‘Ridiculously Resilient Ridge’ formed by human-caused climate change has left California crippled by a withering record drought while diverting life-giving rainstorms to the north, Stanford scientists explain in a new study.

A team led by Stanford climate scientist Noah Diffenbaugh used a unique combination of computer simulations and statistical techniques to show that a “blocking ridge,” a persistent region of high atmospheric pressure over the Pacific Ocean that has diverted storms away from California, was much more likely to form in the presence of modern greenhouse gas concentrations.

Scientists agree that the immediate cause of the drought is a particularly stubborn “blocking ridge” over the northeastern Pacific – known as the ridiculously resilient ridge, or Triple R – that prevented winter storms from reaching California during the 2013 and 2014 rainy seasons.

Blocking ridges are regions of high atmospheric pressure that disrupt typical wind patterns in the atmosphere.

“At its peak in January 2014, the Triple R extended from the subtropical Pacific between California and Hawaii to the coast of the Arctic Ocean north of Alaska,” said Daniel Swain, a graduate student in Diffenbaugh’s lab and lead author of the Stanford study.

Swain coined the term “ridiculously resilient ridge” last fall to highlight the unusually persistent nature of the offshore blocking ridge.

Like a large boulder that has tumbled into a narrow stream, the blocking ridge diverted the flow of high-speed air currents known as the jet stream far to the north, causing Pacific storms to bypass California, Oregon and Washington. As a result, rain and snow that would normally fall on the West Coast was instead re-routed to Alaska and as far north as the Arctic Circle.

“Our research finds that extreme atmospheric high pressure in this region, which is strongly linked to unusually low precipitation in California, is much more likely to occur today than prior to the human emission of greenhouse gases that began during the Industrial Revolution in the 1800s,” said Diffenbaugh, an associate professor of environmental Earth system science at Stanford and a senior fellow at the Stanford Woods Institute for the Environment.

The team first assessed the rarity of the Triple R in the context of the 20th Century historical record. They found that the combined persistence and intensity of the Triple R in 2013 was unrivaled by any event since 1948, when comprehensive information about the circulation of the atmosphere first became available.

To more directly address the question of whether climate change played a role in the probability of the 2013 event, the team collaborated with Bala Rajaratnam, an assistant professor of statistics and of environmental Earth system science.

Using the Triple R as a benchmark, Rajaratnam and his graduate students compared geopotential heights – an atmospheric property related to pressure – between two sets of climate model experiments.

One set mirrored the present climate, in which the atmosphere is growing increasingly warm due to human emissions of CO2 and other greenhouse gases. In the other set of experiments, greenhouse gases were kept at a level similar to those that existed just prior to the Industrial Revolution.

The researchers found that the extreme geopotential heights associated with the Triple R in 2013 were at least three times as likely to occur in the present climate as in the preindustrial climate.

They also found that such extreme values are consistently tied to unusually low precipitation in California and the formation of atmospheric ridges over the northeastern Pacific.