Increased Atmospheric Blocking
Scientists are actively studying trends in large scale atmospheric pressure patterns with little or no movement—what is referred to as atmospheric blocking—and their connection to climate change. Studies have begun to identify an anthropogenic component in recent blocking events, like those responsible for extreme rainfall in the UK and prolonged dry periods in the US Southwest.
Atmospheric blocking events are associated with persistent, slow-moving high-pressure systems that obstruct the prevailing westerly winds in the mid- and high-latitudes and the normal eastward progress of extratropical transient storm systems.
Blocking is an important component of the intraseasonal climate variability in the extratropics and can cause long-lived weather conditions such as cold spells in winter and summer heat waves. However, as is the case for other trends in atmospheric circulation, global assessment of blocking trends is complicated by strong interannual variability in all seasons as well as issues like differences in blocking index definitions.
Scientists have recently observed heavier-than-normal snowfalls in the Midwest and Northeast US in some years, with little snow in other years. These observations are consistent with indications of increased blocking of the wintertime circulation of the Northern Hemisphere.
A 2012 study identifies an eastward shift of blocking events over the North Atlantic (fewer cases of blocking over Greenland and more frequent blocking over the eastern North Atlantic) and the North Pacific. A 2013 study finds an increase in blocking duration year-round over the Northern Hemisphere since about 1990.
An earlier study, from 2008, found a decrease in the overall frequency of blocking events in the Southern Hemisphere, but an increase in the intensity of events.