‘Blocking highs’ becoming more common over Greenland
Just a few weeks after scientists reported record early melting on parts of the Greenland Ice Sheet, a new study helps explain some of the recent dramatic climate shifts in the high latitudes of the northern hemisphere.
Stationary high pressure systems over Greenland have become more frequent since the 1980s, said University of Sheffield geographer Prof. Edward Hanna, adding that the pattern is also linked with extreme weather over northwest Europe, including unusually wet conditions in the UK in the summers of 2007 and 2012.
The study, published in the International Journal of Climatology, looked at large-scale weather patterns over Greenland going back to 1851 using a measure called the Greenland Blocking Index, which marks the how strong the high pressure systems are, how long they last and how often they occur.
Hanna said the research team found that high pressure blocking systems have become more common over Greenland since the 1980s in all seasons, which relates to a significantly strong warming of the Greenland and wider Arctic region compared with the rest of the world. The pattern has become more common in the summer, as a northward-meandering branch of the jet stream enables warmer air to move north into the region more often in recent years.
Other recent studies have come to similar conclusions, showing a link between shifts in the jet stream and increased melting of the Greenland Ice Sheet. The impacts aren’t just limited to the Arctic. Jet stream changes have also been implicated in massive floods in the Balkans in 2014, as well as with persistent patterns of temperature extremes in other regions.
The shifts have had a “significant impact on extreme weather and climate change in the region,” Hanna said. “These weather systems are occurring in the area more often because of strong Arctic warming and changes in the atmospheric jet stream in recent years … This is resulting in an increase in the occurrence of warm air in the region and it is also affecting weather systems downstream of Greenland, such as over the UK.”
The study concluded that, in recent decades, the blocking high pressure systems have become much more variable from year to year in December. This reflects an increasing destabilisation of atmospheric weather systems in late autumn and early winter, which the team believe may be related, at least in part, to dramatic declines in sea-ice coverage in the Arctic region.
“Sea-ice coverage throughout the Arctic has significantly reduced in recent years, which we already know is having an amplifying effect on warming in the region. What this study now tells us is that changes in stationary high pressure over Greenland are adding to the change in polar climate,” Hanna said.
This research has more than doubled the timespan of data analyzed on Greenland ‘blocking’ weather systems and is a useful measure of changes in North Atlantic atmospheric circulation. The results can enable an improved understanding of the links between mid-latitude and high-latitude climate change when combined with other climatological studies.
The research team included climate scientist John Cappelen from the Danish Meteorological Institute in Copenhagen, Denmark.