Last year was the hottest on record for the US. This March was the coldest of the last 50 years for Europe. Last year’s heat spawned a 55 year drought in the US. This year’s cold and wet spawned massive storms that paraded through Europe and wracked the US northeast. What do these two seemingly opposite events have in common? Two words: global warming. Two more words: blocking pattern. And four final words: Arctic sea ice retreat.
Over the past decade, an increased prominence of blocking patterns has emerged. These events happen when the polar jet stream gets stuck in large meanders. These meanders result in a long persistence of weather for regions affected by these blocking patterns. New papers by Jennifer Francis and other polar researchers last year linked the increasing occurence of these new blocking patterns to an erosion of Arctic sea ice.
Ice acts as an insulator. As long as the ice is stable, it keeps cold air locked in the Arctic all while pushing warm air to the south. This strong temperature differential between hot southern air and cold northern air pushed the polar jet stream along at high rates of speed. The result was that weather systems carried along the jet stream tended to move rapidly, rather than staying in one place for a long time.
Enter the present day. We have an 80% loss of end summer sea ice volume, a 62% loss of end summer sea ice area and a 50% loss of end summer sea ice extent all occurring since 1980. The result is that the ice insulator is severely eroded. This allows large volumes of much warmer air to invade the Arctic from the south. It also results in greater cold air incursions from the Arctic into southern regions. This mixing of air, in turn, creates a more moderate temperature difference between polar regions and neighboring temperate regions. The final product is a jet stream that is both far slower and much wavier than previously observed.
(Polar Weather Map. Image source: here.)
To illustrate this point, we can look at the current polar weather map. At first glance it looks innocuous. But when we look at the position of high and low pressure systems and of warm and cold temperatures we will find that they are much the same as they were a week ago.
Low pressure systems keep forming over the Bering and Okhotsk seas. Warmer air temps keep invading the American and Canadian west. The eastern part of the North American continent sees a continuous colder air invasion from the Arctic. The Arctic remains much warmer than average with certain regions in Greenland experiencing average temperatures 10 degrees Celsius above normal for this time of year. And a pool of cold air has been spilling from the Arctic and into Europe throughout much of winter and spring.
This ongoing influx is, at the moment spawning an Easter snowstorm for much of Europe. Also feeding this strong storm is a persistent trough of low pressure to the south of Greenland that has cranked out an almost continuous stream of moisture even as it has pushed much warmer than usual air into Baffin Bay. Looking at a history of these maps, we can see that the position of the most recent low has remained in an almost constant position for five days. The past month shows a constant procession of lows into this region. They remain there for a while. Then they shift out only to be replaced by the next low. This North Atlantic Low has been an almost constant companion to the Greenland and Arctic highs. Yet one more element of the current blocking pattern.
It is the long duration of these weather events that makes them extreme. Europe experiencing a few days to a week of these conditions would be remarkable, but would not carry the same level of impact an entire month or more of cold, wet, snow-filled weather. And a persistent pattern of much warmer weather in the Arctic is likely to have its own extreme impacts, especially if the pattern remains until late spring and on into summer.
The common theme running through all these events is human caused global warming. Without human GHG emissions pushing ocean and air temperatures higher, without the resulting Arctic amplification of temperatures, and without the massive volumes of sea ice melted, these weather patterns wouldn’t be subject to such radical changes. We wouldn’t be seeing regions served large doses of extreme weather for such long periods of time.