More sea level woes ahead?
Scientists are tracking yet another global warming feedback mechanism that will have dire consequences for coastal communities around the world. Melting sea ice and overall rapid warming in the Arctic are factors in the development of so-called blocking high pressure systems — air masses spinning clockwise that block cold, dry Canadian air from reaching Greenland.
The highs tend to enhance the flow of warm, moist air over Greenland, contributing to increased extreme heat events and surface ice melting, according to the study. That circulation pattern leads to more melting of the Greenland Ice Sheet, according to new research published online in the Journal of Climate last month, co-authored by Jennifer Francis, one of the pioneers in studying how global warming is affecting the jet stream.
“I think this study does a good job of pinning down the fact that the [Arctic sea] ice is disappearing for a whole bunch of reasons – and that is causing the surface of Greenland’s melt area to increase,” said Francis, a research professor in Rutgers University’s Department of Marine and Coastal Sciences.
According to Francis, blocking-high pressure systems over Greenland usually form when a lot of warm air is in the Arctic. And so far this year, it’s been extremely warm and Arctic sea ice has been at record low levels for this time of year.
The Arctic warmth tends to weaken the jet stream, which typically flows west to east, allowing it to meander more to the north and south, Francis said.
The jet stream can take “such a big northward swing that it actually kind of breaks off and forms a closed circulation,” she said. Blocking highs tend to be persistent and are hard to forecast, she added.
Greenland Ice Sheet melting has accelerated significantly in the past 50 years, and in recent decades, much of the melting is happening on the surface, contributing runoff that speeds up sea level rise. Last summer, there was melting across more than half of Greenland’s ice sheet for the first time since 2012
Francis warned that sea level rise could happen much faster than many climate models currently anticipate.
“Whenever there’s a big melt year in Greenland, on the surface anyway, it’s usually because there’s either a blocking high or a large northward swing in the jet stream and both of those things tend to be long-lived features in the circulation,” she said. Both transport a lot of heat, moisture and clouds over the Greenland ice sheet, leading to more melting.
The study builds on other research by Francis (and other scientists), including work she described in January at a weather and climate summit in Breckenridge, Colorado.
“We’re starting to make seasonal links, seeing a seasonal slowing in jet stream as the pressure gradient decreases, Francis said at the conference, describing data that measured days with high-amplitude waves in the jet stream. According to Francis, there’s been a big increase in that number since 1979.
“This seems to be a robust linkage, the number of days with blocking highs appears to be increasing,” she said.