Under pressure: Arctic trends sparking extreme weather at large
Laura Nielsen for Frontier Scientists
In September 2012, at the end of last summer, the Arctic sea ice extent reached a record low since satellite measurements began. And, according to the National Snow and Ice Data Center, summer sea ice extent in the Arctic has declined roughly 40 percent in the last three decades.
The Arctic is warming faster than other parts of the planet. This enhanced warming is called Arctic Amplification. It may seem like a far-away problem to many people, yet a study led by National Oceanic Atmospheric Administration scientist James Overland, Ph.D., and published in the American Geophysical Union's Geophysical Research Letters shows that it is a much nearer concern. Amplified Arctic warming is contributing to more frequent and more extreme weather events in North America and Europe, making the Arctic an increasingly important player in shaping regional climate in the coming century.
Climate change and Arctic warming alter the nature of the jet stream, weakening those powerful winds and changing their flow. An altered jet stream can amplify extreme weather events and change the origin, persistence, and severity of regional weather. Let's look at why.
The troposphere is the layer of our atmosphere which lies closest to earth. Bands of powerful winds called jet streams occur in the troposhpere where areas of high and low pressure meet. When the temperature difference between the pressure fronts is particularly pronounced, the resulting winds are fiercer, and jet stream winds can exceed 100 miles per hour. The jet stream in the Northern Hemisphere is known as the Polar jet stream.
Those pressure systems are often dominated in the north by two different forces. One is known as the Aleutian Low, which is an area of low pressure centered near Alaska's Aleutian Islands. It dominates during the winter months, and tends to generate winter storms. Then, during the summer, that low system is pushed north toward the pole by a high pressure system called the North Pacific High which is influenced more by the warmer southerly temperatures. Both systems are semi-permanent and changeable. Where the fronts meet, cold polar air interacts with the warmer air originating in the tropics and pushing north through the mid-latitudes. It is that interaction which influences weather trends in much of the Northern Hemisphere, particularly in North America and Europe.
When the standard low pressure system dominates in the Arctic and a high pressure system pushes northward from the mid-latitudes, the pressure pattern is described as a positive Arctic Oscillation, during which the Polar jet stream behaves in a predictable way. Winds blow swiftly from the west to the east, pushed out of a linear path only slightly by colder low-pressure troughs sliding south and warmer high-pressure air ridges pushing north. The fast, slightly wavy, west-to-east jet stream is called a zonal jet stream flow.
In contrast, a meridional flow describes a jet stream wind that meanders far from its path, blowing much more north-to-south. Its path is more radical because low-pressure troughs and high-pressure ridges penetrate further into the opposite pressure front. The winds follow an extreme winding path, and as a result move more slowly. Meridional flows correspond with a negative Arctic Oscillation, during which a high pressure front dominates in the Arctic. Simulations run on supercomputers foretell more of those intense high-pressure systems dominating the northern Pacific and Atlantic oceans as the 21st century progresses and levels of greenhouse-gasses continue to rise.
This matters because a slow, meandering meridional jet stream can mean weather conditions are more extreme and longer lasting. Cold Arctic temperature troughs reach further south, and hot pressure ridges stab further north. Severe storm fronts often form where the pressure systems with their wild temperature gradients rub against one another. And the meridional flow can make weather conditions dominate in one region for a longer duration. No longer blowing swiftly through along a stronger zonal flow, weather trends linger and sometimes persist. It's called a blocking pattern, and it can inflict prolonged heat spells leading to drought and increased wildfire probability, extreme summer rainfall leading to flooding, or unseasonal cold. These upsets to regional climate spell potential trouble for humans, watershed integrity for crops, and wildlife habitat.
Jennifer Francis, Ph.D., of Rutgers, names "[T]he rapid regional changes and increased frequency of extreme weather that global warming is causing," to be a high-impact symptom of climate change. She adds: "As the Arctic warms at twice the global rate, we expect an increased probability of extreme weather events across the temperate latitudes of the northern hemisphere, where billions of people live." The Earth is one great system, and everything is interconnected. The Arctic, though remote, impacts regional climate patterns in North America and Europe; the amplified warming occurring there deserves our attention and care.
Eyes to the north.
Frontier Scientists: presenting scientific discovery in the Arctic and beyond
'A Rough Guide to the Jet Stream: what it is, how it works and how it is responding to enhanced Arctic warming' John Mason, Skeptical Science http://skepticalscience.com/jetstream-guide.html
'Arctic summer wind shift could affect sea ice loss and U.S./European weather, says NOAA-led study' National Oceanic Atmospheric Administration http://www.noaanews.noaa.gov/stories2012/20121010_arcticwinds.html
'Evidence Linking Arctic Amplification to Extreme Weather in Mid-Latitudes' American Geophysical Union : Geophysical Research Letters http://www.agu.org/pubs/crossref/pip/2012GL051000.shtml
'National Weather Service Glossary' National Oceanic and Atmospheric Administration : National Weather Service http://w1.weather.gov/glossary/
'Weather-Making High-Pressure Systems Predicted to Intensify' Duke University : Nicholas School of the Environment http://www.nicholas.duke.edu/news/weather-making-high-pressure-systems-predicted-to-intensify