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Thu August 29, 2013
Dr. Edward Hanna, University of Sheffield – The Jet Stream and Greenland Warming
In today’s Academic Minute, Dr. Edward Hanna of the University of Sheffield reveals the connection between a pattern in the jet stream and recent warm temperatures in Greenland.
Edward Hanna is a professor of climate change in the Department of Geography at the University of Sheffield. His research is primarily focused on meteorology and climate change in high-latitude regions such at Iceland, Greenland, and the polar region. He received his Ph.D. from University College London.
Dr. Edward Hanna – The Jet Stream and Greenland Warming
Last summer's record surface melt of the Greenland Ice Sheet caught many scientists by surprise but was an inevitable consequence of recent rapid summer warming of about 2-3 degC over Greenland since the early 1990s. This is much greater than global warming over the same time period because of what scientists call the Arctic amplification - the tendency for this warming to be enhanced in high northern latitudes by several processes.
Most notably, extra melting of ice and snow darkens the surface when then absorbs more solar radiation, enhancing the warming. High-latitude warming is also augmented through loss of sea-ice of as much as 50% in late summer since the 1970s, which opens up the Arctic Ocean and causes increased heat loss to the overlying atmosphere from the relatively warmer ocean.
This may well have consequences for the northern hemisphere polar jet stream: a giant river of airflow in the atmosphere which is driven by thermal contrast between tropical and polar air masses and flows more or less from west to east. As the Arctic warms relatively rapidly, the temperature range or heat gradient across latitude decreases, which means less energy is available to drive the jet stream. This then slows down and becomes more meandering, with larger north-south loops developing.
A slower jet stream also means that more fixed weather patterns prevail across mid-latitudes, with the weather at any one place depending on where you are in relation to the jet stream. Greenland in the last six summers has been under a more northward-moving branch of the jet stream, which has dragged much warmer than normal air up over the western flank of the ice sheet, and it is this local change in jet-stream configuration that played a key role in contributing to the record icemelt in summer 2012.