Arctic Melting affects Weather

A high-amplitude jet stream pattern observed over the United States last week (December 13, 2011).  Instead of blowing west-to-east, the jet stream was contorted into a southward-bulging trough of low pressure that brought cold temperatures and a snow storm to Southern California, and a northwards-bulging ridge of high pressure that brought record warm temperatures to portions of the eastern USA.  The axis of the jet stream is marked by the strongest winds (green and light blue colors) at the top of the lower atmosphere (200-300 mb pressure level).

Arctic sea ice loss may be affecting the large-scale atmospheric circulation, slowing its winds and increasing the tendency for contorted high-amplitude atmospheric loops.  Such loops in the upper level wind pattern (and associated jet stream) increase the probability of persistent weather patterns in the Northern Hemisphere, potentially leading to extreme weather due to longer-duration cold spells, snow events, heat waves, flooding events, and drought conditions.

It is of interest to note that this December temperatures in Florida continue in the low 80°s F (27° C) and the persistent drought shows no signs of relenting.

Arctic sea ice is now 40% less extensive than it was when satellite records began in 1979.
See University of Illinois Cryosphere Today

Summertime Arctic sea ice loss:  40% since 1980

The Arctic has experienced a stunning amount of sea ice loss in recent years, due to melting and unfavorable winds that have pushed large amounts of ice out of the region.   40% of the sea ice was missing in September 2007, compared to September of 1980.  That represents an area equivalent to about 44% of the contiguous United States or 71% of the non-Russian portion of Europe.  Such a large area of open water likely causes significant impacts on weather patterns, due to the huge amount of heat and moisture that escapes from the exposed ocean into the atmosphere over a multi-month period following the summer melt.

The extent of Arctic sea ce loss in the summer July-August-September 2007 was about 1.4 million square miles.  A similar amount was 'missing' in 2011.  For comparison, the lost ice coverage (orange colors) was equal to an area about 44% of the United States or 71% of the non-Russian portion of Europe.

Arctic sea ice loss slows the Jet Stream

Recent scientific analysis indicates that since 1948 the extra heat in the Arctic infall and winter have caused the Arctic atmosphere to expand.  As a result, the difference in temperature between the Arctic (60-80° N.) and the mid-latitudes (30-50° N.) has fallen significantly.  It is the difference in temperature that drives the powerful jet stream winds that control much of our weather.  The speed of fall and winter west-to-east upper-level winds circling the North Pole has decreased by 20% over the past decade in response to the extra warmth in the Arctic.  This slow-down of the upper-level winds circling the pole has been linked to a Hot Arctic-Cold Continents pattern that has brought cold, snowy winters to the Eastern USA and Western Europe the past two winters.

Arctic sea ice loss may increase troughs and ridges

The jet stream generally blows from west to east over the northern mid-latitudes, with an average position over the central U.S. in winter and southern Canada in summer. The jet stream marks the boundary between cold polar air to the north and warm subtropical air to the south, and is the path along which rain and snow-bearing low pressure systems ride. Instead of blowing straight west-to-east, the jet stream often contorts itself into a wave-like pattern. Where the jet stream bulges northwards into a ridge of high pressure, warm air flows far to the north. Where the jet loops to the south into a trough of low pressure, cold air spills southwards. The more extreme these loops to the north and south are--the amplitude of the jet stream--the slower the waves move eastward, and consequently, the more persistent the weather conditions tend to be. A high-amplitude jet stream pattern (more than 1000 miles or 1610 km in distance between the bottom of a trough and the peak of a ridge) is likely to bring abnormally high temperatures to the region under its ridge, and very cold temperatures and heavy precipitation underneath its trough.

The mathematics governing atmospheric motions requires that higher-amplitude flow patterns move more slowly. Thus, any change to the atmosphere that increases the amplitude of the wave pattern will make it move more slowly, increasing the length of time extreme weather conditions persist.  During the early 1960s, a natural pattern in the atmosphere called the Arctic Oscillation increased the amplitude of the winter jet stream pattern over North America and the North Atlantic by more than 100 miles, increasing the potential for long-lasting weather conditions. The amplitude of the winter jet fell over 100 miles (161 km) during the late 1960s, remained roughly constant during the 1970s - 1990s, then increased by over 100 miles again during the 2000s. The latest increase in wave amplitude did not appear to be connected to the Arctic Oscillation, but did appear to be connected to the heating up of the Arctic due to sea ice loss. 

A warmer Arctic allows ridges of high pressure to build farther to the north. Since temperatures farther to the south near the bases of the troughs are not changing much by comparison, the result is that the amplitude of the jet stream grows as the ridges of high pressure push farther to the north. Thus it is possible that Arctic sea ice loss and the associated increases in jet stream amplitude could be partially responsible for some of the recent unusual extreme weather patterns observed in the Northern Hemisphere.  

West-to-east jet stream winds speeds at 500 mb (appx. 18,000 feet or 5,600 meters) in the mid latitudes (40-60° N) over North America between 1948 and 2010.  During fall (October-December) and winter (January-March) jet stream winds weakened by about 20%.