Editor's commentsThis article investigates the conjecture that the rapidly warming Arctic is more important in determining Northern Hemisphere weather patterns than conventional climate dynamics theory holds. The authors offer support for this conjecture by proposing a mechanism by which Arctic Amplification affects broader Northern Hemisphere weather patterns. The mechanism that has been proposed involves two hypotheses.

1) As high latitudes warm faster than mid-latitudes as a result of Arctic amplification, the pressure gradient between the Arctic and mid-latitudes weakens. Frances and colleagues presented statistical evidence in support of this in 2009. They argue that a reduced gradient slows west to east winds, increases the waviness of the jet stream (the ridges and troughs extend further northward and southward) and slows the progression of the waves from west to east. This part of the proposed mechanism remains controversial.

The articles, published in Science, listed under Commentary provide a discussion of the controversy. The main criticisms are
a) Scientists have challenged the analysis of historical data. They say there is little evidence that the jet stream's west-to-east winds are slowing and its waviness increasing.
b) Climate modelers say their computer simulations have mostly failed to confirm the hypothesis.
c) Atmospheric dynamicists argue that the Arctic's small influence over the planet's atmospheric energy flow is too small to alter the jet stream. The tropics are a more likely source of the energy required to change the jet stream.

2) The effect of a slower jet stream and increased waviness is to increase the likelihood of certain types of extreme weather, such as drought, prolonged precipitation, cold spells, and heat waves. Recent independent studies have statistically linked large-amplitude Rossby waves in the jet stream to extreme weather in the mid-latitudes.


The jet stream is a high altitude feature that separates warm air to its south from cold air to the north. It follows a wavy path around the Northern Hemisphere between 30 and 60 degrees N as shown in the graphic above. There is independent evidence that the slower progression and north-south elongation of what are called Rossby waves (often referred to as waviness) in the jet stream cause longer lasting weather events and that these can increase the likelihood of certain types of extreme weather, such as drought, prolonged precipitation, cold spells, and heat waves. Examples of weather patterns that seem to have lasted longer during the past few years include deep troughs in the jet stream which hung over the U.S. east coast and Western Europe during the winters of 2009/2010 and 2010/2011, bringing a string of snow storms and abnormal cold. What happened in the US East in 2014 (called the polar vortex) was a known weather pattern caused by a phenomenon known as the "Greenland block". A region of high pressure became stationary over Greenland in the path of the circumpolar jet stream. The jet stream meandered, bending southward into a large loop and shunting cold Arctic air toward the center of the United States.

The authors propose a mechanism by which Arctic Amplification, which refers to the Arctic warming twice as fast as the rest of the planet, changes Northern Hemisphere weather patterns. As a result of this warming the extent of Arctic sea ice in summer has declined by 40 percent in the past three decades. The result of this Arctic warming, they argue, is that as high latitudes warm more than mid-latitudes, the pressure gradient in the poleward (from south to north) direction weakens. The authors argue that in addition to slowing west to east winds, rapid Arctic warming also causes the Rossby waves to get "wavier", slowing and elongating the waves causing them to reach farther north and south. The southern tips of the waves are known to be related to weather events such as storms in mid-latitudes. The progression of the waves in the west to east direction is also slowed making weather events longer lasting. An apt analogy might be to a meandering river on a flat plain which leads to the build up of sandbars, similar to a slow moving jet stream creating slow moving weather patterns such as the 2014 winter storms in the US.

Data sources


In the 2009 article (see Extent of summer Arctic sea ice may help predict Northern Hemisphere weather patterns ) Jennifer Frances and colleagues investigated correlations between summer ice extents in the Arctic and broader Northern Hemisphere atmospheric properties such as surface temperature and pressure. One of the patterns that was identified related to the pressure gradient between the Arctic and mid-latitudes, which appeared to be reduced when summer Arctic ice extents were abnormally low.

Poleward pressure gradient
Poleward pressure gradient
Figure Poleward gradient in the geometric thickness of the 1000–500 hPa layer (m/km) in the North Atlantic and the North Pacific Oceans during years with above- (red) and below-normal (blue) sea ice during summer. Data extend from September of the extreme ice year to the following March.


To support the mechanism they have proposed for the effect of Arctic amplification on Northern Hemisphere weather patterns, the authors compiled pressure and temperature data for a part of the Northern Hemisphere. The region selected for their analysis covers the North Atlantic stretching across North America to the Pacific.

Region of study: 140 degrees W to 0 degrees
Region of study: 140 degrees W to 0 degrees

Figure Region of study from 140 degrees W to 0 degrees. Asterisks show some of the 500 hPa heights used in the analysis.

The data used were 1000 and 500 hectopascal (hPa) pressure surfaces (geopotential heights) which were taken from the NCEP/NCAR 20th Century Reanalysis . These pressure surfaces represent the altitude at which atmospheric pressure reaches a certain pressure level, in this case 1000 hPa and 500 hPa surfaces which are widely used by climate scientists. A pressure of 1000 hPa is about one atmosphere of pressure and corresponds roughly to the pressure at sea level. A pressure of 500 hPa is about half an atmosphere and is reached at an altitude of roughly 20,000 ft with a temperature of about -25 °C (In the upper atmosphere pressure is directly related to temperature.) For climate scientists the contours of the 500 hPa surface are important because they effectively "determine" our weather - low heights indicate troughs and cyclones while high heights indicate ridges and anticyclones. The difference between the 500 hPa and 1000 hPa surfaces, called a thickness, was used by the authors in their analysis.

Effect on the jet stream


The authors analyzed the data looking for evidence linking Arctic warming with three important effects, slower high level winds, increased waviness (elongated Rossby waves stretching further north and south, and the slower progression of the waves in the jet stream.

One effect ascribed to Arctic amplification is a reduced poleward (south to north) gradient in 1000-500 hPa thicknesses. The strength of the poleward thickness gradient determines the speed of upper-level zonal winds. As the gradient has decreased with a rapidly warming Arctic, since 1979 the upper-level zonal winds during autumn have also weakened, with a total reduction in wind speed of about 14% (>95% confidence). Winter winds are more variable but also exhibit a steady decline since the early 1990s.

Seasonal 1000–500 hPa thickness differences and zonal winds
Seasonal 1000–500 hPa thickness differences and zonal winds
Figure The figure on the left shows the time series of 1000–500 hPa thickness differences between 80–60 degrees N and 50–30 degrees N over the study region for autumn (OND), winter (JFM), spring (AMJ), and summer(JAS). The figure on the right shows a time series of zonal mean winds at 500 hPa between 60–40 degrees N over the study region for different seasons. Data is from the NCEP/NCAR reanalysis.


A second effect ascribed to Arctic amplification is ridge elongation or increasing "waviness" of the jet stream. This is expected in response to larger increases in 500 hPa heights at high latitudes than at mid-latitudes. This effectively stretches the peaks of ridges northward. Elongated waves also move eastward more slowly. Evidence for this mechanism was investigated by selecting a narrow range of 500 hPa heights for each season that captures the daily wave pattern in the height field. The authors report that their analysis found a steady northward progression of ridge peaks which supports the hypothesis that Arctic amplification is contributing to ridge elongation. The authors report that confidence in these trends exceeds 99%.

Schematic of ridge elongation
Schematic of ridge elongation

Figure Schematic of ridge elongation in upper-level heights caused by greater warming in the Arctic relative to mid-latitudes. The solid line represents the pattern in the absence of Arctic warming and the dashed line shows the effect of Arctic warming. The arrows indicate that elongated waves progress eastward more slowly.

Independent evidence has shown that the slower progression of upper-level elongated waves causes longer lasting weather conditions and can increase the likelihood of certain types of extreme weather, such as drought, prolonged precipitation, cold spells, and heat waves. For example, in 2013 Vladimir Petoukhov and colleagues correlated summer heat waves in Europe with instances of slow-moving, high-amplitude Rossby waves. James Screen and Ian Simmonds reported a strong statistical correlation between amplified, slow Rossby waves and months from 1979 to 2012 with extreme weather events. They found that more waviness made the western United States more susceptible to heat waves and the eastern United States to extreme cold.

Active research


This research is still in its early stages and the detailed mechanism proposed in this paper has not yet been widely accepted. Some scientists have challenged the analysis of historical data. Climate modelers say their computer simulations have mostly failed to confirm the hypothesis. Atmospheric dynamicists argue that the Arctic's small influence over the planet's atmospheric energy flow is too small to alter the jet stream.

Jennifer Francis, Stephen Vavrus, Evidence linking Arctic amplification and extreme weather in mid-latitudes, GEOPHYSICAL RESEARCH LETTERS, VOL. 39, L06801, doi:10.1029/2012GL051000, 2012

Jennifer Francis, Linking Weird Weather to Rapid Warming of the Arctic

Commentary
The following two articles provide context for the discussion that this paper has engendered within the scientific community.
Carolyn Gramling, Arctic impact, Science 20 February 2015: Vol. 347 no. 6224 pp. 818-821 DOI: 10.1126/science.347.6224.818

Eli Kintisch, Into the maelstrom, Science 18 April 2014: Vol. 344 no. 6181 pp. 250-253 DOI: 10.1126/science.344.6181.250