Global Warming, Arctic Amplification and Extreme Weather

Two research teams have found evidence that enhanced Arctic warming relative to mid latitudes, so called Arctic amplification (AA) is  leading to increased and prolonged extreme weather events, like droughts and heat waves (such as in Russia, summer 2010!), heavy rain with floodings (like in Germany summer 2013) and cold spells (remember persistent chilly conditions in winter 2012713 e.g.): 1) Jennifer A. Francis and Stephen J. Vavrus, “Evidence linking Arctic amplification to extreme weather in mid-latitudes”, (GEOPHYSICAL RESEARCH LETTERS, January 2012) 2) Vladimir Petoukhov, Stefan Rahmstorf, Stefan Petri, Hans Joachim Schellhuber, „Quasiresonant amplification of planetary waves and recent Northern Hemisphere weather extremes“, PNAS, March 2013).

Arctic Amplification is a feature of global warming. It means enhanced arctic warming relative to mid latitudes due to a sort of inverse Ice-Albedo-Feedback. Decreasing bright, highly reflective sea ice is replaced by dark open water, absorbing sunlight effictively. Open water also releases a lot of moisture and latent heat into the atmosphere.
Arctic warming makes the jetstream slowing down and becoming wavier. That´s why jetstream is driven by gradient in air-temperature (leading to a corresponding gradient in air pressure) between polar regions and mid latitudes.

seasonal-anomalies-in-1000-500hpa-thicknesses

Seasonal anomalies in 1000-500 hPa thicknesses (m) north of 40°N during 2000–2010 relative to 1970–1999: (a) autumn (OND), (b) winter (JFM), (c) spring (AMJ), and (d) summer (JAS). White asterisks indicate significance with chance level p < 0.05. 1000-500 hPa thickness reveals aerial vertical thermal expansion by means of distance between 1000 hPa- and 500 hPa isobaric surface. Source: Francis and Vavrus, 2012

As a result high and low pressure systems slow down, too. Both weather systems arise from turbulences in jetstream like white water rapids in a river do. A wavy jetstream makes for cold spells via deep wave troughs (in reverse warm air via high wave crests, respectively), whereas high air moisture generally increases rainfall during warm months (often leading  to flooding) and snowfall during cold months.

Vladimir Petoukhov submitted a interesting physical explanation for jetstream more slowly and wavy: Jetstream is undulating, so called Rossby waves or planetary waves, respectively. A decreased gradient in air-temperature makes for more low frequency Rossby waves. That means a slow jetstream with slowly moving weather systems. If the frequency is low enough to match with jetstream´s natural frequency a resonance phenomenon occurs. Propagation of Rossby waves stops and the amplitude of this standing wave becomes higher and higher.

Two examples of the consequences: If in summer a wave crest (warm air, high pressure, poor in clouds and therefore sunny) continue at one place for a longer time  fair weather is turning into a heat wave.  And if the same happens to a wave trough in summer (cool air, low pressure, cloudy) simple rainy weather  is turning into severe weather with heavy rain and flooding.

Jens Christian Heuer

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