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a core project of
2018WCRPspon col July2018 01 1


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The aim of the workshop was to assess the state of knowledge of Northern Hemisphere polar jet stream changes and mechanisms, focusing on mid-high latitude linkages and occurrence of extreme weather events. Such events are generalized by the recent Warm Arctic-Cold Continents concept, increased meridional flow, and a slowing of the jet occurring in three of the last four previous winters, with a rough tendency for a negative Arctic Oscillation pattern. Of interest is the potential external forcing of hemispheric wave patterns by additional heat provided to the atmosphere and change in geopotential height thickness due to loss of sea ice and late spring-early summer snow cover. Further examples are listed in the Rationale section below, but of keen interest is the potential for Arctic forcing of mid-latitude extreme events such as cold and snowy conditions and heat waves, which have recently received great attention in the popular press. These ideas are controversial, although there are ten recent science articles in support of such linkages. Also such events may be geographically linked, especially occurring in eastern North America, northern Europe and far eastern Asia. Are the mechanisms similar or different in these regions? Of key interest is the interaction of Arctic forcing with the more chaotic flow at midlatitudes, so no two events are exactly similar.

NOAAOver the last decade a tendency has developed for a more meridional Northern Hemisphere polar jet-stream pattern than previously, which has led to unusual regional climate anomalies with severe socio-economic impacts across the Northern Hemisphere. These include in 2010 record floods in Pakistan, intense heat waves and forest fires in Russia, in 2012 record surface melting of the Greenland Ice Sheet and the wettest summer ever recorded in England (which followed a record wet summer there in 2007) as well as further heatwaves in the US and Russia, and in 2010 and 2011 some enigmatically cold winter spells on both sides of the Atlantic and in eastern Asia (e.g. December 2010 was the coldest December since 1890 in the Central England Temperature record). While these regional climate anomalies are directly attributable to changes in the jet stream, the ultimate drivers of these changes remain insufficiently well understood, with an urgent need to properly quantify them in global and regional climate models to improve predictive capability. As a result of the polar amplification of global warming and declining Arctic sea-ice (which reached a record low minimum seasonal extent in summer 2012 following a string of previous unusually low years since 2007), more heat is released from the polar ocean to the overlying atmosphere, which reduces the meridional surface temperature gradient and tends to weaken the polar vortex and mid-latitude westerly winds, with resulting jet-stream changes as noted above.

Although this hypothesis appears to go some way towards explaining the intriguing changes in the Northern Hemisphere polar jet stream and Arctic Oscillation/Arctic Dipole patterns since the mid-2000s (e.g. Francis & Vavrus 2012 Geophys. Res. Lett., Overland et al. 2012 Geophys. Res. Lett.), Arctic sea ice is not the only driver of changes in the jet stream. Contemporaneous changes in solar output, snow cover, the Pacific Ocean's El Nino - Southern Oscillation cycle, the state of the Atlantic Ocean, and especially mid-latitude internal variability are all known to have additional impacts on the northern hemisphere polar jet stream and thereby impact regional climates. Fluctuations in these additional factors are superimposed on top of the recent Arctic sea ice and snow cover decline in the historical record and potentially future model projections, and these varying drivers need to be unravelled for a more complete picture. Further tests based on both observations and modelling studies are clearly needed to make a leap forward in the understanding of the Northern Hemisphere polar jet stream/midlatitude linkages and their drivers.