During winter and spring 2014, waves of unusually cold temperatures hit northeastern regions of North America, noticeably effecting the US economy and hampering growth in the first quarter of 2014 (see CNS news link on a statement by Fed. Chairman Janet Yellen on this topic). A number of research studies proposed mechanisms by which changes in the jet stream strength and location attributed to polar amplification would enhance temperature variability at the surface (Liu et al., 2012; Francis and Vavrus, 2012). Polar amplification is the preferential increase of temperature in high latitudes under global warming, and is found in GCM simulations to be particularly strong in the Northern Hemisphere over the Arctic regions under global warming.
An increase in the meandering of the jet stream was thought to enhance the frequency of “blocking” events over midlatitude regions. These “blocking” events are characterized by unusually cold (or warm) air-masses and weather patterns geographically locked-in over a region for many days, and are often associated with extreme weather patterns (e.g., drought in summer, etc.). This idea was relayed to the public by various media outlets (see weather.con link). But changes in the frequency or intensity of these blocking events have not been observed to increase over the past decades, at least from current observational datasets (Barnes et al., 2014). Instead, Schneider et al. (2014) have recently suggested that cold air outbreaks should become less frequent as climate warms. Schneider et al. (2014) argues that cold air outbreak of the same intensity as in today’s climate will occur less frequently with warmer temperatures at the poles. Their conclusion was that the decreased gradient as a result of polar amplification lessens the chance of outbreaks than if temperature were to warm uniformly across the globe. Other factors could alter this prediction however, such as changes in the storminess of the atmosphere with global warming. Yet the GCM simulations in Schneider et al. (2014) tend to confirm the preponderance of the polar amplification effect and the weakening of cold air outbreaks during winter.
Despite finding opposite conclusions, Schneider et al. (2014) and Francis and Vavrus (2012) both show that global warming will induce significant increases in the seasonal mean and day-to-day variability (e.g., temperature swings) of winters in the midlatitude regions. This could negatively impact society there, straining the economy and infrastructures.
Barnes, E. A., E. Dunn-Sigouin, G. Masato, and T. Woollings, 2014: Exploring recent trends in Northern Hemisphere blocking. Geophysical Research Letters, 41, 1–7. Francis, J. A., and S. J. Vavrus, 2012: Evidence linking Arctic amplification to extreme weather in mid- latitudes. Geophysical Research Letters, 39, L06 801.
Liu, J., J. A. Curry, H. Wang, M. Song, and R. M. Horton, 2012: Impact of declining Arctic sea ice on winter snowfall. Procuration of the National Academy of Sciences, 109, 4074–4079. Schneider T., T. Bischoff and H. Plotka: Physics of changes in synoptic midlatitude temperature variability. Submitted to Journal of Climate, In review.
weather.com link: http://www.weather.com/news/science/environment/arctic-blast-linked-glob… 20140106
CNS link: http://cnsnews.com/news/article/terence-p-jeffrey/yellen-unusually-cold-… cause-pause-economy