Arctic sea-ice decrease may suppress U.S. tornado activity in summer

The observed losses in  Arctic sea ice during the past decades have been linked to the relaxation of poleward thickness gradients (thus weakened zonal winds) and a slower eastward progression of Rossby waves in the upper-level, which help promote prolonged extreme weather conditions, such as heat waves, within the mid-latitudes (e.g., Francis & Vavrus, 2012). However, the background zonal wind and vertical wind shear are important ingredients for tornado formation. An article published in npj Climate and Atmospheric Science (Trapp & Hoogewind, 2018) further explored this idea to report a robust positive correlation between U.S. tornado activity and Arctic sea-ice extent (SIE) in boreal summer during 1990 – 2015.  This statistical relationship is supported by the presence of anomalous regional circulation and storm track that are unfavorable for tornado-bearing thunderstorm formation when SIE is low.

Figure 3 from Trapp & Hoogewind (2018): Probability of a tornado day within the U.S. as a function of calendar day, for tornadoes rated a F/EF3 and greater, and bF/EF1 and greater. The green curve represents a 26-y mean, with the lighter-green shading indicating the daily 95% confidence intervals determined from 10,000 bootstrapped resamples. The blue (red) curve shows the 10-y mean from 1990–1999 (2006–2015). A 15-day Gaussian filter was applied to the raw probabilities over each period. Note that the scales of the y-axis in (a) and (b) are different.

Trapp, R. J., & Hoogewind, K. A. (2018). Exploring a possible connection between US tornado activity and Arctic sea ice. npj Climate and Atmospheric Science1(1), 14.

Francis, J. A., & Vavrus, S. J. (2012). Evidence linking Arctic amplification to extreme weather in mid‐latitudes. Geophysical Research Letters, 39(6).

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