The influence of Arctic amplification on mid-latitude summer circulation

Abstract

Accelerated warming in the Arctic, as compared to the rest of the globe, might have profound impacts on mid-latitude weather. Most studies analyzing Arctic links to mid-latitude weather focused on winter, yet recent summers have seen strong reductions in sea-ice extent and snow cover, a weakened equator-to-pole thermal gradient and associated weakening of the mid-latitude circulation. We review the scientific evidence behind three leading hypotheses on the influence of Arctic changes on mid-latitude summer weather: Weakened storm tracks, shifted jet streams, and amplified quasi-stationary waves. We show that interactions between Arctic teleconnections and other remote and regional feedback processes could lead to more persistent hot-dry extremes in the mid-latitudes. The exact nature of these non-linear interactions is not well quantified but they provide potential high-impact risks for society.

Fig. 1

Summer trends in surface temperature over 1980–2011. a 95th, b 50th, and c 5th quantile of the HadGHCND gridded daily dataset; differences in the trends of different quantiles, plotted in d–f, reflect changes in the width of the distribution. Over most mid-latitude regions, especially over Eurasia, the width of the distribution has broadened and thus variability has increased. (Figure created using R statistical software)

Fig. 2

Schematic figure illustrating the main seasonal differences in upper tropospheric circulation between winter (January) and summer (July). Panels a and b show 250-hPa wind speed (green-to-blue shading) illustrating the jet streams with black arrow lines that follow the zone of maximum wind speed. The wintertime stratospheric polar vortex is outlined with the thick green line following the 30-hPa maximum wind speed. Panels c and d show the 250 hPa meridional wind speed (dark gray-to-dark red shading) depicting the stationary wave features associated with the jet streams. White arrows are added to illustrate wind direction. Basic differences in the summer circulation features, as compared to winter, include shorter stationary waves, more northerly subtropical jet, absence of stratospheric polar vortex and an Arctic front jet forming double jets. Data are 1970–2000 climatology of NCEP Reanalysis (downloadable: https://www.esrl.noaa.gov/psd/data/gridded/data.ncep.reanalysis.html). (Figure created using Panoply and Apple’s Keynote software)

Fig. 3

Schematic representation of proposed dynamical mechanisms in summer. a Weakening of storm tracks, b latitudinal-shift in jet positions, and c amplification of quasi-stationary waves (Figure created using Apple’s Keynote software)

Fig. 4

Observed and projected changes in the mid-latitude Northern Hemisphere summer storm tracks and westerlies. The percentage change in summer storm tracks (vertical axis) and westerlies (horizontal axis) in future (2081–2100, under scenario RCP8.5) relative to 1981–2000 for individual CMIP5 climate models is shown, and their linear fit (solid black line). Observed changes based on ERA-Interim data are given for the 1979–2013 period. Taken from (Coumou et al..)

Fig. 5

Enhanced circumglobal wave train embedded in the summer jet. Linear trends from 1979 to 2010 in the July 250 hPa stream function in the short-wave regime (blue-red shading) computed with the long wavenumbers (1–4) removed. The change in the short waves is embedded in the climatological July-mean 250-hPa wind speed depicting the jet stream (black contour lines). Adapted from (Wang et al..)