Weakening storm tracks

Theoretical, observational and modeling evidence supports the hypothesis that summer storm tracks weaken with enhanced Arctic warming26,71,72. The theoretical basis underlying AA and resultant weakening of the mid-latitude storm track is straightforward: The thermal-wind balance relates vertical shear in the westerly flow to the magnitude of the poleward temperature gradient. In the lower troposphere, a reduction in the temperature gradient equates to a similar reduction in the shear, weakening the thermally driven jet and reducing the low-level baroclinicity63. A reduced low-level baroclinicity implies less or weaker synoptic-scale cyclogenesis and thus leads to overall weakening of the storm tracks. Note, that the thermal-wind balance does not give a direction of causality per se: The causality could be the other way around, whereby a change in mid-latitude circulation alters the poleward heat transport giving rise to more rapid warming in the Arctic73.

Empirical evidence based on multiple datasets shows that over the satellite-covered period (i.e., since 1979), the mid-latitude summer circulation has indeed weakened in conjunction with a reduction in the poleward temperature gradient in the lower troposphere. This weakening has been detected in the westerly jet (following the thermal-wind balance), the total kinetic energy of synoptic storm systems (by about 15%) and the number of strong cyclones26,71,74. Similarly, strong Arctic sea-ice melting years are characterized by a weakened circulation75. While, the satellite era is most reliable when analyzing wind field characteristics, its limited timespan compromises long-term trend analyses. Natural variability on multi-decadal time scales, either due to changes in SSTs or from internal atmospheric variability, are thus likely to have a role in the observed trends.

There is modeling evidence indicating that these observed trends are at least partly attributable to AA. CMIP5 coupled model simulations of the twentieth century show that the observed changes in the zonal-mean temperature gradient in summer (characterized by AA and enhanced high-latitude land warming) are likely attributable to anthropogenic forcing ("likely" according to IPCC lexicon)76. Idealized modeling experiments support storm track weakening when sea-ice is reduced but also indicate that sea-ice changes by itself can explain only part of the observed weakening77. Modeling studies indicate that the effects of historic sea-ice reductions can explain up to one-third of the magnitude of the observed anomalies, with an additional role for changes in SSTs72,78. Thus, other factors including natural variability likely had a role in the recently observed summer circulation changes, but a substantial share of it is likely attributable to AA.

For future high-emission scenarios, models robustly project storm track weakening, supporting the hypothesis that AA is associated with weakened summer storm tracks (see Fig. 4)26,71,79,80. The changes at the end of the century in the high-emission scenario are comparable to the observed changes over the past decades26. This suggests that either the models underestimate the future changes (models also underestimate historic changes in the Arctic itself) or that a substantial part of the observed trend is associated with multi-decadal natural variability81. For the North American sector, the amount of AA in models by the end of century is negatively correlated with the changes in jet speed and wave phase speed in summer28. Models robustly project a weakening of summer storm tracks by the end of the century (Fig. 4), but this is not the case for the upper-level jet: There is large inter-model spread with some models projecting a strengthening and some a weakening of the upper-level jet (Fig. 4)26,28.

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.26.) Full size image

Shift in jet-position

Understanding jet shifts in the Northern Hemisphere is a challenging task due to several competing processes. Theoretically, the change in jet stream position can be divided into a relatively small direct radiative-induced shift and a larger indirect SST-mediated effect. The indirect effect comes from mid-latitude dynamical feedbacks, involving enhanced irreversible mixing due to wave breaking of high-frequency transient eddies82, and can explain most of the expected jet shifts83. When considered in isolation, AA should theoretically cause a southward shift in the mid-latitude jet stream.

Idealized dry atmospheric model simulations indeed indicate a southward shifted jet coming from AA by itself84. This is also confirmed by more complex models with reductions in sea-ice imposed31,72,78,85. Despite the process linking AA to a more equatorward jet, the zonal-mean jet streams are projected to migrate poleward by about one degree by the end of the twenty-first century under a high-emission scenario27,86. Thus in the long run, i.e., at the end-of-century, the tropics likely dominate the tug-of-war, at least in models. While in winter both the Atlantic and the Pacific jets are projected to migrate poleward, in summer this shift is seen only for the North Atlantic jet28. AA may therefore exert a stronger opposing influence to the expected poleward shift of the Pacific jet in summer. Different state-of-the-art climate models employ different simplifying parameterizations (e.g., for clouds) and those can, in a complex, non-linear system, lead to very different outcomes87. The inter-model spread of the poleward shift therefore tends to be larger than the signal itself27,46,86.

Observed jet shifts in the Northern Hemisphere are generally small compared to those in the Southern Hemisphere, but still generally indicate a poleward migration88. For the Northern Hemisphere, some evidence of a poleward shift of jet streams has been identified in reanalysis products88,89 and satellite observations of clouds90,91 and is most significant in winter. This is thus largely in agreement with model projections pointing at the important role of the tropics in shifts in the jet position.

Amplification of wave trains

Limited evidence from theory, observations and some model simulations suggests that AA may amplify synoptic-scale, quasi-stationary waves embedded in the summer jet (Fig. 5). Theory of the dynamics of a dry atmosphere suggests that a lower troposphere diabatic heating source in the mid-latitudes will have a larger stationary wave response (in terms of a meridional stream function displacement) when the background baroclinicity and zonal winds are reduced, as a direct response of AA63. In a more complex atmosphere of an aquaplanet model (i.e., an Earth covered by water only), quasi-stationary synoptic-scale wave trains are enhanced when the meridional temperature gradient is reduced and the westerly winds weaken92. Thus, as the background flow becomes weaker, the same heating source in the lower troposphere can trigger a stronger stationary wave response especially for synoptic-scale waves. The increased moisture content in a warmer atmosphere, and the tendency for increased latent heat release in the tropics and over warm ocean currents in higher latitudes, can provide further heating to perturb more or stronger CGWTs.

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.95.) Full size image

Some observational evidence suggests that the quasi-stationary component of mid-latitude summer circulation has become wavier since 1979, in particular over the North American sector60,62,93,94. Figure 5 plots linear trends over 1979–2010 in the short-wave regime showing enhanced CGWTs over both the North American and Eurasian sectors. For the American sector, this is further supported by detected increases in waviness metrics60,62 (see Box 3). Using three different climate models, the summer time amplification of quasi-stationary short waves over the American sector appears to be attributable to greenhouse gas forcing95. As the CGWT is linked to the summer North Atlantic Oscillation (SNAO, Box 2)39, an AA-like circulation anomaly (i.e., negative phase of the SNAO) could modulate the CGWT and vice versa. However, the summer CGWT can also be triggered by heating sources associated with the Indian monsoon44.

The few modeling studies with historically observed low sea-ice concentration show a stationary wave-train response in summer emerging from AA78,95. Still, future model projections of mid-latitude quasi-stationary short-wave patterns are generally inconsistent with recent observations. Future CMIP5 projections under high-emission scenarios show an overall decrease in blocking both for winter and summer96. The reasons behind these divergent findings are not well understood and may result from competing effects from the tropical monsoons44, changes in land-sea thermal contrast46, model biases in representing summer time Rossby waves39 and the use of different diagnostics to quantify waviness (see Box 3).

BOX 2 Atmospheric teleconnections in summer Remote climate effects, known as atmospheric teleconnections, consist of three main types: (i) regional long-wave pattern confined in a longitudinal sector of the globe, like the Pacific-North America pattern and the North Atlantic Oscillation135; (ii) a hemisphere-wide pattern with a prominent zonal-mean component like the Arctic Oscillation or the Annular Modes70,136,137; and (iii) the trapping and focusing effects of the seasonal jet streams on low-frequency tropospheric waves39, known as the waveguide effect54. The figure below shows schematic representations of these three types of teleconnection patterns. The summer North Atlantic Oscillation (SNAO) can be regarded as the counterpart of the more robust winter NAO. The centers of action of the SNAO exhibit a more northerly location, have a smaller geographical extent and a weaker dipole pattern compared to the winter NAO. Panel a schematically shows the surface pressure patterns of the positive SNAO phase, i.e., when pressure differences are strong, together with the position of the warmer/drier and colder/wetter regions. Generally, the negative phase shows a reverse pattern. Like the winter NAO, the impact of the SNAO on climate extremes such as heavy rainfall and flooding is profound, especially for Europe. Post-2007 summers have seen increasingly robust negative SNAO associated with a persistent anticyclonic anomaly over Greenland and a cyclonic anomaly over Northwest Europe. This pattern caused rapid melting of the Greenland ice sheet and brought unusually wet summers to Northwest Europe, including the massive flooding of U.K. in summer 2012. Future projections of climate models suggest an increasingly positive SNAO in warmer climates138. The Northern Hemisphere Annular Mode (NAM) is an internally driven atmospheric mode maintained by both stationary and transient waves. The NAM is defined as the first EOF of monthly 500 hPa geopotential height fields and coincides with the definition of the Arctic Oscillation during the winter months137. Winter and summer NAM patterns present both a diverse geopotential height field, mean meridional circulation and eddy structure. The summer NAM has a smaller latitudinal extent and has a stronger link to surface air temperature over Eurasia. The positive phase of the summer NAM is associated with negative geopotential height anomalies over Greenland and the Arctic Ocean and an annual band of positive anomalies comprised between 40° and 60° N and particularly extended over Eurasia, as schematically shown in panel b. When the summer NAM is strongly positive, the storm tracks follow the Arctic front137. Anomalously positive summer NAM phases are associated with double jets favoring blocking between the polar and the subtropical jets. During a positive summer NAM phase, surface temperatures over Eurasia show a dipole pattern with warmer conditions over Europe and colder conditions over East Asia70. The energy of waves trapped in a waveguide is not dispersed as broadly as in teleconnections of type (i) and therefore it can propagate farther before being dissipated. In summer, when an efficiently trapping waveguide becomes (almost) circumglobal, then resonant interactions between free and forced waves (typically of synoptic scale, wavenumbers 6–8) might lead to wave-amplification and persistent, high-amplitude waves. Waveguide formation is favored during double-jet regimes and thus linked to strongly positive NAM phases. Panel c shows a schematic representation of a wave-resonance event with an amplified wave 7. The orange and green areas represent regions of positive and negative upper-level meridional winds. Such events are associated with alternating hot-dry and cold-wet conditions, following the ridges and troughs. Such a situation is prone to blocking weather systems and deepened troughs, and their relative longevity is key to making severe weather extremes19. Wave-resonance periods have therefore been linked to both persistent heatwaves and severe flooding events57,64. Finally, blocking itself is also distinctly different in summer compared to winter. Upstream latent heat release has been identified as an important contributor to persistent blocking and this mechanism is especially important in summer139. Figure Box 2: Panel a shows a schematic of the positive SNAO indicating the anomalously low (blue) and high (red) sea level pressure regions together with cold/wet and dry/warm regions. Panel b shows a schematic of the 500 hPa geopotential height configuration during the positive phase of the NAM. Rainy clouds mark the position of the storm tracks during a strongly positive NAM phase. Panel c shows a schematic of an amplified wave 7. Green and orange regions show the position of pronounced northward and southward wind anomalies. (Figure created using Python, GIMP, Powerpoint and Inkscape software)