The jet stream is a narrow, meandering current of air near the tropopause, the boundary between the troposphere and the stratosphere, typically located several kilometres above the surface and flowing from west to east at speeds that can exceed 50 metres per second in winter. Over the Atlantic–European sector, this current is maintained by the temperature contrast between the warm subtropical ocean and the cold polar region, a contrast that generates pressure gradients at upper levels of the atmosphere. Because the jet acts as a kind of atmospheric conveyor belt, guiding storm systems from the ocean toward the continent, even modest shifts in its strength or position can reshape winter weather across Britain, Scandinavia, central Europe and the Mediterranean.

Understanding how the jet responds to greenhouse warming has been one of the more contested questions in climate dynamics. One influential line of argument holds that rapid Arctic amplification—the fact that the high northern latitudes are warming several times faster than the globe as a whole—reduces the equator-to-pole temperature gradient near the surface. A weaker gradient, in this view, should sap the energy available to the jet, producing weaker winds, larger meanders and a greater tendency toward persistent blocking patterns that lock cold air over Europe for weeks at a time. Another line of argument emphasizes that the upper troposphere warms more strongly than the surface in the tropics and mid-latitudes, which increases the vertical shear of the winds and can actually strengthen the jet aloft while shifting it poleward.

The new analysis addresses this debate by looking not only at the average behaviour of the Atlantic–European jet in winter but at the full distribution of its variability, including the extreme tail. Using observational and reanalysis datasets—long records of atmospheric conditions reconstructed from weather observations and satellite data—the researchers tracked jet strength over recent decades and examined how frequently the jet reached its most intense values. The central result is a nuanced one: the typical winter jet has become stronger, consistent with the expectation that upper-level warming and changes in the temperature gradient enhance the mean flow, but the extremes of jet strength have not kept pace. In relative terms, the jet is becoming a steadier current rather than a more violent one.

This distinction between the mean and the extremes matters for how scientists and society interpret jet stream change. A stronger mean jet is generally associated with a more vigorous storm track, which can bring more frequent passages of Atlantic low-pressure systems and the mild, wet, windy weather they deliver to northwestern Europe. Extreme jet episodes, by contrast, are often linked to exceptional weather: an unusually intense jet can coincide with explosive cyclogenesis, in which storms deepen rapidly and cause damaging windstorms, while an unusually weak or displaced jet can coincide with prolonged cold spells or drought. If the mean strengthens while the extremes moderate, the net effect on hazardous winter weather may be less dramatic than either a simple strengthening or a simple weakening story would suggest.

The physical reasoning behind such a divergence can be traced to how different components of the climate system respond to warming. The mean jet strength is governed largely by broad-scale temperature gradients and by the vertical structure of tropospheric warming, both of which evolve smoothly and predictably with rising greenhouse gas concentrations. Extreme jet episodes, however, often depend on transient processes: the phasing of planetary-scale Rossby waves, the life cycles of individual baroclinic storms, and episodic coupling with the stratospheric polar vortex. Some of these transient drivers may weaken or become less variable under warming, damping the upper tail of the jet distribution even as the background flow intensifies. The study’s results are consistent with such a mechanism, in which variability about the mean contracts even as the mean itself rises.

The findings also speak to a long-running scientific controversy about Arctic influence on mid-latitude weather. Over the past decade, a vigorous debate has played out in the journals over whether sea-ice loss and Arctic warming make European winters more prone to blocking and severe cold. Proponents of this view point to episodes such as the cold winters of recent decades and to modelling experiments in which reduced sea ice favours a wavier jet. Critics counter that the observational record is short, that internal variability is large, and that model projections more often show a strengthened, poleward-shifted jet with reduced waviness in winter. By documenting a strengthening yet less extreme jet, the new work lends weight to the second camp, while underscoring that the answer may differ by season, by region and by which aspect of the jet is measured.

Methodologically, the study illustrates the value of going beyond simple averages when assessing climate change signals. Jet stream behaviour is notoriously noisy: individual winters can differ enormously, and the Atlantic–European sector in particular is influenced by modes of variability such as the North Atlantic Oscillation, which swings between phases that strengthen or weaken the westerlies on timescales from weeks to decades. Separating a forced climate signal from this internal noise requires careful statistical treatment, long datasets and, in many cases, large ensembles of climate model simulations in which many parallel realizations of the same warming scenario are run to isolate the common response. The researchers’ focus on the distribution of jet strength, rather than a single metric of average speed, allows a more complete picture of how the flow is evolving and reduces the risk of drawing conclusions from a few memorable extreme winters.

The implications extend to practical forecasting and adaptation. Winter storm risk assessment for Europe depends on assumptions about how often severe windstorms strike, and those assumptions are typically based on historical statistics. If the character of the jet is changing—stronger on average but with moderated extremes—then the historical record may be an imperfect guide to the coming decades. Insurers, infrastructure planners and emergency services all have an interest in knowing whether the tail risks of winter weather are growing or shrinking. The new results suggest that for jet-driven wind extremes, at least, the most catastrophic outcomes may not intensify as rapidly as the mean conditions do, although the researchers caution that other hazards, such as precipitation extremes associated with a moister, warmer atmosphere, continue to worsen independently of the wind field.

There remain open questions. The observational record of the upper atmosphere is only a few decades long, and reanalysis products carry uncertainties, particularly in the earlier satellite era and in the data-sparse regions of the North Atlantic. Disentangling the roles of tropical upper-tropospheric warming, Arctic amplification, stratospheric variability and ocean circulation in shaping the jet will require further modelling work. Nevertheless, the study offers a clear and somewhat reassuring refinement of the picture: the Atlantic–European winter jet is not collapsing into weakness and chaos, nor is it becoming uniformly more ferocious. Instead, it is strengthening as a background current while its wildest excursions become relatively less extreme—a reminder that climate change rarely moves every aspect of a system in the same direction, and that the most useful science is often found in the details between the averages and the extremes.

Subject of Research: Winter strengthening and reduced extremity of the Atlantic–European jet stream

Article Title: A strengthening yet less extreme Atlantic–European jet during winter

Article References: Brönnimann, S., Brugnara, Y., & Kallabis, P. (2026). A strengthening yet less extreme Atlantic–European jet during winter. Nature Geoscience. https://doi.org/10.1038/s41561-026-02069-z

Image Credits: AI Generated

DOI: 10.1038/s41561-026-02069-z

Keywords: jet stream, Atlantic–European jet, winter climate, North Atlantic Oscillation, Arctic amplification, storm track, climate variability, reanalysis, stratosphere, extreme weather, climate change, geoscience