The Jet Stream: When the Atmospheric River Overflows Its Banks

The Great Atmospheric River

The jet stream is a fast air current (150 to 320 km/h outside exceptions) circulating at high altitude (between 8 and 12 kilometers) around the Northern Hemisphere. It is a veritable frontier generated by the thermal contrast between cold polar air and warm subtropical air. This flow is a powerful engine of the climate machine. Its position and strength directly influence the trajectory of low-pressure systems and the distribution of surface temperatures. Since the beginning of the 21st century, several extreme weather events have been associated with unusual jet stream behavior, which has sparked a lively scientific debate about the evolution of its behavior.

The jet stream in the Northern Hemisphere meanders naturally

It does not follow a perfectly zonal straight line (parallel to the equator). It forms meanders (undulations) called Rossby waves (planetary waves) that snake southward (troughs) and northward (ridges). This sinuosity has always existed: the scientific question under debate concerns a possible increase in its amplitude with global warming.

The jet stream in the Southern Hemisphere

Because there are fewer continental landmasses to disturb its flow, the Southern Hemisphere jet stream is often more powerful, more regular and straighter than its northern counterpart. It swirls around Antarctica and helps isolate the frozen continent.

A link with global warming still debated

The Arctic amplification hypothesis

From 2012 onwards, climatologists Jennifer Francis and Stephen Vavrus proposed a mechanism that has become influential: Arctic amplification (faster warming of the Arctic compared to the rest of the planet, with the Arctic warming nearly four times faster than the global average) would reduce the thermal contrast between the North Pole and mid-latitudes. The very engine of the jet stream would be weakened, making it more sinuous, slower and more likely to become blocked over a region for several days or weeks.

Recent studies that qualify this mechanism

This hypothesis, long presented as settled in the media, does not achieve consensus among researchers. A study from the University of Exeter (2023, Blackport and Screen) found no long-term trend towards a more sinuous jet despite the observed Arctic amplification. An in-depth modeling study published in Weather and Climate Dynamics (2025) concludes that future Arctic sea ice loss would have only a minimal influence on the morphology of the North Atlantic jet stream. A Dartmouth University study (2025) on North American winter storms shows that the recent sinuosity of the jet remains comparable to that observed before Arctic amplification, suggesting that other mechanisms (a warmer atmosphere retaining more moisture, for example) would better explain the intensification of storms.

What remains solid despite the debate

This debate concerns the cause of a possible long-term trend, not the existence of the blockings themselves. Whether or not the jet becomes structurally more sinuous due to warming, persistent atmospheric blockings remain a well-documented meteorological phenomenon, whose consequences on extreme events are, for their part, largely established.

How do atmospheric blockings form?

A complex and synchronized wave system

Far from being a single wave, the jet stream resembles an atmospheric channel in which several undulations coexist, evolving independently. These meanders can occasionally synchronize their phases and combine with other Rossby waves (large-scale atmospheric waves linked to the Earth's rotation). The jet's speed sometimes determines the appearance of quasi-stationary resonances, freezing these undulatory configurations. It is this dynamic synchronization that transforms a normal fluctuation into a lasting weather event.

Weather systems that remain blocked

This slowdown has a major consequence: weather systems (anticyclones or low-pressure systems) remain blocked longer over the same regions. This explains the persistence of certain extreme phenomena, such as the historic heatwave in Europe in 2019 or the devastating floods in Germany in 2021. Climatologist Michael E. Mann (born 1965) proposed that these atmospheric blocking situations be reinforced by a phenomenon of planetary wave resonance: the jet stream's meanders enter into phase, slow their propagation and lead to an abnormal persistence of weather regimes.

The role of the stratospheric polar vortex

Another actor, distinct from the jet stream but interacting with it, is the stratospheric polar vortex. When it is strong and compact, it keeps the coldest air around the Arctic. When it weakens or splits in two — a phenomenon called sudden stratospheric warming — the westerly winds at altitude slow down, or even reverse, and this disturbance can propagate downward and reorganize the jet stream, favoring descents of Arctic air towards temperate latitudes. It is this mechanism that was closely monitored by meteorological services during the winter of 2025-2026, marked by an early stratospheric warming as early as November 2025.

Visible consequences and extreme events

The influence of atmospheric blockings is not limited to a few heatwaves or a few episodes of intense rainfall. It manifests as a cascade of effects across the entire Northern Hemisphere.

How do forecasters monitor these phenomena?

Indices and seasonal models

Meteorological services continuously monitor the state of the stratospheric polar vortex (strength of westerly winds at altitude), the extent of Arctic sea ice and climate indices such as El Niño or La Niña, which can favor the propagation of atmospheric waves towards the stratosphere and help destabilize the vortex. Seasonal models from major centers such as the ECMWF make it possible to anticipate, several weeks in advance, an increased risk of vortex disturbance, without however guaranteeing the occurrence or the precise location of a cold spell.

Predictability limited to a few weeks

Even with these tools, the detailed predictability of an atmospheric blocking remains limited to about two to three weeks. Signals such as sudden stratospheric warming give a probable trend (increased risk of cold or persistent heat over a region), but the exact intensity and location remain uncertain until a few days before the event.