The jet stream is a fast-moving air current (150 to 320 km/h, with exceptions) circulating at high altitude (8 to 12 kilometers) around the Northern Hemisphere. It is a true boundary created by the thermal contrast between cold polar air and warm subtropical air. This flow is a powerful engine of the climate system. Its position and strength directly influence the trajectory of depressions and surface temperature distribution. However, since the early 21st century, scientists have observed increasingly erratic behavior.
It does not follow a perfectly zonal (parallel to the equator) straight line. It forms meanders (waves) called Rossby waves (planetary waves) that snake southward (troughs) and northward (ridges).
With fewer continental landmasses to disrupt its flow, the Southern Hemisphere jet stream is often stronger, more regular, and more linear than its northern counterpart. It swirls around Antarctica and helps isolate the frozen continent.
The primary cause of this disruption is the rapid amplification of climate warming in the Arctic. This region is warming nearly four times faster than the global average, a phenomenon climatologists call Arctic amplification. By reducing the thermal contrast between the North Pole and mid-latitudes, the very engine of the jet stream is weakened. Less powerful, it becomes more sinuous, and its meanders tend to stagnate.
Far from being a single wave, the jet stream resembles an atmospheric channel where multiple undulations coexist and evolve independently. These meanders can occasionally synchronize their phases and combine with other Rossby waves. The speed of the jet sometimes determines the appearance of quasi-stationary resonances, freezing these wave patterns. It is this dynamic synchronization that transforms a normal fluctuation into a lasting weather event.
This slowdown has a major consequence: weather systems (high-pressure or low-pressure) remain blocked over the same regions for longer. This explains the persistence of certain extreme events, such as the historic heatwave in Europe in 2019 or the devastating floods in Germany in 2021. As climatologist Michael E. Mann (b. 1965) explains, these atmospheric blocking situations are reinforced by a resonance phenomenon of planetary waves: the jet stream's meanders align, slow their propagation, and lead to an abnormal persistence of weather patterns.
The influence of a disrupted jet stream is not limited to a few heatwaves or intense rainfalls. It manifests as a cascade of effects across the entire Northern Hemisphere.
| Event | Year | Region | Jet Stream Mechanism Involved |
|---|---|---|---|
| Russian heatwave and fires | 2010 | Western Russia | Blocking of a high-pressure system by a persistent meander (stationary Rossby waves) |
| Floods in Pakistan | 2010 / 2022 | South Asia | Interaction between an abnormally sinuous jet and the monsoon |
| Drought and fires in California | 2012-2015 | Western United States | Persistent anticyclonic ridge ("Ridiculously Resilient Ridge") blocking precipitation |
| Floods in Germany and Belgium | 2021 | Western Europe | Omega block trapping rainy depressions for several days |
| "Texas Freeze" cold wave | 2021 | Southern United States | Deep plunge of the polar vortex due to jet stream weakening |
| Heat dome in Canada | 2021 | British Columbia | Omega block creating extreme adiabatic compression (49.6°C in Lytton) |
| Historic drought in Europe | 2022 | Western and Central Europe | Subtropical high-pressure system moving abnormally northward with weakened jet |
| Multiple heatwaves in Europe | 2023 | Mediterranean Basin | Successive heat waves linked to amplified jet stream meanders |
| Storm Boris | 2024 | Central Europe | Omega block trapping a depression with torrential rainfall |
| Cold wave in North America | 2024 | United States and Canada | Polar vortex descent to subtropical latitudes |
Source: NOAA Climate.gov, World Weather Attribution, and World Meteorological Organization.
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