Quasi-Stationary Rossby Waves

Quasi-stationary Rossby waves are large-scale atmospheric waves in the mid-latitudes that remain nearly fixed relative to geographical features for extended periods. They are a form of planetary wave driven by the rotation of the Earth and the variation of the Coriolis effect with latitude. These waves significantly influence weather patterns, climate variability and the persistence of extreme events such as heatwaves, floods and droughts.

Background and Origin

Rossby waves, also called planetary waves, were first described by Carl-Gustaf Rossby in the 1930s. They are a fundamental feature of atmospheric and oceanic dynamics, arising from the conservation of potential vorticity on a rotating sphere.
Quasi-stationary Rossby waves are a special subset that appear almost “locked” in place due to interactions with topography, land–sea contrasts, or resonance conditions in the jet stream. While ordinary Rossby waves propagate westward relative to the mean flow, quasi-stationary ones exhibit very slow or negligible phase speed, meaning they persist for days or even weeks over the same regions.

Characteristics

Key features of quasi-stationary Rossby waves include:

• Scale: Wavelengths of thousands of kilometres, often spanning entire continents.
• Speed: Very low phase velocity compared to transient Rossby waves.
• Structure: Alternating ridges (high-pressure areas) and troughs (low-pressure areas) aligned along the mid-latitude westerlies.
• Persistence: Their stationary nature leads to prolonged weather conditions over affected regions.

They are typically associated with wave numbers 5 to 8 in the mid-latitude circulation, meaning there are about 5 to 8 wave crests encircling the globe.

Forcing Mechanisms

Quasi-stationary Rossby waves are sustained and amplified by several mechanisms:

• Topographic forcing: Large mountain ranges such as the Himalayas, Rockies and Andes deflect atmospheric flow, anchoring wave patterns.
• Land–sea thermal contrasts: Differential heating between continents and oceans enhances stationary wave activity.
• Jet stream dynamics: Resonant amplification of waves can occur when atmospheric flow speeds align with the natural frequency of Rossby waves.
• Tropical-extratropical interactions: Convective activity in the tropics, such as monsoons or El Niño events, can excite stationary wave patterns extending into higher latitudes.

Role in Weather and Climate

Quasi-stationary Rossby waves exert strong control over mid-latitude weather systems:

• Persistent weather extremes: Their stationary ridges and troughs can lock regions into prolonged heatwaves, cold spells, or wet and dry conditions.
• Heatwaves: Amplified ridges trap warm air masses, leading to extreme heat events such as those in Europe in 2003 and 2019.
• Floods and droughts: Stationary troughs can produce extended rainfall and flooding, while ridges may block storm systems, causing drought.
• Teleconnections: They link weather patterns across continents, such as connections between tropical Pacific variability and Eurasian climate.

Because of their persistence, quasi-stationary Rossby waves are often associated with high-impact weather anomalies.

Interaction with Climate Change

Recent research has suggested possible links between climate change and changes in quasi-stationary Rossby waves:

• Arctic amplification: Faster warming in the Arctic relative to the tropics may reduce the equator-to-pole temperature gradient, slowing jet stream flow and favouring stationary wave patterns.
• Wave resonance: Some studies indicate an increased likelihood of wave resonance conditions under global warming, enhancing the risk of persistent extremes.
• Feedback loops: Prolonged extremes, such as heatwaves, can reinforce atmospheric circulation anomalies, further stabilising Rossby wave patterns.

Although scientific debate continues, the potential role of quasi-stationary Rossby waves in amplifying climate extremes is increasingly recognised.

Significance

Quasi-stationary Rossby waves represent a key link between atmospheric dynamics, geography and climate variability. Their ability to “lock in” weather patterns makes them central to understanding persistent climate extremes and improving long-range forecasting. As global warming intensifies, their study has become critical in assessing the risks of recurring and long-duration heatwaves, droughts and floods, which have profound societal and ecological impacts.
By bridging the scales between planetary circulation and local weather events, quasi-stationary Rossby waves highlight the interconnectedness of atmospheric processes and the growing challenges of predicting and mitigating climate-related hazards.