3.3 Jet streams and Rossby waves

Jet streams and Rossby waves

Jet streams and Rossby waves are the primary drivers of mid-latitude weather. Jet streams are fast-moving rivers of air high in the atmosphere that steer storms and separate warm and cold air masses. Rossby waves are large-scale undulations in those winds that create the familiar pattern of alternating warm and cold spells across the mid-latitudes. Together, they explain why weather at temperate latitudes is so changeable and why certain extreme events persist for days or weeks.

Jet streams and their formation

Characteristics and structure of jet streams

Jet streams are narrow bands of strong wind found in the upper troposphere and lower stratosphere, roughly 9–16 km above the surface. Wind speeds in the jet core typically reach 120–250 km/h, though they can exceed 400 km/h in extreme cases.

Each hemisphere has two main jet streams:

• Polar jet stream: Located roughly between 50°–60° latitude. This is the one that most directly affects day-to-day weather in the mid-latitudes.
• Subtropical jet stream: Found near 30° latitude, associated with the poleward edge of the Hadley cell.

Both jet streams are stronger in winter, when the temperature contrast between the equator and the poles is greatest.

Formation and driving factors

The fundamental driver of jet streams is the temperature gradient between warm equatorial regions and cold polar regions. Here's how that gradient translates into fast upper-level winds:

1. Warm tropical air creates a thicker atmosphere (air expands when heated), so pressure surfaces slope downward toward the poles at upper levels.
2. This pressure gradient at altitude pushes air poleward.
3. The Coriolis effect deflects that poleward-moving air to the right (in the Northern Hemisphere), turning it into a westerly flow.
4. The result is a thermal wind balance: the stronger the temperature gradient at the surface, the faster the winds aloft.

Several factors modify jet stream strength and position:

• Topography: Major mountain ranges like the Rockies and Himalayas deflect the jet and help anchor wave patterns.
• Land-sea temperature contrasts: Coastlines create sharp thermal boundaries that intensify the jet locally.
• Tropical convection and teleconnections: Patterns like El Niño and La Niña shift tropical heating, which in turn repositions the subtropical jet and alters wave patterns globally.

Role in atmospheric circulation

Jet streams serve as boundaries between contrasting air masses. The polar jet, in particular, separates cold polar air to the north from warmer subtropical air to the south. When the jet dips southward, cold air plunges into lower latitudes; when it bulges northward, warm air pushes poleward.

Beyond acting as air mass boundaries, jet streams:

• Steer mid-latitude cyclones and anticyclones along their path, determining storm tracks across continents
• Create upper-level divergence and convergence that drives surface low and high pressure development
• Transport heat, moisture, and momentum across latitudes, redistributing energy from the tropics toward the poles
• Set the stage for Rossby waves, the large-scale undulations covered in the next section

Rossby waves: Characteristics and types

Fundamental properties of Rossby waves

Rossby waves are large-scale meanders in the upper-level westerly winds. Their wavelengths span thousands of kilometers, so a single wave might stretch from the Pacific coast of North America to the Atlantic.

The key physics behind Rossby waves is the conservation of absolute vorticity. As air moves poleward, the planetary vorticity (from Earth's rotation) increases, so the air's relative spin must decrease, curving it back equatorward. The reverse happens when air moves equatorward. This back-and-forth creates the wave pattern.

A distinctive feature of Rossby waves is their propagation behavior:

• Longer waves propagate westward relative to the mean flow (and can become nearly stationary or even retrograde against the westerlies).
• Shorter waves propagate eastward relative to the mean flow, carried along by the jet.

This dispersion means that different wavelengths travel at different speeds, which matters for how weather patterns evolve and persist.

Types and generation mechanisms

There are two broad categories:

• Free Rossby waves arise from the internal dynamics of the atmosphere. Any disturbance to the westerly flow can trigger them through conservation of potential vorticity.
• Forced Rossby waves are generated and maintained by external features. The most important forcing mechanisms are:
  • Topography: The Rocky Mountains and the Himalayas physically deflect the jet stream, creating semi-permanent troughs and ridges downstream.
  • Land-sea thermal contrasts: Large temperature differences between continents and oceans (especially in winter) force persistent wave patterns.
  • Tropical convection: Patterns like the Madden-Julian Oscillation (MJO) generate wave trains that propagate into the mid-latitudes.
  • Baroclinic instability: Strong horizontal temperature gradients in the mid-latitudes cause growing disturbances that develop into Rossby waves and cyclones.

Factors influencing wave characteristics

Several factors control how Rossby waves behave:

• Beta effect: The change in the Coriolis parameter with latitude ($\beta = \frac{df}{dy}$) is what allows Rossby waves to exist. Without it, there would be no restoring mechanism.
• Background zonal flow: Stronger westerlies increase the eastward advection of waves, affecting their phase speed and whether they remain stationary.
• Atmospheric stability and vertical wind shear: These determine the preferred wavelength and vertical structure of the waves.
• Stratospheric conditions: Waves can propagate upward into the stratosphere and, under certain conditions, be reflected back down, influencing surface weather.
• Seasonal temperature gradients: Stronger winter gradients produce more intense wave activity.

Rossby waves and jet stream meandering

Interaction between Rossby waves and jet streams

Rossby waves and jet streams are not separate phenomena; they're deeply intertwined. The meanders you see in the polar jet stream are Rossby waves. When the jet dips southward, that's a trough. When it bulges northward, that's a ridge.

The wavelength of the Rossby waves determines the spatial scale of these meanders. Typically, 4–6 major waves encircle the hemisphere at any given time. When wave amplitude is small, the jet flows in a relatively zonal (west-to-east) pattern and weather changes quickly. When amplitude grows, the jet develops deep troughs and tall ridges, and weather patterns become more persistent.

The phase speed of the waves relative to the jet determines how fast weather systems move. If waves slow down or become stationary, the same weather can park over a region for an extended period.

Mechanisms of wave-jet stream coupling

The interaction between waves and the jet involves several reinforcing processes:

1. Potential vorticity conservation drives the basic wave motion, as described above.
2. Baroclinic instability feeds energy from the temperature gradient into growing waves, which deform the jet stream.
3. As waves amplify, they can undergo wave breaking, where the wave crest overturns and folds over. This is analogous to an ocean wave breaking on a beach, but at a continental scale.
4. Wave breaking causes irreversible mixing of warm and cold air masses, which can cut off pools of cold or warm air (forming cut-off lows or blocking highs).
5. The momentum fluxes carried by the waves feed back on the jet, accelerating it in some locations and decelerating it in others.
6. Thermal advection by the waves (warm air pushed poleward in ridges, cold air pulled equatorward in troughs) modifies the very temperature gradients that support the jet.

Atmospheric blocking and extreme events

When Rossby waves amplify strongly and then stall, the result is an atmospheric block: a persistent, nearly stationary pattern that diverts the normal westerly flow.

Two common blocking configurations:

Blocks are responsible for many extreme weather events. The region under the block gets stuck with the same conditions, whether that's a heat wave, a cold spell, drought, or persistent flooding from repeated storms following the same track. Blocks can last from several days to several weeks.

Jet streams and Rossby waves: Impact on weather

Influence on mid-latitude weather systems

Jet streams act as steering currents for surface weather systems. Cyclones and anticyclones generally track along the jet, so the jet's position determines where storms go.

Within the jet, localized wind maxima called jet streaks are especially important. The entrance and exit regions of jet streaks create patterns of upper-level divergence and convergence that can intensify or suppress surface storms. A developing cyclone positioned in the right-exit region of a jet streak, for example, gets a boost from upper-level divergence that enhances rising motion.

Rossby wave patterns create alternating regions of favorable and unfavorable conditions for storm development. When waves amplify into blocking patterns, weather becomes stationary. When the jet is strong and zonal, storms move through quickly and no single weather pattern dominates for long.

Strong vertical wind shear near the jet stream also supports severe convective weather. Supercell thunderstorms, which produce tornadoes and large hail, thrive in environments with strong shear, which is why severe weather outbreaks in the U.S. often occur near the jet stream's position.

Regional climate and extreme events

The position and behavior of the jet stream and its embedded Rossby waves have direct consequences for regional climate:

• A persistently southward-displaced jet brings cool, wet conditions to regions that would normally be warmer.
• Amplified wave patterns can lock in prolonged heat waves (under ridges) or extended cold spells (under troughs).
• Storm tracks shift with the jet, so changes in jet position alter where and how often precipitation falls.

Arctic amplification is a current area of active research. The Arctic is warming roughly 2–4 times faster than the global average, which reduces the equator-to-pole temperature gradient. Some research suggests this weakens the polar jet stream and favors more amplified, slower-moving Rossby waves, potentially increasing the frequency of persistent extreme weather events. This connection remains an area of scientific debate, but it's a major topic in modern climatology.

Global teleconnections and long-term variability

Rossby waves don't just affect local weather; they connect distant regions through teleconnections. A change in tropical convection can launch a Rossby wave train that alters weather thousands of kilometers away.

Key teleconnection patterns linked to Rossby waves and jet streams:

• Pacific-North American (PNA) pattern: A Rossby wave train extending from the tropical Pacific across North America, strongly influenced by ENSO.
• North Atlantic Oscillation (NAO): Variations in the jet stream and pressure patterns over the North Atlantic that affect European and eastern North American weather.
• El Niño-Southern Oscillation (ENSO): Shifts in tropical Pacific sea surface temperatures that reorganize global Rossby wave patterns, altering jet stream positions worldwide.

Rossby waves also connect the troposphere and stratosphere. During stratospheric sudden warming events, amplified Rossby waves propagate upward and break in the stratosphere, rapidly warming it by tens of degrees. This disruption then propagates back downward, often weakening the polar vortex and shifting the jet stream equatorward, bringing cold outbreaks to mid-latitudes weeks later.

The Quasi-Biennial Oscillation (QBO), an alternating pattern of easterly and westerly winds in the tropical stratosphere with a roughly 28-month cycle, also modulates how Rossby waves propagate vertically, influencing jet stream behavior and surface weather patterns. Long-term changes in Rossby wave characteristics affect monsoon systems across South and West Africa and the Indian subcontinent as well.