Rossby Waves and Jet Streams

Imagine you are flying high in the upper troposphere, tracing the path of the strong westerly winds that encircle the Earth. Unlike a neatly drawn circle, these winds do not move in straight lines along latitudes. Instead, they take a wavy path, bending and twisting like a giant cosmic river. These vast atmospheric undulations are called Rossby Waves, and they play a crucial role in shaping global weather patterns.

Why Do Rossby Waves Form?

To understand Rossby waves, we need to recall an important concept—angular momentum conservation.

1. Earth’s Rotation Effect:
   • As air moves poleward, its radial distance (r) from Earth’s axis decreases, just like a figure skater pulling their arms in to spin faster.
   • To conserve angular momentum (L = Iω = mr²ω), the air current starts taking a meandering path instead of moving straight.
2. Meandering Becomes More Pronounced Near the Poles:
   • Closer to the poles, the effect becomes more intense, making the winds curve more sharply.
   • Sometimes, the meandering path stretches deep into the low latitudes, influencing tropical weather patterns.

How Rossby Waves Impact Energy Transfer

Rossby waves are not just beautiful patterns in the atmosphere; they are vital for Earth’s energy balance.

• These waves transfer heat from low latitudes (warmer regions) to higher latitudes (colder regions), preventing extreme temperature imbalances.
• This transfer process occurs as warm air moves poleward and cold air shifts equatorward, setting the stage for weather variations.

The Index Cycle of Rossby Waves

Rossby waves do not always maintain the same shape. They go through a cycle known as the index cycle, which consists of:

1. Straight Path Phase: When the waves are weak and closer to a straight-line flow, the weather remains relatively stable.
2. Sharp Meandering Phase: When the waves become wavier and more pronounced, extreme weather events can occur.
3. Oscillatory Nature: The waves shift between these phases irregularly, making mid-latitude weather highly unpredictable.

Local Pressure Cells and Climatic Instability

Rossby waves don’t just move air horizontally; they also create vertical movements of air in the atmosphere:

• Where the wave rises, low pressure systems form, leading to cloud formation and rainfall.
• Where the wave descends, high pressure systems form, bringing clear skies and dry conditions.

These high and low-pressure systems shift from west to east, influencing the weather of mid-latitude regions like Europe, North America, and parts of Asia.

Impact on Mid-Latitude Weather (Especially British-Type Climate)

Rossby waves are the primary reason for the climatic instability in mid-latitude regions such as Western Europe (Britain).

• When the waves are gentle: The westerlies bring steady, moist, oceanic air, leading to a mild and rainy climate.
• When the waves become extreme: Cold polar air plunges southward, or warm tropical air moves northward, leading to stormy or unusually dry conditions.

This is why weather in regions like the UK is highly unpredictable—Rossby waves are constantly shifting, altering pressure patterns, and changing wind directions.

Now, Just hypothetically imagine yourself soaring at 10-14 km above the Earth’s surface, where commercial aircraft fly. As you look out, you notice something extraordinary—bands of air currents racing at astonishing speeds of 300-350 km/h. These are the jet streams, the invisible highways of the upper troposphere that shape global weather patterns.

What Are Jet Streams?

Jet streams are fast-moving, narrow belts of strong winds that blow from west to east in both hemispheres. These air currents are found at the upper limit of the troposphere, roughly between 7.5 km to 14 km above the Earth’s surface.

• They were first discovered during World War II, when fighter planes encountered unexpected high-speed tailwinds.
• Today, jet streams are crucial for air navigation and play a vital role in weather and climate.

Why Do Jet Streams Exist?

To understand their formation, we need to recall two key atmospheric forces:

1. Temperature Contrast:
   • Jet streams form at the boundaries between air masses of contrasting temperatures (e.g., warm tropical air and cold polar air).
   • The sharp temperature difference leads to strong pressure gradients, making winds accelerate.
2. Earth’s Rotation (Coriolis Effect):
   • The rotation of Earth deflects these winds, forcing them to move west to east instead of a straight north-south direction.

Properties of Jet Streams

• Super-fast winds: Speeds range from 300 to 350 km/h.
• Located in the upper troposphere.
• Follow a wavy, meandering path.
• Shift with the apparent movement of the Sun—moving northward in summer and southward in winter.

Types of Jet Streams

1. Polar Front Jet Stream

• Found between 40° to 60° N/S latitudes.
• Forms due to the collision of cold polar winds with warm westerlies.
• Westerly in direction and influences temperate cyclones.

2. Subtropical Westerly Jet Stream (STWJ)

• Develops at the northern boundary of the Hadley Cell (30° N/S).
• More regular than the polar front jet stream.
• Plays a crucial role in Indian monsoon circulation.

3. Tropical Easterly Jet Stream (TEJ)

• Develops over India and Africa due to excessive heating of the Tibetan Plateau.
• Easterly in direction (unlike most other jet streams).
• Key player in monsoon formation—strong TEJ enhances the Southwest Monsoon.

4. Polar Night Jet Stream

• Forms at around 30 km altitude in the stratosphere during winter.
• Develops due to a steep temperature gradient near the poles.
• Strongest during winters and weakens in summers.

Special Case: Somali Jet Stream

• A temporary jet stream that significantly impacts the Indian monsoon.
• Originates near Mauritius and Madagascar, flows over Kenya and Somalia, and strengthens the Southwest Monsoon.

Why Do Jet Streams Move from West to East?

The west-to-east movement of jet streams is primarily due to:

1. Earth’s Rotation (Coriolis Effect)
   • As air moves toward the poles, the Coriolis force deflects it to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.
   • This causes the winds to move west to east instead of north-south.
2. Temperature Gradient and Pressure Differences
   • Jet streams form at the boundaries of air masses (e.g., polar cold air meeting warm tropical air).
   • The sharp pressure difference creates fast-moving winds along these boundaries, which the Coriolis force steers from west to east.

Significance of Jet Streams in Weather

1. Storm Intensification:
   • When a mid-latitude cyclone aligns with a jet stream, the storm intensifies and produces heavy precipitation.
2. Indian Monsoon Influence:
   • Subtropical Westerly Jet Stream (STWJ) shifts northward in summer, allowing the Southwest Monsoon to advance.
   • Tropical Easterly Jet Stream (TEJ) strengthens the monsoon by pushing moisture-laden winds toward India.
3. Air Travel Impact:
   • Airlines use jet streams for faster eastward flights, reducing fuel consumption.
   • Conversely, flying against jet streams (westward) requires more fuel and time.

Final Thought

Jet streams are the invisible architects of global weather, shaping monsoons, storms, and climate zones. Their ever-changing patterns m