Sudden Stratospheric Warming (SSW)

A meteorologist has recently warned of a possible Sudden Stratospheric Warming (SSW) event, which may result in below-average temperatures across parts of the United States later this month. Such events are known to trigger severe winter conditions in mid-latitude regions.

What is Sudden Stratospheric Warming?

Definition

Sudden Stratospheric Warming refers to a rapid and abnormal increase in temperatures in the stratosphere, especially over the polar regions. This warming leads to a weakening or distortion of the polar vortex, a large mass of cold air usually confined near the poles.

Mechanism of Sudden Stratospheric Warming

Role of Rossby Waves

The process begins when large-scale atmospheric waves, called Rossby waves, are pushed upward from the lower atmosphere into the stratosphere.

Disruption of the Polar Vortex

When these Rossby waves become strong enough, they break over the polar vortex, similar to ocean waves breaking on a shore. This causes:

• Weakening of the westerly winds circulating around the polar vortex

• In extreme cases, a reversal of wind direction from westerly to easterly

As a result, the stratosphere warms rapidly.

Movement of Cold Air

The weakening of the polar vortex can lead to its:

• Displacement away from the pole, or

• Splitting into multiple vortices

This allows cold Arctic air to spill southward into the mid-latitudes, leading to cold waves.

Polar Vortex: Key Features

Definition

The polar vortex is a large region of low pressure and extremely cold air that surrounds both the North and South Poles.

Vertical Extent

The polar vortex extends from the tropopause (the boundary between the troposphere and stratosphere) through the stratosphere and into the mesosphere, reaching altitudes of more than 50 km.

Seasonal Behaviour

The polar vortex:

• Exists throughout the year

• Weakens during summer

• Strengthens during winter

During winter, particularly in the Northern Hemisphere, it often expands and influences weather patterns.

Role of Jet Streams

Jet Stream Interaction

The polar vortex interacts with the jet stream, which consists of narrow bands of strong winds flowing at about 30,000 feet (9,100 metres) above the Earth’s surface.

This interaction frequently causes southward movement of cold Arctic air, resulting in extreme winter weather.

Global Impact of SSW Events

Affected Regions

Although commonly associated with the United States, Sudden Stratospheric Warming events also affect:

• Europe

• Asia

These regions may experience cold surges, snowfall, and prolonged winter conditions following an SSW event.