Atmospheric Waves and Instabilities

The atmosphere is a restless fluid that rings with waves, from the planetary meanders of the jet stream to the ripples behind a mountain, and grows weather systems wherever those waves tap into stored energy.

Definition

Atmospheric waves are propagating oscillations of the flow restored by forces such as gravity or the variation of the Coriolis parameter, while instabilities are conditions under which small perturbations extract energy from the mean flow and amplify.

Scope

This topic covers the major classes of atmospheric waves, including Rossby waves, gravity waves, and inertia-gravity waves, together with the barotropic and baroclinic instabilities that allow small disturbances to grow into the eddies and cyclones of the weather map.

Core questions

• What restoring forces give rise to the different kinds of atmospheric waves?
• Why do planetary Rossby waves propagate westward relative to the flow?
• How do baroclinic and barotropic instabilities generate weather systems?
• How do waves transport energy and momentum through the atmosphere?

Key theories

Rossby wave dynamics

The poleward increase of the Coriolis parameter acts as a restoring mechanism for conserved potential vorticity, producing large-scale Rossby waves that meander the jet stream and steer surface weather systems.

Baroclinic instability

In a vertically sheared flow with horizontal temperature gradients, certain wavelengths grow by converting available potential energy into kinetic energy, which Charney showed to be the dynamical origin of midlatitude cyclones.

Mechanisms

Gravity provides the restoring force for buoyancy oscillations and gravity waves, while the latitudinal change of the Coriolis parameter restores displaced columns of air and supports slow, westward-propagating Rossby waves. When the background flow has strong vertical shear and temperature gradients, or sharp horizontal shear, perturbations of favored wavelengths draw on the available potential or kinetic energy of the mean state and grow into the developing eddies and cyclones of midlatitude weather.

Clinical relevance

Wave and instability theory explains the genesis and tracks of extratropical storms, the planetary patterns that lock weather into prolonged spells, and the gravity waves whose breaking drives stratospheric circulations and aviation turbulence, all of which matter directly to forecasting.

History

Rossby's 1939 identification of large-scale planetary waves and Charney's and Eady's late-1940s theories of baroclinic instability transformed the qualitative picture of weather development into a predictive, mathematical framework that underpins modern dynamic meteorology and numerical forecasting.

Key figures

• Carl-Gustaf Rossby
• Jule Charney
• Eric Eady

Seminal works

• rossby1939
• charney1947

Frequently asked questions

What is a Rossby wave?

A Rossby wave is a large-scale, slowly westward-propagating undulation of the flow that arises because the Coriolis effect strengthens toward the poles; these waves are the meanders of the jet stream that steer high- and low-pressure systems.

Why do midlatitude storms keep forming in the same regions?

Baroclinic instability lets disturbances grow wherever there are strong temperature contrasts and vertical wind shear, conditions found along the storm tracks downstream of the major continents and oceans, so cyclones repeatedly develop there.