Upper-Level Waves and Jet Streams

High in the troposphere, ribbons of fast wind and the great waves they carve are the steering currents of weather, guiding surface storms and concentrating the rising motion that makes them deepen.

Definition

Upper-level waves are the troughs and ridges of the upper-tropospheric flow, and jet streams are the narrow bands of strong wind embedded within them, both of which steer and modulate the development of surface weather systems.

Scope

This topic covers the troughs and ridges of the upper-tropospheric flow, the subtropical and polar-front jet streams, the localized wind maxima called jet streaks and their transverse circulations, and the way these features steer and intensify surface weather systems.

Core questions

• What sets the location and strength of the jet streams?
• How do upper-level troughs and ridges influence surface pressure systems?
• How do the circulations around jet streaks favor storm development?
• How are upper-level patterns linked to extended spells of weather?

Key theories

Jet-streak transverse circulations

Air entering and exiting a jet streak becomes unbalanced, driving four-cell vertical circulations whose ascent regions favor the development of surface cyclones and precipitation.

Upper-level steering of surface systems

Surface lows tend to move with and deepen beneath regions of upper-level divergence east of troughs, so the upper-tropospheric wave pattern acts as the steering and forcing for surface weather.

Mechanisms

Because the geostrophic wind increases with height above strong temperature contrasts, the fastest winds concentrate into jet streams near the tropopause. The jet stream meanders into troughs and ridges, and where it accelerates or decelerates through a jet streak, the flow departs from balance and sets up transverse circulations with regions of rising and sinking air. Surface cyclones preferentially develop and deepen beneath the divergent, ascending zones east of troughs and in jet-streak entrance and exit regions.

Clinical relevance

Forecasters track upper-level waves and jet streaks to anticipate where surface storms will form and intensify, where heavy precipitation will fall, and when blocking ridges will lock weather into prolonged heat waves, cold snaps, or droughts.

History

Routine upper-air observations after World War II revealed the jet streams and the planetary waves that Rossby had identified; the work of Palmen, Newton, and others established their three-dimensional structure and their controlling role in cyclone development, central themes of modern synoptic-dynamic meteorology.

Key figures

• Carl-Gustaf Rossby
• Erik Palmen
• Chester Newton

Seminal works

• bluestein1993
• carlson1991

Frequently asked questions

What is the jet stream?

The jet stream is a narrow band of very strong winds near the top of the troposphere, formed above sharp temperature contrasts; it steers weather systems and separates colder polar air from warmer subtropical air.

Why do meteorologists watch upper-level troughs so closely?

Upper-level troughs create divergence aloft to their east, which promotes rising air and falling surface pressure, so they mark where surface storms are most likely to develop and strengthen.