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Zonal flow (or zonal wind) is air circulation parallel to the equator and latitudes, i.e., east-to-west or west-to-east. This type of circulation promotes stable temperatures along latitudes. Conversely, north-to-south or south-to-north air flow is termed meridional flow. The terms are used to mean regular winds (prevailing winds) in such direction, such as the Earth's trade winds and westerlies and are also used to mean the east-west component of any given wind.
A common type of zonal flow results from the meridional flow that constitutes Hadley cells and similar circulation-pattern cells, in conjunction with the Coriolis effect due to the planet's rotation. This can create a zonal flow in either direction (including super rotation, a zonal flow moving atmosphere faster than the rotation of the planet, such as the Earth's westerlies) depending upon whether the meridional flow generating it is toward or away from its nearer pole. The faster a planet's rotation, the more pronounced are such zonal flows and the Coriolis effect turns the wind toward a zonal flow over less distance, resulting in more cells between equator and pole and more individual zonal flows. Jupiter, for example, rotates faster than Earth and has a much greater distance between equator and pole, resulting in more cells and individual zonal flows, which form the visible striped pattern on its surface.
In the study of other worlds, e.g., gas giants, the phrase zonal flow is often a synonym for jet stream or jet for short. A model for the number of jets on a planet is:
2ΩR
NJets ∝ ( ——— )1/2
NH
A giant planet's atmosphere may have layers that are electrically conductive, e.g., with pressure sufficiently high that hydrogen becomes metallic, and zonal flows in such layers can form dynamos and associated magnetic fields. Conductive fluid layers/regions (gas or liquid) are sometimes called ionic oceans.
The term differential rotation (often used for the Sun to describe how its apparent rotation at its equator differs from near its poles) effectively means the same thing as the presence of zonal flows. In giant planets, and stars, such flows may be at the surface, or may be deep within, or may span from the surface to deep within, e.g., in cylinder-shaped flows encircling the body's axis of rotation.
In (Earth) meteorology, the term zonal flow can refer to the east-west component of the wind or wind patterns, but also is often used to characterize an observed flow that is very close to east-west, i.e., lacking much of a north-south component.