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Atmospheric circulation Created 2026-10-24 Updated 2026-11-05
Atmospheric circulation redistributes heat, water vapour and momentum through the atmosphere. Unequal radiative heating creates temperature and pressure contrasts; gravity, planetary rotation, friction and the evolving flow constrain the response. A simple direct overturning circulation is modified strongly by the Coriolis force on a rotating planet.
The tropical Hadley cell has rising air in strongly heated regions and a returning lower branch. Midlatitude eddies transport heat and momentum and help maintain the Ferrel cell. These statistical cells summarize a time-varying flow containing waves and storms. They explain why a local radiation imbalance can be balanced by horizontal transport rather than by local radiation alone.
Past exam of the geophysical fluid dynamics course of the University of Oxford 2023 1 d Solution 2026-10-25
The low-latitude regions of Earth absorb more incoming solar radiation than they emit as outgoing thermal radiation, whereas high latitudes have a net annual radiation deficit. The obliquity of solar incidence and the distribution of daylight create a strong latitude dependence of absorbed solar energy; emitted thermal radiation varies less strongly because terrestrial temperature contrasts are smaller. The terrestrial atmosphere and ocean therefore carry energy from the tropical surplus toward the polar deficits. Otherwise the low latitudes would keep warming and the high latitudes cooling until a different local radiative equilibrium was reached.
For a steady, zonally averaged energy balance, let be net downward radiation at the top of the terrestrial atmosphere and the total northward energy transport across latitude . A narrow spherical band and conservation of energy giveThe poleward direction is northward in the Northern Hemisphere and southward in the Southern Hemisphere. Seasonal internal energy storage modifies the instantaneous balance, but not the annual need for poleward redistribution.
Several circulation structures contribute in addition to the transient eddies produced by baroclinic instability of the middle latitudes:
- The Hadley cells, mainly between the equator and about latitude, export energy in their upper poleward branches and return air equatorward near the surface. The transported quantity includes enthalpy, gravitational potential energy and latent heat; the upper and lower branches have different thermal properties. The cross-equatorial Hadley cell is especially important in the seasonal cycle.
- Quasi-stationary Rossby waves, associated with mountains and continental heating contrasts, carry heat in the extratropical terrestrial atmosphere. Their meridional velocity and temperature anomalies can have a poleward correlation. These are an additional time-mean contribution alongside travelling weather disturbances.
- The ocean gyres in tropical and subtropical basins redistribute warm water poleward in strong western boundary currents and return cooler water equatorward over broader regions. The thermal difference between the branches produces net heat transport despite little net meridional volume transport.
- The oceanic overturning circulation, including the Atlantic meridional overturning circulation, combines warm upper water moving poleward with colder deep return water. It transports substantial heat through the tropical and subtropical Atlantic toward the northern high latitudes. Wind forcing, buoyancy exchange and mixing all help maintain this global circulation.
The Brewer-Dobson circulation additionally redistributes energy in the stratosphere, with tropical ascent and extratropical descent, although most total atmospheric heat transport occurs lower down. An Eulerian mean middle-latitude Ferrel cell is thermally indirect and can oppose the poleward transport by eddies; listing every meridional circulation as a direct poleward heat carrier would be incorrect.
William Ferrel Created 2026-10-25 Updated 2026-11-05
William Ferrel analysed atmospheric circulation on a rotating Earth. The Ferrel cell carries his name and describes the middle-latitude branch of the idealized meridional circulation.[ref]
Ferrel cell in the Glossary of Meteorology by American Meteorological Society. Definition and historical attribution
Its modern interpretation requires care: this mean circulation is thermally indirect and strongly affected by eddy transport. It is not simply a second thermally direct Hadley cell. The distinction links the historical three-cell picture to the conservation and transport arguments used in geophysical fluid dynamics.