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Incoming links: Ferrel cell
Atmospheric circulation 2026-10-24
The large-scale movement of the atmosphere, including the tropical Hadley cell and the midlatitude Ferrel cell.
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.