M1 General Atmospheric Circulation

Topic

General atmospheric circulation is the average flow pattern—calculated over long periods—that transports heat, moisture, and momentum from the tropics toward the poles. This transport allows the atmosphere to respond to the radiative imbalance existing between different latitudes; this imbalance causes the general atmospheric circulation to organize into three distinct meridional circulation cells: the Hadley cell, occupying the tropics; the Ferrel cell, occupying the mid-latitudes; and the polar cell, occupying the highest latitudes of all.

The fundamental driver setting this entire general atmospheric circulation in motion is precisely that latitudinal radiative imbalance: the tropics absorb more solar energy than they emit back into space, whereas the poles emit more energy than they receive from the sun. This difference necessitates the atmospheric transport of five to six petawatts of energy from the tropics to the mid-latitudes—a task not handled by a single mechanism but shared between two: the mean circulation of the Hadley cell itself, which dominates transport within the tropics, and the baroclinic eddies forming in the mid-latitudes, which dominate transport once the heat leaves the tropical zone.

These same mid-latitude baroclinic eddies, together with the temperature gradient, are also responsible for sustaining the wind jets that meander through those latitudes: a jet's speed results from the balance between the meridional momentum transport produced by baroclinic eddies and the meridional temperature gradient that fuels them. This dependency follows what is known as the thermal wind relationship: the greater the meridional temperature gradient between the tropics and the poles, the stronger the jet that develops as a result of that gradient. While mid-latitude jet streams depend on the temperature gradient, the poleward extent of the Hadley cell relies on a different condition: a stability criterion for high-altitude winds near the top of the troposphere, which determines how far the cell can reach before becoming unstable. As the planet warms, this stability limit is projected to shift toward higher latitudes, causing the Hadley cell to expand and pushing its edge further away from the tropics than it currently reaches.

Climate warming does more than just shift the edge of the Hadley cell; it is also expected to intensify mid-latitude jet streams—an effect driven indirectly by the very meridional temperature gradient upon which those jets depend. When warming is uneven across latitudes—such as when the pole warms differently than the rest of the atmosphere—the resulting difference alters the meridional temperature gradient. Through the thermal wind relationship, this modified gradient then determines whether the associated jet stream becomes stronger or weaker than before.

Thus, from the radiative imbalance that drives the engine of general atmospheric circulation—through the three meridional cells organized by that engine and the jet streams sustained by the temperature gradient between them, to the shifting edge of the Hadley cell and the intensification of jets due to climate warming—the entire system is governed by a single chain of causality. At its core, the differential heating between the tropics and the poles drives the movement of air across the planet, dictating the patterns of motion we observe.

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