Vorticity and General Circulation of the Atmosphere

Atmospheric general circulation and energy

The general circulation of the atmosphere is the large-scale, long-term pattern of winds and pressure that moves energy from the tropics toward the poles. Its driving force is the uneven distribution of radiation. Because the tropics absorb much more shortwave radiation than they lose as longwave radiation, they have a net radiative surplus, while the polar regions have a net deficit. Every body above absolute zero emits longwave radiation, so polar regions emit too; what matters is the net balance. Polar regions receive less shortwave radiation on an annual average, so the statement that they receive more shortwave radiation than the tropics is false.

The atmosphere can be treated as a heat engine. It takes in heat from absorbed radiation and rejects heat at lower temperatures, and a small fraction of this heat input is converted into kinetic energy of the winds. In steady state, the rate of kinetic energy generation equals the rate of kinetic energy dissipation by friction, so the efficiency is $\eta = \frac{\text{rate of kinetic energy generation}}{\text{rate of heat input}} \times 100\%$ For example, if a planet absorbs 150 W m^{-2} and friction dissipates 6 W m^{-2} of kinetic energy, the efficiency is \eta = 6/150 \times 100 = 4\%. The atmospheric heat engine is therefore very inefficient, converting only a few percent of the absorbed heat into motion.

The Hadley cell is the direct meridional overturning circulation of the tropics. Air rises near the equator in the intertropical convergence zone (ITCZ), flows poleward aloft, sinks in the subtropics, and returns equatorward near the surface. The ascending branch is narrow because it is confined to the ITCZ, whereas the descending branch covers the broad subtropics. Deep convection and thunderstorms are concentrated in the ascending region, while subsidence in the subtropics suppresses cloud and rain.

The low-level return flow forms the trade winds. Because of the Coriolis deflection, these winds are north-easterly in the Northern Hemisphere and south-easterly in the Southern Hemisphere, not north-westerly. Moisture evaporated from the subtropical ocean is carried equatorward by these winds and released as latent heat in the ascending region. Thus latent heat is transported from the subsiding to the ascending region of the Hadley cell.

In the midlatitude troposphere, the main poleward transport of energy is carried by baroclinic waves, the synoptic-scale cyclones and anticyclones that grow where horizontal temperature gradients exist. Their eddies carry sensible heat poleward through the eddy correlation \overline{v'T'}, and they carry moist static energy in a similar way. Gravity waves, atmospheric tides, and boundary-layer turbulence contribute little to the net meridional energy flux, since gravity waves and turbulence act mainly in the vertical or on small scales, and tides are global-scale forced oscillations.

Key terms:

• subsidence — sinking air, which warms adiabatically and suppresses cloud formation. Example: subsidence over the subtropical oceans keeps the skies clear.
• baroclinic wave — a wave that grows in a fluid with horizontal temperature gradients, and the main carrier of midlatitude heat transport. Example: a midlatitude cyclone moves heat toward higher latitudes.
• efficiency of the heat engine — the ratio of kinetic energy generated to heat input. Example: a generation rate of 2 W m^{-2} from 80 W m^{-2} of heat input gives 2.5%.