Atmospheric Circulation

08 Jun 2005 - Articles, Atmosphere & Space, Arctic, Antarctic

Diagrammatic representation of the earth's atmospheric circulation cells as well as broad directions of surface winds

Atmospheric circulation is one of the key factors driving regional changes in wind, temperature, precipitation, moisture and other climatic variables. This large-scale movement of air (together with ocean circulation) is the means by which heat is distributed across the Earth's surface, particularly northward from the equator towards the poles. Without atmospheric circulation, average winter temperatures at the poles would be around -100°C rather than -30 °C as at present.

The large-scale structure of the atmospheric circulation varies slightly from year to year but the basic pattern remains fairly constant. Individual weather systems, however, occur "randomly" and it is accepted that weather cannot be predicted beyond a fairly short timeframe.

Circulation "cells" in the two hemispheres

Earth's weather results from the interactions of three large circulation cells in each hemisphere. The wind belts and the high altitude jet streams surrounding the planet are driven by three convection cells: the Hadley, Polar and Ferrel "cells".

The Hadley cell mechanism is a closed heat loop, where warm, moist air rises around the equator to the upper reaches of the troposphere (the atmospheric layer which stretches from the ground to the stratosphere) and thereafter moves toward the poles. At about 30° north (and south) of the equator, the air descends in a cooler high pressure area and then returns to the equatorial regions. The Hadley cell is made up of multiple mini-cells within the equatorial zone which merge and separate randomly over time but contribute to the general circulation within the larger cell framework.

The Polar cell is a similarly simple system. Though cool and dry relative to equatorial air, air masses at the 60th parallel are still sufficiently warm and moist to drive a thermal loop between there and the poles (similar to the Hadley cell loop). When the air reaches the polar areas, it has cooled considerably, and descends as a cold, dry high pressure area, twisting eastward as a result of the earth's rotation to produce the strong Polar Easterlies winds. By acting as a heat sink (ie it absorbs heat), the Polar cell is crucial to balancing the Hadley cell's transport of heat northwards (from the equator) in maintaining the Earth's overall temperature equilibrium.

The Hadley cell and the Polar cell are similar in that they arise directly as a result of surface temperatures. As well, their thermal characteristics override other weather effects within the cells. The passing highs and lows which form part of daily life for mid-latitude dwellers are unknown above the 60th and below the 30th parallels.

The Ferrel cell is a secondary circulation system, acting as a counterbalance between the Hadley and Polar cells. While the Hadley and Polar cells are truly closed predictable loops, the Ferrel cell is not. Localised weather systems can overcome the general trend for the winds to run from west to east.

Differences between the Polar Cells

The considerable differences between the two polar regions mean that there are differences in the way atmospheric circulation operates at each pole (though still in a broadly similar way). For example, the presence of extensive land and mountains around the Arctic Ocean makes for considerably more disrupted weather patterns than in the Antarctic where the vast Southern Ocean and its circumpolar current buffers the polar continent. The air of the Northern polar cell therefore mixes much more with its adjacent Ferrel cell - this in part contributes to the phenomenon of an "ozone hole" being much less pronounced in the Arctic than in the Antarctic.

Longitudinal circulation features

While the Hadley, Ferrel, and Polar cells are the major drivers of global heat transport, they do not act alone. Disparities in temperature also drive a set of long-term longitudinal circulation cells, and the overall atmospheric motion is known as the zonal overturning circulation.

Latitudinal circulation is a consequence of the fact that energy from the sun per square unit is highest at the heat equator, and decreases as the latitude increases, reaching its minimum at the poles. Longitudinal circulation, on the other hand, comes about because water absorbs and releases heat more readily than land.