Introduction: The source of the wind is the sun. The winds come from the suns energy falling on the earth’s surface. Due to the orientation of the earth’s surface to the sun’s rays near the equator the rays strike the surface at more optimum angles. The effect is that the air near the surface in tropical regions is heated more than the air near the surface of the polar regions. This leads to convection currents in the atmosphere, ie the movement of air due to changes in its density and pressure. This air movement is the principal cause of the winds.
Global Scale Circulation of the Atmosphere – A Simple Model of Global Circulation
We can gain an understanding of how global (or planetary) circulation works by developing two simplified graphical models of processes that produce this system. The first model will be founded on the following simplifying assumptions:
- The Earth is not rotating in space.
- The Earth’s surface is composed of similar materials.
- The global reception of solar insolation and outgoing longwave radiation cause a temperature gradient of hotter air at the equator and colder air at the poles.
Based on these assumptions, air circulation on the Earth should approximate the patterns shown on Figure 1. In this illustration, each hemisphere contains one three-dimensional circulation cell.
Figure 1: Simplified one-cell global air circulation patterns.
As described in the diagram above, surface air flow is from the poles to the equator. When the air reaches the equator, it is lifted vertically by the processes of convection and convergence. When it reaches the top of the troposphere, it begins to flow once again horizontally. However, the direction of flow is now from the equator to the poles. At the poles, the air in the upper atmosphere then descends to the Earth’s surface to complete the cycle of flow.
Three Cell Model of Global Circulation
If we eliminate the first assumption, ie the Earth is not rotating in space, the pattern of flow described in the model above would be altered, and the mesoscale flow of the atmosphere would more closely approximate the actual global circulation on the Earth. Planetary rotation would cause the development of three circulation cells in each hemisphere rather than one (see Figure 2). These three circulation cells are known as the: Hadley cell; Ferrel cell; and Polar cell.
In the new model, the equator still remains the warmest location on the Earth. This area of greater heat acts as zone of thermal lows known as the intertropical convergence zone (ITCZ). The Intertropical Convergence Zone draws in surface air from the subtropics. When this subtropical air reaches the equator, it rises into the upper atmosphere because of convergence and convection. It attains a maximum vertical altitude of about 14 kilometers (top of the troposphere), and then begins flowing horizontally to the North and South Poles. Coriolis force causes the deflection of this moving air in the upper atmosphere, and by about 30° of latitude the air begins to flow zonally from west to east. This zonal flow is known as the subtropical jet stream. The zonal flow also causes the accumulation of air in the upper atmosphere as it is no longer flowing meridionally. To compensate for this accumulation, some of the air in the upper atmosphere sinks back to the surface creating the subtropical high pressure zone. From this zone, the surface air travels in two directions. A portion of the air moves back toward the equator completing the circulation system known as the Hadley cell. This moving air is also deflected by the Coriolis effect to create the Northeast Trades (right deflection) and Southeast Trades (left deflection). The surface air moving towards the poles from the subtropical high zone from 30 latitude to 60 is also deflected by Coriolis acceleration producing the Westerlies. Coriolis force deflects this wind to cause it to flow west to east forming the polar jet stream at roughly 60° North and South. On the Earth’s surface at 60° North and South latitude, the subtropical Westerlies collide with cold air traveling from the poles. This collision results in frontal uplift and the creation of the subpolar lows or mid-latitude cyclones. Most of this lifted air is directed to the polar vortex where it moves downward to create the polar high.