Global Climatology — Page 369, Lesson 343

I want to walk you through the global circulation system that drives our planet's weather. We're starting with a key point: if the Earth didn't rotate, the simple convective circulation I described earlier would be straightforward. But the Earth does rotate, and that rotation modifies the circulation into the pattern shown in Figure 20.1. Let me explain what that pattern looks like. Around 30° North and 30° South latitude, the Hadley cells — those are the large-scale convective loops of rising air at the equator and sinking air at about 30° — produce surface high-pressure systems. These are called anticyclones, and specifically, because they form at those latitudes, they're known as subtropical anticyclones. At the surface, these anticyclones produce an outflow of warm air. Some of that warm air moves poleward — toward the nearer pole in each hemisphere. Now, that poleward-moving warm air eventually meets cold air flowing out from the polar regions. That cold flow is also anticyclonic — it's a cold high-pressure outflow from the poles. Where these two air masses meet, you get areas of frontal activity — the boundary between warm and cold air creates weather fronts. The diagram in Figure 20.2 shows the Hadley cell and the polar front, along with the vertical airflows that cause them. So you have rising air at the equator, sinking air at about 30°, and then a separate circulation near the poles. But the subtropical anticyclones don't just send air poleward. From those same subtropical anticyclones in each hemisphere, there's also a surface outflow toward the Equator. When that air from the Northern Hemisphere and the Southern Hemisphere converges near the equator, it causes rising air and a lot of instability in the equatorial zone. This convergence zone is called the Intertropical Convergence Zone — abbreviated ITCZ. Now, think of the Earth's climate as an engine. The airflow pathways of the world clearly show how the climatic zones are interrelated. Along the ITCZ, moisture-laden air rises. That rising, moist air causes masses of cumulonimbus thunderclouds to develop, which produce the heavy rains we see in tropical regions. At upper levels, the air from the Hadley cell and from another circulation called the Ferrel cell — both indicated in Figure 20.2 — meet aloft. That air is cooled and undergoes radiative sinking. That sinking air produces the Subtropical High Pressure zones at the Earth's surface, which give settled, fair weather. Once the air streams separate at the surface in the Northern Hemisphere, one branch flows south as the NE trade winds — the northeast trade winds — while the other branch flows north to become the temperate latitude westerlies. The flow is mirrored in the Southern Hemisphere, so you get SE trade winds and temperate westerlies down there as well. So to summarise: the Hadley cell drives subtropical anticyclones at 30°; those anticyclones send air both poleward — where it meets polar air and creates fronts — and equatorward, where convergence at the ITCZ produces rising air, thunderstorms, and heavy tropical rainfall. The upper-level return flow from the Hadley and Ferrel cells sinks to reinforce those subtropical high-pressure belts, and the surface winds then diverge into the trade winds and the westerlies. That's the big picture of how the global circulation works.