Atmospheric Circulation (Southern Hemisphere)

Atmospheric circulation is how much and in which direction air moves through the atmosphere across the earth. The sun affects the earth’s atmospheric circulation, both in terms of temperature and in terms of axis rotation. Worldwide, a difference in pressure in the atmosphere causes the movement of air. In general, air circulates away from the equator, towards the poles. Certain factors affect how fast and in what direction the air goes. In this way, both the Northern Hemisphere and the Southern Hemisphere are the same. Other similarities also exist. Some differences, however, are pronounced. The Northern Hemisphere has more land, and the Southern Hemisphere has more water. Those differences create different climates, weather patterns and storm severity. The climate of northern Australia is strongly affected by trade winds, whereas southern Australia and New Zealand are more affected by westerlies.

Background

Wind patterns and direction have long fascinated explorers in general and mariners in particular. Knowing in which seasons and from which direction storms would generally come helped shape the patterns of civilisation. Reliable wind patterns were critical to oceanic navigation, and in the Mediterranean and in Southeast Asia, seafarers used wind as a navigational instrument, thousands of years before the Age of Exploration. People first arrived by sea in Australia between 40,000 and 65,000 years ago. Mariners preserved routes by recording and transmitting navigational information such as wind in songs.

During the Age of Exploration, transoceanic voyages depended on a growing knowledge of wind behaviour and the “streams” of air that flowed around the world. The trade winds, which could be picked up in subtropical waters, blew vessels north or south towards the equator; at other latitudes, easterly and westerly air currents could, barring gales and other weather phenomena, reliably be found to carry a ship in the correct direction. The Dutch and Spanish reached Australia in the early seventeenth century and the English in the eighteenth century. Such exploration yielded global records of wind patterns so that in the eighteenth-century scientists gained significant understanding of wind and atmospheric circulation.

Three circulation belts exist between the equator and the poles. The English scientist George Hadley first described two of those three circulation belts in 1753. An American meteorologist named William Ferrell discovered the third in 1856. Another important scientific innovation was the discovery of the Coriolis Effect, by the French mathematician Gaspard-Gustave de Coriolis, in 1835.

It was another century before jet streams (as their name suggests, a particularly strong pattern of air movement in the upper atmosphere) were discovered. The difference in direction of prevailing winds factors into the flight paths and flight time estimates of airplanes. For example, a flight from San Francisco to Sydney might take thirteen hours while the same flight in the opposite direction might take twelve hours. Modern satellite technology has vastly improved scientific understanding of weather; however, some elements remain a mystery.

Overview

Temperatures change more quickly on land than in the water. Geographic barriers on land — such as hills and mountains and skyscrapers — serve to divert or even obstruct wind flow; with few exceptions, ocean winds are not so affected. This means that in the Southern Hemisphere, atmospheric circulation tends to be much stronger than at similar Northern Hemisphere latitudes. This can drive weather systems more quickly and fiercely and can have a sometimes pronounced effect on ships and planes traveling along southern latitudes.

At the same time, the greater time it takes for water to heat and cool means that on balance, Southern Hemisphere climates, with their greater preponderance of water, are more moderate than Northern Hemisphere climates, with their greater preponderance of land. The seasons are reversed from one hemisphere to the next, because of the tilt angle of the earth toward the sun, and so a particularly cold winter in January in the United States might be, at the same time, a less warm summer in New Zealand. Stronger winds in the Southern Hemisphere generally contribute to this.

The equator is not a physical barrier, and so the circulation of air from the Northern Hemisphere to the Southern Hemisphere, and vice versa, is altogether common and expected. What does not tend to switch hemispheres is any kind of high-pressure or low-pressure system; this is because of the Coriolis Effect. Technically, the Coriolis Effect is what happens to an object moving relative to the surface of the earth when that object is affected by the earth’s rotation. Such things as airplanes and ocean currents encounter the Coriolis Effect and are “deflected”; the faster the object is going, the more it is deflected. Another thing that is so affected is wind.

Because the earth is round, its rotational path creates a curved path for wind. The earth rotates more quickly at the equator than at latitudes further north or further south. The faster the rotation, the more pronounced the Coriolis Effect and the more the airplane or ocean current or wind is affected. A tropical storm, for example, will gravitate towards a northern latitude because the storm needs strong amounts of rotation in order to form and maintain strength. The direction of the deflection is different on either side of the equator: Northern Hemisphere objects are deflected to the right, and Southern Hemisphere objects are deflected to the left. Therefore, Southern Hemisphere weather systems appear to rotate clockwise because ocean currents are deflected to the left. In the same way, cyclones in the Southern Hemisphere spin clockwise, the opposite of their Northern Hemisphere counterparts.

Wind patterns differ depending on latitude. In areas from the equator to 30 degrees latitude north or south can be found the trade winds (also known as Hadley cells), which blow toward the equator. In areas from 30 degrees latitude north or south are the prevailing westerlies (also known as Ferrell cells). At the remaining latitudes are the polar easterlies (also known as polar Hadley cells). Because of the Coriolis Effect, Northern Hemisphere surface winds (both Hadley cells) blow from the northeast to the southwest and the Southern Hemisphere trade winds do the opposite, blowing from the southwest to the northeast. For Ferrell cells, the directions are reversed.

Bibliography

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