The Web of Life (18-4) Atmospheric Circulation

Atmospheric Circulation

Warmth is spread through the troposphere by atmospheric circulation. While chaotic turbulence plays a large role locally, wind belts girdle the Earth and create global patterns.

Solar heating primarily drives atmospheric circulation. The hot equator and cold poles move thermal energy poleward.

The Coriolis effect caused by planetary rotation tends to move air mass flow latitudinally, in contrast to the thermal longitudinal inclination. Thus, 2 countervailing dynamics affect the atmospheric circulation gyre.

The uneven heat distribution of solar radiation and Earth’s rotation cause differences in air pressure. Aiming at equalization, wind flows from high- to low-pressure areas.

The Earth’s wind belts are organized into 3 different cell types, based upon latitude. These cells are characterized by the bands of pressure which drive them.

The equator is a low-pressure area. Prevailing winds are calm: the doldrums.

The tropical Hadley cell carries the vast bulk of vertical circulation in Earth’s atmosphere. The Hadley cell mechanism characterizes the trade winds.

Low- and high-pressure areas on the surface are balanced by their opposite relative pressures in the upper troposphere; hence a circulating cell is defined.

Elevated temperatures at the equator cause surface air to expand and become lighter. This warm, moist air at the equator is lifted aloft, and carried poleward 10–15 km above the surface. The warm air is replaced by cooler, heavier air, flowing in from the north and south.

At the high-pressure Horse Latitudes, between 30°–35° north and south, the air descends. Some of the descending air travels along the surface, closing the Hadley cell loop and creating the trade winds.

The trade winds generally flow in the same direction due to the Coriolis effect: from the northeast in the northern hemisphere, and from the southeast in the southern hemisphere.

Owing to chronic high pressure, land in the Horse Latitudes receives little precipitation. Winds there are variable, mixed with calm.

Over the oceans, the Westerlies have the strongest winds, especially between 40°–50° latitude, and particularly in the southern hemisphere, where there is less land in the middle latitudes. The Westerlies blow from west to east.

The Polar cell is the belt between latitude 60° and the pole. Like the Hadley cell, air masses at the 60th parallel are still warm and moist enough to undergo convection, and thereby drive a thermal loop. By the time relatively warm air masses reach the polar region they have cooled considerably, and so descend as a dry high-pressure area. This wind moves away from the pole near the surface, but twists westward because of the Coriolis effect, producing the polar easterlies.

The polar front is the boundary between the polar cell and the Ferrel cell in each hemisphere. A secondary mechanism, the Ferrel cell depends upon the Hadley cell and Polar cell for its existence, acting like a ball bearing between the 2.

While the Hadley and Polar cells are closed-loop systems, the Ferrel cell is not. The Westerlies are created from eddy circulations between high- and low-pressure regions in the middle latitudes.

Just as the trade winds blow beneath the Hadley cell, the Westerlies are found under the Ferrel cell. The base of the Ferrel cell has moving air masses which are influenced by the jet stream. Thus, winds created by the Ferrel cell are highly variable.

Jet streams are fast-flowing, narrow air currents near the tropopause, caused by solar radiation and Earth’s rotation. The tropopause is the transition layer between the troposphere, where the temperature drops with altitude, and the stratosphere, where temperature rises with altitude.

Earth’s major jet streams are westerly winds: flowing west to east. Jet stream paths may meander. Their shapes are highly variable.

The strongest jet streams are the polar jets, 7–12 km above sea level. At 10–16 km, subtropical jets are higher and somewhat weaker. Both the northern and southern hemispheres each have a polar jet and a subtropical jet.

While the polar jet in the southern hemisphere mostly circles Antarctica year-round, the northern hemisphere polar jet flows over middle to northern latitudes in North America, Europe, and Asia, and their intervening oceans. A weakened polar jet from climate change has delivered bursts of frigid winter weather to northern continents in the 2010s.