Chapter 7 Notes

Circulation of the Atmosphere

Satellite Image of Swirling Wind Pattern Lee of an Island

Global atmospheric circulation can be thought of as a series of deep rivers of air that encircle the planet. Embedded in the main currents are vortices of various sizes including hurricanes, tornados, and mid-latitude cyclones. These rotating wind systems develop and die out with somewhat predictable regularity. The smallest eddies, like dust devils, last only a few minutes, while larger and more complex systems, such as hurricanes, may survive for several days.

Winds are generated by pressure differences caused by unequal heating of Earth’s surface. Because the tropics receive more solar radiation than Earth’s polar regions, Earth’s winds blow in an unending attempt to balance inequalities in surface temperatures.

Macroscale, Mesoscale, and Microscale Circulation

Scales of Atmospheric Motion

Macroscale winds – include large planetary-scale flow that blow consistently for weeks or longer as well as smaller feature like hurricanes. Examples of macroscale winds include the prevailing westerlies that dominate the airflow in the United States, and the trade winds

Mesoscale winds – circulation associated with tornadoes, thunderstorms, other local winds that generally last from minutes to hours. Mesoscale winds are usually less than 100 kilometers across, and some—thunderstorms and tornadoes—also have a strong vertical component.

Microscale winds – have short life spans, usually a few seconds to minutes, and include dust devils, gusts, and general atmospheric turbulence.

Tornado, Mesoscale Circulation

Local Winds – Mesoscale Winds

Winds are named for the direction from which they blow, example: a north wind blows from north to south.

Land and Sea Breezes – A sea breeze develops as cooler air over water moves inland, while a land breeze is the result of cooler air over land moves out to the water. Sea breezes tend to moderate coastal temperatures.

Mountain and Valley Breezes – During the day, air along mountain slopes is heated more intensely that air at the same elevation over the valley floor. This warmer air glides up along the mountain slope and generates a valley breeze. The reverse happens after sunset. Rapid cooling along the mountain slopes cools the air, which drains into the valley below and a mountain breeze.

Chinook (Foehn) Winds - These winds are created when a strong pressure gradient develops in a mountainous region. As the air descends the leeward slopes of the mountains, it is heated adiabatically, and can result in rapid warming. Chinooks winds usually occur in the winter and early spring.

Katabatic (Fall) Winds – Winds that originate when cold air, situated over a highland area is set in motion. Under the influence of gravity, the slides over the highland rim and descends into the lowlands.

Country Breezes – A wind associated with urban areas. This circulation pattern is characterized by a light wind blowing into the city from the surrounding country-side. The breeze is initiated as warm, less-dense air in the city rises and is replaced by cooler air from outside the city.

Global Circulation

Knowledge of global winds comes from two sources: the patterns of pressure and winds observed worldwide, and theoretical studies of fluid motion.

Idealized Three-Cell Circulation Model

Classical Circulation Models

1)Single-Cell ModelModels global circulation on a non-rotating Earth. It is a simple convection system produced by unequal heating of the atmosphere. Hot, less dense air rises at the equator until it reaches the top of the troposphere where it then begins to spread toward the poles. At the poles, the now dense air sinks and moves toward the equator.

2)Three-cell Circulation Model (pictured)In the zones between the equator and roughly 30O latitude north and south a convection system similar to the single-cell model exists. Circulation between 30O and 60O latitude is more complicated. The net surface flow is pole-ward, but because of the Coriolis force the winds have a strong westerly component. At the poles, air subsidence produces a surface flow that moves toward the equator and is deflected into polar easterlies.

Idealized Pressure Belts; Belts Broken Up by Real Pressure Cells

Observed Distribution of Pressure and Winds

1) Idealized Zonal Pressure BeltsIf Earth had a uniform surface, two latitudinally oriented belt of high and two of low pressure would exist. Near the equator, the warm rising air is associated with the pressure zone know as the equatorial low. In the belts about 20O to 35O where the westerlies and trade winds originate are the pressure zones known as the subtropical highs. Another low pressure region is situated at about 50O to 60O

latitude where the polar easterlies and prevailing westerlies clash to form a convergence zone known as the subpolar low. Finally, at Earth’s poles are the polar highs, from which the polar easterlies originate.

Average Surface Pressure and Global Circulation in January and July

Observed Distribution of Pressure and Winds

2)Semipermanent Pressure systems: The Real WorldBecause the Earth’s surface is not uniform, the only true zonal distribution of pressure exists along the subpolar low in the Southern Hemisphere. In reality, Earth’s pressure patterns vary in strength or location during the course of the year. Factors that influence the location and strength of pressure patterns include amount of solar heating and the proportion of land compared to ocean

Monsoon Circulation

Monsoons

The greatest seasonal change in Earth’s global circulation is the monsoon. Monsoon refers to a wind system that exhibits a pronounced season reversal in direction. In general, winter is associated with winds that blow predominantly off the continents and that produce a dry winter monsoon. In the summer warm moisture-laden air blows for the sea toward the land. As a result, the summer monsoon is usually associated with abundant moisture.

Prevailing Westerly Winds Aloft

Prevailing Westerlies

After World War II, one of the most important meteorological discoveries was the existence of the westerlies, a circulation of air aloft in the middle latitudes that has a strong west-to-east component.

Two forces interact to produce the westerlies. First, there is a strong pressure gradient from high pressure at the equator to low pressure at the poles. The pressure gradient results in air flowing south to north (in the Northern Hemisphere). Once air aloft starts moving toward the north the Coriolis force acts upon it and bends the motion to the right. The balance between the pressure gradient force and the Coriolis force generates a wind with a dominant west-to-east component.

Jet Streams

Embedded within the westerly flow aloft are narrow ribbons of high-speed winds that travel for thousands of kilometers. These ribbons of winds are called jet streams, and they have velocities between 200 km/hr to 400 km/hr.

Jet streams are located in regions of the atmosphere where large horizontal temperature differences occur over short distances. These large temperature contrasts occur along linear zones called fronts.

1)The polar jet streamThe polar jet stream occurs along a major frontal zone called the polar front. The polar jet occurs mainly in the middle latitudes and migrates with the seasons. During the summertime the polar jet moves north while the opposite happens in the winter.

2)The subtropical jet streamA semipermanent jet exists over the subtropics and occurs mainly in the wintertime

Ocean Currents

Global Winds and Ocean Currents

Where the atmosphere and ocean are in contact, energy is passed from moving air to the water through friction. The drag exerted by winds blowing steadily across the ocean causes the surface layer of water to move. The movement of water like this is called ocean currents., and a relationship exists between the oceanic circulation and the general atmospheric circulation.

Ocean currents have an important effect on climate. Poleward-moving warm currents moderate the climate on land near (relatively) the poles. Also, cold ocean currents impact conditions on the land by reducing the amount of rainfall in coastal regions alongside the cold current.

Ocean currents also lay a role in maintaining Earth’s heat balance. They transfer heat from the tropics to the polar regions.

Southern Oscillation and El Nino

El Niño and La Niña

Typically, atmospheric circulation in the tropical Pacific is dominated by strong trade winds which generate a strong equatorial current that flows westward from South America toward Australia and the Philippines. In addition, a cold oceanic current flows equatorward along the coast of Ecuador and Peru. Near the end of each a warm countercurrent, flowing eastward along the equator develops, and warm water begins to accumulate along the coasts of Ecuador and Peru.

Usually the warm countercurrent lasts for a few weeks, but occasionally—every three to seven years—the countercurrent is unusually strong. The result is that the normally cold offshore waters are replaced with warm equatorial waters. These episodes are called El Niño.

La Niña is the opposite of El Niño, that is the surface temperatures of the Pacific are colder than normal. Like El Niño, La Niña has distinctive weather patterns.

Major El Niño events are related to the large-scale atmospheric circulation. Each time an El Niño occurs, the barometric pressure drops over large portions of the southeastern Pacific , while in the western Pacific the pressure rises. When the El Niño ends, the pressure difference in the two regions swings back in the opposite direction. This see-saw pattern of atmospheric pressure is called the Southern Oscillation.

The effects of El Niño are somewhat variable depending in part on the temperatures and size of warm pools. During most El Niños, warmer-than-normal winters occur in the northern United States and Canada, and colder-than-normal winters are experienced in the Southwest and Southeast. Also, the eastern United States experiences wetter-than-normal conditions.

Global Distributions of Precipitation; Annually, in July, and in January

Global Distribution of Precipitation

In general, regions influenced by high pressure, with its associated subsidence and divergent winds, experience dry conditions. Conversely, regions under the influence of low pressure and its converging winds and ascending air receive ample precipitation.

However, air pressure is not the only factor that determines the presence or absence of precipitation. Because cold air has a low capacity for moisture compared with warm air, there should be a latitudinal variation in precipitation with low latitudes (near the equator) receiving the greatest amounts of precipitation and high latitudes receiving the least.

Also, the distribution of land and water complicates the precipitation pattern. Large landmasses in the middle latitudes commonly experience decreased precipitation toward their interiors. Additionally, the effects of mountain barriers alter the idealized precipitation models. Windward mountain slopes receive abundant precipitation while the leeward slopes and adjacent lowlands are usually deficient in moisture.