41 Atmospheric Circulation: General Concepts, Wind System, and Global Atmospheric circulation

General Introduction:

• Atmospheric circulation, the general circulation of the Earth, and the flow of air are used to refer to the movement of air in the area around high and low-pressure systems.
• According to Strahler, 97% of the earth’s atmosphere found at the height of 29km but the limit of atmospheric height goes at the height of 10,000km.
• Earth’s atmosphere works like a big glass ball in which the short wave of radiation coming from the sun is allowed to enter but the terrestrial long wave is absorbed by atmospheric gases such as co2 and much more.

• On average, this cycle corresponds to large wind systems located in seven North-south zones surrounding the Earth.
  • Equatorial Low-Pressure Belt (Doldrum Belt).
  • Subtropical High-Pressure Belt (Horse latitude).
  • Subpolar Low Pressure.
  • Polar high pressure.

Equatorial Low-Pressure Belt (Doldrum Belt)

1. This zone extends up to between 5°N and 5°S, in both hemispheres.
2. In this zone, the trade winds converge, coincide with the world’s latitudinal belt of heaviest precipitation and most persistent cloud cover.
3. This zone is also known as the intertropical convergence zone (ITCZ or ITC) because in this region the trade winds generated from tropics of both hemispheres converge on the equatorial region.
4. The climate, which is very warm and heated by the sun, expands and rises here, retaining the area’s low pressure and high rainfall capacity.

Subtropical High Pressure

1. generally located between latitudes 25° and 35°N and S,
2. the cause of high pressure in this zone is air sinking and settling from higher altitudes.
3. These subtropical belts of variable or calm winds have been called the “horse latitudes” because of the occasional need by sailors to eat their horses or throw them overboard in order to conserve drinking water and lighten the weight when their sailing ships were becalmed in these latitudes.

Subpolar Low-Pressure Belt

1. This zone lies near the arctic and Antarctic circle 60 to 70 ° in both hemispheres.
2.  Poleward of the subtropical highs in both hemispheres large belts of low pressure extend along the upper-middle latitudes, called the subpolar lows. Dynamic factors dominate the formation of the subpolar lows, as opposing winds collide and cause air to rise.

Polar high-Pressure Belt

1. The Arctic and Antarctic regions are dominated by high-pressure systems called the polar highs. Extremely cold temperatures and the consequent sinking of dense cold air create the higher pressures found in the polar regions.

• In the subtropical high-pressure zone near latitude 30° north and latitude 30° south, air sinks, and trade winds are forced to blow westward toward to equator.
• Atmospheric circulation plays a very important role, along with circulation in the ocean is a means of reducing the energy of the Earth’s surface.
• The Earth’s atmosphere varies from year to year, but the size of its atmosphere remains constant.
• Small-central system – depression between cells or tropical convection cells. They meet and rise in the Intertropical Convergence Zone near the equator, blowing eastward and polar at altitudes of 2 to 17 km (1 to 11 miles).
• Some of the currents descend the subtropical ridge, the rest merge at high altitudes, and mid-latitude westerlies head further in the polar direction.
• During the year, the Sun changes the angle of noon from the vertical of the Tropic of Cancer to the vertical of the Tropic of Capricorn.
• Angle changes result in stronger solar radiation in the Northern Hemisphere from April to September and stronger solar radiation in the Southern Hemisphere from October to March.
• Therefore, various pressure regions are formed that cause a large circulation between the equator and the poles.
• The rotation of the earth impedes the direct flow between high pressure (equator) and depression (pole).
• In the Northern Hemisphere, the air mass is deflected to the right, and in the Southern Hemisphere, it is deflected to the left. As a result, three large circulating cells (Hadley cell, Ferrel cell, and polar cell) are generated.

Effects of the Atmospheric circulation:

• Transport of warmer air and humidity from the temperate to the colder zones.
• Continuous transport of humidity from the equator to the north and south tropics.
• Transport of hot air and humidity from the tropics to the temperate zone.

Hadley Cell:

• At the Equator, the wind rises due to the strong heat of the sun. Due to the tropopause (temperature fluctuations 18 km above the ground), air masses tend to north and south.
• Through field correction, the air mass travels down to the poles. Also, through the rotation of the Earth, the edges fall to the 30th degree of latitude and return to the Equator in the form of a tread.
• At the Equator, these winds meet in the Intertropical Convergence Zone (ITCZ). This circulation is called the Headley cell.
• During the ascent, the air cools, the internal humidity condenses, clouds form and it rains very hard. During the landing process, the opposite happens.
• The air heats up and the water begins to evaporate. Desert areas around 30 latitudes (e.g., Sahara or the Namib Desert) are the result of this process.
• There are several anticyclones in this area due to the warm weather. These are attached to subtropical ridges (horse width).

Polar Cell:

• Near ground level, currents of airflow from the poles to the equator.
• They are called Polar Easter because they move around the earth’s crust.
• The winds warm up to latitude 90 degrees and move south.
• This second cycle is called the polar cell.

Ferrell cell:

• Based on the rise of air (60th latitude) and the cooling of air (30th latitude), a third cycle will be created in the area between 60th and 30th latitude.
• This circulation is called the Ferrell cell. Near ground level, there is air traffic to the poles, with air flowing to the equator at higher levels.
• In the northern hemisphere, the air on the ground is obstructed on the right and in the southern hemisphere on the left.
• The winds from the west are called west. The polar front is located on the border between the polar east (cold) and east (warm).
• This boundary is usually between the 60th and 70th latitudes. Depression often occurs in this area.

General Atmospheric circulation and its effects on the Oceans

• The general circulation of the atmosphere also affects the oceans. The warming and cooling of the Pacific Ocean are the most important in terms of the general circulation of the atmosphere.
• The warm water in the central Pacific Ocean is gradually drifting towards the South American coast, replacing the cold Peruvian current.
• Such the presence of hot water off the coast of Peru is known as El Nino.
• El Nino is associated with pressure fluctuations in Australia and the central Pacific.
• This change in pressure conditions over the Pacific Ocean is called the Southern Oscillation.
• The combined phenomenon of El Nino and the Southern Oscillation is known as ENSO.

Global Atmospheric circulation affects Weather Patterns

• The circulation of the earth’s atmosphere creates winds across the planet as air moves from high to low-pressure areas.
• It also leads to heavy rainfall areas, such as tropical rainforests, and to dry air areas, such as deserts.
• In addition to the heat from the equator, which moves toward the poles with air circulation, ocean currents also transfer heat.
• The oceans transfer about 20 percent of the total heat from the tropics to the poles. Each ocean of surface currents called gyres has a rounded pattern.
• They are made according to an assessment of the movement of water from one climate zone to another.
• They are formed by surface winds created by the earth’s atmospheric circulation.