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Unit 10

CIRCULATION PATTERNS OF THE ATMOSPHERE

Unit Overview

This unit examines the global- and regional-scale circulation features of the atmosphere. The main sections are

  • A Model of the surface circulation
  • The actual surface circulation pattern
  • Secondary surface circulation
  • Monsoonal windflows
  • Circulation of the upper atmosphere

Major semi-permanent atmospheric features of the lower latitudes include the intertropical convergence zone (ITCZ), which is also referred to as the "equatorial low," and the subtropical high-pressure cells. The subtropical high pressure cells are origins for winds blowing towards the ITCZ and winds blowing towards the polar front. Those winds are the trade winds and the westerlies, respectively. The polar front is associated theoretically with the interactions of the westerlies and the polar easterlies, which are winds that diverge out of polar high-pressure cells. The converging air produces upper midlatitude low-pressure cells. The major high-pressure cells in the subtropics of the Northern Hemisphere are the Bermuda High and the Pacific High, while the major high-pressure cells in the upper latitudes of the Northern Hemisphere are the Canadian High and the Siberian High. The major high-pressure cells in the Northern Hemisphere besides the ITCZ are the Aleutian Low and the Icelandic Low. The above-surface features are linked closely to upper-atmospheric features and winds. A discussion of upper atmospheric circulation, including jetstreams, is included in the final segment of the unit.

Unit Objectives

  • To develop a simple model of the global atmospheric circulation
  • To discuss the pressure systems and wind belts that constitute that model circulation, and the complications that arise when the model is compared to the actual atmospheric circulation
  • To introduce the basic workings of the upper atmosphere¿s circulation


Glossary of Key Terms

Azonal flow The meridional (north-south) flow of upper atmospheric winds (poleward of 15 degrees of latitude), particularly the subtropical and Polar Front jet streams; periodic departures from the zonal (west-to-east) flow of these air currents are important because they help to correct the heat imbalance between the polar and equatorial regions.
Equatorial low The Inter-Tropical Convergence Zone (ITCZ) or thermal low-pressure belt of rising air that straddles the equatorial latitudinal zone; fed by the windflows of the converging Northeast and Southeast Trades.
General circulation The global atmospheric circulation system of windbelts and semipermanent pressure cells. In each hemisphere, the windbelts include the Trades, Westerlies, and Polar Easterlies. The pressure cells include the Equatorial Low (ITCZ), and, in each hemisphere, the Subtropical High, Upper-Midlatitude Low, and Polar High.
Inter-Tropical Convergence Zone (ITCZ) The thermal low-pressure belt of rising air that straddles the equatorial latitudinal zone, which is fed by the windflows of the converging Northeast and Southeast Trades.
Jet stream The two concentrated, high-altitude, west-to-east flowing "rivers" of air that are major features of the upper atmospheric circulation system poleward of latitude 15 degrees in both the Northern and Southern Hemispheres; because of their general occurrence above the subtropical and subpolar latitudes, they are respectively known as the subtropical jet stream and the Polar Front jet stream. A third such corridor of high-altitude, concentrated windflow is the tropical easterly jet stream, a major feature ofthe upper-air circulation equatorward of 15 degrees North. This third jet stream, however, flows in the opposite, east-to-west direction and occurs only above the tropics of the Northern Hemisphere.
Monsoon Derived from the Arabic word for "season," a regional windflow that streams onto and off certain landmasses on a seasonal basis; the moist onshore winds of summer bring the wet monsoon, whereas the offshore winds of winter are associated with the dry monsoon.
Northeast Trades The surface wind belt that generally lies between the Equator and 30 degrees North; the Coriolis force deflects equatorward-flowing winds to the right, thus recurving north winds into northeast winds.
Polar Easterlies The high-latitude wind belt in each hemisphere, lying between 60 and 90 degrees of latitude; the Coriolis force is strongest in these polar latitudes, and the equatorward-moving air that emanates from the Polar High is sharply deflected in each hemisphere to form the Polar Easterlies.
Polar Front The latitudinal zone, lying at approximately 60 degrees north and south, where the equatorward-flowing Polar Easterlies meet the poleward flowing Westerlies; the warmer Westerlies are forced to rise above the colder Easterlies, producing a semipermanent surface low-pressure belt known as the Upper-Midlatitude Low.
Polar High Large semipermanent high-pressure cell centered approximately over the pole in the uppermost latitudes of each hemisphere.
Southeast Trades The surface wind belt that generally lies between the Equator and 30 degrees South; the Coriolis force deflects equatorward-flowing winds to the left, thus recurving south winds into southeast winds.
Subtropical High The semipermanent belt of high pressure that is found at approximately 30 degrees of latitude in both the Northern and Southern Hemispheres; the subsiding air at its center flows outward toward both the lower and higher latitudes.
Upper-Midlatitude Low The semipermanent surface low-pressure belt, lying at approximately 60 degrees north and south, where the equatorward-flowing Polar Easterlies meet the poleward-flowing Westerlies; at this sharp atmospheric boundary, known as the Polar Front, the warmer Westerlies are forced to rise above the colder Easterlies.
Westerlies The two broad midlatitude belts of prevailing westerly winds, lying between approximately 30 and 60 degrees in both hemispheres; fed by the Coriolis-force deflected, poleward windflow emanating from the Subtropical High on the equatorward margin of the Westerlies wind belt.
Zonal flow The westerly flow of winds that dominates the upper atmospheric circulation system poleward of 15 degrees latitude in each hemisphere.


Unit Outline

  • A model of the surface circulation
    • The Equatorial Low and Subtropical High
      • Equatorial Low (ITCZ) is a region of low pressure and rising air
      • Air from ITCZ rises, expand, cools, and subsides in the high pressure Subtropical Highs at 30 degrees N and S
    • The Trade Winds and the Westerlies
      • Northeast trades of northern hemisphere created by air returning toward Equator, deflected to the right
      • Southeast Trades of Southern Hemisphere created by air returning toward Equator, deflected to the left
      • Westerlies from 30 to 60 degrees N and S produced by air moving poleward from Subtropical Highs
    • The Polar high, Polar Easterlies, and Polar Front
      • Polar Highs are pressure cells over each pole
      • Air moving toward Equator from poles directed to create Polar Easterlies
      • Polar Front (upper mid-latitude low) created by converging air from the Polar Easterlies and the Westerlies (Fig. 10.2)
  • Actual surface circulation pattern
    • Actual circulation more complex than idealized models
    • The Equatorial Low (ITCZ)
      • The ITCZ migrates into the "summer" hemisphere more prominently over land than water
      • ITCZ migrates generally less towards Southern Hemisphere because there are fewer landmasses
    • The Bermuda and Pacific highs
      • Subtropical Highs are actually more cellular than belt-like
      • Bermuda (Azores) High in North Atlantic
      • Pacific High in North Pacific
    • The Canadian and Siberian Highs
      • The Polar High in the Northern Hemisphere actually consists oftwo cells
        • weaker Canadian High
        • stronger Siberian High
    • The Aleutian, Icelandic, and Southern Hemisphere Upper Mid-latitude Lows (Polar Fronts)
      • Polar Front (Upper Mid-latitude Low) in Northern Hemisphere in January is actually two cells
        • Aleutian Low in North Pacific
        • Icelandic Low in North Atlantic
        • Secondary surface circulation: monsoonal windflows
    • Monsoon is a regional wind that blows onto and off certain land areas seasonally
      • In coastal Asia, summer wind reversal brings moist onshore winds: the wet monsoon
      • In coastal Asia, winter offshore winds produce the dry monsoon
  • Circulation of the upper atmosphere
    • Zonal flow maintains westerly (west-to-east) winds
    • Jet streams located at subtropics and polar fronts, due to pressure gradients in upper atmosphere
    • Azonal flow to correct global heat imbalance
      • Troughs of low pressure
      • Ridges of high pressure


Review Questions

  1. Describe the Bermuda and Pacific Highs, using Fig. 10.3 in your textbook as a guide.
  2. Compare and contrast the Polar Easterlies, Polar Highs, and the Polar Fronts.
  3. What would the hypothetical atmospheric circulation patterns look like on an Earth that had no topographic relief and did not rotate? Use Fig. 10.1 in your textbook as a reference.