4.2

• Parallels


  Lines of constant latitude

  
• Zonal flows


  Winds parallel to parallels
• Meridians


  Lines of constant longitude
• Meridional flows


  Winds parallel to meridians
• Mid-latitudes


  Between latitudes of 30° and 60°

  
• High latitudes


  Between latitudes of 60° and 90°
• Low latitudes


  Between latitudes of 30° and 60°

  
• Extratropical


  Regions not in the tropic (i.e., poleward)
• Extratropical cyclones


  Low pressure centers outside of topics
• Atmospheric Circulation


  1. Trade winds at low latitudes blowing towards the equator in both hemisphere

  2. Intertropical convergence zone (ITCZ) with hot, humid air and heavy precipitation

  3. Subtropical highs at 30° latitude with calm winds

  4. Cyclones and anticyclones in mid-latitudes with corresponding weather patterns

  5. Westerlies dominate the general circulation, with variability from highs and lows

  6. Subpolar lows at 60° latitude bring precipitation and cool temperatures

  7. Polar highs near the poles have clear skies and descending air

  8. Polar easterlies lie between the polar highs and subpolar lows

  
• Upper-troposphere


  1. Stratosphere is stable, trapping vertical circulations within the troposphere

  2. Equatorial high pressure creates easterly winds that diverge towards the poles

  3. Subtropical jet of westerly winds forms near 30° latitude

  4. Polar jet with meandering westerly winds creates Rossby waves at mid-latitudes

  5. Low pressure near the poles with cyclonic circulation

  6. Winds generally blow from the west at all latitudes except near the equator

  
• Vertical Circulations


  1. Hadley cells are vertical circulations of warm rising air in the tropics and descending air in the subtropics

  2. Trade winds at the bottom and divergent winds at the top, near the tropopause, form the Hadley cell

  3. Updraft portion often contains thunderstorms and heavy precipitation at the ITCZ

  4. Major Hadley circulation crosses the equator, with rising air in the summer hemisphere and descending air in the winter hemisphere

  5. Major Hadley cell transports significant heat away from the tropics and from the summer to the winter hemisphere

  6. During transition months, Hadley circulation has nearly symmetric Hadley cells in both hemispheres

  7. Strong but reversing major Hadley circulation partially cancels itself, resulting in a weak annual average circulation

  8. In the winter hemisphere, one or more jet streams circle the earth at mid-latitudes while meandering north and south as Rossby waves

  9. In the summer hemisphere, all circulations are weaker due to weaker temperature contrast between the tropics and poles

  
• Monsoonal Circulations


  1. High-pressure centers (anticyclones) over warm oceans, low-pressure centers (cyclones) over hotter continents

  2. Low-pressure centers over cool oceans, high-pressure centers over colder continents

  3. These circulations represent average conditions over a season

  4. Actual weather can vary from these seasonal averages

  5. Seasonally-varying monsoonal circulations superimpose on planetary-scale circulation, resulting in a complex global circulation pattern

  
• Radiative Differential Heating


  • The global circulation is influenced by differential heating, where incoming solar radiation balances outgoing infrared radiation
  • Significant imbalances occur at different latitudes, causing differential heating
  • The flux of solar radiation on the Earth's surface depends on the cosine of the latitude
  • Incoming energy adds heat to the Earth-atmosphere-ocean system
  • Infrared radiation emitted from the Earth-ocean-atmosphere system to space results in heat loss
  • The Stefan-Boltzmann law suggests uniform emission rates around Earth
  • At low latitudes, more solar radiation is absorbed than leaves as IR, causing net warming
  • At high latitudes, IR radiative losses exceed solar heating, causing net cooling
  • This differential heating drives the global circulation
  • The general circulation cannot eliminate all global north-south temperature differences, leaving a meridional temperature gradient
• Polar regions experience significant insolation
• Reflections


  From snow, ice, oceans, light-colored land, cloud top

  
• Outgoing Terrestrial radiation


  • IR emissions to space originate from Earth's surface, cloud top, and middle-altitude air
  • Net IR emissions are based on absolute temperature near the middle of the troposphere

  
• Net Radiation


  Difference between incoming solar radiation and outgoing terrestrial radiation
• Radiative Forcing by Latitude Belt


  Imbalance of net radiation between equator and poles
• General Circulation Heat Transport

  Atmospheric and oceanic circulations that undo the imbalance by removing excess heat from equator and depositing near poles
• The magnitude of the "needed transport" curve peaks at about 5.6 PW at latitudes of about 35° North and South
• Transport Achieved


  • Satellite observations of radiation and heat fluxes provide necessary transport data
  • Ocean currents dominate heat transport at latitudes 0-17° and ±40°
  • Asymmetry of ocean curve across the equator due to different ocean basin shapes and currents
  • Hadley circulation in tropics and subtropics, Rossby waves dominate atmospheric transport at mid-latitudes