THE ATMOSPHERIC CIRCULATION
SYSTEM
GOALS
• Describe the major characteristics of the
atmospheric circulation
• Explain why they occur
• Illustrate the way in which they affect the
transport of energy and materials around the
globe
OBJECTIVES
• Explain why weather and climate vary across
the globe
• Emphasize that the responses to global-scale
processes and changes may not be uniform
across the globe
Why does air move?
• Latitudinal energy distribution uneven
– Tropics – surplus
– Poles – deficit
– Temperature gradient causes density & pressure
differences
• Cause global-scale pattern of wind-belts
– NET EFFECT
• Restore latitudinal energy balance by moving surplus energy
away from the tropics to cancel deficit at poles
– Earth’s atmospheric circulation has a direct impact on
the global distribution of temperature & precipitation
THE GLOBAL CIRCULATORY
SUBSYSTEMS
• Circulation of energy and matter throughout
the Earth system is ordered
• Subsystems work to maintain the planet in
thermal and chemical equilibrium
• All subsystems act to help regulate the global
temperature
CIRCULATION PUMPS
• Short time scale (years to decades)
– Tropical ocean circulation pump
• Moves air and water over globe
• Energy source is the sun
• Atmospheric Circulation
• Longer time scales (1000s of years)
– Deep-ocean circulation pump
• Moves water
• Energy source is the sun
• Thermohaline Circulation
• Longest time scale (millions of years)
– Interior circulation pump
• Moves continents & Earth’s interior
• Energy source is radioactive decay and heat from Earth’s interior
• Convection Currents
THE ATMOSPHERIC CIRCULATION
The Movement of Air
• SYSTEM – Movement of Air
• PRODUCT – Vertical Movement of Air
• PROCESS –
– Change in buoyancy
– Mechanical forcing
Buoyancy
• Due to density differences
• T↑ : D↓ (air expands) – positive buoyancy –
air rises
• T↓ : D↑ (air condenses) – negative buoyancy
– air subsides
The Movement of Air
• SYSTEM – Movement of Air
• PRODUCT – Horizontal Movement of Air
• PROCESS – Difference in Pressure
Horizontal Movement
• Air moves from high-pressure (cool, dry air) to
low-pressure (warm, moist air)
• Air moves down the pressure gradient
• Air moves form high pressure regions to low
pressure regions
Uneven Heating of the
Atmosphere
Distribution of Insolation
• Gradient in absorbed energy single most
important control on temperature
• Most weather/climate is the response of
atmosphere to the unequal distribution of energy
by latitude
GLOBAL SCALE ATMOSPHERIC
CIRCULAITON
• Represent negative feedback loop
– Atmosphere responds to temperature gradient by
latitudinal transfer of energy to reduce gradient
and restore energy balance
– Sun continually adds energy, so balance never
attained
Rising air hits stratospheric barrier and is forced to diverge
(Divergence – movement of air outward from a high pressure region)
ITCZ = Intertropical Convergence Zone
(Convergence – movement of air inward toward a low pressure region
As air rises, it cools and condenses, forms clouds
ITCZ = extensive cloud cover and precipitation
WORLD DESERTS AT 30° N and 30° S LATITUDES
HADLEY CIRCULATION
• Air movement pattern –
– Convergence at tropics
– Divergence
– Subsidence at 30° N & S latitudes
– Dominant tropical circulation
HADLEY CIRCULATION
Hadley Cells and ITCZ
• Globally noncontinuous
• Most obvious in Atlantic and Pacific Oceans
– Large scale circulation in Southeast Asia & Indian
Ocean dominated by monsoon
• Release of latent heat in ITCZ convective
towers drive Hadley Circulation Pump
– Radiation, evaporation, transportation,
condensation
MID &HIGH LATITUDE CIRCULATION
• Cold polar air moves toward equator
• Warm tropical air moves toward pole
• Create Polar Front Zone (steep Temperature Gradients)
MERIDIONAL CIRCULATION
Alternating N & S moving air at surface
EXPECTED
OBSERVED
Coriolis Effect
• The apparent tendency for a fluid moving
across Earth’s surface to be deflected from a
straight path
• Results from observers frame of reference
• Coriolis Effect
Northern Hemisphere – deflected right
Southern Hemisphere – deflected left
RESULT
ACTUAL
MID LATITUDE FLOW PATTERNS
• Cyclonic Flow – air flowing into a
low pressure region
– Hurricane Sandy
• Extratropical cyclones – cyclones
formed outside of the tropics
• Anticyclone – air flowing out of a
high pressure region
• Circulation patterns flow along
polar front
UPPER-LEVEL FLOW
UPPER-LEVEL FLOW
• Air will flow down the pressure gradient
• Wind speed greatest where pressure gradient
greatest
– upper troposphere at mid latitudes
– Jet Stream
UPPER-LEVEL FLOW
• Due to balancing of
pressure gradient
force and Coriolis
force air flow is nearly
geostrophic (air flow
at right angles to the
gradient)
• But flows in wavelike
patterns around globe
ROSSBY
WAVES
Steer high and
low pressure
systems that
produce
weather
SEASONAL VARIABILITY
• Distribution of solar energy varies with the
seasons
SEASONAL VARIABILITY
• Tropics receive large input of solar radiation at
all times
• Seasonal variability of where sun is overhead
affects circulation patterns
SEASONAL VARIABILITY
TEMPERATURE & RAINFALL
DISTRIBUTION
• Atmospheric circulation affects global
temperature & rainfall distributions
– Atmosphere important part of thermoregulatory
system
– Evaporation/precipitation influenced by
temperature/energy
• Transport of water modifies temperature distribution by
modifying radiation budget & feeds back and affects
circulation
• TEMPERATURE, PRECIPITATION & CIRCULATION
LINKED CLOSELY BY FEEDBACKS
OCEAN LAND COMPARISONS
OCEAN
• Low albedo
• Absorb more solar energy
• Rapid downward transfer of
heat (turbulent mixing)
• High Thermal Conductivity
• High Heat Capacity
– Changes temperature slowly
• Solar radiation absorbed
below surface
LAND
• High albedo
• Absorb less solar energy
• Slow downward transfer of
heat
• Low Thermal Conductivity
• Low Heat Capacity
– Changes temperature rapidly
• Solar radiation reflected or
absorbed at surface
SEA BREEZE
CONTINENTALITY
• More extreme climate variability and
temperature changes over land compared to
water
• Winter – land surfaces much colder
• Summer – land surfaces much warmer
• Greatest seasonable variability – interior of
continents
• Least seasonal variability – tropical oceans
CONTINENTALITY EFFECTS
Global Temperature Distribution – Jan
Global Temperature Distribution – Jul
SEA LEVEL PRESSURE VARIABILITY
• Temperature difference affects mean sea level
pressure distribution
– This affects atmospheric circulation
• Seasonal temperature changes cause air flow
patterns to shift north and south
SEA LEVEL PRESSURE - JAN
SEA LEVEL PRESSURE - JUL
ATMOSPHERIC CIRCULATION
• Broad pattern of temperature distributions
determined by latitude distribution of net
radiation
• Seasonal range of temperature is small in the
tropics and increases poleward
• Seasonal variability modified by land-ocean
contrasts
MONSOON or
The Seasonal Reversal of Surface Winds
SUMMER
• Large Asian landmass heats up
– High surface temperatures
– Low pressures
– Intense convection above
surface
• Rising air replaced by moist air
moving in from high pressure
over Indian Ocean
• Clouds & heavy rainfall
WINTER
• Large Asian landmass snow
covered
– Low surface temperatures
– High pressures
– Subsidence of airmass
• Subsiding air moves out and
away from landmass and
towards Indian Ocean
• No clouds, no rain
MONSOON
MONSOONS
GLOBAL PRECIPITATION
• Water (clouds and water vapor)
– Most important substance transported in
atmosphere
• Huge role in global energy balance
• Significant factor in distribution of freshwater
• Exhibit temporal and spatial variability
IMPORTANCE OF WATER
•
•
•
•
•
•
Humans 60% water by weight
All organisms require water to live
71% of Earth’s surface covered with water
Polar ice sheets (sea ice and glaciers)
Clouds cover 50% of Earth’s surface
Water vapor varies
– 0% at poles
– 7% in tropics
WATER IS UNIQUE
• Only naturally occurring substance that exists
in all 3 phases at temperatures found on Earth
– Solid, liquid, gas
• Changes readily
– Cycles easily among Earth’ systems
• Water molecule stores great quantities of
energy
PHASES OF WATER
Latent Heat of Fusion
• Amount of energy required to convert ice to
liquid water (at sea level)
– 330kJ/kg at 0C
– 80 cal/gram
Latent Heat of Vaporization
• Amount of energy required to convert liquid
water to water vapor (at sea level)
– 2260 kJ/kg at 100C
– 539cal/gram
PHASE CHANGES
If these changes occur at different locations, there is a net transfer of energy
• influences global pattern of surface temperatures
WATER
• Water in all its phases is the primary medium
by which energy and matter are circulated
among Earth system components
MAJOR RESERVIORS OF WATER
• OCEANS (97%)
– Liquid seawater
• LAND (3%)
– Surface
• Ice sheets, glaciers, snow (¾ of 3% or 2.25%)
• lakes, rivers (less that 1% of 3% or 0.003%)
– Subsurface
• groundwater (¼ of 3% or .75%)
• ATMOSPHERE (< 0.001%)
– Water vapor & clouds
The Hydrologic Cycle
ANNUAL EXCHANGE IN RESERVOIRS
PRECIPITATION
• Occurs when atmospheric
water vapor condenses to
form small droplets of water
VAPOR PRESSURE
• Atmospheric pressure
– Sum of all partial pressures of individual gases
• the pressure each would exert if it were the only gas
present
• Vapor Pressure – pressure exerted by water
vapor
SATURATION VAPOR PRESSURE
• Vapor pressure when
condensation rate = evaporation rate
& gas is at equilibrium
• As temperature
increases, saturation
vapor pressure
increases
SATURATION VAPOR PRESSURE
• Clouds form when
air is at saturation
vapor pressure
• Air not at
saturation vapor
pressure will not
form clouds
SATURATION VAPOR PRESSURE
• To reach Saturation
Point
– increase evaporation
(and increase vapor
pressure)
– or
– reduce temperature
•
SATURATION VAPOR PRESSURE
• Saturation Vapor Pressure changes with
temperature
– Therefore to determine if clouds will form must
determine relative humidity
• The ratio of actual vapor pressure to the saturation vapor
pressure at that temperature
• When air is fully saturated (100% relative
humidity) clouds form
– Condensation requires Cloud Condensation Nuclei
(CCN)
• Aerosols, dust, pollen, sulfates, ash, etc
UPLIFT & GLOBAL PRECIPITATION
• Rising of air in the troposphere
• Precipitation occurs when air cools as it is
forced to rise
– Large scale uplift when air masses of different
densities mix
– Convective uplift
– Orographic Uplift
LARGE SCALE UPLIFT & GLOBAL
PRECIPITATION
UPLIFT & GLOBAL PRECIPITATION
• Heavy precipitation
– Along polar front
– Within ITCZ
CONVECTIVE UPLIFT
Dominant
rainfall-producing
process over
warm
landmasses in
summer
OROGRAPHIC PRECIPITATION
DESERTS
• Areas with inhibited precipitation
– Areas where uplift is suppressed
• Descending parts of Hadley Cells
– 30 N or S of equator
• Leeward side of mountain ranges
– Sierra Nevadas
• West coasts of continents south or north of mid-latitude low
press systems
– Baja & Namib Deserts
• Low temperature areas
– Antarctica
– Areas with inadequate moisture supply
• Interiors of large landmasses
– Asia
DESERTS