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

ITCZ = Intertropical Convergence Zone(Convergence – movement of air inward toward a low pressure region

Rising air hits stratospheric barrier and is forced to diverge(Divergence – movement of air outward from a high pressure region)

As air rises, it cools and condenses, forms cloudsITCZ = 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

EXPE

CTED

OBS

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• 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

Coriolis Effect

Northern Hemisphere – deflected right

Southern Hemisphere – deflected left

RESU

LT

ACTU

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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 0C– 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 100C– 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 UPLIFTDominant 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

DESE

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