ATMOSPHERE Air Circulation

UNIT 7Meteorology and Climate

Atmosphere-Ocean Coupling

Why study atmospheric circulation? Atmosphere and ocean

processes are intertwined

Atmosphere-ocean interaction moderates surface temperatures, weather and climateWeather: local

atmospheric conditionsClimate: regional long-

term weather Atmosphere drives most

ocean surface waves and currents

Composition of the Atmosphere

Dry Air: 78% Nitrogen, 21% Oxygen

BUT it is never completely dry

Typically contains about 1% water vapor

Chemical residence time of water vapor in the air is about 10 days

Warm air holds much more water vapor than cold air

http://www.nature.com/scitable/knowledge/library/the-global-climate-system-74649049

Density of AirTypical air density ~ 1

mg/cm3

Temperature and pressure affect the density of air

Temperature: Hot air is less dense than cold air

Pressure: Air expands with elevation above sea level

http://www.physicalgeography.net/fundamentals/7d.html

Density and TemperatureRising air expands and

coolsVapor condenses into

clouds and precipitation

Sinking air is compressed and warmsClear air

Expanding Air Cools and Condenses

Like opening a pressurized bottle of soda

Air expands and cools

Water vapor condenses – cloud formation

Solar Heating of the EarthSolar energy absorbed unevenly over Earth’s

surface – why is air rising?

Energy absorbed / unit surface area varies with:Angle of the SunReflectivity of the surface (i.e., ice versus ocean)Transparency of the atmosphere (i.e. clouds)

Solar Insolation Variations with Latitude

Solar Heating of the EarthSunlight heats the ground more intensely in the

tropics than near poles

July

January

Solar Heating and SeasonsSeasons are caused by Earth’s 23.5° tilt

http://astro.unl.edu/naap/motion1/animations/seasons_ecliptic.html

Solar Heat EnergyEquator absorbs more heat from the sun than it

radiates away

Polar regions radiates much more heat than they absorb from the sun

Energy in at equator and heat out at poles

Heat transfer from

E.g. Equator isn’t that hot – Poles aren’t that cold

Evidence that the atmosphere (2/3) and oceans (1/3) redistribute heat (wind and ocean currents)

Convective heat transfer moderates Earth climate

http://oceanmotion.org/html/resources/solar.htm#vishead

Convective HeatConvective heat transfer model’s Earth climate

Heated air expands and rises, then cools and sinks

Equator

Poles

Atmospheric CirculationCold, more dense air sinks near the Poles

Cold, more dense air sinks near the Poles

Warm, less dense air rises near the Equator

Wind from the north

Actual Atmospheric Circulation

Air rises and sinks

More than one convection cell

Earth spins once per day that amounts to a speed for us at the surface of Earth of 100’s of miles/ hour

Coriolis Effect

Coriolis Effect Movies

http://earthsciweb.org/GeoMod/index.php?title=Coriolis

http://ww2010.atmos.uiuc.edu/%28Gh%29/guides/mtr/fw/gifs/coriolis.mov

http://science.nasa.gov/science-news/science-at-nasa/2004/23jul_spin/

The Coriolis Effect on Earth

Surface velocity increases from poles to equator

Points on the equator must move faster than points near the poles to go around once a day

Latitude velocity differences land to curving paths

Northern hemisphere deflected to right

The Coriolis EffectStrength of Deflection varies with

latitude:Maximum at the polesZero(!) at equator

Faster a planet rotates, the stronger the Coriolis effects

The larger the planet, the stronger the Coriolis effects (Jupiter spins once every 10 hours)

Hurricanes A storm with lots of clouds has rising air – thus

low-pressure at the surface!

Converging air sets up counter-rotation (cyclonic)

Spinning counter clockwiseBending to the left

Spinning clockwiseBending to the right

Hurricanes – Low PressureHurricane is rising and already

has moisture in it

Low pressure system at the surface – air rising so that means air is being sucked in at the base

Arrows get defected by Coriolis to the right

Set up a counter clockwise circulation in the northern hemisphere

L

High PressureAir sinks and compresses

and it gets warmer and dry

See clear air not clouds

Pushes air away

Rotate clockwise in northern hemisphere

H

Atmospheric CirculationSinking air at 30° –

deserts

Easterly winds – trade winds

Westerly winds at 30° and 60°

Atmospheric Circulation 3 convection cells in each hemisphere

Each cell: ~30° latitudinal width

Veritcal MotionsRising Air: 0° and 60° LatitudeSinking Air: 30° and 90° Latitude

Horizontal MotionsZonal winds flow nearly along latitude linesZonal winds within each cell band

DUE to DEFLECTIONS BY CORIOLIS!

Sea BreezeLand warms fastest during the day. Air during

the day expands and rises

Ocean surface temperature changes slowly. Air cools and becomes more dense, sinks then begins to rise over the land.

Result – wind from sea towards land

Land BreezeLand cools fastest at night. Low heat capacity. Air

contracts and sinks

Ocean surface temperature changes slowly. Air is pushed away and up by cooler denser land air.

Result – wind from land towards sea

Marine LayerCold waters, warm air: think cloud layer on

ocean surface

Subtropics: H pressure, regional subsidence

Cloud layer flows onto land at night

Evaporates over land by day

LAND OCEAN

Marine LayerCold waters, warm air: think cloud layer on

ocean surface

Subtropics: H pressure, regional subsidence

Cloud layer flows onto land at night

Evaporates over land by day

LAND OCEAN

Marine LayerCold waters, warm air: think cloud layer on

ocean surface

Subtropics: H pressure, regional subsidence

Cloud layer flows onto land at night

Evaporates over land by day

LAND OCEAN

Marine LayerCold waters, warm air: think cloud layer on

ocean surface

Subtropics: H pressure, regional subsidence

Cloud layer flows onto land at night

Evaporates over land by day

LAND OCEAN

Marine LayerCold waters, warm air: think cloud layer on

ocean surface

Subtropics: H pressure, regional subsidence

Cloud layer flows onto land at night

Evaporates over land by day

LAND OCEAN