Circulation in the atmosphere

Circulation in the Atmosphere

Balance of forces in the fluid motion

• Forces due to planetary rotation– Centrifugal force Geoid– Coriolis force Deflection of moving fluid

• Pressure gradient force– From high to low pressure

x (longitude)

y (latitude)

+

_

Surfaces ofconstant pressure

Deflection of air flow due to Coriolis force

p

y

High Pressure

Low Pressure

Fpressure

FCoriolis

1) Coriolis force 90 degrees to the right2) Pressure force down the gradient3) Air flows along the line of constant pressure4) Particles will have the high pressure on their right

(opposite in the southern hemisphere)

Balanced flow: geostrophic balance

High Pressure

Low Pressure

H

L

• What would be the direction of (1) the pressure force and (2) the Coriolis force in geostrophic balance?

• What would be the direction of geostrophic flow?

Geostrophic circulation

Cyclonic Anti-cyclonic

Geostrophic circulation

H

L

Now we are in the Southern Hemisphere, what would be the direction of the geostrophic flow?

Cyclonic Anti-cyclonic

Tropical cyclone: an intense low pressure system

L

H H

H

H

Air circulates around the low pressure

Flow under radial pressure gradient

• A bucket full of water• Open up a hole in the middle

- Generates a low pressure

• What would happen to the water?

- Non-rotating

- Rotating

Tropical cyclones

Energy sourceWarm, moist air from tropical ocean

Coriolis effectAir flows around the low pressurecounter-clockwise

http://www.nhc.noaa.gov/surge/animations/hurricane_stormsurge.swf

Storm surge

Graphic illustration by National Hurricane Center

Atmosphere-ocean interaction

What are the ways that the Earth’s atmosphere and ocean interact?

Wind-driven ocean currents

Atmospheric winds applies frictional force on the surface waters

Ocean waves

Wind-driven circulation(next week)

Water cycle

Implications to the salinity of seawater?

Temperature

Salinity

Sea surface temperature and salinity are controlled by air-sea interaction

ITCZ Warm SST, low SSS Excess precipitation

SubtropicsWarm SST, high SSSExcess evaporation

4 components of air-sea heat flux• Incoming shortwave radiation

– Latitudes, cloud cover

• Outgoing longwave radiation– Temperature, water vapor, cloud cover

• Sensible heat flux– Boundary layer turbulence

• Latent heat flux– Evaporation

Shortwave radiation• Climatology

• Average over long time period (1968-1996)

– Upward positive (positive into the atmosphere)

Factors controlling SW radiation• Latitude• Cloudines (albedo)

Longwave radiation• Climatology (1968-1996)

– Upward positive (positive into the atmosphere)

Longwave radiation• SST, cloud and water vapor

Sensible heat flux• Climatology (1968-1996)

– Upward positive (positive into the atmosphere)

– Turbulent heat exchange between ocean and atmosphere

Sensible heat flux• Driven by surface wind speed and air-sea temperature difference

Latent heat flux• Climatology (1968-1996)

– Upward positive (positive into the atmosphere)

– Proportional to the rate of evaporation

Latent heat flux• Rate of evaporation

- Wind speed and relative humidity

Net heat flux• Climatology (1968-1996)

– Upward positive (positive into the atmosphere)

Ocean heat transport

• Ocean gains heat from the atmosphere in tropics

• Ocean circulation transports heat poleward, and release back to the atmosphere at high latitudes

Biogeochemical cycle(in October after midterm)