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
Deflection of air flow due to Coriolis force
Low Pressure
_
Surfaces of
constant pressure
y (latitude)
p
y
x (longitude)
+
High Pressure
Balanced flow: geostrophic balance
F
pressure
Low Pressure
High Pressure
F
Coriolis
1)
2)
3)
4)
Coriolis force 90 degrees to the right
Pressure force down the gradient
Air flows along the line of constant pressure
Particles will have the high pressure on their right (opposite
in the southern hemisphere)
Geostrophic circulation
• 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?
L
H
Cyclonic
Anti-cyclonic
Geostrophic circulation
Now we are in the Southern Hemisphere,
what would be the direction of the
geostrophic flow?
L
H
Cyclonic
Anti-cyclonic
Tropical cyclone: an intense low pressure system
H
L
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
Coriolis effect
Air flows around the
low pressure
 counter-clockwise
Energy source
Warm, moist air from
tropical ocean
Storm surge
Graphic illustration by National Hurricane Center
http://www.nhc.noaa.gov/surge/animations/hurricane_stormsurge.swf
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
ITCZ
Warm SST, low SSS
Excess precipitation
Salinity
Subtropics
Warm SST, high SSS
Excess evaporation
Sea surface temperature and salinity are controlled by air-sea interaction
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)