Atmospheric Circulation

Scales of Motion: size and duration

Macroscale Winds:
Planetary-scale Winds:
     • major global wind belts
     • 1000 to 40,000 km
     • Weeks or longer
Synoptic-scale Winds:
     • weather scale winds
     • 100 to 5000 km; Days to weeks
     • strong horizontal air movement

Mesoscale Winds:
     • 1-100 km
     • Minutes to hours
     • Strong vertical air movement

Microscale Winds:
     • <1 km
     • Seconds to minutes

Local Winds

Mesoscale winds produced by locally generated pressure gradients

Sea-Land Breezes

Valley and Mountain Winds
Mountain barriers cause variable wind directions, updrafts and downdrafts are common
Valley Winds: Air along mountain slope is heated more intensely than air at the same elevation over the valley.
The warm air rises creating a valley wind.
Mountain Winds: At night, rapid radiation loss creates cool, dense air which sinks along the slope.

Chinook winds
Warm, dry, leeward winds associated with the orographic precipitation.

Katabatic Winds (Fall Winds)
Wintertime winds which develop as cold highland air descends or flow over the edge of the highland area.

Country Breezes (Urban Heat Island Winds)
Dark surfaces in cities absorb more insolation, causing the air in urban areas to warm and rise.
The warm rising air is replaced by cooler air from the surrounding countryside.

Macroscale Winds: Ideal Global Wind Patterns

Start with an ideal globe model - no surface features - no seasons

Intertropical Convergence Zone

Incoming solar radiation will be most intense at the equator - warms air at the equator.
Warm air will rise, creating a low pressure center.
Surrounding air will move into the equatorial region, into the low pressure area - this air in turn is heated by insolation and rises.

Intertropical Convergence Zone

Air converges at the equator (ITC).
This belt is also known as the Doldrums - surface winds are weak and variable - air movement is primarily vertical.

Hadley Cells

Warm air at the equator rises, then moves poleward.
Cools and descends at ˜30° Lat., creating a high pressure area.
Air moves out of the high pressure centers towards the equatorial low pressure centers completing a circulation cell known as a Hadley Cell.

Subtropical High Pressure Belt

The high pressure areas at ˜30° Lat.
Within theses belts, 2 to 4 large, stable anticyclones develop.
There are 2 belts, one in the northern hemisphere and one in the southern.
Winds are weak and variable - air movement is primarily vertical. This area is also known as the Horse Latitudes.

Prevailing Westerlies

Poleward of the Subtropical High Pressure Belt
Air movement out of the high pressure centers into the midlatitudes is out of the west.
Tend to be found between 30-60° Latitude.
This region is complicated by the Polar Front.

Polar Front

Polar regions, with very cold air, tends to be a high pressure center.
Air movement away from the poles towards the midlatitudes.
Air in the Subtropical High Pressure Belt is hot and moving poleward.
Two conflicting air masses, one hot, one cold, meet at the Polar Front.

Ferrel and Polar Cells

Along the Polar Front, the warm air will rise over the cold air.
Some of this air returns at high altitude to the region above the Subtropical High Pressure Belt completing the Ferrel Cell.
Some of this air continues poleward completing the Polar Cell circulation.

Macroscale Wind Belts

Real Wind Patterns

January Wind Pattern
January Polar Wind Pattern
July Wind Pattern
July Polar Wind Pattern

Monsoons

The Monsoons in Southern and Southeastern Asia: 3 factors.
1)
2)
3)
In January:
     •The wind direction -
     •Air mass -
What happens to this air mass?
In July:
     •The wind direction -
     •Air mass -
What happens to this air mass?

North American Monsoon

Not a true "monsoon"
Warm temperatures in the southwestern U.S. due to intense solar radiation.
This creates a ______ pressure area.
Moisture is brought in from the ___________________________.
This air warms; then rises; creating local thunderstorms.

High Altitude Global Circulation

Similar to low altitude but with differences.
1) Equatorial Easterlies: 15°S-15°N Lat.
2) Tropical High-Pressure Belt: 15°-20° Lat.
3) Upper-air Westerlies: 25° Lat. to poles

Rossby Waves

Undulations in the Upper-air Westerlies.
Form along the Polar Front - equalizing temperatures between cold and warm air masses along Polar Front.
Move from west to east.
Shorter waves can be found within the longwaves.
Smaller waves move faster than longwaves.

Jet Streams

Narrow zones of high speed, high altitude air flow.
Three kinds of Jet Streams

Polar-Front Jet Stream
Occurs along the Polar Front
Associated with Rossby Waves
Found between 35°-65° Lat.
10-12 km altitude
Wind speeds of 350-450 kph
Winds: west to east

Subtropical Jet Stream
Occurs just poleward of the Hadley Cell
Found at ˜30° Lat.
~14 km altitude (Tropopause)
Wind speeds of 345-385 kph
Winds: west to east

Tropical Easterly Jet Stream
Occurs only in the Northern Hemisphere
Associated with the Monsoons in southern Asia and southeast Asia
Winds: east to west

Poleward Transport of Heat

Where is the greatest amount of solar radiation received on Earth?
Where is the least amount of solar radiation received on Earth?

Energy Surplus -Deficit

Low latitudes: energy surplus - more energy is gained than lost
High latitudes: energy deficit - more energy is lost than gained
Global Energy Balance on a global scale
Excess energy moved from low latitudes to high latitudes

Heat Transport

2 methods:

Ocean Circulation

Two Methods:

Surface Currents:
Warm tropical water moves to the poles. Cool arctic water returns to the tropics.
Counterradiation from the warm water transfers energy to the atmosphere as it moves poleward.

Vertical Circulation:
Currents created by changes in density
Equatorial heating creates warm water
As the water moves poleward evaporation occurs
Evaporation increases the salinity of the surface water, and cools the water
Salt water is more dense
Colder water is more dense than warm
Increased density causes the water to sink at the high latitudes
Bottom current returns to the tropics

Surface ocean currents are driven by atmospheric circulation due to friction.
Not all of the energy is transferred: slower ocean movement.

Coriolis Effect on Ocean

Ocean currents are deflected to the right in the Northern Hemisphere
To the left in the Southern Hemisphere.

Ocean Gyres: Large circular currents due to Subtropical High Pressure Belt

Ekman Spirals

Ekman Spiral
Upwelling Currents due to Ekman Transport

El Niño

Normally: cold water off the coast of South America
Warm equatorial current flows to the west
El Niño: warm water off the coast of South America
Warm equatorial current flows to the east

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