Atmospheric Circulation

Causes of Atmospheric Circulation

1) Uneven solar energy inputs

2) Rotation of the Earth

Global Circulation Patterns

Local Events and Storms

Uneven Solar Energy Inputs: Earth is heated unevenly by the sun due to different angels of incidence between the horizon and Sun.

This angle of incidence is affected two factors:

1) Latitude: solar inputs are most dense when the sun is overhead in the tropics; reflection is low. The reverse holds true in polar regions.

2) Season: Due to Earth’s annual orbit around the Sun on an axis tilted by 23.5º.

Vernal equinox Mar 22nd

Autumnal equinox Sept 22nd

Summer solstice June 22nd

Winter solstice Dec 22nd

The Global Heat Budget with Latitude:

Losses by long-wave radiation is fairly constant at most latitudes, whereas inputs are variable and greatest in the tropics.

Heat inputs from the tropics are delivered to the poles via atmospheric and ocean circulation, each delivering about 50% of the tropical heat.

These circulation patterns are partly due to heat gradients, or more accurately pressure gradients.

Atmospheric pressure gradients = winds

Low Pressure: warm, moist, low density air, is lighter and rises.

High Pressure: cool, dry, high density air is heavier and sinks

Net force from high to low pressure is called the pressure gradient force and initiates air movement, i.e. wind.

Convection cells establish themselves due to the pressure gradient with latitude and altitude. The flow of air due to pressure differences generates winds.

The intense heating of the tropics by solar radiation sets up a strong convection cell near the equator. As water evaporates from the ocean, the overlying air becomes less dense .

Air looses moisture and cools as it rises and moves poleward. It becomes more dense and sinks at poles

Winds would blow across the surface of Earth from the poles toward the tropics.

Idealized pattern without accounting for Earth’s rotation.

Rotation of the Earth: Coriolis Effect

The idealized model of a single convection cells driving atmospheric circulation in each hemisphere is incorrect.

The eastward rotation of the Earth causes a deflection in wind flow (Coriolis Effect) that results in three smaller atmospheric convection cells in each hemisphere. Perspective of earthbound observer.

Gaspard Gustavede Coriolis, 1835.

The idealized model of a single convection cells driving atmospheric circulation in each hemisphere is incorrect.

The eastward rotation of the Earth causes a deflection in wind flow (Coriolis Effect) that results in three smaller atmospheric convection cells in each hemisphere.

(24,864 miles)

24h per rotation regardless of latitude.

(18,800 miles)

1036 mi/h

Global Circulation Patterns• Again, there are three atmospheric convection cells per

hemisphere (Hadley, Ferrel, and Polar).

• Low pressure belts are where air rises and climate is wet (Intertropical Convergence Zone (0º) and Polar Front (60º)).

• High pressure belts are where climate is dry (Horse Latitudes (30º) and Polar High (90º)).

• Surface winds converge at low pressure belts:– ITCZ: NE & SE Trade Winds (easterlies).– Polar Front: Westerlies and Polar Easterlies.

• There are latitudinal shifts in these patterns with season.

Sea Breezes and Land BreezesThere are localize patterns of air circulation superimposed or superseding those of global scale.

These too are the result of differential heating causing air pressure gradients, but their cause is due to difference in the specific heat of land versus sea, not solar intensity.

Monsoons:

Caused by: 1) difference in specific heat of land and sea; and 2) seasonal shift in the ITCZ.

Winter: weaker NW and NE monsoons of SE Asia and N. Australia. Cool and dry.

Summer: strong SW and SE monsoon of India, SE Asia and parts of Africa. Intense heating of Asian continent and warm moist Indian Ocean air create extremely wet, warm and stormy conditions.

Cyclonic Storms• Extratropical Cyclones (nor’easterns)

– Created at the Polar Front.– Involves two air masses.– Meandering (wave) creates low pressure center.– Air from both sides flow inward and begin rotating due to Coriolis effect

(c.cw. in N.hemi.)– Rising warm moist air condenses, fueling storm energy and creating strong

wind and rains.– Usually during winter when differences in air masses are extreme.– Strongest winds hit northeast US and eastern Canada from the northeast

(hence nor'eastern)

• Tropical Cyclones (hurricanes, typhoons, cyclones, willi-willies)– Created by disturbances within a single tropical air mass.– Genesis involved thunderstorms during periods of low wind shear.

Ash Wednesday Storm, March 7, 1962

Extratropical Cyclone on Mars

Tropical Cyclones: Move westward and poleward.

Willi-willies

Begins as a tropical depression over warm water (>25ºC).  Most of the energy associated with a storm derives from the latent heat of evaporation which is given off during condensation of rising moist air in vertical thunderstorms.

Creates intense low pressure. Coriolis effect initiates rotation of air as it moves inward toward the low pressure center = "eye". Upgrades to a hurricane when the wind velocity increases above 74 mph.  Winds circulate around the eye in a counter-clockwise direction in the northern hemisphere. 

Weakens when they travel over colder water or land.

Low wind shear favors development!

Mitch (Oct 1998)

Tegucigalpa, Hondurus