Atmospheric Circulation

Winds on a Non-Rotating Earth Air at the equator warms and rises. Once aloft, air flows back towards the poles where it cools and sinks back toward the equator. Winds are named for the direction from which they blow.

The Effect of Rotation As the earth rotates, parcels that are moving are deflected by apparent forces ~ Coriolis Force C.F. – the apparent deflection or moving of the air relative to the earth’s surface. In the Northern Hemisphere, deflection is to the RIGHT. In the Southern Hemisphere, deflection is to the LEFT.

Effect of Rotation

Wind Bands

The Jet Streams Jet streams – high speed winds of the upper troposphere Polar jet streams (50° - 60° N & S)- WESTERLIES Subtropical jet streams (30° N & S) – EASTERLIES Oscillations in the jet streams are caused by strong high-pressure systems to the south and intense low-pressure systems to the north.

Hurricanes When the trade winds develop variations in speed and direction, air converges, oscillates and diverges creating an easterly wave. This easterly wave will form a tropical depression when: Sea-surface temperature is above 27° Atmospheric pressure decreases & evaporation increases The wave develops into an intense, isolated low-pressure cell

Hurricanes If the tropical depression builds in strength, a tropical storm develops; if it continues to build strength, a hurricane is formed. Northern Hemisphere – depressions flow counterclockwise Southern Hemisphere – depressions flow clockwise

Hurricanes Gain energy when: Hurricanes extract water vapor, thus heat, from the sea-surface A large amount of heat is liberated from condensing water vapor Lose energy when: They move over colder water or land

Global Air Circulation

El Nino Trade winds blow surface water away from the coast and upwelling occurs (cool water surfaces)

El Nino Sometimes the cell circulation does not follow the expected pattern. In 3 to 8 year cycles, the circulation patterns change significantly. This is caused by the Southern Oscillation (SO). a reversal in the usual westward flow of air between the low pressure area over the eastern Pacific north of Australia and the high pressure area over the eastern Pacific near Easter Island.

El Nino The reversal of atmospheric pressure causes the trade winds to weaken or reverse.

El Nino A strong El Nino counter current forms. It brings with it large mass of warm water, that moves east along the coast of the Americas. It usually arrives around Christmastime, thus the name “the Christ Child.”