GEO 130 - Winds and Global Circulation

Air pressure

Force exerted by air per unit area, determined by the weight and motion of air

Barometer (mercury or aneroid)

Instrument that measures air pressure

Wind

Typically horizontal motion of air across Earth's surface, driven by pressure differences from uneven heating

Two properties of wind

Speed and direction

Advection

Horizontal movement of air or liquid

Convection

Vertical movement of air or liquid

Anemometer

Instrument that measures wind speed

Wind vane

Instrument that measures wind direction

Wind direction naming convention

Winds are named for the direction they blow FROM

Pressure gradient force (PGF)

Air pressure difference over a distance; the fundamental driving force of wind, affecting both speed and direction

Gravitational force (GF)

Pull on gas molecules that limits winds to the troposphere and works in the background

Coriolis effect

Deflection of wind caused by Earth's rotation: to the right in the NH, to the left in the SH

Frictional force

Drag from surface roughness that slows wind and, with Coriolis, can change its direction

Convection loop

Circulation made by PGF and gravity: surface and upper-level horizontal flow joined by vertical motion

Cause of spiraling winds

The pressure gradient pulls air toward lows, while Coriolis deflects it and friction slows it, so air spirals around pressure centers

Cyclone

Converging wind system around a LOW; counterclockwise in the NH, clockwise in the SH

Anticyclone

Diverging wind system around a HIGH; clockwise in the NH, counterclockwise in the SH

Weather associated with cyclones (lows)

Rising air that cools, forming clouds and precipitation; often stormy

Weather associated with anticyclones (highs)

Sinking air that warms, bringing clear, dry, fair skies

Single-cell model

Global circulation model that assumes Earth does not rotate

Single-cell circulation pattern

On a non-rotating Earth, air rises at the equator, flows aloft to the poles, sinks, and returns along the surface to the equator

Three-cell model

Global circulation model for a rotating Earth

ITCZ (Intertropical Convergence Zone)

Equatorial low-pressure trough where surface winds converge and air rises

Polar high

High-pressure ridge over the poles

Subtropical high

Semi-permanent high-pressure cells at about 30° latitude

Subpolar low

Semi-permanent low-pressure cells at about 60° latitude

Driving force of global circulation

Uneven heating: the pressure difference between the equatorial low and the polar high

Trade winds

Prevailing surface winds blowing from the subtropical highs toward the ITCZ

Westerlies

Prevailing surface winds blowing from the subtropical highs toward the subpolar lows

Polar easterlies

Prevailing surface winds blowing from the polar highs toward the subpolar lows

Hadley cell

Circulation cell between the equator and about 30° latitude

Ferrel cell

Circulation cell between about 30° and 60° latitude

Polar cell

Circulation cell between about 60° latitude and the pole

Pressure and wind belts, equator to pole

ITCZ (0°) → trade winds → subtropical high (30°) → westerlies → subpolar low (60°) → polar easterlies → polar high (90°)

Local winds

Small-scale winds driven by local heating differences and terrain, easily overwhelmed by larger air motions

Land–sea breeze

Local wind caused by the heating difference between land and water

Sea breeze

Daytime local wind blowing from water to land, because land heats faster

Land breeze

Nighttime local wind blowing from land to water, because land cools faster

Mountain–valley breeze

Local wind caused by heating differences along slopes and valleys

Valley breeze

Daytime upslope wind as mountain slopes heat up

Mountain breeze

Nighttime downslope wind as cooled air sinks off the slopes

Chinook wind

Warm, dry wind descending the leeward side of a mountain range

Surface ocean currents

Ocean flow driven by frictional drag of prevailing winds, deflected by Coriolis

Thermohaline circulation

Deep ocean circulation driven by density differences from temperature and salinity

Tides

Ocean movement caused by astronomical/gravitational forces

Gyre

Large circular ocean current system formed around a high-pressure cell

Gyre rotation direction

Clockwise in the NH, counterclockwise in the SH

Which coasts get cold vs. warm currents

Cold currents run along the west coasts of continents; warm currents run along the east coasts

Upwelling

Rising of deep, cold water toward the surface

Downwelling

Sinking of surface water toward the deep ocean

Atmosphere–ocean interaction

Winds drive surface currents, while ocean heat and moisture feed back into the atmosphere

ENSO (El Niño Southern Oscillation)

Reversal of normal ocean currents and wind patterns in the tropical Pacific

El Niño

Warm phase of the tropical Pacific current and wind reversal

La Niña

Negative phase that mirrors El Niño

El Niño pattern

Trade winds weaken, warm water shifts east toward South America, and upwelling off Peru weakens

La Niña pattern

Trade winds strengthen, the eastern Pacific turns cooler than normal, and upwelling increases

El Niño impacts

Heavy rain and flooding in western South America; drought in Australia and Indonesia

Teleconnection

Link by which conditions in one region affect weather in distant parts of the world