Topic 11Global Atmospheric Circulation

GEOL 2503Introduction to Oceanography

Earth’s Sources of Heat

• Internal Heat from Radioactivity– Drives mantle convection and plate tectonics

• External Heat from Solar Radiation– Drives the hydrologic cycle and convection

currents in the atmosphere and oceans

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Why is it warm near the equator and cold near the poles?

•The atmosphere reflects, scatters and absorbs solar radiation. At high latitudes solar radiation travels a longer path through atmosphere.• Equal amounts of sunlight are spread over a greater surface area near the poles than in the tropics.• Ice near the poles reflects much of the energy that reaches the surface there.

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A. vernal equinox

B. summer solstice

C. winter solstice

D. autumnal equinox7

A. EquatorB. Tropic of CapricornC. Tropic of Cancer

D. Arctic CircleE. Antarctic Circle

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Imbalance of heat on Earth surface drives global atmospheric circulation

Winds and ocean currents redistribute heat around Earth

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CONVECTION CURRENTS

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Ascending air cools as it expands.

Cooler air can hold less water, so water vapor condenses into tiny droplets - clouds.

Descending air warms as it compresses – the droplets (clouds) evaporate.

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On a hypothetical non-rotating Earth, atmospheric circulation would be a single convection cell in each hemisphere.

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But because of rotation, the picture is more complex:

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Coriolis Effect

• Because of Earth’s rotation• Deflects moving objects to the right in

northern hemisphere• Deflects moving objects to the left in

southern hemisphere• Caused because of the variation of rotation

speed with latitude

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• Motions must be large enough for Earth’s rotation to have an effect

• Large scale movements only• Will not be the case for cats, nor for water going

down a drain

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What is wind?

• Horizontal movement of air• Rising or sinking air is technically not wind• Winds are named for the direction from

which they blow• Trade winds are belts of fairly steady wind,

so named because of their importance to commerce, or trade

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Can you point out the Intertropical Convergence Zone?

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US weather moves west to east

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Modifying the Wind Bands

• Seasonal—monsoons• Day/night—seabreeze/

landbreeze• Topography—

orographic effect

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Seasonal Changes

• Summer– Land warmer than ocean– Air rises over land—low pressure– Air sinks over ocean—high pressure

• Winter– Land colder than ocean– Air sinks over land—high pressure– Air rises over ocean—low pressure

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Meteorological equator

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Note latitudinal migration of high and low pressure centers with the seasons (equator in red)

Is the ITCZ high or low pressure?

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The Monsoon Effect

• Changing wind direction caused by seasonal changes in heating

• Most pronounced in Southeast Asia• We also have monsoons in U.S.

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Summer Monsoon

• Land warmer• Air rises over land• Moist air flows in from ocean• Rising air cools, moisture condenses• Rainfall on land• Also call wet monsoon

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Winter Monsoon

• Ocean warmer• Air rises over ocean• Dry air flows off land to ocean• Little chance of rain on land• Also called dry monsoon

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Air flows from high pressure to low pressure38

Day/Night Effect

• Daytime:– Land warmer– Air rises over land– Air flows in from ocean– Get an onshore-blowing breeze– Called a seabreeze

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Day/Night Effect

• Night time– Water warmer than land– Air rises over water– Air flows from land to water– Get an offshore-blowing breeze– Called a landbreeze

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Fig. 52-12

Warm airover land rises.1

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Air cools athigh elevation.

Cool air over watermoves inland, replacingrising warm air over land.

Coolerair sinksover water.

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Seabreeze

Landbreeze

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The Topographic Effect

• Also called the orographic effect• Mountains force winds to rise• Moisture condenses• Rainfall on oceanside of mountains—called

orographic rainfall• Dry on opposite side of mountains—called

rain shadow

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