1

Wednesday, Wednesday, April 07,April 07, 20102010

Chapter 7:Chapter 7:Circulation of the AtmosphereCirculation of the Atmosphere

•• Scales of Atmospheric MotionScales of Atmospheric Motion• Large and Small Scale Circulation

•• Local WindsLocal Winds• Land and Sea Breezes• Mountain and Valley Breezes

•• Global CirculationGlobal Circulation• 3-Cell Circulation Model

•• Global Pressure and WindsGlobal Pressure and Winds• Idealized Zonal Pressure Belts• Global Scale Winds

•• MonsoonsMonsoons• Asian Monsoon• North American Monsoon

Chapter 7: Circulation of the AtmosphereCirculation of the Atmosphere

•• The The WesterliesWesterlies•• Jet StreamsJet Streams

• Polar Jet Stream• Sub-Tropical Jet Stream

•• Global Winds and Ocean CurrentsGlobal Winds and Ocean Currents• Ocean Currents Follow Wind• Upwelling (Eastern Shores)

2

•• Large and Small Scale CirculationLarge and Small Scale CirculationAll winds are highly integrated

• Macroscale (2 kinds)(2 kinds)• Planetary

• Can last for weeks or longer• 1,000 - 40,000 km• Westerlies and Trade Winds

• Synoptic • Days to weeks• 100 – 5,000 km• Mid-latitude cyclones,

anticyclones and hurricanes• Mesoscale

• Minutes to hours• 1 – 100 km• Thunderstorms, tornadoes and

land-sea breezes• Microscale

• Seconds to minutes• Less than 1 km• Turbulence, dust devils

and wind gusts

Scales of Atmospheric Motion

•• Land Land –– Sea BreezesSea Breezes• Day

• Land heats quickly, air over land heats

• Warm air Rises• Leaves LOW pressure over ground• Sea is relatively HIGH• Sea Breeze = Sea ���� Land• Brings moist cool air

• Night• Land cools quickly, Sea is warm, air

over sea heats• Warm air Rises• Leaves LOW pressure over sea• Land is relatively HIGH• Land Breeze = Land ���� Sea

Local Winds

•• Mountain Mountain –– Valley BreezesValley Breezes• Day

• Air along slopes is heated• Air over valley (at same elevation) is

cooler (farther from heat source – land)• Valley Breeze = Valley ���� Mountain

• Night• Slopes cool faster

• Mountain Breeze = Slopes ���� Valley

•• Country BreezeCountry Breeze• Urban heat island effect in action

• Cities are warmer than the surrounding countryside

• Warmer air over cities rises, expands, cools, leaves Lower Pressure

• Country Breeze = Country ���� City

3

Land-Sea Breezes

Mountain-Valley Breezes

4

Idealized Pressure• In General, Highs and Lows are found along certain latitudes• These correspond to global circulation

models of rising and sinking air• Polar High (90°N, 90°S)

• Subpolar Low (Polar Front) (60°N, 60°S)

• Subtropical High (30°N, 30°S)

• Equatorial Low (0°)

Global Circulation

Idealized Pressure Zones

90°

60°

30°

0°

30°

60°

90°

HHHH

HHHHHHHHHHHH HHHHHHHH

HHHHHHHHHHHH HHHHHHHH

HHHH

LLLL LLLL LLLL LLLL

LLLL LLLL LLLL LLLL

LLLL LLLL LLLL LLLL LLLLLLLL

5

•3-Cell Circulation Model•Vertical Structure of the Atmosphere•This describes the interconnections between the rising and sinking air patterns

•Hadley Cells

Global Circulation

Idealized Pressure Zones

90°

60°

30°

0°

30°

60°

90°

HHHH

HHHHHHHHHHHH HHHHHHHH

HHHHHHHHHHHH HHHHHHHH

HHHH

LLLL LLLL LLLL LLLL

LLLL LLLL LLLL LLLL

LLLL LLLL LLLL LLLL LLLLLLLL

6

Idealized Pressure Zones

• Global Scale Winds• These are winds aloft (above the surface)

• Driven by the Pressure Gradient Force (H � L) and Coriolis Effect (NH = )(SH = )

• Form consistent and persistent directional global scale winds• Polar Easterlies• Prevailing Westerlies• North East Trades / South East Trades• Also… Areas of notoriously low winds

• Inter-tropical convergence zone &Doldrums (Equatorial Low)

• Horse Latitudes (Subtropical High)

Global Circulation

7

Idealized Pressure Zones

90°

60°

30°

0°

30°

60°

90°

HHHH

HHHHHHHHHHHH HHHHHHHH

HHHHHHHHHHHH HHHHHHHH

HHHH

LLLL LLLL LLLL LLLL

LLLL LLLL LLLL LLLL

LLLL LLLL LLLL LLLL LLLLLLLL

Global Circulation

8

Global Circulation

• Monsoon refers to a SEASONAL change in wind patterns for a large geographic region

• Note: Monsoon does NOT mean heavy rain!• Monsoons are like a land-sea breeze that occurs ann ually

Monsoons

Monsoon(Seasonal)

Land/SeaBreeze(Daily)

Winter Summer

Day

LLLL

LLLLHHHH

HHHH

HHHH

Night

HHHHLLLL LLLL

9

• Concerns the Indian Ocean and Southern Asia• The yearly two-phase change is similar to the daily change

of a land-sea breeze• Winter Conditions

• High mountains and plains cool quickly• Very cold air ���� Dense ���� Sinking air ���� High Pressure• Over the water, relatively lower pressure• Cold, dry air rushes out across Southern Asia• Bringing cold/cool and very dry air

• Summer Conditions• High plains warm quickly (largest landmass)• Warm air ���� Rises ���� Low Pressure• Over the water, relatively higher pressure• Hot, moist air rushes from the ocean

across the Southern Asia• Brings heavy rains and flooding

Asian Monsoon

Asian Monsoon

10

Asian Monsoon

Jet Streams

Jet Streams: Jet Streams: Narrow ribbons of high-speed winds

• Located high in the troposphere (pressure gradient force increases with height = stronger & faster winds)

• Large differences in temperature at surfacelead to large differences in pressure

• Surface pressure is expanded with altitude � Fast Winds

• Winds are NOT straight, but meander following a curved path(Rossby Waves ) which relates to the N/S interactions

11

Jet Streams

•• Polar Jet StreamPolar Jet Stream• Occurs above the Polar Front – between the cold, dry

Northern air and the warm, moist Southern air• Speed usually 50-75 mph, but can exceed 300 mph• Seasonal shift (Winter = South, Summer = North)

•• SubSub --Tropical Jet StreamTropical Jet Stream• Occurs above the Subtropical High• Mainly a winter event• Much weaker than the Polar Jet

Global Winds and Ocean CurrentsGlobal Winds and Ocean Currents

•• Global Scale WindsGlobal Scale Winds– Push water beneath them– Continents deflect and redirect the flows– Move heat from the equator to the poles

•• Ocean CurrentsOcean Currents– follow certain predictable pathways determined by…

• Global Scale Winds• Continental Land Masses

– They bring Climate ConditionsClimate Conditions (mostly temperature) with them– Transfer of Heat – Warm and Cold Currents– Much of this effect is at the surface…– But some vertical movement of ocean water also occurs…

•• Upwelling Upwelling : The rising of deep, cold water from wind action on the surface

– As the wind pushes surface water away, cold water from below is brought to the surface

– This water is often nutrient rich, providing food for many fish communities

12

Global Winds and Ocean CurrentsGlobal Winds and Ocean Currents

Polar Easterlies

Westerlies

Northeast Trades

Southeast Trades

Westerlies

Polar Easterlies

Global Winds and Ocean CurrentsGlobal Winds and Ocean Currents

Polar Easterlies

Westerlies

Northeast Trades

Southeast Trades

Westerlies

Polar Easterlies

13

Vertical Movements: Upwelling & DownVertical Movements: Upwelling & Down--wellingwelling

•• UpwellingUpwelling : How water moves from the deep ocean up to the surface and down again

• UP = UPWELLING–Water is pushed against the side of a continent

FROM BELOW and can go nowhere but UP–This water will be COLD–This will bring COOLER CONDITIONS

• DOWN = DOWN-WELLING–Water is pushed against the side of a continent

FROM THE SURFACE and can go nowhere but DOWN–This water will be Warm–This will bring WARMER CONDITIONS

Ocean CurrentsOcean CurrentsVertical Water Movement