Winds and Global Circulation

• Atmospheric Pressure

• Winds

• Global Wind and Pressure Patterns

• Oceans and Ocean Currents

• El Niňo

Precipitation in Montana

How is Energy Transportedto its “escape zones?”

• Both atmospheric and ocean transport are crucial

• Buoyancy-driven convection drives vertical transport

• Latent heat is at least as important as sensible heat

Atmospheric Circulationin a nutshell

• Hot air rises (rains a lot) in the tropics

• Air cools and sinks in the subtropics

(deserts)

• Poleward-flow is deflected by the Coriolis

force into westerly jet streams in the

temperate zone

• Jet streams are unstable to small

perturbations, leading to huge eddies

(storms and fronts) that finish the job

Atmospheric Circulationin a nutshell

• Winds are initially generated by differences

in heating at the Earth’s surface

• Geostrophic winds result in rotational

movement around high and low pressure

centers.

Ocean Circulationin a nutshell

• Surface winds cause large, clockwise rotating

gyres in the northern hemisphere and

counterclockwise gyres in the southern

hemisphere

• Salinity and temperature differences in water

cause sinking of water (deep water formation) in

the North Atlantic and Southern Ocean

(Antarctic)

• EL Nino is a quasi periodic rocking of the

Ocean-Atmosphere system in the tropical Pacific

Atmospheric Pressure

As the atmosphere is held down by gravity, it exerts a force upon every surface (pressure = force per unit area)

At sea level the force is the weight of 1 kg of air that lies above each square centimeter of the surface

(around 15 lbs per in2)

atmospheric

pressure decreases

rapidly with altitude

near the surface

Therefore a small

change in elevation

will often produce a

significant change in

air pressure

How winds are made

Two columns of air–

same temperature

same distribution of mass

1000 mb 1000 mb

500 mb level

Cool the left column;

warm the right column

1000 mb

500 mb

500 mb

1000 mb

The heated column

expandsThe cooled

column

contracts

original 500 mb level

The level of the 500 mb surface changes;

the surface pressure remains unchanged

1000 mb

new 500 mblevel in warmair

new 500 mblevel in coldair

1000 mb

The 500 mb surface isdisplaced upward in thewarmer column

The level corresponding to500 mb is displaced

downward in the cooler column

original 500 mb level

The surface pressure remains the same since both columns still contain the same mass of air.

1000 mb

new 500 mblevel in warm air

new 500 mblevel in cold air

1000 mb

The 500 mb surface isdisplaced upward in thewarmer column

The 500 mb surface isdisplaced downward inthe cooler column

original 500 mb level

The surface pressure remains the same since both columns still contain the same mass of air.

A pressure difference in the horizontal direction develops above the surface

HighLow

1003 mb 997 mb

original 500 mb level

Air moves from high to low pressure in the middle of the column,

causing the surface pressure to change.

HighLow wind

1003 mb 997 mb

original 500 mb level

Air moves from high to low pressure at the surface…

HighLow

High Low

Where would we have rising motion?

What have we just observed?

• Starting with a uniform atmosphere at rest, we introduced differential heating

• The differential heating caused different rates of expansion in the fluid

• The differing rates of expansion resulted in pressure differences along a horizontal surface.

• The pressure differences induced horizontal flow (wind) in the fluid

• This is how the atmosphere converts heating into motion

• Winds are the result of differential heating

Surface Pressure Maps

• Altitude-adjusted surface station pressures are used to

construct sea level pressure contours

weak pressure

gradient

The pressure gradient forces acts at right

angles to the isobars (90 degrees)

strong pressure

gradient

820

830

840

850

860

870

880

890

820

830

840

850

860

Differences in air pressure = a pressure gradient

Global Circulation

SUNWarm

Low Pressure

Cold

High

Pressure

But heat is

transported from

the Equator to the

Poles - how?

0o

30oN

60oN

30oS

60oS

90oN

90oN

Earth

Fig. 7-6, p. 151

How is Energy Transportedto its “escape zones?”

• Both atmospheric and ocean transport are crucial

• Buoyancy-driven convection drives vertical transport

• Latent heat is at least as important as sensible heat

What a single cell convection model would look like for a

non-rotating earth

• Thermal convection leads to formation of convection cell in each hemisphere

• Energy transported from equator toward poles

• What would prevailing wind direction be over N. America with this flow pattern on a rotating earth?

What’s wrong

with the 1-cell model?

• Neglects effect of rotation

- with rotation, winds would cause earth to spin down

- with rotation, the upper level winds would accelerate to

unphysical speeds near the pole. You would be funneling all

the air from the Equator down at the Poles

- It is not a stable solution for wind circulation

Fig. 8-1, p. 172

Coriolis Force acts to the right

in the Northern Hemisphere

Physics

Coriolis Effect

The Coriolis Effect deflects moving objects to the right in the

northern hemisphere and to the left in the southern.

General Circulation of the Earth’s

Atmosphere

No Deflection

Deflection

Deflection

Deflection is

least at the

equator and

greatest at the

poles

0o

30oN

60oN

30oS

60oS

90oN

90oN

Wind patterns on a rotating Earth3 circulation cells in each hemisphere

Fig. 7-12, p. 154

Wind patterns on a rotating Earth3 circulation cells in each hemisphere

warm air rises at the

equator producing low

pressure (Intertropical

Convergence Zone,

ITCZ) and flows

towards the poles

L0o

30oN

60oN

30oS

60oS

90oN

90oN

Cold air sinks at 30o

N and S latitude

Creating high pressure

(subtropical high

pressure, STH)

L0o

30oN

60oN

30oS

60oS

90oN

90oN

H

H

Northeasterly and southeasterly

surface winds flow from the

subtropical high pressure belts

(30o N and S) to the low

pressure belt (ITCZ) at

the equator (calm winds:

doldrums)

westerly surface winds

flow from the subtropical

high pressure belts towards

higher latitudes

L0o

30oN

60oN

30oS

60oS

90oN

90oN

H

H

westerly surface winds are forced

to rise around 60o N and S latitude

when they encounter cold polar

easterly winds from the poles

resulting in Subpolar Low

pressure (SPL) belts

L0o

30oN

60oN

30oS

60oS

90oN

90oN

H

H

L

L

cold air sinks at the

poles producing

polar high (PH)

pressure regions

L0o

30oN

60oN

30oS

60oS

90oN

90oN

H

H

L

L

H

H

Figure 5.17, p. 163

Jet streams are

streams

of fast

moving

air aloft

that occur

where

atmospheric

temperature

gradients

are strong

L0o

30oN

60oN

30oS

60oS

90oN

90oN

H

H

L

L

H

H polar jet stream

polar jet stream

subtropical

jet streams

Key features of three cell model

• Hadley cell (thermally direct cell)

- driven by meridional gradient in heating

- air rises near equator and descends near 30 degrees

- explains deserts; trade winds; ITCZ

• Ferrel Cell (indirect thermal cell)

- driven by heat transports of eddies

- air rises near 60 degrees and descends near 30 degrees

- explains surface westerlies from 30-60

• Weak winds found near

– Equator (doldrums)

– 30 degrees (horse latitudes)

• Boundary between cold polar air and mid-latitude warmer air is the polar front

Geostrophic Winds

Coriolis Force acts to the right

in the Northern Hemisphere

Physics

Coriolis Effect

The Coriolis Effect deflects moving objects to the right in the

northern hemisphere and to the left in the southern.

low pressure

pressure geostrophic

gradient force winds

992

996

1000

1004

1008

1012

1016

1020

high pressure

Coriolis Force

Gradient Wind

“Geostrophic Wind”

• The Geostrophic wind is flow in a straight line in

which the pressure gradient force balances the

Coriolis force.

994 mb

996 mb

998 mb

Lower Pressure

Higher Pressure

Note: Geostrophic flow is often a good approximation high in the atmosphere (>500 meters)

air descends

High pressure (anticyclone)

From above

H

Side View

surrounding air is

relatively low

L

HL L

air ascends

Low pressure (depressions, cyclone)

From above

L

Side View

surrounding air is

relatively high

H

LH H

Friction forces

Near the surface, friction reduces the speed of the wind,

This reduces the Coriolis Force,

Which changes the direction of the geostrophic wind,

The pressure gradient force over powers the Coriolis effect,

As a result wind flow across the isobars.

Northern

Hemisphere

Southern

Hemisphere

H L

LH

anticyclone cyclone

Global Temperature patterns and

weather

Temperature Patterns

• Stronger seasonal heating and cooling on land produces asymmetry

• Poleward distortion of isotherms over northern high latitude oceans

• Equatorward distortion over subtropics

Seasonal Migration of ITCZ

• Mean position is somewhat north of Equator

• Strong departures from zonal mean position driven by seasonal heating over land

(Especially over Asia, S. America, Africa)

MonsoonsIn July the position of the ITCZ moves North

• low pressure over land causes winds to flow off the ocean

• this brings heavy rainfall

Figure 5.20, p. 167

Monsoons

In January high pressure over the land produces dry winds

Air is flowing towards the ITCZ

Figure 5.20, p. 167

Elevation of the 500 mb isobar

Polar Front Jet Stream

• Polar front jet stream forms along polar front where strong thermal gradient causes a strong pressure gradient

• Strong pressure gradient force and coriolis force produce strong west wind parallel to contour lines

• Polar jet sometimes splits into north and south branches

Jet Streams• Fast air currents, 1000’s of

km’s long, a few hundred km wide, a few km thick

• Typically find two jet streams (subtropical and polar front) at tropopause in NH

• When would you expect the jets to be strongest?

Smooth westward

flow of upper air

westerlies

Develop at the polar

front, and form

convoluted waves

eventually pinch off

Primary mechanism

for poleward heat

transfere

Pools of cool air

create areas of low

pressure

Rossby Waves

The “dishpan” experiment

• A tank of water with a hot equator and a cold pole is

rotated

– Troughs, ridges and eddies are produced, similar to

patterns observed in earth’s general circulation

http://jrscience.wcp.muohio.edu/coriolis/satmovies.html#anchor1386282

movies

The Earth’s Oceans

Ocean currents produced by:

1) winds

2) density differences in sea water

3) Coriolis force

4) shape of ocean basins

5) astronomical factors (TIDES)

Ocean Currents

driven mostly by wind

blowing over the surface

however, currents move

slowly

lag behind wind speed so

often called drifts

wind

Ocean currents

• large continuously moving loops (gyres)

• produced by winds, Coriolis force and land masses

Figure 5.32, p. 175

Fig. 8-2, p. 172

Each hemisphere contains a tropical and subtropical gyre

EQUATOR

North Tropical Gyre

South Tropical Gyre

N. Subtropical Gyre

S. Subtropical Gyre

Surface Currents redistribute heat

Upwelling

where cold water rises from deep ocean areas and where

the Coriolis forces prompts ocean currents to diverge

from coastlines

Figure 5.37, p. 180

Deep-sea currents

• driven by differences in temperature and salinity

• much slower than surface currents

Figure 5.32, p. 180

The Ocean Conveyor Belt

Deep-sea currents

• driven by differences in temperature and salinity

• much bigger and slower than surface currents

El Niño Southern Oscillation (ENSO)

• Trade winds promote cold water upwelling in eastern tropical Pacific

– Cool, deep water is nutrient rich and supports rich ecosystem

(plankton, fish, birds,…)

• Weaker trades lead to weaker upwelling. Warm nutrient-poor tropical water replaces the

cold, nutrient-rich water .

– called El Niño (boy child)

• Every few years this El Niño (surface warming) persists and is widespread

– Huge ecosystem and economic losses

– Alters weather patterns over much of the world

+3

+2

-1

+1

-2

0

1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 2005

El Niño

La Niña

57-58 65-66

72-73

76-77

82-83

86-87

97-98

El Niño

La Niña

Normal

Sea Surface Temperatures

(oC)

La Niña: cold surface water

moves over central and

eastern Pacific.

El Niño

La Niña

Normal

Sea Surface

Temperature

Anomalies oC

Normal conditions – equatorial Pacific

ENSO (El Nino - Southern Oscillation)

conditions

Idealized ENSO wave

http://www.cdc.noaa.gov/people/joseph.barsugli/anim.html

For animation of most recent anomaly asee

http://www.cdc.noaa.gov/map/clim/sst_olr/sst_anim.shtml

Animations on the web

Why do we care about ENSO?

• Global impacts on weather.

• Long timescale (months) yields improved

seasonal prediction.

• Provides insight into coupled behavior of

oceans and atmosphere … may lead to

better overall understanding of climate

Weather Variation: ENSO cycle

winter

summer

Impacts of El Niño

• Droughts– Fires

– Agricultural productivity

– Water supply

• Extreme Precipitation– Floods

– Erosion

– Disease

– Transportation

• Impacts through marine food chain– Natural ecological responses

– Economic