Chapter 5Chapter 5

Winds and Global Circulation

IntroductionIntroduction

Reason for winds goes back to concepts of insolation and radiation

Differential (unequal) heating of Earth’s surface by latitude and at smaller scales leads to variations in pressure gradient between locations air motion

Like opening a bottle or can of beerEarth’s rotation plays role in direction of flow

How Pressure Leads to How Pressure Leads to Air MotionAir Motion

So What is Pressure?So What is Pressure?

Force per unit areaDad says, “Have you checked the air in

your tires lately?” – can measure because higher pressure in tire exerts force on gauge

In day-to-day life, why do we feel atmospheric pressure?

Air has mass and is being pulled downward by Earth’s gravity

Measurement of PressureMeasurement of Pressure

Various units – in tire example, use pounds per square inch; also kg/cm2, pascals (Pa), millibars (mb), cm or mm of Mercury, inches of Mercury

Standard sea level pressure = 101,320 Pa = 1013.2 mb = ~ 76 cm Hg = 760 mm Hg = 29.92 in Hg

Measure using barometer – mercury or aneroid

Mercury BarometerMercury Barometer

Very accurate! Hg in a glass tube Tube placed in dish of Hg Hg flows out of tube due to

gravity a vacuum at top Hg stops flowing when air

pressure pushing on Hg in dish = pull of gravity on Hg in tube

As pressure on Hg in dish increases, Hg in tube rises

Pressure Change with AltitudePressure Change with Altitude

What do you notice if you swim to the bottom of a pool?

Same concept with atmosphere – less of atmosphere above you as altitude increase, so less pressure

On weather maps, use a formula to adjust pressure readings to sea level, which enables pressure analyses (isobars)

WindWindAir motion or flow of air with respect to

surface, usually only horizontal movementsMeasured in meters per second or miles per

hour (1 m/s approximately = 2 mph)Speed determined using anemometer (most

common is cup anemometer)Direction determined with wind vane and

always given as direction from which wind is coming (wind blowing from NW to SE = northwesterly wind); also use degrees

Measuring Wind Speed Measuring Wind Speed and Directionand Direction

Wind vane and anemometer Compass directions/

degrees

Forces Affecting WindForces Affecting Wind

Many features affect wind (buildings, hills, valleys, etc.); think of swirling motion in a football/baseball stadium

Three main forces determine wind speed and direction on larger scales:

1. Friction 2. Pressure gradient force 3. Coriolis effect

FrictionFriction

“Drag” due to a surfaceCauses wind speeds to decrease and has

minor influence on wind directionIf looking at a profile of wind vs. height,

would see a half-U shape, with slower speeds near the surface and increasing values higher in atmosphere

Negligible at high levels

Pressure Gradient ForcePressure Gradient Force

Due to difference in pressure (a gradient) between locations

PGF “pushes” air from areas of higher to lower pressure

A stronger gradient (greater difference in pressure) between locations produces stronger winds

Therefore, affects both direction and speed If only PGF, then wind is perpendicular to isobars,

but of course this is too simple....

Pressure Gradient ForcePressure Gradient Force

Pressure Gradient ForcePressure Gradient Force

Coriolis EffectCoriolis Effect

Deflection of motion of an object (including wind) from its path

So mainly affects direction Due to rotation of Earth In Northern Hemisphere, deflection is to the right

of pressure gradient force, while in Southern Hemisphere, deflection is to the left of PGF

Most deflection at poles, least at equator

Coriolis EffectCoriolis Effect

Local WindsLocal Winds

Friction and PGF have greatest effect on small-scale winds; Coriolis negligible

Some local winds include:– Convective winds: heating causes pressure

gradient; at low levels, wind flows toward warm region (convergence), and at high levels, wind flows away from warm region (divergence)

– Mountain and valley breezes: during day, mountainsides heated causing air flow up valleys; opposite effect at night

Local Winds (cont’d)Local Winds (cont’d)– Land and sea breezes: due to specific heat

attributes (land heats and cools faster than water); during the day, land heats, and air flow from cooler water surface towards land (sea breeze); opposite at night, because water warmer than land surface (land breeze)

Cyclones and AnticyclonesCyclones and Anticyclones Cyclones

– Areas of low pressure– Air spirals inward and upward (convergence)– Due to Coriolis, air moves counterclockwise in NH and

clockwise in SH– Associated with cloudy weather and precip

Anticyclones– Areas of high pressure– Air spirals outward and downward (divergence)– Due to Coriolis, air moves clockwise in NH and

counterclockwise in SH– Associated with fair weather

Winds in Cyclones (L) and Winds in Cyclones (L) and Anticyclones (H)Anticyclones (H)

SUNWarmLow Pressure

ColdHigh Pressure

How is heat transported from the Equator to the Poles?

0o

30oN

60oN

30oS

60oS

90oN

90oN

Earth

Warm air rises at the equator producing Low pressure 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)

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

IG4e_05_19

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

Three-cell ModelThree-cell Model

2-D Glance at Surface2-D Glance at Surface

Semi-permanent pressure cells around globeCalled semi-permanent, because location and

intensity vary with season (Fig. 5.17, p. 164)Seasonal shifts lead to

– Movement of Intertropical Convergence Zone (ITCZ) – moves northward in July, southward in January – why?

– Monsoon: shift in wind direction from offshore flow in winter (dry season) to onshore flow in summer (wet season)

Semi-permanent Pressure CellsSemi-permanent Pressure Cells

Winds AloftWinds Aloft As you move farther from the Earth’s surface,

friction has less impact, so can focus mainly on PGF and Coriolis

Geostrophic wind: wind that moves parallel to isobars and at a right angle to the PGF (Fig. 5.22a)

Flow is initially in direction of PGF, but in Northern Hemisphere, Coriolis deflects motion to the right

At some point, PGF = Coriolis (sum = 0), so speed and direction of air flow no longer changes

See Fig. 5.22b for illustration of final two points

Winds AloftWinds Aloft

Other Characteristics of Other Characteristics of Winds AloftWinds Aloft

General pattern: – Weak easterly winds in

tropics (Equatorial Easterlies)

– High pressure at Tropics of Cancer and Capricorn, westerly winds to Arctic and Antarctic Circles (Westerlies)

– Spiraling motions from Circles to poles

Specific Features of Specific Features of Winds AloftWinds Aloft

Rossby waves– Undulations or waves in Westerlies which move cold air

toward equator and warm air toward poles– Primary mechanism for poleward heat transfer

Polar front– Sharp boundary between cold polar air and warm tropical air

Jet streams– Narrow zones (tube-like) of very fast wind speeds (center

has highest speeds)– Occur along strong pressure gradients– Two affecting US – polar jet and subtropical jet

•Smooth westward flow of upper air westerlies

•Develop at the polar front, and form convoluted waves eventually pinch off

•Primary mechanism for poleward heat transfer

•Pools of cool air create areas of low pressure

Rossby WavesRossby Waves

Ocean CirculationOcean Circulation

Why are we talking about the oceans during the same class at winds?

Atmospheric circulation drive the direction of surface ocean currents

Ocean currents also act as heat transfer mechanisms (help global energy balance)

General pattern is warm currents on eastern flank of continents and cold currents on western flank (Gulf Stream vs. California current)

Ocean CurrentsOcean Currents Upwelling is vertical movement in the oceans Important because it brings nutrients to upper

levels of ocean Occurs along western edges of continents – one of

strongest is along South America What does this have to do with weather and

climate? Teleconnection – relationship between circulations

in one region and weather/climate in another region; impacts vary across globe and even on same continent

Ocean CurrentsOcean Currents

El NinoEl Nino

El Niño-Warmer than normal waters in the Equatorial Pacific

La Niña-Cooler than normal waters in the Equatorial Pacific

Normal vs. El Nino and Associated Normal vs. El Nino and Associated Weather EffectsWeather Effects