Winds and the Global Circulation System

Objectives:•Describe the measurement and variation of atmospheric pressure•Explain the processes that impact upon wind direction and weather systems•Describe the pressure and circulation patterns of the atmosphere•Evaluate the role of upper atmospheric circulation•Describe the circulation patterns of the oceans

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 inch)

-Atmospheric pressure decreases rapidly with altitude near the surface

-Therefore a small change in elevation will often produce a significant change in air pressure

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

The planet Earth also rotates

• in the northern hemisphere air appears to be deflected to the right

• in the southern hemisphere, deflected to the left

• this deflective force = Coriolis force

• because the wind is deflected it now flows parallel to the isobars = geostrophic wind

Imagine a turntable

• when not turning, a ball traces straight line

• when moving, ball traces a curved line

low pressure

pressure geostrophic

gradient force winds992

996

1000

1004

1008

1012

1016

1020

high pressure

Friction forces

• As wind flows over the surface friction reduces the speed

• Friction also changes the direction of the geostrophic wind

• The pressure gradient force over powers the Coriolis effect

• As a result wind flow across the isobars

• In Northern hemisphere – cyclones spiral counter clockwise

• Anticyclones spiral clockwise

• In Southern hemisphere – opposite

• Cyclones spiral clockwise

• Anticyclones spiral counter clockwise

Land and Sea breezes

During the day, air over land heats up and the sea is relatively cool (sea breeze)

land = low pressure and sea = high pressure

At night air over land cools and the sea is relatively warm (land breeze)

land = high pressure and sea = low pressure

air descends

High pressure (anticyclone)

From above

H

Side View

surrounding air is relatively low

LHL L

air ascends

Low pressure (depressions, cyclone)

From above

L

Side View

surrounding air is relatively high

HLH H

General Circulation of the Earth’s Atmosphere

Deflection is least at the equator and greatest at the poles

• In Northern hemisphere – cyclones spiral counter clockwise

• Anticyclones spiral clockwise

• In Southern hemisphere – opposite

• Cyclones spiral clockwise

• Anticyclones spiral counter clockwise

SUNEARTHWarmLow Pressure

ColdHigh Pressure

But heat is transported from the Equator to the Poles - how?

LOW PRESSURE

Warm air rises and flows polewards

LSo, around the equator = (ITCZ) Inter-Tropical Convergence Zone

= low pressure because solar heating

Doldrums• Intense heating

causes air to rise, leading to little horizontal motion at times

• Very calm conditions

Samuel Taylor Coleridge“The Rime of the Ancient Mariner”

“Down dropt the breeze, the sails dropt down,'Twas sad as sad could be”

“Day after day, day after day,We stuck, nor breath nor motion ;As idle as a painted shipUpon a painted ocean.”

Cooled air sinks at 30 degrees N&S

HIGH PRESSURE

H

Subtropical High

• Trade winds are predictable

• North East in Northern hemisphere

• South East in Southern hemisphere

• At subtropical high conditions are calm

• known as the “horse latitudes”

Cooled air sinks at 30 degrees N&S

HIGH PRESSURE

Air flows northwards towards the Poles

Some air flows back towards the Equator

Warm air meets cold air and rises

LWarmer air rises

Cold air sinks

Subpolar low pressure because the warmer air of the mid-latitudes rises as it meets cold polar air

HAt the Poles the air is cold and dense• this produces an area of high pressure

From the subtropical high to the subpolar low = “westerlies”

• Includes variable low and high pressure systems

• At the Polar Front - storms form

From the Polar High to the subpolar low are the Polar easterlies - variable

HadleyCells

Monsoons-January high pressure over the land produces dry winds.-Air flows towards the ITCZ.

MonsoonsJuly - position of the ITCZ moves North • Low pressure over the land causes winds to flow off the ocean.• This brings heavy rainfall.

Permanent area of high pressure over Antarctica

Seasonal pressure changes over the Arctic

Upper Atmosphere

• At 5-7 kms above the surface

• Influenced only by pressure gradient force and Coriolis force

• Geostrophic winds that flow parallel to isobars

-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 Waves

Jet Streams

• Narrow bands of high velocity

• Form along the polar front and above the Hadley cell in the subtropics

Circulation patterns (CURRENTS) produced by:

• Winds

• Density differences in sea water

• Coriolis force

• Shape of ocean basins

• Astronomical factors (TIDES)

Ocean Temperatures• Surface warms – especially in the summer

• Layer of rapid temperature drop - thermocline

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

Gyres

• Large circular currents

• Subtropical gyre corresponds to the subtropical high pressure

• N. and S. Equatorial currents corresponds to the trade winds

• Equatorial countercurrent corresponds to the ITCZ

• West winds around Antarctica create circumpolar gyre

• Portions branch off toward the equator

Deep-sea currents

• Driven by differences in temperature and salinity

• Much slower than surface currents

• Thermohaline circulation

• Depends on conditions in the North Atlantic

El Nino/Southern Oscillation (ENSO)

• Every year warming occurs off the coast of Peru (~2˚/C)

• Suppresses upwelling

• But every 4 or 5 years it is much more pronounced = El Nino

• Because there is no upwelling fish die

• May have far wider effects

In normal year, low pressure dominates in Malaysia and northern Australia.

In an El Niño year, low pressure moves east to the central part of the western Pacific