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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