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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
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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)
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-Atmospheric pressure decreases rapidly with altitude near the surface
-Therefore a small change in elevation will often produce a significant change in air pressure
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Differences in air pressure = a pressure gradient
The pressure gradient forces acts at right angles to the isobars (90 degrees) 820 830 840 850 860 820 830 840 850 860 870 880 890 strong pressure gradient weak pressure gradient
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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
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Imagine a turntable when not turning, a ball traces straight line when moving, ball traces a curved line
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low pressure high pressure gradient force winds pressure geostrophic
992 996 1000 1004 1008 1012 1016 1020 high pressure
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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
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In Northern hemisphere – cyclones spiral counter clockwise
Anticyclones spiral clockwise In Southern hemisphere – opposite Cyclones spiral clockwise Anticyclones spiral counter clockwise
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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
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At night air over land cools and the sea is relatively warm (land breeze)
land = high pressure and sea = low pressure
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H H L L L High pressure (anticyclone) air descends
Side View From above H H L L L air descends surrounding air is relatively low
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L L H H H Low pressure (depressions, cyclone) air ascends
Side View From above L L H H H air ascends surrounding air is relatively high
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General Circulation of the Earth’s Atmosphere
Deflection is least at the equator and greatest at the poles
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In Northern hemisphere – cyclones spiral counter clockwise
Anticyclones spiral clockwise In Southern hemisphere – opposite Cyclones spiral clockwise Anticyclones spiral counter clockwise
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EARTH SUN But heat is transported from the Equator to the Poles - how?
Cold High Pressure Warm Low Pressure EARTH SUN
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L Warm air rises and flows polewards
PRESSURE L So, around the equator = (ITCZ) Inter-Tropical Convergence Zone = low pressure because solar heating
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Doldrums 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 ship Upon a painted ocean.” Intense heating causes air to rise, leading to little horizontal motion at times Very calm conditions
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Cooled air sinks at 30 degrees N&S
H HIGH PRESSURE Subtropical High
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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”
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Cooled air sinks at 30 degrees N&S
Air flows northwards towards the Poles Some air flows back towards the Equator HIGH PRESSURE
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Warm air meets cold air and rises
Warmer air rises L Cold air sinks Subpolar low pressure because the warmer air of the mid-latitudes rises as it meets cold polar air
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H At the Poles the air is cold and dense
this produces an area of high pressure H
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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
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Hadley Cells
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Monsoons -January high pressure over the land produces dry winds. -Air flows towards the ITCZ.
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Monsoons July - position of the ITCZ moves North Low pressure over the land causes winds to flow off the ocean. This brings heavy rainfall.
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Permanent area of high pressure over Antarctica
Seasonal pressure changes over the Arctic
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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
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Rossby Waves -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
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Jet Streams Narrow bands of high velocity
Form along the polar front and above the Hadley cell in the subtropics
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Circulation patterns (CURRENTS) produced by:
Winds Density differences in sea water Coriolis force Shape of ocean basins Astronomical factors (TIDES)
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Ocean Temperatures Surface warms – especially in the summer
Layer of rapid temperature drop - thermocline
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Ocean Currents wind Driven mostly by wind blowing over the surface
However, currents move slowly Lag behind wind speed so often called drifts wind
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Ocean currents Large continuously moving loops (gyres) Produced by winds, Coriolis force and land masses
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Gyres Large circular currents
Subtropical gyre corresponds to the subtropical high pressure
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N. and S. Equatorial currents corresponds to the trade winds
Equatorial countercurrent corresponds to the ITCZ
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West winds around Antarctica create circumpolar gyre
Portions branch off toward the equator
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Deep-sea currents Driven by differences in temperature and salinity Much slower than surface currents Thermohaline circulation Depends on conditions in the North Atlantic
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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
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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
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