atmospheric & ocean circulation-mdmccar/migrated/ocea80b/public/lectures/... ·...
TRANSCRIPT
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Atmospheric & Ocean Circulation-
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Overview: Atmosphere & Climate
• Atmospheric layers• Heating at different latitudes• Atmospheric convection cells (Hadley, Ferrel,
Polar)• Coriolis Force• Generation of winds• Low pressure, wet, convergence• High pressure, dry, divergence• Climate zones
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Recall: Atmospheric temp. vs. height
Heated atthe bottom:where the land iswarm
Heated from top:ozoneabsorbsenergy
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Atmospheric layers have different stability
TROPOSPHERE:Unstable becauseatmosphere is heated from below:CONVECTINGCONDITIONS~80% of mass of atm
STRATOSPHERE:Stable because cool dense air is beneath warm dense air:STRATIFIEDCONDITIONS~19.9% of mass ofatm
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overall: TROPOSPHERE
=the Weather Zone
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Also in TROPOSPHERE:CONVECTION due to heating from below
STRATOSPHERE:STABLE because cool dense air is beneath warm dense air:STRATIFIEDCONDITIONS
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STEPPING BACK:
Fundamentally, Why does the Atmosphere circulate at all?
What is energy source that sets it in motion?
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Oceanic motion ultimately derives from the Sun’s rays
© Calvin J. Hamilton
NASA
http://www.solarviews.com/raw/misc/ss.gif
http://starchild.gsfc.nasa.gov/docs/StarChild/questions/question31.html
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Lights Please!
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Uneven solar heating with latitude
• Solar energy in high latitudes:– Has a larger “footprint” – Is reflected to a greater
extent– Passes through more
atmosphere• Therefore, less solar
energy per square meter is absorbed at high latitudes than at low latitudes
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Uneven solar heating with latitudeanother way to visualize: how “high” is sun in sky?
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Recall: basic radiation budget
Earth
Incoming visible
• Energy arrives as visible (shortwave) sunlight
Reflected solar
• About 30% is reflected “albedo”
• Higher reflection “albedo” at higher latitudes
Outgoing infrared• The remainder leaves
as outgoing infrared (longwave) radiation
*+ GREENHOUSE GASSES : In atm. trap that outgoing long-wave radiation
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This basic model does a good job of predicting AVG global temp..
BUT: Do you think it does a good job of predicting the temp at Equator vs. Santa Cruz?
Incoming visible
Outgoing infrared
Earth
Reflected solar
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In a word: “NO”
Incoming visible
Outgoing infrared
Earth
Reflected solar
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1. Curvature of the Earth’s surface (flashlight effect..)
2. Albedo increases with latitude (more snow in N..)
Recall Solar heating imbalance with latitude:
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Predicts WAY too much heat at Equator (ie, predicts mid-latitudes warmer than they are..)
WAY too little heat at poles (ie, predicts them to be colder than they are)
• Means: a net heat gain is experienced in low latitudes
• A net heat loss is experienced in high latitudes
• ?
Actual- much more EVEN
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How do we explain the global Heat transfer that must be happening?
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Recall..
Convection:the soup analogy.!
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As with earth’s crust: its still all about Density
If air mass WARMS•molecules move more quickly•air mass expands•DENSITY DECREASES•AIR MASS RISES
If air mass COOLS•molecules move less quickly•air mass contracts•DENSITY INCREASES•AIR MASS SINKS
Up in atm
osphere
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Implications of differential warming: Convection in Troposphere*!
Figure 6-5
• Warm, low density air rises
• Cool, high density air sinks
• Creates circular-moving loop of air (convection cell)
* remember, this is lower layer that is heated from BELOW
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One more thing: High vs. Low air Pressures
Figure 6-6
• A column of cool, dense aircauses high pressure at the surface, which will lead to sinking air
• A column of warm, less denseair causes low pressure at the surface, which will lead to rising air
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A big result of Different “Pressure” zones: moisture.
Equator 30˚ N
LOW Pressure HIGH Pressure
As air rises, it cools, water condenses, lots of rain
As air sinks, it warms,lots of evaporation
Warm rising airWIND
WIND
Cold sinking air
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What does water this transport have to do with heat?
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Water phase changes require enormous energy (in part due to H-bonding)
Figure shows “latent heat” of each water phase change
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Evaporation (liq.=> gas) removes heat fromatm.
Rain (gas => liquid) releases heat to atm.
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So, thinking about a circulation cell,
WHAT WOULD YOU EXPECT THEM TO BE LIKE?
So: Basic Global wind patterns RESULT differential heating/cooling.
They then redistribute heat due
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To review:Basic Convection Cell
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“Ideal” Circulation for a non-rotating Earth
Fig. 6-7
• Warm air would rise at the equator
• Cold air would sink at the poles
• Single circulation cell with equator-ward flow
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But, of course , it doesn’t work in the “ideal” way.
Fig. 6-7
• Why NOT?
• Density and pressure differences create smaller “cells” of circulation!
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AtmosphericCells
90˚N
30˚N
60˚N
Equator L
H
HL
Rising air•Low pressure zone•Convergence•Wet - Tropical
Sinking air•High pressure zone•Divergence•Dry - Sub-tropical
Rising air•Low pressure zone•Convergence•Wet - Temperate / Sub-Polar
Sinking air•High pressure zone•Divergence•Dry - Arctic / Polar
Hadley Cell
Ferrel Cell
Polar Cell
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Overview: Atmospheric Circulation
• 1) Think of density differences driving vertical movements
• Warm air rises• Cool air sinks
• 2) Think of pressure differences driving horizontal movements
• Air moves fromHIGH TO LOW pressure
• 3) Think of evaporation/ precipitation of water carried by winds as transporting heat.
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Major circulation cells global moisture
bands
Equator 30˚ NLOW Pressure HIGH PresWarm rising air
WIND
WIND
Cold sinking a
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Fig. 13.2
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Rainy equator?ITCZ – Inter-tropical Convergence Zone
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ITCZ - Intertropical Convergence Zone
More evidence of “Hadley” cell” Cooling as the air rises causes the water Cooling as the air rises causes the water vapor to condense as clouds andvapor to condense as clouds and rain rain -- releasing its latent heat. The heat can then releasing its latent heat. The heat can then transported to highertransported to higher latitudes by the Hadley cells (directly as warmer air, as well latitudes by the Hadley cells (directly as warmer air, as well
as indirectly as water vapor)as indirectly as water vapor)
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BUT :
THERE IS YET ANOTHER COMPLICATION
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Big Complication #2: The Earth rotates The “Coriolis” Effect
• Accounts for how things move relative to the earths surface (which is rotating underneath them!)
• Causes objects in motion to curve (relative to the earth!)– To right in the North– To left in the south
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N. HemisphereDeviate to Right(relative to direction of motion)
S. HemisphereDeviate to Left (relative to direction of motion)
Coriolis effect
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Coriolis EffectConsequence of something moving over a turning object..
Figure 6-9
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The Earth rotates The Coriolis Effect
"Image/Text/Data from the University of Illinois WW2010 Project."
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Flow around LOWCyclonic flowCounter-clockwise in NH(Clockwise in SH)
Flow around HIGHAnti - Cyclonic flowClockwise in NH(Counter-clockwise in SH)`
Also explains direction that storm winds circulate
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Resulting Atmospheric
Cells & Winds90˚N
30˚N
60˚N
Equator L
H
HL
Hadley Cell
Ferrel Cell
Polar Cell
H HHHHH
L LL L LL
L LL L L
Northeasterly Trade Winds
Prevailing Westerlies
Polar Easterlies
Southeasterly Trade Winds
H HHHHH30˚S H
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Resultant cells
Hadley Cell
Ferrel Cell
Polar Cell
Major ”Circulation cells”- start with idealCells, then add the “twisting” of coriolis!
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IntertropicalConvergence Zone
Horse latitudes
Polar Front
Note: BOUNDARIES BETWEEN WINDBELTS
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Finally, the real world naturally deviates even more from this ideal
“three cell” model
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Real world Complications:
Regional or local pressure gradients can be influenced by:
• Seasons: Tilt of earth’s axis - latitude of max. heating changes through the year
• Land:– Variations in land topography and albedo– Land - Sea contrasts
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Heat flux in July (W/m2)
Heat flux in January (W/m2)
Why does solar heating change seasonally?
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Fig 6-1
Seasonal Heating Differences Due to TILT
Tilt = 23.5 ˚ , CAUSES SEASONS
PerihelionAphelion
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• Heat capacity of rock is much less than that of water, so land heats up more quickly during the day than the water.
• Air above land warms and rises.
Land-driven Sea Breezes ( Very near to shore.)
• At nighttime, no solar influx, but outgoing radiation remains. So both land and sea cool.
• However, land cools more rapidly than water because of a lower heat capacity. Circulation reverses.
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Can experience this on our Coast: leads to afternoon onshore sea breezes
(even better example is S. Cal coast)
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Real world: complicated and varies by region
• Seasonal heating changes• Variations in land topography and albedo
These factors produce:- some strong High and Low Pressure Zones- lots of change from one Season to another
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Next: Main wind bands lead to Ocean Circulation!