Atmospheric & Ocean Circulation-
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
Recall: Atmospheric temp. vs. height Heated from top: ozone absorbs energy Heated at the bottom: where the land is warm
Atmospheric layers have different stability STRATOSPHERE: Stable because cool dense air is beneath warm dense air: STRATIFIED CONDITIONS ~19.9% of mass of atm TROPOSPHERE: Unstable because atmosphere is heated from below: CONVECTING CONDITIONS ~80% of mass of atm
overall: TROPOSPHERE =the Weather Zone
Also in TROPOSPHERE: CONVECTION due to heating from below STRATOSPHERE: STABLE because cool dense air is beneath warm dense air: STRATIFIED CONDITIONS
STEPPING BACK: Fundamentally, Why does the Atmosphere circulate at all? What is energy source that sets it in motion?
Oceanic motion ultimately derives from the Sun s rays http://www.solarviews.com/raw/misc/ss.gif Calvin J. Hamilton http://starchild.gsfc.nasa.gov/docs/starchild/questions/question31.html NASA
Lights Please!
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
Uneven solar heating with latitude another way to visualize: how high is sun in sky?
Recall: basic radiation budget Energy arrives as visible (shortwave) sunlight Reflected solar About 30% is reflected albedo Higher reflection albedo at higher latitudes Earth Incoming visible The remainder leaves as outgoing infrared (longwave) radiation Outgoing infrared *+ GREENHOUSE GASSES : In atm. trap that outgoing long-wave radiation
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? Reflected solar Earth Incoming visible Outgoing infrared
In a word: NO Reflected solar Earth Incoming visible Outgoing infrared
Recall Solar heating imbalance with latitude: 1. Curvature of the Earth s surface (flashlight effect..) 2. Albedo increases with latitude (more snow in N..)
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) Actual- much more EVEN Means: a net heat gain is experienced in low latitudes A net heat loss is experienced in high latitudes?
How do we explain the global Heat transfer that must be happening?
Recall.. Convection: the soup analogy.!
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 atmosphere
Implications of differential warming: Convection in Troposphere*! Warm, low density air rises Cool, high density air sinks Creates circularmoving loop of air (convection cell) Figure 6-5 * remember, this is lower layer that is heated from BELOW
One more thing: High vs. Low air Pressures A column of cool, dense air causes high pressure at the surface, which will lead to sinking air A column of warm, less dense air causes low pressure at the surface, which will lead to rising air Figure 6-6
A big result of Different Pressure zones: moisture. WIND As air rises, it cools, water condenses, lots of rain Cold sinking air As air sinks, it warms, lots of evaporation Warm rising air LOW Pressure Equator WIND HIGH Pressure 30 N
What does water this transport have to do with heat?
Water phase changes require enormous energy (in part due to H-bonding) Figure shows latent heat of each water phase change
Evaporation (liq.=> gas) removes heat from atm. Rain (gas => liquid) releases heat to atm.
So: Basic Global wind patterns RESULT differential heating/cooling. They then redistribute heat due So, thinking about a circulation cell, WHAT WOULD YOU EXPECT THEM TO BE LIKE?
To review: Basic Convection Cell
Ideal Circulation for a non-rotating Warm air would rise at the equator Cold air would sink at the poles Single circulation cell with equator-ward flow Earth Fig. 6-7
But, of course, it doesn t work in the ideal way. Why NOT? Density and pressure differences create smaller cells of circulation! Fig. 6-7
90 N Sinking air High pressure zone Divergence Dry - Arctic / Polar H 60 N 30 N Polar Cell L Ferrel Cell H Atmospheric Cells Rising air Low pressure zone Convergence Wet - Temperate / Sub-Polar Hadley Cell Sinking air High pressure zone Divergence Dry - Sub-tropical Equator L Rising air Low pressure zone Convergence Wet - Tropical
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 from HIGH TO LOW pressure 3) Think of evaporation/ precipitation of water carried by winds as transporting heat.
Major circulation cells global moisture bands WIND Cold sinking a Warm rising air WIND LOW Pressure Equator HIGH Pres 30 N
Fig. 13.2
Rainy equator? ITCZ Inter-tropical Convergence Zone
ITCZ - Intertropical Convergence Zone More evidence of Hadley cell Cooling as the air rises causes the water vapor to condense as clouds and rain - releasing its latent heat. The heat can then transported to higher latitudes by the Hadley cells (directly as warmer air, as well as indirectly as water vapor)
BUT : THERE IS YET ANOTHER COMPLICATION
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
Coriolis effect N. Hemisphere Deviate to Right (relative to direction of motion) S. Hemisphere Deviate to Left (relative to direction of motion)
Coriolis Effect Consequence of something moving over a turning object.. Figure 6-9
The Earth rotates The Coriolis Effect "Image/Text/Data from the University of Illinois WW2010 Project."
Also explains direction that storm winds circulate Flow around HIGH Anti - Cyclonic flow Clockwise in NH (Counter-clockwise in SH)` Flow around LOW Cyclonic flow Counter-clockwise in NH (Clockwise in SH)
90 N 60 N H L Polar Cell L L Polar Easterlies L L L Ferrel Cell Resulting Atmospheric Cells & Winds Prevailing Westerlies 30 N H H H H H H H Hadley Cell Northeasterly Trade Winds Equator L L L L L L L 30 S H H H H H H H Southeasterly Trade Winds
Major Circulation cells - start with ideal Cells, then add the twisting of coriolis! Polar Cell Ferrel Cell Resultant cells Hadley Cell
Note: BOUNDARIES BETWEEN WINDBELTS Polar Front Horse latitudes Intertropical Convergence Zone
Finally, the real world naturally deviates even more from this ideal three cell model
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
Why does solar heating change seasonally? Heat flux in January (W/m 2 ) Heat flux in July (W/m 2 )
Seasonal Heating Differences Due to TILT Aphelion Perihelion Fig 6-1 Tilt = 23.5, CAUSES SEASONS
Land-driven Sea Breezes ( Very near to shore.) 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. 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.
Can experience this on our Coast: leads to afternoon onshore sea breezes (even better example is S. Cal coast)
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
Next: Main wind bands lead to Ocean Circulation!