Atmospheric & Ocean Circulation- I

Transcription

1 Atmospheric & Ocean Circulation- I

2 First: need to understand basic Earth s Energy Balance 1) Incoming radiation 2) Albedo (reflectivity) 3) Blackbody Radiation

3 Atm/ Ocean movement ultimately derives from the Sun s rays Calvin J. Hamilton

4 But this picture would just imply that atmosphere/ ocean gets HOTTER Why would things CIRCULATE?

5 One Key: Uneven incoming solar energy with latitude Solar energy in high latitudes: Has a larger footprint Is reflected to a greater extent Passes through more atmosphere Therefore, less energy per square meter is absorbed at high latitudes

6 Uneven solar heating with latitude another way to visualize: how high is sun in sky?

7 2) ALBEDO ( Reflectivity )

8 Technically... Albedo: percentage of incident radiation that can reflect back to the atmosphere The Earths reflects about 30% of the sun s incident radiation Relative Albedo...

9 Albedo Much higher at poles! Mainly due to: Snow and Ice Cloud-cover

10 So is the Earth just getting hotter and hotter? Where does the energy go?

11 3) Black Body Radiation

12 As we continuously heat the ball, its color changes. Why?

13 Recall Light Spectrum- Recall that visible light is only small part of whole spectrum. Higher-Energy UV light Wavelength Lower-Energy Infra-red light

14 Constant temp (T) Radiation flux vs temperature Stefan-Boltzmann Law: High Temp = High Total Energy output (Total radiation flux emitted as a function of temperature) (T in Kelvin) Wien s Law: High energy = LOW wavelength (wave-length of maximum intensity as a function of Temp) 2898 T (T in Kelvin) max E T W m K 8-2 4

15 Sun emits primarily in visible (short wave radiation, SWR) Note: Sun emissions MUCH LARGER than Earth emissions (Stefan-Boltzmann Law) Earth emits primarily in IR (long wave radiation, LWR)

16 Overview : Blackbody radiation the major sink for incoming Solar Energy 1) All objects (with temperatures above absolute zero) radiate energy sun, light bulbs, earth, people (those cool IR cameras..) 2) The amount of energy & wavelength related to temperature Earth and people (~ cool ) * * lower energy in the (invisible) infrared Why does it get cool at night? because earth is radiating away energy it absorbed during day! For example: Sun and incandescent light bulbs (HOT) emit largely visible light

17 Now, we can create basic energy balance..

18 Simplified radiation budget Reflected solar 1) Energy arrives as visible (shortwave) sunlight About 30% is reflected albedo Higher reflection albedo at higher latitudes Earth Incoming visible The remainder leaves as outgoing infrared (longwave) radiation Outgoing infrared

19 But.you may be asking... Why does the ocean circulate again? Still have not answered this question!

20 Problem: this simple model does not Quite work out Reflected solar Earth Incoming visible Outgoing infrared

21 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 Means: a net heat gain is experienced in low latitudes A net heat loss is experienced in high latitudes BUT HOW?

22 How to explain Heat Loss/Gain that must be happening?

23 Basis of Global Wind Bands:

24 Recall.. Convection: (the soup analogy..)

25 Implications of differential warming: Convection in Atmosphere! Warm, low density air rises Cool, high density air sinks Creates circularmoving loop of air (convection cell) Figure 6-5

26 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

27 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

28 A big result of Different Pressure zones: moisture. WIND As air rises, it cools, water condenses, lots of rain As air sinks, it warms, lots of evaporation LOW Pressure Equator WIND HIGH Pressure 30 N

29 So: Basic Global wind patterns RESULT differential heating/cooling. They are a redistribution of heat due to convection cells! Thinking about a circulation cell, WHAT WOULD YOU EXPECT THEM TO BE LIKE??

30 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

31 But, of course, it doesn t work in the ideal way. Why NOT? Density and pressure differences, coupled with rotation of earth create smaller cells of circulation!

32 90 N Sinking air High pressure zone Divergence Dry - Arctic / Polar H 60 N 30 N L H Atmospheric Cells Rising air Low pressure zone Convergence Wet - Temperate / Sub-Polar Sinking air High pressure zone Divergence Dry - Sub-tropical Equator L Rising air Low pressure zone Convergence Wet - Tropical

33 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

34 Major circulation cells global moisture bands WIND WIND LOW Pressure Equator HIGH Pres 30 N

35 Rainy equator? ITCZ Inter-tropical Convergence Zone

36 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 is then transported to higher latitudes by the Hadley cells.

37 So what creates the REAL wind bands.. Big Complication #2: The Earth rotates..

38 BREAK

39 Review: 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

40 Coriolis effect N. Hemisphere Deviate to Right (relative to direction of motion) S. Hemisphere Deviate to Left (relative to direction of motion)

41 Coriolis Effect Consequence of something moving over a turning object.. Figure 6-9

42 The Earth rotates The Coriolis Effect "Image/Text/Data from the University of Illinois WW2010 Project."

43 Major Circulation cells - start with ideal Cells, then add the twisting of coriolis! Polar Cell Ferrel Cell Resultant cells Hadley Cell

44 90 N 60 N H L L L Polar Easterlies L L L Resulting Atmospheric Cells & Winds Prevailing Westerlies 30 N H H H H H H Equator L L L L L L H L Northeasterly Trade Winds 30 S H H H H H H H Southeasterly Trade Winds

45 BOUNDARIES BETWEEN WINDBELTS Polar Front Horse latitudes Intertropical Convergence Zone

46 The real world deviates even from from ideal cell model 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 These factors produce some persistent features (strong Highs and Low)

47 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.

48 Can experience this on our Coast: leads to afternoon onshore sea breezes.

49 Next: Main wind bands lead to Ocean Circulation!

50 END

51 Quiz! On top: name / TA & Section day/time Briefly answer following : 1) sketch and label: what are 3 main components of earth s radiation balance. 2) Why are mid-latitudes (30 N and S) relatively dry? 3) what was coolest organism you saw in tidepools on weekend?

52 Brief REVIEW of Atmospheric Circulation

53 Uneven heat balance 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

54 Heat Transport: How to explain Heat Loss/Gain that must be happening?

55 Convection: Ideal Circulation for a non-rotating Earth

56 90 N Sinking air High pressure zone Divergence Dry - Arctic / Polar H 60 N 30 N L H Atmospheric Cells Rising air Low pressure zone Convergence Wet - Temperate / Sub-Polar Sinking air High pressure zone Divergence Dry - Sub-tropical Equator L Rising air Low pressure zone Convergence Wet - Tropical

57 Coriolis Effect Consequence of something moving over a turning object.. Figure 6-9

58 Coriolis effect: Take home Info N. Hemisphere Deviate to Right (relative to direction of motion) S. Hemisphere Deviate to Left (relative to direction of motion)

59 The Earth rotates The Coriolis Effect "Image/Text/Data from the University of Illinois WW2010 Project."

60 Major Circulation cells - start with ideal Cells, then add the twisting of coriolis! Polar Cell Ferrel Cell Resultant cells Hadley Cell

61 90 N 60 N H L L L Polar Easterlies L L L Resulting Atmospheric Cells & Winds (note: winds are named by where they blow from) Prevailing Westerlies 30 N H H H H H H Equator L L L L L L H L Northeasterly Trade Winds 30 S H H H H H H H Southeasterly Trade Winds

62 BOUNDARIES BETWEEN WINDBELTS Polar Front Horse latitudes Intertropical Convergence Zone

63 The real world deviates even from from ideal cell model 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 These factors produce some persistent features (strong Highs and Low)

64 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.

65 Can experience this on our Coast: leads to afternoon onshore sea breezes.

66 Next: Main wind bands lead to Ocean Circulation!