The Atmospheric Circulation System

1
The Atmospheric Circulation System

• Geos 110 Lectures Earth System Science
• Chapter 4 Kump et al 3rd ed.
• Dr. Tark Hamilton, Camosun College

2
Overall the Earths Climate is in Balance

• In Balance Kind-of
• But you have to average over night and day
• It helps to average for many seasons or years
• And we need to overlook trivialities like burning
  all of Earths fossil Carbon from the past 350
  Ma in lt 3 centuries!
• However
• Region to region there are hot and cold spots,
  wet and dry places, rain forests and deserts,
  mountains and plains, seas and glaciers, tropics
  and polar climes a whole lot of weather!

3
The Ideal Gas Law Relationships of Pressure,
Temperature, Volume Moles

• Ideal Gas Law
• P V n R T PPressure, TTemperature, nmoles
  of gas particles (with mass), Rideal gas
  constant
• Special Case 1 Boyles Law (_at_ Tconstant)
• Pinitial Vinitial Pfinal Vfinal
• PV has units of work e.g. F/d2 x d3 F x d
• At constant E, a P increases V decreases
• Special Case 2 Charles Law (_at_ Pconstant)
• Vinitial / Tinitial Vfinal / Tfinal

4
There are Big Latitudinal Differences
Fig 4.1

• The Tropics have Energy Surplus
• The Poles run a Deficit
• Temperate zones have transitory seasonal swings

5
There are Big Latitudinal DifferencesIR emission
doesnt match? How does heat move?
Fig 4.2

• The Tropics have a Net Radiation Surplus
  (SingtEout)
• The Poles run a Net Radiation Deficit (SinltEout)
• Temperate zones have transitory seasonal swings

6
There has to be a Global Circulation System
Fig 4.3
Ferrel Cell
Ferrel Cell

• IR conversion to Latent Heat (Liquid??Vapor)
• Convection driven by density and pressure
  differences between different air masses

7
Convergent versus Divergent Windsat Earths
Surface

• Rising light warm air of the Tropic Lows is
  replaced laterally by denser air flowing in from
  higher latitudes converges towards the Equator
• This position changes seasonally by 5 of
  Latitude
• Descending cold dense air from the Horse Latitude
  Temperate Highs hits the Earths surface and
  gently diverges
• This position is fixed by the stable Tropopause

8
Weather Climate Vary Across the Globe
Fig 4.4

• Wind Ocean Currents Redistribute Solar Heating
• Solid Earth processes buffer CO2 levels by
  weathering rocks over few hundred Ka to Ma
• Eddies on all spatial temporal scales prevent
  the heat redistribution from being complete or
  even.

9
Eastern Pacific Central America w/ ITCZ
Intertropical Convergence Zone
Fig 4.5

• NOAA Satellite Image
• Cloud Band marks ITCZ at top of Troposphere
• The Troposphere, heated from below convects

10
Convective Towers Cumulonimbus drive Hadley Cells
of ITCZ
Fig 4.5

• Cloud Band marks ITCZ at top of Troposphere
• Solar Evaporation Latent Heat from Condensation
  make the heat pump that drives the Convection

11
Horizontal Vertical Air Movements result from
Temperature Pressure differences driving
Buoyancy

• Buoyancy is due to density contrasts,
  ?mass/volume
• Fast molecules, more collisions more F/A
  Pressure
• Temperature increase ? Pressure increase
• Pressure increase ? Volume increase, buoyancy
• Air columns heated from below expand and rise
• Other denser air moves in laterally to replace it
• Cooling upper Troposphere cools air shrinks
  sinks

12
Mid-latitude Convective Mixing
Fig 4.6

• Cold fronts descend from higher latitudes
• Replacing/passing beneath tropical warm fronts
• This rapid mixing of air masses is an ever
  changing recipe for weather

13
N-S Meridional Mixing of Troposphere
Fig 4.7

• Tropics to Horse Latitudes - Hadley Cells
• Mid-Latitudes Ferrel Cells
• High Latitude - Polar Front

14
Hadley Cells

• Individual atmospheric cells
• Between the Equator and 30-35 N
• Over the ocean in Atlantic and Pacific
• Driven by heat from below absorbed by ocean
• The rocky planet rotates faster than the
  atmosphere
• Hadley Cells are broken up by continents

15
The Horse Latitudes
The Dead Horse Shanty Oh, poor old man your
horse will dieAnd we say so, and we know soOh,
poor old man your horse will dieOh, poor old
man We'll hoist him up to the main yardarmWe'll
hoist him up to the main yardarm Say, I old man
your horse will dieSay, I old man your horse
will die We'll drop him down to the depths of
the seaWe'll drop him down to the bottom of the
sea We'll sing him down with a long, long
rollWhere the sharks'll have his body and the
devil have have his soul

• Spanish ships bound for the New World became
  becalmed w. Hi Pressure, no wind and Horses died
• English Dead Horse Shanty, working off advance

16
Idealized Tropospheric Circulation

• ITCZ Polar Front Storm Belts Hi Precipitation
• Horse Latitude Polar Deserts

17
A Simple Pressure Model for Winds
Fig 4.8

• Winds blow out of descending High Pressure limbs
  30-35 NS between Hadley Ferrel Cells
• Winds blow towards rising Low Pressure limbs on
  equatorial edge of Hadley Cells at ITCZ also PCZ

18
Coriolis Rotational Effects on a Sphere
0 m/s
Fig 4.9a
4.64 m/s

• Since the Earth revolves once a day.
• Bantus and Guajirans move a lot faster and
  further
• Than Innu or Lapplanders!

19
Apparent Wind Deflection to the Right N in N.Hem.
(rotating reference frame)
Fig 4.9b
The curved paths are relative to fixed points on
the ground which revolves.

• While the Earth revolves from A?A B?B
• The N flowing Air moves from P1 ? X,
• This is really in a straight line viewed from
  Space

20
Coriolis (Centrifugal) Force acts on East or West
moving Winds (increasing w/Latitude)
Fig 4.10

• A Vector with 2 components in a plane defined by
  the spin axis and the location on the Earths
  surface
• 1 Component is vertical, 1 horizontal-tangent
  away
• In N Hem. E moving wind deflects Right to South

21
A More Realistic Model for Surface
WindsPressure Differences, Buoyancy Coriolis
Effects
Fig 4.11
Big Seasonal Changes
Tropic of Cancer 23.5N
1 Season
Tropic of Capricorn 23.5S
Big Seasonal Changes

• The same divergence convergence zones are shown
• Coriolis force effects are shown
• Permanent Peri-equatorial Trades Winter Polar
  Easteries

22
High Pressure Systems tend to be Localized

• Descending limbs of Hadley-Ferrel Cells in Mid
  latitudes tends to be fixed
• Trade Winds blow from the Equator-ward side of
  these Sub-Tropical Highs
• Temporary passing fronts of High or Low pressure
  form near the edge of the Polar Front affecting
  these
• 1000 km wide Low Pressure systems form from T
  gradients and convective winds in upper
  troposphere
• Inwards directed wind deflects to right in
  Northern hemisphere (Cyclonic Flow)
• Outwards directed flow from Highs creates
  Anticyclones

23
Tropical Cyclones Hurricanes Monsoons
Box Fig 4.1

• The Circle is an Isobar line of constant
  pressure
• High Pressure winds deflect to the right
• Hurricane flow is set by P gradient Centripetal
  Acceleration
• Cyclonic storm rotate counterclockwise in North
  hemisphere
• Cyclonic storms from at 26-27C gt 5 Latitude
  from the Equator

24
Causes of Tropical Cyclones
Box Fig 4.1

• Low Vertical Wind Shear or the storms tear apart
  as they build
• Maximal humidity in lower Troposphere, builds
  latent heating
• Steep vertical thermal gradient, promotes upwards
  buoyant convection
• Initial atmospheric disturbance from ordinary
  Trade wind flow old frontal boundaries, easterly
  waves (off Africa or S. Pacific), usually late
  summer fall when ITCZ is furthest from equator

25
Extratropical Cyclones
Box Fig 4.1

• From outside the tropics gt 23.5 N or S latitude
• Flow of Warm air from Equator hits cold air from
  High Latitudes
• These air masses do not mix well so
• Warmer less dense humid air rises above a cold
  front
• Lots of Mid-latitude rain or snow
• Lots of daily weather variations due to transient
  fronts

26
Flow of Troposphere
Fig 4.7

• Surface Flow is dominated by latitudinal belts ??
• Upper Level Flow is Dominantly Polewards!

27
Upper Level Tropospheric Flow
Fig 4.12a

• The troposphere is warmer and thicker in the
  tropics
• Colder and thinner at the Poles

28
Tropospheric Pressure Surfaces
Fig 4.12b

• Tropics more expanded lt vertical pressure
  gradient
• Poles are more compressed w/ gt vertical pressure
  gradients

29
Mid-latitude Upper Level Jet Streams
? Flow ?
Fig 4.12c

• At any elevation there is Hi P towards the
  Equator
• Flow naturally moves from High to Low Pressure
• These control the paths of Low Pressure Storms

30
Geostrophic Wind
Fig 4.13

• Pressure Gradient decreases upwards (less mass)
• Coriolis Force decreases downwards, net
  Geostrophic Right/Left flow
• Centrifugal Centripetal Forces contribute
  around Lows/Highs
• (Similar curved flow occurs across mid latitude
  continental shelves)

31
Friction acts near surface at High Pressure
Fig 4.20
Fig 4.13

• Slows and deflects wind lt 90 from coriolis
• Causes winds to spiral in cyclonic storms

32
Height of the 300 mb Geopotential Surface in
January (Winter N. Hem.)
Fig 4.14

• As per the previous 3 figures, this show the
  Polar Low
• Equatorial High

33
Seasonal Variation in Insolation
Fig 4.15
Perhelion
Aphelion

• Obliquity (tilt) affects vertical incidence
  heating
• More than Eccentricity (elliptical orbit)
• At Spring-Fall equinoxes Sun is Overhead

34
The Analemma Equation of Time

• The maximum noon shadow And Elevation of the Sun
  trace out the Figure of 8 or Analemma over the
  year.
• More heat at top and less at bottom

35
Seasonal Migration of Atmospheric Circulation
Patterns
Fig 4.16

• The ITCZ shifts to the summer hemisphere side of
  the Equator and the weaker circulation cells
  shift Polewards
• Discreet Subtropical Highs mark descending Hadley
  Cells

36
Diurnal Wind Changes on Arid CoastsStrong
Onshore breeze by dayWeak Offshore breeze by
night

• Ships sail in by day and out by night

37
Fig 4.17a
Water has 3-4X the heat capacity of dry land.
1cal/gC Counterintuitively, this makes the land
heat 3-4 times faster than the sea!

• Coastlands heat by day creating Low Pressure
• This sets-up Onshore Adiabat winds
• As denser High Pressure Cool Air flows in to
  replace

38
Fig 4.17b

• Land cools faster than sea, less water thermal
  mass
• Cool high pressure air falls on land flows to
  the Bay

39
Continentality Land Heats Cools Faster than
the Ocean Winter in North
Fig 4.18a
Greenland Siberia hit -48C While Australia,
Madagascar Brazil pass 24C The Thermal
Equator shifts to about 10South

• January
• Isotherms deflect Southwards in Northern
  Hemisphere
• in the Southern Hemisphere too!

40
Continentality Land Heats Cools Faster than
Ocean Summer up North
Greenland Siberia hit a balmy 12C While
Australia, Johannesburg Brazil dip to a
frigid 12C Much smaller climate variation in
the southern hemisphere, zonal air flow over
southern oceans. The Thermal Equator shifts to
about 10North
Fig 4.18b

• July
• Isotherms deflect Northwards in Northern
  Hemisphere
• in the Southern Hemisphere too!

41
Annual Temperature DifferenceBetween Summer and
Winter
Fig 4.18c
Bumpy ?
Flat ?

• The Tropics and Southern Oceans Experience little
  ?T While the Southern Continents get a little
  more Northern Continents Oceans Get more ? T

42
Average Sea Level Pressure January
Fig 4.19a

• Pressure in mbar, 1 atm 1.013 bar
• 2 Belts of Highs /-30 from Equator ITCZ
• Lows at 60-70South

43
Average Sea Level Pressure July
Fig 4.19b

• Northern Belts of Highs 40 from Equator ITCZ
• Southern Belts of Highs -25 from Equator ITCZ
• Lows still at 60-70South Cape Stiff Blows!

44
Wind Field _at_ 00Z Aug 1, 1999Radar Satellite Data

• ITCZ 10N of equator
• Where NE SW trade winds converge
• Left curves to south right to north
• Subtropical highs as spirals

Fig 4.20
45
Reversing Monsoon Flow

• Summer High over Tibetan Plateau but Winter Low

46
Fig 4.21a
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Fig 4.21b
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Fig 4.22
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Fig 4.23
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Fig 4.24
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Fig 4.24
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Fig 4.24b
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Fig 4.25
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Fig 4.26
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Fig 4.26a
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Fig 4.26b
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Fig 4.27
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