The Atmospheric Circulation System

Geos 110 Lectures: Earth System ScienceChapter 4: Kump et al 3rd ed.

Dr. Tark Hamilton, Camosun College

Overall the Earth’s Climate is in BalanceIn 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 Earth’s fossil Carbon from the past ~350 Ma in < 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!

The Ideal Gas Law: Relationships of Pressure, Temperature, Volume & Moles

Ideal Gas Law:

• P V = n R T; P=Pressure, T=Temperature, n=moles of gas particles (with mass), R=ideal gas constant

Special Case 1 – Boyle’s Law: (@ T=constant)

• 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: (@ P=constant)

• Vinitial / Tinitial = Vfinal / Tfinal 2

There are Big Latitudinal Differences

• The Tropics have Energy Surplus

• The Poles run a Deficit

• Temperate zones have transitory seasonal swings

Fig 4.1

There are Big Latitudinal DifferencesIR emission doesn’t match? How does heat move?

• The Tropics have a Net Radiation Surplus (Sin>Eout)

• The Poles run a Net Radiation Deficit (Sin<Eout)

• Temperate zones have transitory seasonal swings

Fig 4.2

There has to be a Global Circulation System

• IR conversion to Latent Heat (LiquidVapor)

• Convection driven by density and pressure differences between different air masses

Ferrel Cell Ferrel Cell

Fig 4.3

Convergent versus Divergent Windsat Earth’s 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

Weather & Climate Vary Across the Globe

• 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.

Fig 4.4

Eastern Pacific & Central America w/ ITCZ Intertropical Convergence Zone

• NOAA Satellite Image

• Cloud Band marks ITCZ at top of Troposphere

• The Troposphere, heated from below convects

Fig 4.5

Convective Towers Cumulonimbus drive Hadley Cells of ITCZ

• Cloud Band marks ITCZ at top of Troposphere• Solar Evaporation & Latent Heat from Condensation

make the heat pump that drives the Convection

Fig 4.5

Horizontal & Vertical Air Movements result from Temperature & Pressure

differences driving Buoyancy

• Buoyancy is due to density contrasts, Δmass/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

Mid-latitude Convective Mixing

• 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

Fig 4.6

N-S Meridional Mixing of Troposphere

Tropics to Horse Latitudes - Hadley Cells

Mid-Latitudes – Ferrel Cells

High Latitude - Polar Front

Fig 4.7

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

The Horse Latitudes

• Spanish ships bound for the New World became becalmed w. Hi Pressure, no wind and Horses died

• English “Dead Horse Shanty”, working off advance

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

Idealized Tropospheric Circulation

ITCZ & Polar Front Storm Belts – Hi Precipitation

Horse Latitude & Polar Deserts

A Simple Pressure Model for Winds

• Winds blow out of descending High Pressure limbs ~30-35° N&S between Hadley & Ferrel Cells

• Winds blow towards rising Low Pressure limbs on equatorial edge of Hadley Cells at ITCZ & also PCZ

Fig 4.8

Coriolis Rotational Effects on a Sphere

• Since the Earth revolves once a day….

• Bantu’s and Guajiran’s move a lot faster and further

• Than Innu or Lapplanders!

Fig 4.9a

4.64 m/s

0 m/s

Apparent Wind Deflection to the Right N in N.Hem. (rotating reference frame)

While the Earth revolves from AA’ & BB’

The N flowing Air moves from P1 X,

This is really in a straight line viewed from Space

Fig 4.9b

The curved paths are relative to fixed points on the ground which revolves.

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

• A Vector with 2 components in a plane defined by the spin axis and the location on the Earth’s surface

• 1 Component is vertical, 1 horizontal-tangent away

• In N Hem. E moving wind deflects Right to South

Fig 4.10

A More Realistic Model for Surface Winds:Pressure Differences, Buoyancy & Coriolis Effects

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

Fig 4.11

Tropic of Cancer 23.5°N

Tropic of Capricorn 23.5°S

~1 Season

Big Seasonal Changes

Big Seasonal Changes

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

Tropical Cyclones: Hurricanes & Monsoons

• 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 26-27°C & > 5° Latitude from the Equator

Box Fig 4.1

Causes of Tropical Cyclones

• 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

Box Fig 4.1

Extratropical Cyclones

• From outside the tropics > 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

Box Fig 4.1

Flow of Troposphere

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

Fig 4.7

Upper Level Tropospheric Flow

The troposphere is warmer and thicker in the tropics

&

Colder and thinner at the Poles

Fig 4.12a

Tropospheric Pressure Surfaces

• Tropics more expanded & < vertical pressure gradient• Poles are more compressed w/ > vertical pressure gradients

Fig 4.12b

Mid-latitude Upper Level Jet Streams

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

Fig 4.12c

Flow

Geostrophic Wind

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)

Fig 4.13

Friction acts near surface at High Pressure

Slows and deflects wind < 90° from coriolis

Causes winds to spiral in cyclonic storms

Fig 4.13

Fig 4.20

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

As per the previous 3 figures, this show the Polar Low

& Equatorial High

Fig 4.14

Seasonal Variation in Insolation

Obliquity (tilt) affects vertical incidence & heating

More than Eccentricity (elliptical orbit)

At Spring-Fall equinoxes Sun is Overhead

Fig 4.15

PerhelionAphelion

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

Seasonal Migration of Atmospheric Circulation Patterns

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

Fig 4.16

Fig 4.17a

Fig 4.17b

Fig 4.18a

Fig 4.18b

Fig 4.18c

Fig 4.19a

Fig 4.19b

Fig 4.20

Fig 4.21a

Fig 4.21b

Fig 4.22

Fig 4.23

Fig 4.24

Fig 4.24

Fig 4.24b

Fig 4.25

Fig 4.26

Fig 4.26a

Fig 4.26b

Fig 4.27