The Atmospheric Circulation System Cumulonimbus “Thunderhead” Storm Cloud with Anvil Flatiiron Top Lots of vertical convection and mixing Usually Vapour, liquid droplets and ice crystals coexist in this mixed dynamic system Geos 110 Lectures: Earth System Science Chapter 4: Kump et al 3rd ed. Dr. Tark Hamilton, Camosun College

Overall the Earth’s 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 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 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

There are Big Latitudinal Differences Fig 4.1 The Tropics have Energy Surplus The Poles run a Deficit Temperate zones have transitory seasonal swings

There are Big Latitudinal Differences IR emission doesn’t match There are Big Latitudinal Differences IR emission doesn’t match? How does heat move? Fig 4.2 The Tropics have a Net Radiation Surplus (Sin>Eout) The Poles run a Net Radiation Deficit (Sin<Eout) Temperate zones have transitory seasonal swings

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

Convergent versus Divergent Winds at 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 The troposphere is well mixed by convection and is heated from below. The overlying stratosphere is warmer, heated from above and puts a lid on this mixing 7

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. Very long term ~100’s of Ma heat redistribution in the core and mantle affect the shapes and positions of continents and ocean basins via solid state convection and plate dynamics.

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

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

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 Fig 4.6 This calls for steep lateral temperature gradients and abrupt storm fronts with all kinds of wind and weather blowing through, especially at the polar front zone ~ 60°N & S. 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

N-S Meridional Mixing of Troposphere Fig 4.7 Tropics to Horse Latitudes - Hadley Cells Mid-Latitudes – Ferrel Cells High Latitude - Polar Front 13

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 The Dead Horse Shanty Oh, poor old man your horse will die And we say so, and we know so Oh, poor old man your horse will die Oh, poor old man We'll hoist him up to the main yardarm We'll hoist him up to the main yardarm Say, I old man your horse will die Say, I old man your horse will die We'll drop him down to the depths of the sea We'll drop him down to the bottom of the sea We'll sing him down with a long, long roll Where 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

Idealized Tropospheric Circulation Rising moist air cools and engenders rain or snow, especially Tropical Lows Descending cold dry air dessicates underlying landscapes making deserts & drylands, especially under Polar Highs. This idealized image would work best on an all water covered world or at least one with longitudinal oceans! If you are a convecting fluid or a fish, land kind of gets in the way! ITCZ & Polar Front Storm Belts – Hi Precipitation Horse Latitude & Polar Deserts 16

A Simple Pressure Model for Winds Fig 4.8 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 17

Coriolis Rotational Effects on a Sphere 0 m/s Fig 4.9a N-S motions are deflected in apparent curved paths due to the rotating reference frame. E-W motions are subjected to centrifugal force due to the velocity of rotation and momentum mv 4.64 m/s Since the Earth revolves once a day…. Bantu’s and Guajiran’s move a lot faster and further Than Innu or Lapplanders! 18

Apparent Wind Deflection to the Right N in N. Hem 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. This is the same right wise rotation for S flowing air in the northern hemisphere This is the opposite way, leftwards deflection for S flowing air in the Southern hemisphere 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 19

Coriolis (Centrifugal) Force acts on East or West moving Winds (increasing w/Latitude) Fig 4.10 This Coriolis effect increases towards higher latitude due to decreasing rotational velocity! It is at its maximum at the poles and zero at the equator. This is like the NW Trade Winds in the Northern Hemisphere or the opposite, The SE Trade Winds in the Southern Hemisphere 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 20

A More Realistic Model for Surface Winds: Pressure Differences, Buoyancy & Coriolis Effects Fig 4.11 Big Seasonal Changes Tropic of Cancer 23.5°N ~1 Season Tropic of Capricorn 23.5°S Winds are names by where they blow from rather than where they blow to. This works for Sailors and Meteorologists as they really need to pay attention to where the weather is coming from. In detail the divergence zones are weak and wind speed and direction vary considerably on a day to day basis. Big Seasonal Changes The same divergence & convergence zones are shown Coriolis force effects are shown Permanent Peri-equatorial Trades & Winter Polar Easteries 21

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 Box Fig 4.1 Centripetal acceleration is experienced by objects rotating at constant tangential speed. The acceleration is inwards along the radius vector of rotation at ω2 r. This and the pressure gradient and the coriolis force all interact to determine cyclone wind speed and rotational velocity. 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 23

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 Centripetal acceleration is experienced by objects rotating at constant tangential speed. The acceleration is inwards along the radius vector of rotation at ω2 r. This and the pressure gradient and the coriolis force all interact to determine cyclone wind speed and rotational velocity. 24

Extratropical Cyclones Box Fig 4.1 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 While polar easterlies are stable all winter, in summer this flow breaks down and the storms can migrate to the Arctic. 25

Flow of Troposphere Fig 4.7 Surface Flow is dominated by latitudinal belts  Upper Level Flow is Dominantly Polewards! 26

Upper Level Tropospheric Flow Fig 4.12a The troposphere is warmer and thicker in the tropics & Colder and thinner at the Poles 27

Tropospheric Pressure Surfaces Fig 4.12b Tropics more expanded & < vertical pressure gradient Poles are more compressed w/ > vertical pressure gradients

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 29

Geostrophic Wind Fig 4.13 There are also curved Geostrophic currents from storm set up across broad shallow continental shelves. 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) 30

Friction acts near surface at High Pressure Fig 4.20 Fig 4.13 There are also curved Geostrophic currents from storm set up across broad shallow continental shelves. Slows and deflects wind < 90° from coriolis Causes winds to spiral in cyclonic storms 31

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 32

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 33

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 circulation is weaker on the Summer hemisphere because of less thermal gradient 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 35

Diurnal Wind Changes on Arid Coasts Strong Onshore breeze by day Weak Offshore breeze by night Ships sail in by day and out by night 36

Coastlands heat by day creating Low Pressure Fig 4.17a Water has 3-4X the heat capacity of dry land. 1cal/g°C Counterintuitively, this makes the land heat 3-4 times faster than the sea! Land, especially dark or forested land heats quickly but transfers this heat up by evaporation and convection. This sets up small thermal lows, ideal for birds along the coast. Oceans with lower albedo heat better at the same latitude. They are also more transparent than rock or dirt so they absorb heat below the surface too, but thy also transfer this heat both ways by convection in the top of the sea and convection of overlying air. Especially large flat layers like shallow bays have high Renyolds’ numbers favouring turbulent mixing downwards. This and the larger thermal mass of the ocean makes for very small temperature changes. Great for sailboarding! Coastlands heat by day creating Low Pressure This “sets-up” Onshore Adiabat winds As denser High Pressure Cool Air flows in to replace 37

Land cools faster than sea, less water & thermal mass Fig 4.17b As the weak daytime lows on shore collapse by rapid cooling, the air falls bringing high pressure down especially along arid coastlines. The Ocean cools relatively little so it now gets to play the low as weak breezes flow out to sea. Land cools faster than sea, less water & thermal mass Cool high pressure air falls on land & flows to the Bay 38

Continentality: Land Heats & Cools Faster than the Ocean: Winter in North Fig 4.18a Greenland & Siberia hit -48°C While Australia, Madagascar & Brazil pass +24°C The Thermal Equator shifts to about 10°South This deflection of the isotherms at sea makes the Northern Continents colder and the Southern continents warmer. This is because in the North the cold of the seas is felt preferentially further south (arctic fronts), weather from the Gulf of Alaska. By contrast the tropic equatorial air bends further south across the southern lands warming them and giving them summer. The interior of Antarctica warms to a balmy -30°C! January Isotherms deflect Southwards in Northern Hemisphere & in the Southern Hemisphere too! 39

Continentality: Land Heats & Cools Faster than Ocean: Summer up North Greenland & Siberia hit a “balmy” +12°C While Australia, Johannesburg & Brazil dip to a “frigid” +12°C Much smaller climate variation in the southern hemisphere, zonal air flow over southern oceans. The Thermal Equator shifts to about 10°North This deflection of the isotherms at sea makes the Northern Continents warmer and the Southern continents colder. This is because in the North the warmth of the tropic seas is felt further north inland. Mild summer southerly breezes. By contrast the cold southern seas from the Antarctic convergence zone lend cold air on land. cooling the southern lands and giving them winter. The interior of Antarctica chills like a step mother’s breath to -66°C! Polar and Continental is seemingly a real recipe for an ice age no matter what the sun and earth are doing. Fig 4.18b July Isotherms deflect Northwards in Northern Hemisphere & in the Southern Hemisphere too! 40

Annual Temperature Difference Between Summer and Winter Fig 4.18c Bumpy  Flat  The Tropics and Southern Oceans Experience little ΔT While the Southern Continents get a little more Northern Continents & Oceans Get more Δ T 41

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 60-70°South 42

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 60-70°South & Cape Stiff Blows! 43

Wind Field @ 00Z Aug 1, 1999 Radar Satellite Data ITCZ ~ 10°N of equator Where NE & SW trade winds converge Left curves to south & right to north Subtropical highs as spirals Fig 4.20 Which way does the wind blow at the equator? So which way should you sail around the world? 44

Reversing Monsoon Flow Summer High over Tibetan Plateau but Winter Low 45

Fig 4.21a 46

Fig 4.21b 47

Fig 4.22 48

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Fig 4.24 50

Fig 4.24 51

Fig 4.24b 52

Fig 4.25 53

Fig 4.26 54

Fig 4.26a 55

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Fig 4.27 57

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