Transcript Document
The Atmospheric Circulation 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 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 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? 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 (LiquidVapor) • 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 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. 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 • 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 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 ITCZ & Polar Front Storm Belts – Hi Precipitation Horse Latitude & Polar Deserts 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 Coriolis Rotational Effects on a Sphere 0 m/s Fig 4.9a 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! 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 AA’ & BB’ The N flowing Air moves from P1 X, This is really in a straight line viewed from Space 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 Earth’s surface • 1 Component is vertical, 1 horizontal-tangent away • In N Hem. E moving wind deflects Right to South 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 Big Seasonal Changes • The same divergence & convergence zones are shown • Coriolis force effects are shown • Permanent Peri-equatorial Trades & Winter Polar Easteries 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 • • • • • 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 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 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 Flow of Troposphere Fig 4.7 Surface Flow is dominated by latitudinal belts Upper Level Flow is Dominantly Polewards! Upper Level Tropospheric Flow Fig 4.12a The troposphere is warmer and thicker in the tropics & Colder and thinner at the Poles 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 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) Friction acts near surface at High Pressure Fig 4.20 Fig 4.13 Slows and deflects wind < 90° from coriolis Causes winds to spiral in cyclonic storms 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 Seasonal Variation in Insolation Fig 4.15 Aphelion Perhelion Obliquity (tilt) affects vertical incidence & heating More than Eccentricity (elliptical orbit) At Spring-Fall equinoxes Sun is Overhead 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 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 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 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! Coastlands heat by day creating Low Pressure This “sets-up” Onshore Adiabat winds As denser High Pressure Cool Air flows in to replace Fig 4.17b Land cools faster than sea, less water & thermal mass Cool high pressure air falls on land & flows to the Bay 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 January Isotherms deflect Southwards in Northern Hemisphere & in the Southern Hemisphere too! 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 Fig 4.18b oceans. The Thermal Equator shifts to July about 10°North Isotherms deflect Northwards in Northern Hemisphere & in the Southern Hemisphere too! Annual Temperature Difference Between Summer and Winter Bumpy Fig 4.18c Flat The Tropics and Southern Oceans Experience little ΔT While the Southern Continents get a little more Northern Continents & Oceans Get more Δ T 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 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! Wind Field @ 00Z Aug 1, 1999 Radar Satellite Data Fig 4.20 • ITCZ ~ 10°N of equator • Where NE & SW trade winds converge • Left curves to south & right to north • Subtropical highs as spirals Reversing Monsoon Flow Summer High over Tibetan Plateau but Winter Low 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