Presentation on theme: "Atmospheric Circulation"— Presentation transcript:
1Atmospheric Circulation Introductory OceanographyInstructor: Ray Rector
2Atmospheric Circulation Key TopicsComposition and StructureSolar Heating and ConvectionThe Coriolis EffectGlobal Wind PatternsStorm SystemsClimate PatternsEffects on Ocean
3Atmospheric Circulation Key ConceptsEarth’s atmosphere is stratified and consists mostly of N2 and O2Differential heating of Earth’s curved surface by incoming solar radiation produces an over-heated equator and under-heated polesHeating and cooling creates regions of low and high atmospheric pressure, respectively. Coupled with gravity, this drives atmospheric convection, i.e. global-scale, circulation system of windsThe Earth has three major atmospheric circulation cells in each hemisphere – a total of six around the planetEarth’s rotation causes moving air masses to curve – left in the northern hemisphere and right in the southern hemisphere; this is called the Coriolis EffectAtmospheric circulation is responsible for the transfer of 2/3rds of Earth’s surface heat from the equator to the polesSpinning storm systems are of two types: Tropical and Extra-tropicalCirculation of atmosphere and ocean moderates Earth’s surface temperatures, and shapes weather and climateSurface winds and storms generate ocean currents and wind waves
4Atmosphere Composition Proportion of Gases in the AtmosphereKey IdeasMostly consist of nitrogen (78%), oxygen (21%) and argon (1%)Water vapor and carbon dioxide important minor componentsWater vapor can be as high 4% by volumeAir has mass: 1 sq. foot column of the vertical atmosphere weighs 1 ton
5Vertical Structure of Atmosphere Key IdeasDensity-stratified air columnMost of air found in troposphereWeather occurs in troposphereJet stream at top of troposphereOzone found in stratosphereTemperature inversions at the layer boundaries
6Annual Solar Energy Striking Earth Incoming Solar Radiation atTop of Earth’s AtmosphereAnnual Solar Radiation at Earth’s Surface (kcal/cm2/year)Key Idea:Global variation in the amount of solar energy striking Earth’s surface is controlled by the latitude, season, atmospheric conditions, and altitude.
7Variable Solar Radiation at Earth’s Surface Key IdeaInsolation is incoming solar radiation. The amount of insolation received at the surface of the earth is primarily controlled by the sun angle. Sun angle is a function of latitude and season.
8Atmospheric Conditions Affect Amount of Sunlight Striking Earth’s Surface Key IdeaThe amount of sun energy received at the surface of the earth can also be affected by cloud cover, dust, and other particulates that reflect and/or absorb incoming sunlightSea surfaces are typically much more prone to cloud cover than land surfaces.
9Human-Scale Convection Process Uneven Solar Heating of Earth’s Surface Causes Global-Scale Atmospheric ConvectionHuman-Scale Convection ProcessGlobal-Scale Convection ProcessHeat difference causes pressure differences in the overlying atmosphereOverheating of equatorial regions forms belt of low pressureUnder-heating of polar regions creates centers of high pressurePressure differences in lower atmosphere cause air masses to movesAir masses move from regions of high pressure to regions of low pressure
10Pressure gradient -- Isobars --- Wind strength Air masses move from regions of high pressure to regions of low pressureSeverity of pressure gradient between adjacent regions of high and lows controls how strong of wind will blow between the high and low
11Atmospheric Circulation Model of a Non-Spinning Earth Key IdeasOne cell per hemisphereOverheated equatorial air rises and moves horizontally aloft toward the polesOvercooled polar air sinks and moves horizontally at surface towards equatorSingle-Cell Hemispheric Convection Model
12Atmospheric Circulation Model of a Spinning Earth Key IdeasThree cells per hemisphereHadley, Ferrel, and PolarSimilar convection processSmaller convective cellsTwo surface convergence zonesTwo surface divergence zonesTriple-Cell Hemispheric Convection ModelSpinning causes the Coriolis effectCoriolis effect deflects air currents
13Deflection of Flying Projectiles Deflection of Moving Air Masses The Coriolis EffectDeflection of Flying ProjectilesDeflection of Moving Air MassesKey IdeasObjects deflect to the right in Northern HemisphereObjects deflect to the left in Southern HemisphereMoving air masses appear to have curved paths
14Earth’s Surface WindsLand-free Circulation Model Landmass Circulation Model1) Polar Easterly belts2) Mid-latitude Westerly belts3) Low-latitude Tradewind belts4) Subpolar low and equatorial low pressure belts5) Subtropical high pressure belt1) Landmasses break up the wind and pressure belts2) High to mid-latitude pressure centers replace pressure beltsEquatorial low pressure beltSeasonal shift of pressure centers
16Convergence and Divergence Zones Equatorial ConvergenceConvergence = Orange L’sDivergence = Blue H’s
17Earth’s Atmospheric Pressure and Circulation Belts Latitudinal belts dominated by either precipitation or evaporation1) Polar high pressure centers2) Sub-polar low pressure centers = polar fronts3) Sub-tropical high pressure centers = horse latitudes4) Equatorial low pressure belt = ITCZ or duldrums
18Divergence Versus Convergence Air Flow at High and Low Pressure Centers 1) Pressure differences cause air masses to moves = Wind2) Air masses move from regions of high pressure to regions of low pressure
19Pressure gradient force only See pg in ChristophersonPressure gradient force onlyEffect of coriolisupper atmosphereEffect of coriolis and frictionlower atmosphere
20Physical Effects of Vertical Movement of Air Masses Temperature and Volume Changes as a Function of Vertical Movement of Air Mass (adiabatic lifting or falling)Water Vapor Saturation Level as a Function of Changing Temperature of Air MassKey IdeasAscending (rising) air expands, cools, and becomes less denseDescending (falling) air contracts, heats, and becomes more denseWater vapor in rising and cooling air will condense into cloudsFurther rising and cooling of cloud-rich air will lead to precipitation
21Atmospheric Humidity versus Temp Relative Humidity for Air Masses of Different Temperatures with a Given Absolute Amount of Water VaporMaximum Vapor Pressure (Absolute Humidity Limit) for Air Masses of Different TemperaturesKey IdeasAscending (rising) air expands, cools, and becomes less denseDescending (falling) air contracts, heats, and becomes more denseWarm air can hold more water than cold airWater vapor in rising and cooling air will condense into clouds
22Weather at Divergence and Convergence Zones Polar DivergenceHigh evaporationVariable windsCold, harsh, dry weatherSubpolar ConvergenceHeavy precipitationWinter storm frontsStormy, wet, cool weatherSubtropical DivergenceHigh evaporationVariable winds and CalmsWarm, mild, dry weatherTropical ConvergenceHeavy precipitationLight winds and CalmsTropical cyclone nurseryStormy, wet, hot weather
23The Jet Stream Key Ideas Narrow fast-moving ribbons of wind Travel west to east between cellsControls position and movement of high and low pressure (storm) systems
25Seasons, Surface Winds and Weather Key IdeaSeasonal changes in Earth’s axis in respect to the sun cause latitudinal migrational shifts in several atmospheric elements:Pressure centersWind beltsJet streamsIntertropical Convergence Zone (ITCZ)Large-scale weather patterns
26Pressure Systems and Wind Patterns Averaged January Pattern Key IdeasThe ITCZ is shifted to its maximum southward positionPolar lows dominate the northern Pacific and Atlantic OceansSubtropical highs dominate south Pacific and Atlantic OceansWinter in the Northern Hemisphere; summer in the S. Hemi.
27Pressure Systems and Wind Patterns Averaged July Pattern Key IdeasThe ITCZ is shifted to its maximum northward positionSubtropical highs sit over the north Pacific and Atlantic OceansSouthern Ocean westward wind belt at maximum strengthSummer in the Northern Hemisphere; Winter in the S. Hemi.
28Weather SystemsLocal atmospheric conditions that prevail over a period of days to weeks
29Spinning Air Mass Disturbances Tropical CyclonesExtratropical Cyclones
30Solar Energy Powers the Both the Atmospheric and Hydrologic Cycles Solar Energy Causes Evaporation of the Ocean Surface Waters1 meter of ocean surface is evaporated each year!Most precipitation falls back into the oceanPrecipitation over land plays huge part in weathering and erosion
33Tropical Cyclones1) Tropical Cyclones are known as hurricanes in the Atlantic Ocean, typhoons in the Pacific Ocean and cyclones in the Indian Ocean.2) Very extensive, powerful, and destructive type of storm.3) This type of storm develops over oceans 8° to 15° North and South of the equator.4) Hurricanes draw their energy from the warm water of the tropics and latent heat of condensation.
34Necessary Conditions for Cyclone Development: 1) Must originate over ocean water that is least 26.5 °C.Hurricanes feed off the latent heat of water – hotter the better! 2) Have an atmosphere that cools quickly with height.This creates potentially unstable conditions that builds storms.3) Low vertical wind shear.Winds at all levels of the atmosphere from the ocean up to 30,000 feet or higher are blowing at the same speed and from the same direction. 4) No closer than 500 kilometers to the equator.The Coriolis Force is too weak close to the equator.It is the Coriolis Force that initially makes the cyclone spiral and maintains the low pressure of the disturbance.5) An upper atmosphere high pressure area above the growing storm.The air in such high pressure areas is flowing outward. This pushes away the air that is rising in the storm, which encourages even more air to rise from the low levels.6) Hurricanes will not always form in these conditions. However, a willhurricane only form if these conditions are present.
35Anatomy and Behavior of a Hurricane 1) Warm, humid surface winds spiral towards eye.2) Strongest winds occur in the eye wall at the surface.3) Air in the eye sinks which inhibits wind and cloud formation4) Body of hurricane divided into concentric rain bands5) Surface rotation direction depends on hemisphere6) All hurricanes move toward the west
36Life Cycle of Tropical Cyclones FormationTropical Disturbance to DepressionWeak to moderate windsLittle to no rotation2) PrematurityTropical StormModerate to strong windsModerate rotation3) Full MaturityHurricaneVery Strong windsRapid rotation with eye4) DecayDissipation into weaker and weaker systemEntire cycle typically lasts between 1 to 2 weeks
37Global Tropical Cyclone Tracks Which ocean basin has the most tracks? Why?Which ocean basin has the least tracks? Why?
41Mid-Latitude Cyclonic Systems What is a Mid-Latitude Cyclone?A high- to mid-latitude cyclonic low pressure systemTypically form from fall to springIT IS NOT A HURRICANE OR TROPICAL STORMForms from the swirling convergence of a cold and warm air mass along the polar frontAssociated with high winds, clouds and precipitationTypical size of mid-latitude cyclone = km in diameter
42Development of Mid-Latitude Storm Systems 1) Frontal Wave Develops2) Instability “Kink” Forms3) Fully-developed CycloneExample4) System Begins to “Occlude”5) Advanced “Occlusion”
43Cold and Warm Fronts Cold Front Warm Front Cross Section View Map View
44Spot the Three Frontal Systems - 1) Where are the centers of systems? 2) Centers of low pressure?3) Sense of rotation? 4) Regions of warm and cold air masses?
46Wind – Wave Connection Surface Wind Speeds Wave Heights Key Points Highest = WesterliesLowest = Doldrums/ ITCZWave HeightsHighest = High LatitudeLowest = Low LatitudesKey Points1) Ocean seas and swell are direct result of ocean surface winds2) The stronger the wind, the larger the seas and swell
48Atmospheric Circulation Review of Key ConceptsEarth’s atmosphere consists mostly of N2 and O2Differential solar heating of Earth’s surface produces an over-heated equator and under-heated polesDifferential solar heating of Earth’s surface, coupled with gravity, drives atmospheric convection, i.e. global-scale, circulation system of windsThe Earth has three major atmospheric circulation cells in each hemisphere – a total of six around the planetEarth’s rotation causes moving air masses to curve – left in the N. Hemisphere and right in the S. Hemisphere, a.k.a. the Coriolis EffectAtmospheric circulation is responsible for the transfer of 2/3rds of Earth’s surface heat from the equator to the polesSpinning storm systems are of two types: Tropical and Extra-tropicalCirculation of atmosphere and ocean moderates Earth’s surface temperatures, and shapes weather and climateSurface winds and storms generate ocean currents and wind waves