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ESS15 Lecture 11 Coriolis force, atmospheric circulation, ocean circulation, El Nino and the thermohaline circulation. Today’s lecture will give you an in depth appreciation for: • Why it doesn’t rain much in the O.C. • Origins of a mysterious geographic pattern in where Earth’s deserts are located. • As promised last week, a rap about this. Cool fact: there’s a planetary scale “latitudinal” pattern to where the Earth’s deserts occur! You are here ~ 30 deg N dry dry dry equator ~ 30 deg S wet wet dry wet dry It all starts with Earth’s energy imbalances. At low latitudes, near the equator, sun rays strike the earth surface more perpendicularly, so more Watts are absorbed there than at the poles. This is called “differential forcing” —> different solar radiation absorbed at different latitudes. Energy Imbalances I-clicker exercise: In this graph of Earth’s energy imbalances… the red line is and the blue line is A. . solar heating, terrestrial cooling B. Terrestrial cooling, solar heating C. Evaporative heating, solar cooling D. Solar heating, evaporative cooling Energy Imbalances What do air currents do when there is differential heating? Example of a thermal circulation cell – theday(mecoastalseabreeze. The circulation cell moves heat energy from the hotter to the colder place. Anatomy of a thermal circulation cell. Low pressure Subsiding High pressure Rising High pressure Low pressure COLDER HOTTER Differential heating of air sets up circulation cells. i-clicker: At 4am the land is usually ocean so the surface wind tends to blow Upper atmosphere Surface A: hotter, towards the land B: colder, towards the land C: hotter, towards the ocean D: colder, towards the ocean than the . Forces acting on the air. Pressure gradient force Gravity (falling) Friction (rubbing against the surface) Lastlecturewelearnedhowcircula1oncells resultedfromdifferen1alhea1ngasaresultof thepressuregradientforce Earth’s differential heating drives a planetary scale pattern of moving air. IftheEarthdidn’trotate,theglobalairflow pa=ernwouldlooklikeathermalcircula(on cell,justlikethecoastalseabreeze. IftheEarthdidn’trotate,theglobalairflow pa=ernwouldlooklikeathermalcircula(on cell,justlikethecoastalseabreeze. Iftherewerenorota(ononplanetEarth… • Thermal convection would lead to formation of convection cell in each hemisphere • Rising air over the relatively energy rich (hot) equator • Subsiding air over the relatively energy poor poles • Energy transported from equator toward poles • Surface wind in Irvine would always blow from the North What it would look like if the planetary wind patterns looked like a coastal cell. I-clicker:Iftherewerenorota(ononplanet Earth… • Surface wind in Irvine would always blow from the: • • • • A. North B. South C. West D. East What it would look like if the planetary wind patterns looked like a coastal cell. But the Earth does rotate. This really matters to the planetary air flow pattern. Forces acting on the air. Pressure gradient force Gravity (falling) Apparent forces: Coriolis force Centrifugal force Friction (rubbing against the surface) Coriolis force — the “apparent” force of being in a rotating world. Coriolis force • Magnitude • Depends upon the latitude and the speed of movement of the air parcel • The higher the latitude, the larger the Coriolis force • • • zero at the equator, maximum at the poles The faster the speed, the larger the Coriolis force Direction • The Coriolis force always acts at right angles to the direction of movement • • To the right in the Northern Hemisphere To the left in the Southern Hemisphere Iftherewerenorota(ononplanetEarth… • Thermal convection would lead to formation of convection cell in each hemisphere • Rising air over the relatively energy rich (hot) equator • Subsiding air over the relatively energy poor poles • Energy transported from equator toward poles • Surface wind in Irvine would always blow from the North What it would look like if the planetary wind patterns looked like a coastal cell. ButtheEarthdoesrotate–aCoriolisforce.Andthis reallyma=erstotheglobalcircula(on! We live underneath a descending branch of The Hadley Cell The pattern of the Hadley Cell explains why the deserts are where they are. Pattern: 30S EQ 30N dry wet dry Increasing latitude Why it doesn’t rain in the O.C. (science) • Differential forcing (different solar radiation at different latitudes) drives energy imbalances in the Earth system. • The air tries to move the heat poleward via a thermal circulation cell. • But because the Earth is spinning and the result is a Hadley cell with descending, dry air near 30S & 30 N. • That’s why it doesn’t rain much in the OC. • That’s why most deserts are located at similar latitudes. Why it doesn’t rain in the O.C (hip hop) • Best believe my science tight i’m talking differential forcing • (Different solar radiation that the latitudes absorbing) • - add the fact the Earth be spinning Hadley circulation soaring • Up in tropics, down in O.C. that’s the reason it ain’t pouring. Winds on the rotating Earth. wavy westerlies • Deep convective cells confined to tropics • Condensation heating in rising branch of Hadley Cell lifts the center of mass of the atmosphere (latent -> potential energy) • Downhill slope toward winter pole produces jet streams in middle latitudes • Jet is unstable to small perturbations, breaks down in waves we call winter storms doldrums easterly Trade Winds Global atmospheric circulation • Hadleycell(thermallydirectcell) - driven by N-Sgradientinhea.ng - air risesnearequatoranddescends near 30 degrees - explains deserts; trade winds; doldrums • FerrelCell(indirectthermalcell) - driven by heat transports of eddies - air risesnear60degrees and descends near 30 degrees - explains surface westerlies from 30-60 • Weak winds found near – – • Equator (doldrums) 30 degrees (horse latitudes) Boundary between cold polar air and mid-latitude warmer air is the polarfront Surface winds and pressure January July Atmospheric water Annual mean precipitable water (mm) • • • • • • Source http://www.cdc.noaa.gov/ Reanalysis for 1968-1996 Mean ~ 25 mm (1 inch) Mean precip rate is about 2.6 mm/day Residence time ~ 9 days Very steady E (evaporation) ~ P (precipitation) ~ 2.6 mm/day globally. Most near equator Jet streams Annual mean precipitable water (mm) Waves on the polar vortex. Polar vortex Changing vortex Feb 1, 2016 From http://earth.nullschool.net Atmospheric circulation In a nutshell. • Radiation energy imbalances require Earth’s fluid to move energy from where it accumulates (tropics) to the poles. • Hot air rises (rains a lot) in the tropics • Air cools and sinks in the subtropics (deserts) • Poleward-flow is deflected by the Coriolis force into eastward moving jet streams in the temperate zone • Jet streams are unstable to small perturbations, leading to huge eddies (storms and fronts) that finish the job The oceans and their global circulation I. Wind-driven gyre circulations Map of global surface ocean currents. gyre gyre gyre gyre gyre Balance of forces, at the ocean surface. fri c tio i r o c Wind stress accelerates surface water • Friction couples surface to underlying water • Friction always acts exactly opposite current motion • Coriolis acceleration is always perpendicular to current motion n re is l o • wind stress su lta nt cu rre nt RESULT: Surface current directed about 45º (right) of wind in Northern Hemisphere Ekman flow Force balance within the upper ocean. • Combined effects of Coriolis and friction on “stack” of thin layers • Each layer moves more slowly and further right than layer above; Average motion of upper ocean is 90º totherightof wind (in NH) Ekman pumping. • Ekman flow in NH is 90º to the right of the wind stress • Cyclonic wind forces divergence in water, and upwelling • Anticyclonic wind forces convergence and downwelling Idealized gyre • Convergence of Ekman flow raises sea surface • Rotating “dome” results Asymmetric gyres. • Real gyres aren’t symmetrical! • Boundary currents are strong in west, weak in east Is there a Gulf Stream? Trajectories of derelict (drifting) ships (19th century) Sea surface temperature from space. • Gulf Stream flows along the boundary of warm and cold water • Maximum current is about 4 “knots” (~ mph) • Note eddies and “rings” in Gulf Stream Other important current patterns. east-west flow in tropics Antarctic circumpolar current. Next time: Tropical oceans and El Nino Thermohaline circulation