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El Niño: the Ocean in Climate El Niño: the Ocean in Climate

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El Niño: the Ocean in Climate El Niño: the Ocean in Climate El Niño: The ocean in climate We live in the atmosphere: where is it sensitive to the ocean? ➞ The tropics! El Niño is the most spectacular short-term climate oscillation. It affects weather around the world, and illustrates the many aspects of O-A interaction Most of the poleward heat transport is by the atmosphere First hint that this may all be myth comes from using observations to estimate atmosphere and ocean(except heat in the tropics) transports Ocean and atmosphere heat transports Heat transport is poleward in &'()*('&+,-.,/01,2%""34,-5.67/.-5,89:,;9:.<=/:>,<-/.,.:/;5?9:.5 # both ocean and atmosphere: )D(B,E('1,F&G (DGCH,E('1,F&G )D(B,E('1,ID(F) Atmosphere (DGCH,E('1,ID(F) The role of the O-A system is $ Net radiation at Atmosphere (line) to move excess heat from the top of atmosphere tropics to the poles, where it Ocean (Dashed) Ocean = divergence of % is radiated to space. (AHT + OHT) " BC Net radiation Northward transport (PW) !% from satellites, AHT from weather obs !$ and models, OHT from residual Trenberth et al (2001) !# 80°S!!" 60°S!#" 40°S!$" 20°S!%" 0°" 20°N%" 40°N$" 60°N#" 80°N!" @/.6.A>- Most of the poleward transport occurs in mid-latitude winter storms near 40°S/N. Only in the tropics is the ocean really important. Much recent interest in the “global conveyor belt” But the timescales are very slow Does the Gulf Stream warm Europe? An example of the (largely) passive effect of the ocean Why is Ireland warmer than Labrador in winter? Because of need to conserve angular momentum, Rockies force a stationary wave in the westerlies with northerly flow (cooling) over eastern N. TR OPOPAUSE America and southerly flow (warming) over E. Atlantic and western Europe Cold winter Mild winter 2OCKY-OUNTAINS 3 2 The Rocky Mts force &IGURE4HEWAVINESSINTHEFLOWOFTHEMID LATITUDEWESTERLIESTHATISRESPONSIBLEFORKEEPING%UROPEANWINTERSMILDRESULTSFROMA FUNDAMENTALPRINCIPLEOFPHYSICSTHECONSERVATIONOFANGULARMOMENTUM"ECAUSETHETOPOFTHETROPOSPHEREACTSASSOMETHINGOFALID AIR FLOWINGFROMTHE0ACIFICOVERTHE2OCKYthe-OUNTAINS jetMUST COMPRESSstreamVERTICALLYAND toASACONSEQUENCE makeEXPAND aHORIZONTALLY #ONSERVATIONOF ANGULARMOMENTUMDEMANDSTHATAPACKAGEOFAIRDEPICTEDASWHITECYLINDER UNDERGOINGSUCHAHORIZONTALEXPANSIONMUSTDEVELOPACOM PONENTOFCLOCKWISESPINTOREDUCETHEPREDOMINANTLYCOUNTER CLOCKWISESPINITHASBYVIRTUEOFITSLOCATIONINTHE.ORTHERN(EMISPHERE 4HELENGTHOFTHEREDARROWSINDICATES5000-kmRELATIVEAMOUNTOFSPIN WHICHbendISDERIVED FROMtoBOTH theLOCALAIR MOVEMENTSsouth.ANDTHEREVOLUTIONOFTHE PLANET 4HENEWCOMPONENTOFCLOCKWISESPINMANIFESTSITSELFASAGENTLESWERVETOTHESOUTHINWHATISPREDOMINANTLYWEST TO EASTFLOW 7HENTHISPACKAGEOFAIRTHENMOVESOVERTHEEASTERNSIDEOFTHECONTINENTANDONOVERTHE!TLANTIC ITDOESTHEOPPOSITE EXPANDINGVERTI CALLYANDCONTRACTINGHORIZONTALLY WHICHAsALLOWS aIT TOresult,VEERBACKTOWARD mid-latitudeTHENORTH4HEWAVELIKEPATTERNSENDS AIRHEATEDOVERTHE!TLANTIC TOTHENORTHEAST WHEREITWARMS%UROPE TOEXPLAINTHESEVEREWINTERCOOLING) 4HE GERM OF TRUTH ON WHICH SUCH CATASTROPHICEFFECTONCLIMATE4HETEM AGREETHATSOMETHINGOFTHISSORTwesterliesPROB HYPEISBASEDISTH AapproachTMOSTATMOSPHERE PE RAEuropeTURE DIFFERENCE BE TWEEN %UROPE ABLYHAPPENED BUTITSNOTATALLCLEAR OCEAN MODELS SHOW A SLOWDOWN OF AND,ABRADORSHOULDREMAIN4EMPER TOMEHOWSTOPPINGTHE!TLANTICfromCON THE RMtheOHALINE Csouthwest.IRCULATION IN SIMULA ATURESWILLNOTDROPTOICE AGELEVELS VEYOR COULD CAUSE A SUFFICIENT REDIS TIONSOFTHESTCENTURYWITHTHEEX NOTEVENTOTHELEVELSOFTHE,ITTLE)CE TRIBUTIONOFHEATTOBRINGONTHISVAST PECTEDRISEINGREENHOUSEGASES4HE !GE THERELATIVELYCOLDPERIODTHAT%U ACHANGE CONVEYER SLOWS BECAUSE THE SURFACE ROPESUFFEREDAFEWCENTURIESAGO4HE )NANYEVENT THESTILL TENTATIVECON WATERSOFTHESUBPOLAR.ORTH!TLANTIC .ORTH!TLANTICWILLNOTFREEZEOVER AND NECTIONSINVESTIGATORSHAVEMADEBE WARMANDBECAUSETHEINCREASEDTRANS %NGLISH#HANNELFERRIESWILLNOTHAVE The east-west temperature contrast depends on three phenomena:TWEEN THERMOHALINE CIRCULATION AND PORTOFWATERVAPORFROMTHESUBTROP TOPLOWTHEIRWAYTHROUGHSEAICE! ABRUPTCLIMATECHANGEDURINGGLACIAL ICSTOTHESUBPOLARREGIONSWHEREIT SLOWDOWNINTHERMOHALINECIRCULATION TIMESHAVECOMBINEDWITHTHEPOPULAR FALLS AS RAIN AND SNOW FRESHENS THE SHOULD BRING ON A COOLING TENDENCY 1) Summer heating regained by the atmosphere during winterPERCEPTI ONTHATITISTHE'ULF3TREAM SUBPOLAR.ORTH!TLANTICANDREDUCES OF AT MOST A FEW DEGREES ACROSS THE THATKEEPS%UROPEANCLIMATEMILDTO THEDENSITYOFSURFACEWATERS WHICH .ORTH!TLANTICONETHATWOULDMOST CREATE A DOOMSDAY SCENARIO 'LOBAL MAKESITHARDERFORTHEMTOSINK4HESE LIKELYBEOVERWHELMEDBYTHEWARM carried east by the mid-latitude westerly winds; WARMING MIGHT SHUT DOWN THE 'ULF SeagerPROCESSESCOUL DetBEA UGalME N(2001,TEDBYTHE IN GQJRMS) CAUSED BY RISING CONCENTRATIONS 3TREAM WHICHCOULDhPLUNGEWESTERN MELTING OF FRESHWATER RESERVES GLA OFGREENHOUSEGASES4HISMODERATING %UROPEINTOAMINIICEAGE vMAKING CIERS PERMAFROSTANDSEAICE AROUND INFLUENCEISINDEEDWHATTHECLIMATE 2) Stationary atmospheric waves forced by the Rocky MountainsWINTERS hAS HAR SH; AS THOSE IN .EW THE.ORTH!TLANTICAND!RCTIC MODELSSHOWFORTHESTCENTURYAND FOUNDLAND vORSOCLAIMS FOREXAMPLE "UT FROM WHAT SPECIALISTS HAVE WHATHASBEENSTATEDINREPORTSOFTHE ARECENTARTICLEIN.EW3CIENTIST4HIS LONGKNOWN )WOULDEXPECTTHATANY )NTERGOVERNMENTAL 0ANEL ON #LIMATE GENERAL IDEA BEEN REHASHED IN HUN SLOWDOWN IN THERMOHALINE CIRCULA #HANGE)NSTEADOFCREATINGCATASTRO 3) Heat transport by the Gulf Stream. DREDSOFSENSATIONALNEWSSTORIES TIONWOULDHAVEANOTICEABLEBUTNOT PHE IN THE .ORTH!TLANTIC REGION A http://la.climatologie.free.fr/ocean/ocean1.htm!MERICAN3CIENTIST 6OLUME ©3IGMA8I 4HE3CIENTIFIC2ESEARCH3OCIETY2EPRODUCTION WITHPERMISSIONONLY#ONTACTPERMS AMSCIORG However, even the small effect of Gulf Stream warm advection is important for the ice edge And the salty surface water carried into the Arctic affects the convection that controls the formation of deep water Climatological March sea ice (NASA/SSMI) Warm advection Western boundary currents do matter regionally Winter continental cold air flows over warm WBC water: significant heat flux to atmosphere Yu and Weller (2007) Rio (2004) What’s special about the tropics? (Sea Surface Temperature) Close relation among SST, convection and winds High cloud fraction (deep convection) and surface winds In the tropics, → → ← ←ITCZ convection coincides with warm SST and surface wind convergence. All three define the West Pacific warm pool and the Intertropical SST and surface winds Convergence Zone (ITCZ). Tropical winds blow Warm Pool directly towards warm SST. East Pacific Cold Tongue (Reynolds SST, ISCCP high clouds, Quikscat winds) Currents and temperature on the equator Currents on the Equator Frictional currents decay with depth Sea level Trade winds ~ 50 cm higher Warm SEC 50m Pool 200m EUC Observed mean temperature (colors) and currents (white contours) Thermocline Cool lower water along the Pacific equator The trade winds 1) Pile up water in the west. SEC 2) Drive the SEC by direct friction. Below the frictional layer (25m?) pressure due to the EUC high sea level in the west pushes the EUC eastward below the surface. SEC = South Equatorial Current ( EUC = Equatorial Undercurrent ) Johnson et al (2001) Close connection between thermocline depth ⟺ SST The tropical climate is coupled Vertical slice along the equator Walker Circulation Trade Winds New Guinea Warm Water heated by the sun Pool Upwelling South America Thermocline Cool lower water Cool lower water W.S.Kessler, NOAA/PMEL A positive feedback! Why are there trade winds? Because the warmest water is in the west. Why is the warmest water in the west? Because there are trade winds. Bjerknes (1965) What’s special about the tropics? • Surface air temperature is always near the threshold for convection (~27°C). • Equatorial zonal winds respond rapidly to the SST gradient. • The close connection between thermocline depth and SST means the tropical ocean varies rapidly. • The high speed and large zonal scale of equatorial waves allows efficient basinwide transmission of signals. Thus, small SST changes can greatly affect the atmosphere in the tropics, and conversely small changes in the wind can greatly affect the SST pattern: coupling. ➢ All this is less true of the extra-tropics: other mechanisms by which SST affects the atmosphere are less effective. Signals spread efficiently across the Pacific equator Anomalies The response to episodic westerly wind anomalies in the western Pacific: • Winds blow into warm SST. • Westerly winds come in bursts lasting ~30 days. • Each wind event cools locally (mostly by evaporation). • Each event also generates an Time eastward-propagating Kelvin wave. Although eastern Pacific winds remain easterly throughout, the thermocline deepens due to persistent remote forcing. • Shallow eastern thermocline cools local SST. Red = westerly wind Red = deep thermocline Red = warm SST Signals spread efficiently across the Pacific equator Anomalies The response to episodic westerly wind anomalies in the western Pacific: • Winds blow into warm SST. • Westerly winds come in bursts lasting ~30 days. • Each wind event cools locally (mostly by evaporation). • Each event also generates an Time eastward-propagating Kelvin wave. Although eastern Pacific winds remain easterly throughout, the thermocline deepens due to persistent remote forcing. • Shallow eastern thermocline cools local SST. Red = westerly wind Red = deep thermocline Red = warm SST Signals spread efficiently across the Pacific equator AnomaliesAnomalies The response to episodic westerly wind anomalies in the western Pacific: • Winds blow into warm SST. • Westerly winds come in bursts lasting
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