““We walk away from our dreams,We walk away from our dreams,afraid that we might fail,afraid that we might fail,or worst yet,or worst yet,afraid that we might succeedafraid that we might succeed”” Anonym o u sAnonym o u s ““Whatever you can do, or dream you can,Whatever you can do, or dream you can,begin it.begin it.Boldness has genius, power and magic in it.Boldness has genius, power and magic in it.Begin it now!Begin it now!”” GoetheGoetheOCNG 251: OceanographyOCNG 251: OceanographyTuesdayTuesday, Nov., Nov. 4 4thth, , 20082008Ocean Circulation & Short-term Climatevariability• El Niño-Southern Oscillation• North Altlantic Oscillation Global Circulation Patterns:AtmosphereNSAtmospheric circulation results from1) Earth’s rotation and2) differential heating at surface and inatmosphere! this produces prevailing winds andlatitudinal belts of high/lowpressure: Hadley Cells (north-south)Equatorial Pacific Climate••Superimposed on the North-South Hadley circulation, there are three mainSuperimposed on the North-South Hadley circulation, there are three maincenters of convection in the inter-tropical zone.centers of convection in the inter-tropical zone.••These centers induce a significant East-West circulation in the troposphereThese centers induce a significant East-West circulation in the tropospherethat is most prevalent over the equatorial Pacific (Walker Circulation).that is most prevalent over the equatorial Pacific (Walker Circulation).••The centers of South America and Africa are stable to movement east andThe centers of South America and Africa are stable to movement east andwest.west.••Because it is over the ocean, the convection center over the oceanic regionBecause it is over the ocean, the convection center over the oceanic regionof Southeast Asian is not stable to movement in the east-west direction.of Southeast Asian is not stable to movement in the east-west direction.Equatorial Pacific Climate••In the tropical Pacific, the surfaceIn the tropical Pacific, the surfacewinds (trade winds) flow westward -winds (trade winds) flow westward -converging towards the equator.converging towards the equator.••The SST climatology displays aThe SST climatology displays arelatively cold eastern half and arelatively cold eastern half and awarm west - warm west - zonal asymmetry.zonal asymmetry.••The sea level (atmospheric) pressureThe sea level (atmospheric) pressureis lower over the warm water thanis lower over the warm water thanover the cold water, providingover the cold water, providingadditional forcing of the easterlyadditional forcing of the easterlywinds on the equator.winds on the equator.••Convergence in the west results inConvergence in the west results inintense convection over Southeastintense convection over SoutheastAsia and Australia, which results inAsia and Australia, which results inlow pressure and abundant rainfall.low pressure and abundant rainfall.Why is the eastern Pacific cold?••Constant easterlies cause the general westward movement of (warm)Constant easterlies cause the general westward movement of (warm)surface water towards the western Pacific.surface water towards the western Pacific.••The accumulation of water pushes down the thermocline in the west andThe accumulation of water pushes down the thermocline in the west andshallows it in the east.shallows it in the east.••Esterlies cause Eckman divergence in the west leading to upwelling alongEsterlies cause Eckman divergence in the west leading to upwelling alongthe Peruvian coast.the Peruvian coast.••Upwelling waters from intermediate depths decrease the SST of theUpwelling waters from intermediate depths decrease the SST of theeastern equatorial Pacificeastern equatorial PacificThe mean state••The contrast in SST leads to aThe contrast in SST leads to acontrast in sea level (atmosphericcontrast in sea level (atmosphericpressure) pressure) !! higher pressure in the higher pressure in theeast and low pressure in the westeast and low pressure in the west••The pressure gradient maintains theThe pressure gradient maintains thesurface easterlies.surface easterlies.••The convergence of moist air leadsThe convergence of moist air leadsto massive convection and upwardto massive convection and upwardmotion (sinking air in the east -motion (sinking air in the east -Walker Circulation)Walker Circulation)••This situation is unstable...This situation is unstable...El Niño••When a perturbation occurs andWhen a perturbation occurs andthe trade winds weaken, thethe trade winds weaken, thepiled waters in the westpiled waters in the west““collapsecollapse”” and the thermocline and the thermoclineequilibrates (becomes level).equilibrates (becomes level).••Waters from the west moveWaters from the west movetoward the east and the toward the east and the ““coldcoldtonguetongue”” is weakened. The is weakened. Thehighest temperature are near thehighest temperature are near thedateline (central Pacific).dateline (central Pacific).••The convection migrates with theThe convection migrates with thewarm waterswarm waters••The low pressure is in the centerThe low pressure is in the centerof the basin, further weakeningof the basin, further weakeningthe trade winds.the trade winds.La Niña••When the trade winds are stronger than usual,When the trade winds are stronger than usual,••The water in the eastern half of the Pacific is colder than normal and theThe water in the eastern half of the Pacific is colder than normal and thewarmest waters are pushed further westward (Australia and Southeastwarmest waters are pushed further westward (Australia and SoutheastAsia).Asia).••The pressure increases further in the east.The pressure increases further in the east.••Convection is pushed westward..Convection is pushed westward..Southern Oscillation••A sea level pressure A sea level pressure ““seesawseesaw””,,between Darwin and Tahiti,between Darwin and Tahiti,accompanies the El Niño SSTaccompanies the El Niño SSTvariation.variation.••The variation in pressure in these twoThe variation in pressure in these twoplaces is almost perfectly correlatedplaces is almost perfectly correlated(back to 1880).(back to 1880).Time series of anomalies in seaTime series of anomalies in sealevel pressure at Darwin (dashed)level pressure at Darwin (dashed)and Tahiti (solid) from 1970 toand Tahiti (solid)