Lecture 18 - oceans• Ocean gyres. Boundary currents (e.g. Gulf Stream)• The thermohaline circulationTextbook error : Fig.5-4(a) should be labeled “pressuregradient” where it incorrectly says “geostrophic current”Coriolis force, P gradient balanceRESULT in gyre:CONCEPTFlow at depth: Ekman spiralLike a deck of cards: Touch the top and you can moveunderlying cards by frictional coupling.Water movementaveraged over all layersis 90° to wind direction(this is NOT the direction ofcurrents seen at the surface)Energy is used up bythe water motion untilnone is left, so water speedgets smaller down the spiral~100 mWhy water piles up at the center of asubtropical gyre1) The net movement of water BENEATH the surfaceis at right angles to the wind.2) In the N.H. tropics, subsurface horizontal flow of watergoes northwards in the tropics (90° to the trade winds) andsouthwards from the midlatitude westerlies. Causes pile up.trade windswesterlies30 Nequatorsubsurface watergyre (surface oceancurrents in red)Convergence in N.H. subtropical gyreSubsurface,Ekman currents cause convergencein the gyre’s center.This createsa pressure gradient.This is balancedby the Coriolis force.Just like flowaround a “high”pressure in the atmos.Flow around a gyre in N.H.gf = pressure gradient forcec = Coriolis forceF = resultant flow direction of current“geostrophic flow” = when pressure gradient force and Coriolisforce balance, and flow is perpendicular to eachCenter of thegyre is only about50 cm higherthan at edgesFig. 5-3BBoundary currentsGyres are actually asymmetric. East side is more spread out.West side is narrower, faster. Apex of “hill” is displaced to west.Coriolis force increases with latitude. (It’s zero at the equator).So when water turns at high lat. eastern boundary, you get a strongerturning force equatorward, causing flow over a broader area.East side of gyre: broad eastern boundary current (EBC)West side of gyre: narrow western boudary current (WBC)WBCs are faster currents than EBCs (about 10 ¥ faster)Best known WBC is the Gulf Stream.Gulf Stream: The fastest large-scale ocean currentWarm (20-22°C) current, originating in the Gulf of Mexico.Up to 9 km/h (5.6 mi/h) speed. Beyond Newfoundland, it is broader and calledthe N. Atlantic Drift.High covergence point inside the gyre is the “Sargasso Sea”. Here, downwelling(lack of nutrients) makes it devoid of plankton, food for fish. But lots of floatingSargassum seaweed gets trapped in this convergence zone.Climatic effect: Norwegianbranch of the Gulf Stream warmswestern Europe 5-15ºC.Thermo-haline circulation (THC)Ocean water density - 2 controlling factors: 1) temperature (cold seawater is more dense)2) salinity (more saline water is more dense)• warm, "fresh" water stays on the surface• cold, saline water sinksDeep-ocean circulation is driven by differences inwater density, NOT by windsVariations in salinity:- river input- formation of sea-ice-differences in evaporation/rainfall- depth (more saline deeper down)thermos = Gk. for heathals = Gk. for salt“thermohaline”Thermo-haline circulation (THC): Fig 5-12• causes Deep Ocean mixing • very long timescale: ~500-1000 years• driven by density variationsThermo-haline circulation (THC): Fig 5-12• Northern Atlantic Ocean is a key region where cold, saline water develops• this may be what initiates the THC• possible "trigger" point for global climate (i.e. suppose it were hotter)Thermo-haline circulation (THC):50% deep water from N. Atlantic, 50% from Weddell Sea• North Atlantic sea-ice forms and excludes salt from the ice crystalstructure• Cold water becomes salty + dense, and sinks• This deep water is North Atlantic Deep Water (NADW)• A similar process forms cold water that sinks in the Weddell Sea,Antarctica, called Antarctic Bottom Water (AABW)Worlddrainedto 4000 mdepth.Fig 5-10THC: Upwelling and DownwellingIf water sinks, water must rise elsewhere. Upwelling,much along western continental margins, returns coldbottom water to the surface.This sequence (downwelling, circulation and upwelling)is called the “thermohaline conveyor belt”Connections to the BiosphereNear-surface organisms sink into the ocean when they die, andrelease nutrients upon decomposition. Upwelling along the coastsreturns nutrients back to the surface for marine life.If upwelling ceased/slowed, coastal fish stocks would plummet,because plankton (at the base of the food chain) would diminish.THC also transports O2 to the deep ocean, where it supports lifeand helps decompose the “rain” of organic material from above.Phytoplankton = free-floating minute organisms (algae,photosynthetic bacteria, etc.). Responsible for 1/2 of theplanet’s photosynthesis.light color = high chlorophyll = high concentration of phytoplanktonSummarySURFACE / SURFACE-LAYER CURRENTSWinds drive surface currents. These deviate from wind direction due to theCoriolis effect by about 20-25°.In the 100 m deep surface layer as a whole, the deviation compared towind direction is 90°. Gyres are set up, with high water at the gyre center.Geostrophic balance applies to surface currents around the gyre.Boundary currents flow along coastlines of the continents, withfast western boundary currents and sluggish eastern boundary currentsDEEP WATER CURRENTS: THERMOHALINE CIRCULATIONDensity variations (due to temperature + salinity) drive deep circulation.Dense water in the N. Atlantic and Antarctic Weddell Seasinks and forms “bottom water”, which slowly flows along the ocean floorat great depth. This is balanced by upwelling elsewhere.Ocean currents redistribute Earth’s heat: Warm currents flow from tropics to poleand cold currents flow from pole to tropics.Downwelling oxygenates the deep ocean and upwelling replenishes