class 27 deep circulation and water masses origin of atlantic water masses/currentsorigin of...

Download Class 27 DEEP CIRCULATION AND WATER MASSES Origin of Atlantic water masses/currentsOrigin of Atlantic water masses/currents –Bottom water –Deep water –Intermediate

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  • Class 27DEEP CIRCULATION AND WATER MASSESOrigin of Atlantic water masses/currentsBottom waterDeep waterIntermediate waterWater masses and circulation in Pacific and Indian Oceans

    THE GLOBAL CONVEYOR BELTCoupling of deep and surface circulation

  • Driving force: creation of dense water masses (cold, saline) at the surface of high-latitude oceans (particularly the Atlantic)


  • General vertical structure

    Mixed layer = Surface zone (top 100 m)

    Thermocline (100 - 1,000 m)

    Deep Ocean (> 1,000 m)Cold, high S water

  • Intermediate, deep, and bottom water massesAll waters in todays oceans are always movingBottom Water: The very densest waters at bottomDeep Water: Very dense water sinks and moves along the ocean floor or not far above it.Intermediate Water: Moderately Dense- sinks part way downEach water mass sinks until it reaches a depth where it has less dense water on top and denser water below

  • Why the Atlantic?Extends to highest latitudesNorth Atlantic is relatively high salinity

    The most important deep water masses:North Atlantic Deep Water (NADW)Antarctic Bottom Water (AABW)

    The Most Important Deep Water Masses Form In The Atlantic

  • Sinks near Antarctica Sea ice forming thereWinter cooling Flows north on west side of Atl. (channeled by mid Atlantic ridge and the coriolis effect)

    Antarctic Bottom Water (AABW)

  • Formed near GreenlandN. Atl. surface water has high salinity + Winter cooling, sea-ice formationSinks and flows southwardEventually joins the denser AABW North Atlantic Deep Water (NADW)

  • Upwelling NADW + Antarctic watersFlows eastward around Antarctica in a complete circleSome of it spills north to become the deep water of the Indian and Pacific OceansAntarctic Circumpolar Water (ACW)

  • 1) Mediterranean Intermediate Water (MIW)Saline (35.5 g/kg), warm (10C) outflow from Mediterranean SeaSinks to ~1 km in the North AtlanticINTERMEDIATE WATER MASSES

  • 2) Sub-polar water masses: Antarctic Intermediate Water (AAIW) [in all oceans]Arctic Intermediate Water (AIW) [in the Pacific]

    Sinking of cold, low-salinity surface watersTypically stop sinking and spread out at ~1 km depthINTERMEDIATE WATER MASSES

  • Deep Waters:ACW becomes deep water for these!

    Intermediate Waters:Pacific Ocean: sub-polar intermediate waters (N + S)Indian Ocean: sub-polar intermediate waters, south;warm, highly saline outflow from the Red Sea in north.PACIFIC AND INDIAN OCEANS

  • Combination of deep currents and surface currents forms a huge loopWater takes a few thousand years to complete the circuit- very fast compared to geologic processesMoves Heat- Important Climate ImplicationsAlso moves salt- keeps the oceans well mixed; no pockets of stagnant water in the modern oceans


  • 1. NADW sinks2. Southward flow; mixing w/ AABW3. Upward flow to join ACW circles around Antarctica4. Northward into Indian & Pacific Oceans5. Eventual Upwelling to join surface currents 7. Gyres eventually move water to where NADW and AABW sink.From Pacific: Through Indonesia, around Indian Ocean and tip of AfricaAtlantic surface gyres move water around, some goes to NADW/AABW formation zones

    Currents that make up the global conveyor belt

  • Northern European Climate: NADW sinking draws warm water farther northwardNorthern Europe receives warm air from this- westerliesIf NADW flow stopped, warm water would not go as far northNorthern Europe would become cooler

  • Past 10,000 years -- yes. Brings warm water north and warms northern Europe.10-15,000 years ago -- it shut down!Dilute glacial meltwaters- low density prevented sinking of NADWNorthern Europe was cold, glaciers advancedMany Millions of years ago - Continents in different positions- lack of deep flow and stagnant oceans (?)Has the global conveyor belt always worked like it does now?


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