Class 27 DEEP CIRCULATION AND WATER MASSES Origin of Atlantic water masses/currentsOrigin of Atlantic water masses/currents –Bottom water –Deep water –Intermediate water Water masses and circulation in Pacific and Indian OceansWater masses and circulation in Pacific and Indian Oceans THE GLOBAL “CONVEYOR BELT” Coupling of deep and surface circulationCoupling of deep and surface circulation

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

General vertical structure Mixed layer = Surface zone (top 100 m) Thermocline ( ,000 m) Deep Ocean (> 1,000 m) Cold, high S water

Intermediate, deep, and bottom water masses All waters in today’s oceans are always movingAll waters in today’s oceans are always moving Bottom Water: The very densest waters at bottomBottom Water: The very densest waters at bottom Deep Water: Very dense water sinks and moves along the ocean floor or not far above it.Deep Water: Very dense water sinks and moves along the ocean floor or not far above it. Intermediate Water: Moderately Dense- sinks part way downIntermediate Water: Moderately Dense- sinks part way down Each water mass sinks until it reaches a depth where it has less dense water on top and denser water belowEach 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 latitudesExtends to highest latitudes North Atlantic is relatively high salinityNorth Atlantic is relatively high salinity The most important deep water masses: North Atlantic Deep Water (NADW)North Atlantic Deep Water (NADW) Antarctic Bottom Water (AABW)Antarctic Bottom Water (AABW) The Most Important Deep Water Masses Form In The Atlantic

Sinks near AntarcticaSinks near Antarctica –Sea ice forming there –Winter cooling Flows north on west side of Atl. (channeled by mid Atlantic ridge and the coriolis effect)Flows north on west side of Atl. (channeled by mid Atlantic ridge and the coriolis effect) Antarctic Bottom Water (AABW)

AABW

Formed near GreenlandFormed near Greenland –N. Atl. surface water has high salinity – + Winter cooling, sea-ice formation Sinks and flows southwardSinks and flows southward Eventually joins the denser AABWEventually joins the denser AABW North Atlantic Deep Water (NADW)

NADW

AABW ACW

Upwelling NADW + Antarctic watersUpwelling NADW + Antarctic waters Flows eastward around Antarctica in a complete circleFlows eastward around Antarctica in a complete circle Some of it spills north to become the deep water of the Indian and Pacific OceansSome of it spills north to become the deep water of the Indian and Pacific Oceans Antarctic Circumpolar Water (ACW)

ACW

NADW AABW ACW

1) Mediterranean Intermediate Water (MIW) Saline (35.5 g/kg), warm (10˚C) outflow from Mediterranean SeaSaline (35.5 g/kg), warm (10˚C) outflow from Mediterranean Sea Sinks to ~1 km in the North AtlanticSinks to ~1 km in the North Atlantic INTERMEDIATE 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 waters Typically stop sinking and spread out at ~1 km depth INTERMEDIATE 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

ACW

Combination of deep currents and surface currents forms a huge loopCombination of deep currents and surface currents forms a huge loop Water takes a few thousand years to complete the circuit- very fast compared to geologic processesWater takes a few thousand years to complete the circuit- very fast compared to geologic processes Moves Heat- Important Climate ImplicationsMoves Heat- Important Climate Implications Also moves salt- keeps the oceans well mixed; no pockets of stagnant water in the modern oceansAlso moves salt- keeps the oceans well mixed; no pockets of stagnant water in the modern oceans THE GLOBAL CONVEYOR BELT

1. NADW sinks 2. Southward flow; mixing w/ AABW 3. Upward flow to join ACW – circles around Antarctica 4. Northward into Indian & Pacific Oceans 5. 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 Africa Atlantic 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 northwardNADW sinking draws warm water farther northward Northern Europe receives warm air from this- westerliesNorthern Europe receives warm air from this- westerlies If NADW flow stopped, warm water would not go as far northIf NADW flow stopped, warm water would not go as far north Northern Europe would become coolerNorthern Europe would become cooler

Past 10,000 years -- yes.Past 10,000 years -- yes. –Brings warm water north and warms northern Europe ,000 years ago -- it shut down!10-15,000 years ago -- it shut down! –Dilute glacial meltwaters- low density prevented sinking of NADW Northern Europe was cold, glaciers advancedNorthern Europe was cold, glaciers advanced Many Millions of years ago - Continents in different positions- lack of deep flow and “stagnant” oceans (?)Many 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?