Greenland-Norwegian Sea Deep er ° at 60 N W ep circulation e D ic ) nt W la D At A Med. Intermed. h (N Mediterranean Deep waters form in the rt Water (MIW) No Sea Gulf of high North Atlantic and Mexico 30 °N around Antarctica. North Atlantic Intermediate waters form Caribbean Sea in the Mediterranean Sea Density -Driven Downwelling (MIW) and the Southern Hemisphere polar front 0° (Equator) A nt ac rc region (AAIW). itc and Thermohaline Circulation In te r (A rm Deep water masses, like e t AI ed a W ia ) te W near -surface currents, are W a m t AAIW ) er o t influenced by the Coriolis W influenced by the Coriolis t o B South Atlantic B A effect The Great Ocean Conveyor c A effect The Great Ocean Conveyor 30 °S i ( t c r a – As deep waters flow t n A equatorward , they hug the lower continental slopes and Circumpolar Water (CPW) rises on the western sides of 60 °S the ocean basins. – They are also channeled by Weddell Sea topographic features on the (Antarctica) seafloor. 60 °N Net import of near-surface water rrent The Global Conveyor c Cu Surface vs. deep into the Atlantic tlanti rth A No t n Mediterranean e m r ea r Gulf of tr u Sea S North Atlantic circulation f C circulation Mexico ul y 30 °N G r a n Subtropical a C Greenland-Norwegian Sea Caribbean Gyre Sea er ° at N. Equatorial Current 60 N W t ep Eq. Countercurrent urren e ° (Equator) al C c D 0 atori ti ) t n W . Equ la D S n At A Med. Intermed. Mediterranean e (N r rth r o Water (MIW) Sea u N t Subtropical n C ° e 30 N r a r l Gyre e u North Atlantic C u l g i z South Atlantic n a e Caribbean r B 30 °S B Sea A (Equator) n 0° ta cr West Wind Drift ci tc ° r In 60 S e t t e East Wind Drift ( r a A m A e W I d W ia ) te m ) W o a Near -surface waters move AAIW t W t t er o B South Atlantic B A Net export of deep water 30 °S c A independently of deep and i independently of deep and ( t c r out of the Atlantic a intermediate waters because of the t intermediate waters because of the n A pycnocline that separates them. Circumpolar Water (CPW) The global conveyor describes the complete circuit of global ocean 60 °S The global conveyor describes the complete circuit of global ocean Downwelling & upwelling link the circulation involving the horizontal & vertical flow of near -surface surface & deep ocean. Weddell Sea (Antarctica) and deep waters. 1 deep waters lose O 2 over time due to ColdWhy water is there holds so much more animal respiration and the decomposition of organic matter; the older the water mass, variationdissolved in dissolved oxygen O 2 acrossthan the warm ocean water surface? the oldest deep waters the longer it has been away from the surface, are in the North Pacific the lower its O 2 content and the higher its CO 2 content deep & intermediate waters acquire their physical & chemical characteristics at the surface Better views of the conveyor NADW formation and clihmate Note the Gulf Stream in its glacial period (“ice age”) interglacial (warm) period proper position. Note upwelling of NADW in the Weddell Sea and subsequent formation of AABW. Note formation of bottom heat released to atmosphere water in the Ross Sea also. Can you translate the terms in this diagram (even without BODENWASSER ? knowing German)? Hint: think “deep” OBERFLACHENWASSER TIEFENWASSER A few decades at most – YIKES! 2 Oceanic gateways and climate Drake Passage opening prior to ~40 Ma … long -term changes (millions -of -years time scale) – isthmus of land existed which connected southern tip of South which re -organized global ocean circulation America to Antarctic Peninsula – Brazil Current acted like Gulf Stream today 3 oceanic gateways (remember Ma = “Mega -annums ” – Brazil Current acted like Gulf Stream today » swift, strong, warm, poleward -flowing = millions of years ago) » kept Antarctica warm 1) Drake Passage (opened ~34 Ma) after 34 Ma (Eocene -Oligocene boundary) … 2) Central American seaway (closed ~5 Ma) – Drake Passage opened, land bridge disappeared 3) Indonesian seaway (moved north ~3 -4 Ma) – Antarctic Circumpolar Current began to flow » isolated Antarctic continent » allowed glacial ice sheets to form on Antarctica 2 3 or was it something else? – maybe changes in atmospheric CO 2 did it 1 – DeConto and Pollard, 2003 modeling paper in Nature Central American Seaway closure Gulf Stream and Northern prior to ~5 Ma … Hemisphere Glacial Cycles – open passage existed between North and South America intensified Gulf Stream carried not only more heat, but also – allowed exchange of tropical water between ancient Pacific and A tlantic » salinity in both was similar more moisture to the Northern Hemisphere » no (or very weak) Gulf Stream existed rain, snow fell on Eurasia after ~2.7 Ma … great rivers flowed into – Isthmus of Panama fully formed, closed Central Am. Seaway Arctic Ocean, freshening – salinity increased in Atlantic, water and forming sea ice decreased in Pacific – fresh water short -circuited – Gulf Stream intensified – fresh water short -circuited ocean conveyor » warm, salt ier waters no rth » ocean conveyor started – ice reflected sunlight » onset of Northern Hem. – both cool the N. Hem., ice sheets & glacial cycles resulting in an ice age wait a second … – when conveyor on again, – Gulf Stream warms N. Hem. get a warm (and fuzzy) – so why ice sheets??? interglacial period See http://www.whoi.edu/oceanus/viewArticle.do?id=2508 3 Indonesian Seaway switch So the question is … prior to ~4 Ma … – New Guinea farther south What happens next ??? – allowed warm South Pacific water to flow through Indonesian Seaw ay – persistent El Ni ño-like state » moved warm water from tropics to high latitudes » kept N. Hemisphere ice -free after ~3 Ma … – northward displacement of New Guinea – switched the source of flow through Indonesia – relatively cold North Pacific today waters – persistent La Ni ña-like state – persistent La Ni ña-like state ~4 Ma OR… » reduced transfer of heat from tropics to high lat’s ~3 Ma » onset of Northern Hem. ice sheets & glacial cycles Cane and Molnar, 2001 paper in Nature 4.