Ocean Circulation II

I. Thermohaline Circulation


 
 


 
 
  • occurs in the Weddell Sea (bay on Antarctica opposite south end of the Atlantic Ocean); forms the cold, high salinity water mass (-0.5°C; 34.65) called Antarctic Bottom Water which sinks to the deep ocean floor and is transported eastward around Antarctica; as it sinks it moves northward into each of the three major ocean basins up to 45°N in N. Atlantic and 50°N in Pacific
  • also occurs in the Arctic Ocean, but topography of that basin prevents most of it from escaping -- can't flow into Pacific because of a shallow sill between the Aleutian Islands and the shallow, narrow Bering Strait (which connects the Bering Sea and the Arctic Ocean) -- can't flow into Atlantic Ocean either because of shallow ridges between Scotland and Greenland
  • however, get cold water masses in the Norwegian Sea and Greenland Sea which forms the North Atlantic Deep Water (NADW) that sinks and flows south in vast quantities --is one of the most voluminous masses
    • NADW has water of higher than average salinity because large amounts of high salinity water are introduced from the Mediterranean Sea (where more water is evaporated and less is added via precipitation) and are carried into high latitudes by the Gulf Stream; this makes NADW distinguishable from Antarctic Bottom Water, and, because it is less dense, it flows over Antarctic Bottom Water


 
 



  • ocean sediment records show that the NADW conveyor belt has been turned off and on during the past tens of thousands of years and that these switches have coincided with abrupt changes in climate and extinction of marine forms
    • the hypothesis for this is related to postglacial periods where glaciers melted and their freshwater runoff emptied into the ocean -- the cold melted "fresh" water formed a stable, low density surface layer over a large area of the North Atlantic Ocean; this formed a cap, severely restricting formation of the NADW current and slowing the conveyor belt; last ice age ended 13,500 years ago, but 11,000 years ago there was a short cold period where the northwestern part of Europe's climate became colder within a matter of decades and then rewarmed to its former condition
      • once the glaciers finished melting, runoff was reduced, low density surface water eventually became mixed, and the NADW conveyor was able to start up again
      • the turn off of the conveyor belt causes a new ice age as northern climates cool without the input of warmer Indian and Pacific Ocean water
      • it is unknown if changes in the conveyor belt cause climate changes or if climate changes cause conveyor belt changes; BUT in 1980's, the annual rate of formation of the Greenland Sea Deep Water (one source of NADW) decreased by about 90 percent (which suggests that climate change may drive conveyor belt changes since greenhouse gases increased dramatically during the same time)

II. Ancient Current Patterns


 
 
  • as the basin widened, winds blowing along the north-south axis would weaken to form gyres via the Ekman spiral



  • after the Americas separated from Eurasia and Africa, the connection between N. and S. America was still submerged -- once the Tethys Sea opened between Africa and Eurasia (~100 mya), there was a globe-circulating equatorial current system
    • this equatorial current system continued until the ends of the Mediterranean Sea closed when Africa and Eurasia came together about 30 mya
    • then the land connecting N. and S. America emerged and further disrupted the equatorial current system
  • 40 mya Antarctica separated from Australia and Australia moved northward and helped to demark the Indian Ocean
    • 30 mya the Drake Passage between S. America and Antarctica deepened permitting the Circum-Antarctic Current to flow around Antarctica, which isolated Antarctica initiating the formation of present ice cap and causing the present glacial climate


 
 

III. Ancient Subsurface Currents