Cold water on the Great Ocean Conveyor

by John D. Cox | May 22, 2009

If you want to see the blood drain from the face of a serious oceanographer, just start waxing disputatiously about the Great Ocean Conveyor, the pattern of discrete flows that transports warm and cold seawater around the world. Researchers in the field have always known -- and have doggedly pointed out -- that the real pattern of global ocean circulation is vastly more complicated than Wally Broecker's 1987 magazine article cartoon that has become the iconic depiction for people who are not oceanographers.

A key component of this conception is the sinking of surface water to the depths of the far North Atlantic as it becomes colder and saltier and more dense -- a vertical overturning that is a driving force. Monitoring these sluggish but vital flows is an important goal in climate science, because changes imply far-reaching climatic effects. For starters, the northbound surface loop of the conveyor is the warm Gulf Stream, so a weakening of this overturning, a slowing of the conveyor, would send an unwelcome chill over Europe.    

Now along come researchers at Duke University and Woods Hole Oceanographic Institution to confirm studies in the 1990s that found a major loop in the great conveyor concept -- the so-called Deep Western Boundary Current that hugs the eastern North American continental shelf -- is not really transporting cold water south from the Labrador Sea, one of the sources of overturning cold water.

(The top panel is the conventional depiction of the North Atlantic loops of northbound warm surface water, the Gulf Stream (orange), and southbound cold deep water, the Western Boundary Current (blue).  The bottom panel is a map of the movement of floats embedded in southbound cold water flows that the oceanographers deployed and tracked in their study.)

As Duke oceanographer Susan Lozier put it, "what we are saying is that the concept doesn't hold anymore. So it's going to be more difficult to measure these climate change signals in the deep ocean."  Instead of flowing south in the deep ocean along the Western Boundary Current, most of the cold sea water from between Greenland and Canada apparently finds more diffuse pathways south in the open ocean.

Writing in the journal Nature, Lozier and WHOI scientist Amy Bower reported the results from 76 ocean sounding floats that were launched into the North Atlantic between 2003 and 2006.  The floats were configured to submerge to depths of 700 or 1,500 meters, recording their location and other measurements once a day.  After drifting two years, they resurfaced and transmitted their data to scientists by satellite.  Only 8 percent followed the pathway defined by the conventional conveyor model.

- John D. Cox
IMAGES:  Archana Gowda, Duke; Nature