-LuAnne Thompson, Associate Professor, School of Oceanography and Interim Director, UW Program on Climate Change
Two weeks ago, 4000 oceanographers met in Orlando Florida for the American Geophysical Union Ocean Sciences meeting. The halls of the School of Oceanography at UW were very quiet that week as many of us attended and presented at the meeting. There were many concurrent sessions that made it difficult to determine where to go at any given time, so I will focus on the exciting research that was presented at the meeting related to the Atlantic Meridional Overturning Circulation (AMOC) in several different sessions, highlighting the work of UW Researchers. The bottom line is that transport associated with the Atlantic Meridional Overturning Circulation is more complex the closer we look, and the cartoon presented of the thermohaline circulation with the warm water returned in an effective pipe by the Gulf Stream to the subpolar North Atlantic and the Nordic Seas must be revised. In addition, the pathways of the deep water created in the Nordic Seas is only now being mapped out completely with new technologies, some of which are being developed at UW.
The focus on the Gulf Stream as a conduit for warm water transports was examined in detail using satellite observations and high resolution ocean models in two talks, one by Kathryn Kelly of the UW Applied Physics Laboratory, and the second by LuAnne Thompson of UW Oceanography. In both talks, we describe the Gulf Stream as less of a pipe for warm water, and more of a switch whereby water either returns to the south at the end of the Gulf Stream, or travels further North, in an unpredictable way. The challenges of modeling the Gulf Stream were also highlighted, with even high resolution models not able to reproduce the heat transport anomalies. Whether this is owing to transport errors, or because there is intrinsic unpredictable variability in the ocean remains to be seen.
While much recent work in monitoring the AMOC has focused on mid-latitudes, it is becoming clearer that the transports at 20-30N are not directly related to sinking and deep water formation in the Nordic or Labrador Seas. James Girton of the UW Applied Physics Laboratory, presented work suggesting that a better proxy for the AMOC is to monitor the amount of eddies in the Denmark Strait through satellite observations of sea surface height and sea surface temperature.
Charlie Eriksen and Peter Rhines, using new glider technologies developed at UW, have given us a new three-dimensional view of the overflows from the Norweigian Sea to the subpolar North Atlantic.
Rhines, and his colleague from NASA, Sirpe Hakkinen, have shown by examining surface drifters that the Gulf Stream can have a direct pathway to the Nordic seas, but that this pathway is extremely intermittent, and more to the east than previously thought. The increase of floats making their way North also coincides with a reduction in the strength of the subpolar gyre circulation.
Susan Bates along with Cecilia Bitz and David Battisti in Atmospheric Sciences explored the link between the AMOC and sea-ice cover in the modern climate and during the last glacial maximum. The AMOC is much more sensitive to changes in sea-ice cover during glacial times, and the modes of variability of the AMOC are distinctly different during the two epochs.
Unlike the scenario suggested in the movie “The Day After Tomorrow” the picture emerging from this highlighted research as well as the work of many others has shown that the AMOC is extremely variable, that ocean currents on relatively small scales play an important role in controlling the pathways of both fresh water and heat, and that the simple cartoons that served us in the past to understand the response of the ocean circulation to climate change will no longer work. In addition, the lessons that we have learned about the system from data describing the climate system during glacial times may not be a good proxy for what may happen in the future. We are just now, through new technologies and satellite data, beginning to be able to accurately measure and model this important aspect of the climate system. Full Story