Retooling the Ocean Conveyor Belt

June 16, 2010

Tim Lucas, 919-613-8084,

DURHAM, N.C. – For decades, oceanographers have embraced the idea that Earth’s ocean overturning currents operate like a giant conveyor belt, continuously transporting deep, cold polar waters toward the equator and warm equatorial surface waters back toward the poles along narrow boundary currents.

This conveyor belt has assumed to be driven by changes in the temperature and salinity of the surface waters at high latitudes.

In a paper in the June 18 issue of Science, a Duke University oceanographer reviews the growing body of evidence that suggests it’s time to rethink the conveyor belt model.

“The old model is no longer valid for the ocean’s overturning, not because it’s a gross simplification but because it ignores crucial elements such as eddies and the wind field. The concept of a conveyor belt for the overturning was developed decades ago, before oceanographers had measured the eddy field of the ocean and before they understood how energy from the wind impacts the overturning,” explains Susan Lozier, professor of physical oceanography and chair of the Division of Earth and Ocean Sciences at Duke University’s Nicholas School of the Environment.

“It is important to understand that there is clear and convincing evidence that the ocean waters overturn and that this overturning impact’s the Earth’s climate.  Recent studies, however, have cast doubt on our ability to describe this overturning as a conveyor belt.  From these studies we now understand that the overturning waters are not restricted to narrow boundary currents, that the overturning may vary from one ocean basin to the next and that the winds may create variability in the amount of water that overturns and in the pathways for the upper and lower limbs of the overturning”.

Despite these recent advances in our understanding of how the ocean’s overturning operates, the Science article also reviews what remains unknown about the ocean’s overturning. A major unknown question is: How do changes in the production of deep water masses at high latitudes affect overturning changes?  As the surface waters warm and/or freshen due to climate variability and climate change, how might the overturning change?  Though modeling studies have addressed this question, there has been no observational study to date that has addressed this crucial question.

A new international research program in the planning stages, led by Lozier, aims to address this question. The initiative will bring together researchers from the United States, Germany, Canada, France and the United Kingdom to study overturning in the northern North Atlantic over a 5-to-10-year period.

In her Science article, Lozier reviews the emerging view of the overturning circulation within a historical framework that chronicles significant scientific developments in the field, from the first reported measurement of ocean overturning in 1751 through the present.

“Basically, our ability to refine our understanding of the ocean’s overturning stems in large part from our ever increasing ability to measure the ocean at finer and finer scales and at depths previously unmeasured.  Because the ocean waters are corrosive, at high pressure and generally inaccessible, the ocean has historically been a sparsely observed system.  Recent technological advances are rapidly expanding the ocean’s observational database and with it, our understanding of ocean circulation.”