In present-day climate, high-latitude oceans produce cold and heavy water that sinks and circulates equatorward, eventually flowing over topographic constrictions and ridges. Such dense overflow plumes maintain the deep part of the ocean circulation, which in return carries heat toward the Arctic. Due to their small-scale and the shortcomings in understanding the mechanisms by which they mix, overflows are very poorly represented in climate models.
During the IPY-project ”Bipolar Atlantic Thermohaline Circulation” (BIAC) a group of scientists, lead by Ilker Fer at the Geophysical Institute and the Bjerknes Centre, have surveyed the dense overflow plume from the Faroe Bank Channel in detail. Cold water overflow from the Faroe Bank Channel alone is significant for the circulation of the abyssal North Atlantic Ocean. The observations, reported in the renowned scientific journal Geophysical Research Letters, are the first detailed measurements of the turbulence maintaining this part of ocean circulation and climate.
The authors collected full depth vertical profiles of temperature, salinity, currents and turbulence levels during a cruise of RV Håkon Mosby in June 2008. The data reveal a swift, bottom-attached plume of cold water of about 200 m thick in water depths of 800 – 1200 m. The internal structure of the plume consists of about 100 m thick well-mixed bottom layer underlying a comparably thick layered interface. The flow is found to be exceptionally turbulent, comparable to tidally active fjords, close to the bottom layer and in the interface, but it is very quiet in the interior core of the plume. A transverse circulation expected from the structure of the plume influenced by the rotation of Earth actively dilutes the bottom layer of the plume. The thick interfacial layer supports waves which can cause further mixing, a phenomenon which presently lacks in parameterizations of overflows.
Turbulent deep waters
New observations reveal spectacular mixing rates and turbulence, and new insights into circulation structure and dynamics in deep waters spilling down to the North Atlantic.
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