The meridional overturning circulation is split into two connected cells. The upper cell is wind-diven and adiabatic and consists of density surfaces that outcrop in the northern and southern hemispheres of the Atlantic. The abyssal branch requires turbulent mixing to return dense water formed around Antarctica back up towards the surface. Observations of turbulent microstructure show that mixing is enhanced around topographic features. However, this leads to downwelling in the ocean interior rather than the upwelling required to close the overturning circulation. This downwelling was directly observed during the Brazil Basin Tracer Release experiment. Microstructure observations from this same study suggested that water upwelled close to the rough topography of the mid-Atlantic ridge. Theory predicts that upwelling must occur in the near-boundary region.
The BLT experiment aimed to test this theory and directly measure upwelling close to topography for the first time. The project was a large collaboration funded by NSF and NERC, including research groups from Scripps Institution of Oceanography, Woods Hole Oceanographic Institution, Massachusetts Institute of Technology, the University of Southampton / National Oceanographic Centre, the University of Exeter, IFREMER and the Royal Netherlands Institute for Sea Research. 

The field work for BLT focused on a canyon in the Rockall Trough region of the North Atlantic. Most of the measurements were done within a canyon on the east side of the trough. The experiment included a dye release, a chemical tracer release, moorings measuring temperature, velocity and buoyancy flu and microstructure surveys.