Movie S1.

A virtual fly-through of the second of ALR’s two missions on the slope of Orkney Passage main sill (mission 44, 18 - 22 April 2017). Bathymetry is shown by blue shading. The red line indicates the track followed by ALR. Black vectors show horizontal velocity averaged in the range 125-150 m above the ocean floor. Purple circles denote the neutral density measured by the ALR at ∼90 m above the ocean floor.Each velocity and density value represents an average of measurements over 0.5 km of ALR track.