Almost within earshot of the bagpipes of Scotland, we deployed the last of the 20 moorings slated for this leg of R/V Knorr Voyage 221. It was an exciting finish to this phase of the cruise. Instead of the “normal” mooring deployment strategy where we pay out the top of the mooring first and finish with the drop of the anchor, we lowered a single instrument to within one meter of the sea floor on a heavy wire, and then sent an acoustic signal to release the instrument and let it drop to the bottom. It is an upward-facing Acoustic Doppler Current Profiler (ADCP), which will constantly emit sound signals up to the sea surface and use the Doppler shift of the return signals to measure the currents from top to bottom. As described in a previous post by technician Karen Wilson, the ADCP is encased in a trawl-resistant frame (which our Scottish colleagues call “the Spaceship” for obvious reasons) so that fishing gear pulled along the sea floor will roll over the cage and not snag it.
The exciting part of the deployment was the final approach of the Spaceship to the sea floor. No one can see with their eyes what is happening 400 meters down—so we “see” with our ears, relying on sound signals from the Knorr and from the Spaceship to tell us how close to the bottom it is. The goal was to not release the frame until it was within a few meters of the seafloor. If we release it too early, it could flip upside down before it hits the bottom. If we hold on too long, the package might slam into the bottom and be damaged. And remember that the ship is rolling a bit, which lifts the frame up and down slightly with every swell. After much suspense, Principal Investigator Stuart Cunningham “pulled the trigger” and the frame dropped to the sea floor. Next year we will return to this site to recover the instrument and its precious cargo—the first long-term continuous measurements of the shelf edge current off Scotland.
Now the science crew is changing gears from mooring to CTD operations. For the rest of the cruise, we will be slowly making our way back along the same cruise track, stopping every so often to lower the CTD package to the bottom in order to measure the temperature, salinity, dissolved oxygen content and other seawater properties. A pair of ADCP’s mounted on the CTD package will also record the ocean current profile at each station. These stations will be spaced much more closely together than the moorings—this will give us the opportunity to get an initial high-resolution snap-shot of all the currents and water properties along the OSNAP line to compare with the measurements from the moored instruments.