We finished off the work at the shelf edge station with a set of samples collected using the Marine Snowcatcher. Quite a long process, with a good few misfires of the sampler, but all of the samples needed were eventually collected. At 0200 this morning we did another of the “pre-dawn” samples, collecting water from different depths to measure plankton growth rates and nutrient requirements, and the nutrients dissolved in the seawater. This was earlier than we would normally carry out pre-dawn work, but we need to get back up to the moorings further on the continental shelf with sufficient daylight to recover them all. We are due at the mooring site just after 0900. We’ll first collect a CTD profile of data adjacent to the moorings, which can later be used to help calibrate the mooring data, and then we’ll begin what will likely be a full day of manoeuvring and collecting the 3 mooring components.
We have a couple of hitchhikers aboard. Two storm petrels were found resting in the hangar by the CTDs last night. Both are currently having a sleep in a cardboard box, and Clare Davis is hopeful one of them will be OK to fly off later today. A few days ago we had an owl flying round the ship. Very exotic – cruises out here usually only attract tired homing pigeons.
Here’s a question for the year 3 ocean dynamic students back in Liverpool University. The water out here by the moorings will soon be completely vertically mixed, and I want to estimate the date when that will happen. The water is 150 metres deep, with a surface layer 60 metres thick and density 1025.6 kg m-3, and a bottom layer 90 metres thick and density 1026.0 kg m-3. The average tidal current amplitude is 0.45 m s-1, average wind speed is 12 m s-1, and the heat flux across the sea surface is 100 W m-2 (a heat loss to the atmosphere). That’s all the information you need, along with a handful of constants that are in your notes!