by Penny Holliday
We are here to measure the ocean currents in the Labrador Sea. The temperature and salinity data that we collect with the CTD will tell us about the strength of the currents through an indirect route; we calculate the gradient of density between stations, and high gradients mean strong currents. But we also have instruments that directly measure the currents below us. They make use of the presence of tiny biological particles being swept along in the water. The instruments (ADCPs) send a series of short bursts of sound, called “pings”, and wait for the sound reflected by the particles to return. The reflections have a different frequency to the pings, and that change tells us how fast the particles are moving. You might be familiar with this effect if you have heard a train whistle as it passes you; the change in the note of the whistle as it passes by is caused by the same phenomenon (called the Doppler effect).
The currents measured in this way are shown in the graphic below (made by Felicity Williams). The big arrows show the general location of the two main boundary currents of the Labrador Sea; the Labrador Current in the south west and the West Greenland Current in the north east. The latter is very fast, reaching a speed of about 1 m/s (or 2 knots); this is where we saw ice racing past us while we did a CTD station, and the ship had to steam gently forwards to stay in position. Between the two main currents there is a perfect eddy; a circulating ring of slightly warmer water. You might be able to see that there is quite a lot of to-ing and fro-ing in the currents, with arrows pointing in all directions, not just the main direction of the current. This is because water never flows in a straight line, but is always spinning and twirling, having been given energy by things like tides and winds.
Nice plots like this one make us scientists feel pleased with the way things are going, because they are simple illustrations of the progress we are making. But it’s not just data that make the ship’s company feel good. I dont think I should try to speak for everyone on board, but I know that another day in the presence of the dramatic Greenland coast certainly made me feel pretty good. It was less sunny than Thursday, but we were able to creep closer to the mountains and it was beautifully calm again. There was less ice, but the jagged triangular peaks were fantastic, and I could even see a glacier in the distance. Having taken another few hundred photos, it can be hard to drag yourself away from the views and back to the computer again. Right now however, we are heading back out into deeper water again. Our plans are to work back to the Greenland coast twice more before heading east across the open North Atlantic – I cant wait to see it again!
Images: surface currents of the Labrador Sea, and more views of Greenland and ice