MSM 83 – Weekly Report 3

In the third week of the “Maria S. Merian” cruise MSM83 we crossed the deep Western European Basin to the west and continued our course along 47°/48°N. Since 2016, there are three so-called PIES at the bottom of this basin, inverted bottom echo-sounders, which are equipped with an additional pressure sensor. Classic echo-sounders are typically installed in the bottom of a ship and serve to determine the respective water depth with the aid of an acoustic signal, which is sent by the ship’s echo-sounder to the seabed and whose echo is received again. Inverted bottommounted echo-sounders are similar in principle but serve a different purpose. They are installed at the bottom of the sea, sending an acoustic signal from the seabed to the sea surface at regular intervals and measuring the time it takes to send the signal and receive the echo. This signal transit time is dependent on the speed of sound in the ocean, and this in turn is shaped by the temperature, salinity and pressure distributions in the water column. Taking the measurement data from two PIES, these run time measurements can be converted into density time series for the respective PIES position under different assumptions and with the help of additional temperature and salinity measurements from the surroundings of the respective PIES. If the water density changes from one PIES to another at the same depth, this is an indication of a current that flows between the two PIES, in our case either from north to south or vice versa from south to north. Also the pressure sensors of the PIES provide important information for this, because also pressure changes between two devices indicate that larger or smaller amounts of water are moved, which is done by currents that can be determined from these pressure changes. We therefore want to examine these currents and use the time series obtained from the PIES to determine the strength and fluctuations of the currents in the deep ocean basins across 47°/48°N.

After receiving data from the easternmost PIES BP-32 at the end of previous week, we visited the two PIES positions BP-33 and BP-34 this week. The data recording can be established via a kind of underwater microphone (the hydrophone). Here, the PIES are communicated with acoustically at depth and asked to send the recorded data. These are divided into different frequency ranges in which the hydrophone listens. On board, they are then recorded with a receiver. The device remains at the bottom of the sea. In bad weather like last year, this method called telemetry is difficult. This year we were able to extract the data from all the East Atlantic PIES, partly by telemetry or by recovering the device, which was then replaced by a device brought along.