This is post 2/3 in a series on our experiences and tasks as ArcTrain PhD students during the research cruise M164 (GPF 19-1-105) in summer 2020 in the subpolar North Atlantic. Click here to see part 1/3 and here to see part 3/3.
A research cruise offers the great opportunity to escape your usual office routine and experience your science in the field. In my PhD project, I analyze the temporal evolution of the current system in the North Atlantic, the most prominent being the extension of the Gulf Stream, the North Atlantic Current, in the past decades. So, in summer 2020 I had the chance to go out to the North Atlantic on the research vessel Meteor and experience my study region in the real world.
Every person on board of a research vessel is responsible for a specific task. For this research cruise I was responsible for the PIES, similar as Hannah, an ArcTrain colleague, was a few years ago. However, this was not just a piece of cake. PIES is an acronym for Pressure Inverted Echo Sounder. They are about yoga-ball-sized, sealed glass spheres with a lot of measuring equipment inside. Being deployed on the ocean floor for up to several years they autonomously measure the travel times of a sound pulse to the ocean surface and back. From these measurements oceanographers can infer the hydrographic properties and ocean velocities in the ocean. My job on the research vessel Meteor was to take care of the data readout and recovery of the PIES. Since PIES measure and communicate with acoustic signals, this often involved sitting in the lab with headphones on, listening closely to the sounds that these instruments make in the water, because using the right language we can actually talk to them.
Systematically investigating sound and its properties in the ocean and using it for seafaring and oceanography has started about a century ago. Initially sound was used to make obtaining depth estimates of the ocean more feasible. In the early days of seafaring a weight on a string was used to measure the water depth. This meant that the ship had to be very slow to measure the depth and that these measurements required a lot of time and effort. This changed drastically when the echo sounder was invented. A sound source was mounted under the ship together with a receiver. This device was used to determine the travel time of a sound pulse sent from the ship towards the ocean bottom until the signal reached the ship again. Knowing the speed of sound in seawater, this allows to calculate the water depth. Such an echo sounder can determine the water depth in only seconds with great accuracy and is therefore still commonly used in ocean science and navigation. However, the measurements still did not completely agree with what the classical weight on a string measured. Though there can be other influences, e.g. drag on the line by currents, in the early 20th century it was found that the travel time of the sound signal varied not only with depth but also with temperature and salinity. Tables to look up the sound velocity for specific values were published to correct for these variations. This dependency should prove valuable for the working principle of the PIES when oceanographers became more interested in the changes rather than only the state of the ocean.
In the western North Atlantic, the former Gulf Stream continues northward as the North Atlantic Current. Since this is still a warm current, it transports a lot of heat northeastward and thus contributes to maintaining comparatively mild temperatures in Europe. Changes in this current system have direct effects on the European weather and climate. Thus, oceanographers are particularly interested in measuring the changes in volume transports, i.e. how much water is transported by the currents over a fixed amount of time. Volume transports are calculated by measuring the velocity in a confined vertical area; the multiplication of area and velocity yields the volume transport. The measurements to determine these transports are usually provided by repeated ship surveys and specifically designed current meters – recording devices mounted on a rope, so-called deep-sea moorings – that are installed in the ocean for several months or even years. Either from direct current speed measurements or indirectly, from the vertical distribution of temperature and salinity and the resulting density at two different locations, it is possible to estimate the oceanic volume transport between them.
But with ships oceanographers cannot cover every region with the desired temporal resolution, and putting a lot of instruments at various depths into the ocean is not always possible. So, oceanographers often have to live with large spatial and temporal data gaps. Also the young researcher and MIT PhD graduate Thomas Rossby was facing these problems in the 1960s. He remembered the relation between the speed of sound in seawater and its temperature and salinity and had the idea to make use of this in the opposite way. If the relation of the speed of sound in seawater to its temperature and salinity is known, it is possible to develop a technique allowing to estimate the temperature and salinity from the known speed of sound. He developed an affordable device, measuring the travel time of a sound pulse to the ocean surface and back. This is what Inverted Echo Sounders (IES) do. They work just like echo sounders, but are inverted, standing on a metal frame, which serves as a bottom weight, on the ocean floor and pointing towards the surface. When they are equipped with an additional pressure sensor, they are called Pressure Inverted Echo Sounders (PIES). At a predefined rate they emit sound pulses from a speaker and measure the time it takes the signal to travel to the ocean surface, be reflected there and get back down to the instrument. With profiles of temperature and salinity, measured from ships at the same or nearby locations as reference measurements (and the resulting speed of sound) the approximate profile of temperature and salinity can be inferred from this travel time. This is done by comparing the travel time measured by the PIES with the travel time computed from the reference profiles. In the ocean, temperature and salinity together determine the density of seawater, and ocean currents are evoked by horizontal density differences, just as winds in the atmosphere revolve around high or low pressure systems. Thus, it is now possible to calculate the volume transport between two PIES. In our case, with eight PIES in the water, the volume transport across at least seven segments in the North Atlantic can be calculated and added to existing transport timeseries of more than a decade in some cases. These timeseries deliver important information about the past evolution of the circulation in the North Atlantic.
But why would I sit there with headphones on and listen to the sounds of the PIES during our research cruise when we are supposed to recover them? It is because these devices come with a special feature: You can actually “talk” to them. By lowering a hydrophone – basically a waterproof microphone and speaker – into the water you can send acoustic commands to the device and listen to its answers. The PIES understands different commands, including CLEAR (“stop whatever you are doing and tell me if you can hear me”), TELEMETRY (“tell me what you have recorded since the last data readout”) and RELEASE (“detach from your bottom weight and ascend to the ocean surface”). The TELEMETRY command is used to receive a compressed form of the recorded data via acoustic signals, similar to morse code, before recovering the instrument. In the unusual case that something goes wrong during recovery and you unfortunately lose the instrument, at least you have the data. When the PIES receives the TELEMETRY command it starts sending acoustic signals at different frequencies and intervals which can be translated by a computer into the different parameters measured by the instrument. When the data transmission via acoustic telemetry is finished, the PIES can be recovered. The recovery is mostly done during the night since the PIES are equipped with a bright flashing light that can be seen best during the night.
It is close to midnight now and I am sitting in the lab with headphones on. The acoustic telemetry of our last PIES for this research cruise has finished successfully and the data have been safely stored on a hard disk. I have sent the RELEASE command to the PIES, and the device has confirmed that it has detached from its bottom weight. Since it is now buoyant it ascends to the surface while constantly pinging. From the traveltime of these pings to the hydrophone of the ship I can infer the depth to which the device has ascended until now. I can see that the device is close to surfacing, and thus almost everyone who is awake right now is standing on the bridge looking out for a flashing light in the darkness of the night.
TO BE CONTINUED