5 September 2018

PIES

(Pressure Inverted Echo Sounders)

Yesterday, the ship spent most of the day and night transiting and positioning to deploy three small moorings in a small notch of the Chukchi continental shelf slope, where the Canada Basin forms almost a 90 degree angle. Tom Peacock, a professor at MIT and one of the Principal Investigators of the SODA cruise is interested in studying the deep wave energy in this area. In particular, he’s looking for Near Inertial Waves, which are generated at the surface by winds and propagate down toward the seafloor.

Historically, the Arctic Ocean has been a quiescent place relative to other oceans. The ice cover at the surface hinders the ability of the atmosphere to affect the water deep below the surface. As the summer ice coverage shrinks, this is changing. The passage of storms over open water creates a lot of wave energy, some of which Tom hopes his moorings will see 1000 to 2000 m below the sea surface.

The moorings carry instruments called PIES which stands for Pressure Inverted Echo Sounders (oceanographers love their acronyms), mounted on a domed structure that stands about knee to waist height. The PIES measure acoustic travel time to the surface, which is affected by the temperature structure of the water, and also by the presence of sea ice. Compared to open water, sea ice sitting over the mooring locations will return a strong acoustic signal back to the PIES. Tom can also gather some information about the thickness of the sea ice because ice with a deeper draft will return the signal slightly sooner than ice with a shallow draft.

The mooring is equipped with a current meter which will measure ocean velocities in the lower part of the water column. The goal is to be able to correlate deep ocean currents with seasonal variations in sea ice cover. Are there more inertial motions at depth when there is less ice cover? If that’s true, it could have important implications for the distribution of heat in the Canada Basin. Water masses with different properties are stacked like pancakes in the basin. The surface water, where ice grows and melts, sits on top of Pacific-origin water which comes in from the Bering Sea. Below that is Atlantic-origin water which is significantly warmer. Near Inertial Waves have the ability to mix heat from Atlantic Water up into the overlying, cooler Pacific Water. The amount of heat that is currently contained in the deep Atlantic Water is enough to melt all the sea ice in the Arctic; its movement closer to the surface could have serious consequences for Arctic sea ice.

The deployment: how can you tell that a PIES successfully reached the bottom?

The PIES acoustic signal is great for collecting data, but it’s also useful for communicating with the ship during deployment. It’s nice to feel confident that the instrument is on and functioning at the bottom of the ocean before driving away and leaving it for several years.

Our ship, the Sikuliaq, has several hydrophones mounted on the hull. These can be used for listening to acoustic signals that are sent to and received from the PIES. During deployment, a spectogram was displayed on a big monitor in the lab. The y-axis is the frequency of the signal and the x-axis is time. As the PIES is descending, you can send an acoustic signal at a known frequency and see it show up on the spectrogram. Some time later (farther to the right), you can see it bounce off the PIES. Some time after that, it bounces off the seafloor. You know it’s reached the bottom when the return signal from the PIES and the seafloor come at the same time.

Before leaving it for good, Tom did a short test to make sure it was working. He sent an acoustic signal that contained a command for the PIES to send two pings. You can see the command as a series of several vertical lines on the spectrogram (the deepest blue panel) followed by two distinct pings with a very clear frequency signature. It’s safe and collecting data!

The recovery: how do you get the data back?

Tom’s PIES are designed to stay out for three years, but that’s an awfully long time to wait for data! To get it faster without having to take a ship back to the mooring location, four pop-up data shuttles were also mounted to the mooring. The PIES broadcasts its data to the data shuttles every hour. Next summer, at the end of the first deployment year, two of these shuttles will detach from the mooring, float to the surface and make a satellite connection to send the data back to MIT. The following summer, two more will be released, and the summer after that Tom will return on a ship to collect the PIES itself. In the meantime, the shuttles, equipped with GPS, will act as surface drifters, following ocean currents providing some bonus information about currents in the Arctic.