OSNAP GDWBC 07 July 2022

by Ellen Park

On our eight-day transit to the Irminger Sea, we completed a few calibration casts for instruments that will be deployed on the OSNAP moorings and saw some pretty incredible sunsets (see Figure 1).

Figure 1. OOI mooring buoys on the deck at sunset.  (Photo by Ellen Park)

Once we reached our target region, we got to work right away. First, we deployed all of the new components of the OOI Global Irminger Sea Array, which consists of a surface mooring, three subsurface moorings, and gliders. Then we moved on to recover the OSNAP Greenland Deep Western Boundary Current (GDWBC) moorings, which are all subsurface moorings. One of the key differences between the OOI and OSNAP moorings is how collected data are stored and recovered. OOI data are stored locally and also telemetered so that data can be accessed by the public in near real time. OSNAP data, on the other hand, are stored locally and cannot be accessed until the sensor is recovered and the data are downloaded.

Before recovering each mooring, we do a CTD cast roughly 0.5 nautical miles away from the mooring to provide an endpoint for each timeseries (see Figure 2). Not only can sensors drift overtime, but they also can become biofouled, which will affect the measured value (see Figure 3). Having an endpoint value is important because it allows us to correct the sensor data for these phenomena and provide more accurate results.

Figure 2. CTD going into the water. (Photo by Ellen Park)
Figure 3. Biofouled oxygen optode recovered from M1. (Photo by Heather Furey)

Many of the casts that we have done so far went below 2,000 m depth because we are interested in the GDWBC. Sensors deployed at these depths must be able to withstand harsh conditions and extremely high pressures. 10 m of water is roughly equivalent to 10 decibar of pressure (~1 atm). Therefore, at 2,000m depth, sensors experience 200 times the pressure that they experience at the surface. We could see the pressure effects on Styrofoam cups that we sent down with one of the deep CTD casts (see Figure 4).

Figure 4. Styrofoam cups before and after being sent to ~2,600 meters (Photo by Ellen Park)

We have roughly another week to finish up all of our operations before heading to Reykjavik, where we will leave the ship and fly home. During this time, we will finish recovering and deploying moorings and taking a preliminary look at the recovered two years’ worth of data!

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