by Ryan Peabody
Imagine that you were inclined to make an observation about the ocean. As someone reading an oceanography blog, there is a very good chance that you are exactly the kind of person who would be. In this hypothetical situation, you would walk down to your hypothetical dock, and record some property of the water – let’s say current speed. If you were to repeat this activity over time, we could then build a time series, a record of these speeds over time at a fixed location. This is what an oceanographic mooring does, with multiple types of information. It records data continuously at one location, giving a very good sense of what is going on at that spot over time.
Our record of observations would only tell us what was going on at that one point. We might be able to guess what was happening elsewhere, but we would not be able to verify our guesses. We can overcome these limitations by deploying more moorings and instruments, but each is still fixed in location. Our ability to guess at what is happening elsewhere improves, but we still only know what is happening where we choose to look. When tracking the pathways that ocean currents take throughout the ocean, we run into an issue: if we do not know the pathways that these currents take, how can we decide where to look in the first place?
Besides in situ observations, there are other ways to examine oceanographic currents. We can use satellite altimeters to examine ocean height and derive surface currents, or we can physically put objects in the water and watch where they go. However, what do we do when these currents are far beneath the surface of the water? We can place instruments at those depths on moorings, or observe how sound waves change as they bounce back from depth, but we run into the issue discussed earlier: we limit ourselves to observations constrained in space. It is in our interest to know where and how these water masses move, not simply what the speed is at a handful of points.
The ocean is layered, with denser waters filling the deep ocean basins, and “lighter”, less dense water masses floating on top of one another subsequently to the surface. An oceanographic instrument with a certain density would then float on top of a dense water mass, but sink through the one overlying, riding the interface as a surface float would between the ocean and atmosphere. RAFOS floats are 2 m-long glass tubes that do exactly that. Each has a specially made steel weight that allows it to sit at a certain height above the bottom of the ocean. In this way, we can follow the pathways that deep ocean currents take.
2000 m below the surface of the ocean, it is pitch black. Light does not transmit very far through water, attenuating in the surface waters high above. Similarly, signals from GPS satellites, radio waves, and most of the mediums we use for communication do not propagate well. It is one thing to deploy a RAFOS float beneath the surface, but knowing where it goes and then recovering that information require additional processes. Sound sources deployed throughout the North Atlantic emit a pulse of sound every 24 hours. Unlike light, sound can travel for great distances underwater. By calculating the difference in time between emission and reception from these sound sources, the position of the float can be calculated as it moves underwater.
Each float looks like a long, glass thermometer, sitting atop a metal weight. The weight connects on the bottom, screwing into a plastic socket, connected to the main tube with a thin wire, next to a hydrophone and a temperature and pressure sensor. Above the sensors lies a battery pack, satellite transmitter, microprocessor, and a connecting rod that runs through the middle of the glass tube to the antenna, located in the top of the glass tube. Once they are turned on, they are lowered (dropped) off the stern of the ship, where they sink underwater, and record their positions for two years. After two years, a current is passed through the connection point for the wire, heating it to the point that the wire shears, dropping the weight to the bottom of the ocean and allowing the float to truly float, up through the water column until it reaches the surface where it can upload its data to a satellite.
Of the thirty-four floats we started with, fourteen have been deployed, on a transect perpendicular to the Reykjanes Ridge south of Iceland. Two will surface after eight and eighty days, to verify that the sound sources are working as expected. Twelve will stay under for the full two-year deployment. The remaining twenty will be deployed on a transect further west, perpendicular to the coast of Greenland. The data gathered from the deployments at different depths and different locations will provide insight into the movement of deep water in this part of the North Atlantic.
It was initially intimidating to plan out the timing of my deployments in the context of the larger cruise mission. Our course can be simply visualized at two east to west transects: one off Greenland and the other off Iceland. Each transect is performed three times, to recover permanent moorings, perform CTD casts and deploy the RAFOS floats, and to redeploy the moorings (with fresh batteries and instruments, stripped of data from the previous deployment). If I had my way, all the RAFOS deployments would occur over the course of a couple days, between 11:00 AM and 3:00 PM, and then everyone else could figure out their stuff while I relaxed with a cup of coffee. Unfortunately, my scientific plan has been rejected in favor of one more “economically efficient” and that “actually makes sense,” so I deploy floats when we arrive at my stations, regardless of the hour.
Fortunately, there is plenty of coffee and just-shy-of-unbearably salty Dutch licorice on board. With night a barely perceptible dimming of the sun behind clouds, and my ability to avoid an eight-hour CTD shifts, it is sometimes hard to keep track of what time it is. Maybe a structured work schedule makes it easier, but I exist in a gray limbo, punctuated by meals and float deployments. At eye level, the horizon is approximately 5 km away. Raising or lowering the viewpoint changes the distance accordingly. We will be able to see Greenland soon, giving at least one boundary to the horizon that is not more water, tucked away from view by the curvature of the earth. Until then, we will all just keep looking out at the horizon.