Drifting Buoys

Project purpose

The principal purpose of this project is to demonstrate the feasibility of instrumenting heavily icecovered fjords to obtain real-time data of the upper ocean.
Greenland's ice-covered fjords are the connections between the Greenland Ice Sheet and the open ocean. These dynamic environments enable access of warm ocean water to outlet glaciers, causing large amounts of melting under floating tongues (e.g. Rignot and Steffen, 2008; Motyka et al., in press). On the other hand, deep fjords also enable ice to break up mechanically through the process of calving. These icebergs are then transported away from the glaciers, where they eventually melt. The interactions between the ocean, its ice cover (the melange), the glacier ice, and the atmosphere remain poorly understood, mostly due to the extremely difficult conditions for direct observations (e.g. Amundson et al., 2010). Yet, it is increasingly clear that the dynamic behavior of the ice sheet is dominated by its interaction with the surrounding oceans (e.g. Rignot and Kanagaratnam, 2006; Joughin et al., 2008; Holland et al., 2008). It is therefore imperative to gain a better understanding of the physical processes that determine the heat and mass exchange between ocean and ice. This is an issue not only for Greenland, but at all the larger glaciated areas of the planet. The current inability of predicting changes at marine-terminating glaciers is responsible for the lack of a reliable estimate of the future cryospheric contribution to sea level rise (IPCC, 2007; Truffer and Fahnestock, 2008).
To make progress in the task of predicting the behavior of outlet glaciers, a better understanding of physical processes in glacier-fed fjords is necessary. This will require direct observations. The physical environment for this type of work is extremely challenging. The inner fjords are often covered in brash ice and large ice bergs, sometimes mixed with sea ice. Large ice bergs can roll, creating hazards to boats. The area very close to glaciers can have turbulent upwelling with fast currents, the proximity of the glacier is too dangerous to work in due to calving activity, and calving events can send meter-scale waves through the fjord. Moorings are difficult to deploy, and have to be deeply submerged to avoid interaction with the keels of the bigger ice bergs, making it impossible to measure processes and exchanges at the critical atmosphere-ice-ocean boundary. Here we propose to measure the properties of the upper water column using drifting buoys. The proposed experiment carries a certain risk, as the equipment could get destroyed. We will attempt to minimize this risk by letting the buoys drift, and by constructing them more solidly, so they are better able to absorb impacts. Also, they will be equipped with Iridium satellite modems, so that data can be uploaded on a regular basis and will not be lost should the buoys fail.

Expected results

We expect to obtain a record of up to one year length of temperature, salinity and currents in the upper water column (down to ~30m) of the inner Godthabsfjord, near the main outlet glacier Kangiata Nunata Sermia (KNS). We propose to deploy four Lagrangian drifters; two on the glacier side and two on the outer side of a sill that was created by a previous glacier advance (Mortensen et al., subm. to JGR). The deployments in the heavily ice-covered inner fjord are considered higher risk. The deployments on either side of the sill balance the risk of deploying in heavy ice with the desire to obtain data at those locations. The other expected result is to gain experience with instrumenting these difficult areas, where many details of physical processes have remained elusive. For example, if drifting buoys prove to be successful, one could develop these further into profiling instruments that are capable of sampling the entire water column. Another possible application is to develop drifting depth sounders to obtain geometric observations where boats cannot penetrate. Before such plans are implemented, it is imperative to gain some experience with lower cost instruments.