Novel Ocean Observations by μFloats at a Towering Glacier

Swarms of small, inexpensive buoyancy-controlled floats are a critical technology to study the melting retreat of LeConte Glacier, Alaska. A trigger for tidewater glaciers’ retreat is the melting caused by the inundation of warm ocean water against the glacier’s submerged face. During summer, rain and surface meltwater drain to the bottom of the glacier and emerge at the grounding line – 100–250 m deep in LeConte Bay – as a freshwater plume that rises and spreads down the bay. This energetic, buoyant discharge – an upside-down waterfall – entrains comparatively warm ocean water as it rises in a turbulent plume along the underwater face of the glacier, melting ice.

Despite the central importance of this discharge plume to the glacier’s total melt rate, observations at the glacier face are scarce because it is simply too dangerous for scientists to work there. APL-UW investigators and colleagues at Oregon State University (OSU) designed observational campaigns using the unique capability of μFloats to target and ride the multilayer circulation in the bay, become entrained in the buoyant discharge plume, all while their movements are tracked acoustically. 

Before μFloats were deployed, five surface buoys were set out in LeConte Bay to create a network of acoustic beacons. Dropped in small swarms about 300 m from the terminus, μFloats were programmed to descend to depths ranging from 45 to 90 m, targeting the layer of ocean water drawn toward the glacier face. As they were entrained into the discharge plume, they accelerated to speeds up to 2.5 m/s and rose quickly toward the surface with the buoyant plume. "With all the upward momentum, the floats cannot maintain their programmed depth," explains Senior Oceanographer Trevor Harrison. "Now on the surface, they work to go to depth again, become entrained, and take the grand ride again, circling back into the buoyant flow." Typical float drifts played out over four hours. As currents inevitably carried the surface buoys away from the terminus, the team retrieved and relocated them to maintain the acoustic localization network for the floats. At day's end, all assets were collected as they drifted down the bay with the freshwater plume.

Unlike well-known oceanographic floats built for five-year deployments in the open ocean that communicate their data through a satellite network, μFloat missions are no longer than one day and conducted in shallow, turbulent environments. Its fast, high-volume buoyancy control engine can move the float to depth quickly, overcoming moderate currents and strong density gradients, and their onboard acoustics system, working in tandem with a multi-node tracking network, allows reconstructions of their position and velocity in three dimensions.

μFloat measurements of the discharge plume highlight how the theoretical layered, two-dimensional flow – with warm, salty deep inflow toward the glacier and cold, fresh outflow away – is an oversimplification. "The floats caught in the discharge plume tumble up and down multiple times before entering the shallow outflow," remarks Principal Oceanographer Zoltan Szuts. Tossing in this turbulent eddy adjacent the glacier face, the μFloats' temperature and salinity sensors detected the signatures of fresh discharge plume water, the salty inflow water, and the melting of submerged ice.

A swarm of small, inexpensive instruments filled a gap in sampling capabilities at LeConte Glacier. With them, researchers gained new insights into the complexity of the ocean circulation and a new basis for estimates of melt rates at this and other tidewater glaciers.