Collaborative Research: How fast do tidewater glaciers melt? Quantifying the processes that control boundary layer transport across the ice-ocean interface

About This Grant

Sea-level rise will affect millions of people in coastal communities within the next several decades. Accurate predictions of how quickly it will rise is challenging because it depends on many different processes and how these processes interact with and feedback on each other. One process that may play a surprisingly large role is the effect of small swirls and eddies (only a few feet across) of warm water that control the rate of ice melt at the near-vertical cliff faces of the world’s marine-terminating (tidewater) glaciers. At these glaciers, ice flows directly into the ocean and melts underwater or calves icebergs. Melting of the ice produces freshwater that flows out near the ocean surface and drives a return flow that draws in deep warmer ocean water toward the glacier. According to current theory, increasing the rate of ice melt increases the strength at which warmer ocean water is pulled in towards the ice face, which further enhances the melting. The details of this process - particularly the small-scale dynamics near the ice face - have never been measured because the calving ice cliffs are too dangerous to make measurements. Here we propose to use a highly specialized underwater robot (a remotely operated vehicle, or “ROV”) with state-of-the-art optical and acoustic instruments to observe the melt rate and the processes that control it. One of the novel aspects is the use of “melt stakes” - 6 ft long rods that will be driven into the glacier face by the ROV and monitored continuously to determine the melt processes. These stakes then provide a frame of reference for our ROV to make a suite of detailed measurements of the shape of the glacier face, the dynamics of the currents adjacent to it, and how the ice-water interface evolves. At the same time, we will observe the local ocean environment in the fjord - the currents, salinity and temperature - which are the main ingredients we need to predict ice melt in larger-scale and climate models. Our analyses will combine field data with a high-resolution fluid-flow model that recreates the conditions along the ice with realistic water properties. The combination of model and data will be used to refine our melt predictions and verify these directly using our observed measurements. At the end of the project, we will be able to extend our results to estimate how much melt is occurring for tidewater glaciers around the globe, and how this may change in time. Beyond this importance to society and the scientific community, this grant provides broader impacts across several levels: (1) mentorship and support for two early career women (2) support for three graduate students in interdisciplinary ice-ocean studies, (3) experiential opportunities, funding, and mentorship for 45 senior-year undergraduate students, whose capstone projects will directly contribute to this project while being supervised by our gender and culturally diverse team of engineers and technical staff, (4) classroom experiments showing buoyancy and convection to engage K-12 students and the general public, and (5) two teams of high-school women will additionally be involved and make observations through Girls in Icy Fjords expeditions. Melting at the ice-ocean interface of marine-terminating glaciers influences the rate of mass loss from the world's ice sheets. In addition to contributing to sea-level rise, details of the melt process dictate the depth at which fresh meltwater enters the ocean (which in turn affects ocean circulation on a variety of scales) and alters calving rates. Existing theory suggests that the rate of submarine melting along these ice faces is set by the strength of subglacial discharge. However, recent observations find unexpectedly high melt rates over broad sections of glacier termini, even outside discharge plume areas. The observed order of magnitude discrepancies between observed and predicted melt rates suggests the presence of energetic dynamics elsewhere along the ice face that drive near-ice turbulent flows. We hypothesize that this discrepancy arises from differences in the rate-controlling physics within the boundary layers. Current turbulent transfer coefficients were derived from stable boundary layers. Yet on vertical glacier ice faces, boundary layers have strong buoyant forcing and marginal stability that likely produce dynamics not captured by laboratory or idealized models. Because buoyant meltwater fluxes provide kinetic energy for near-boundary outer flows -- and because enhancement of those flows leads to enhanced melting -- there is potential for strong positive feedbacks in the dynamics. As a result, small errors in the melt parameters or the parameterization functional form can have significant consequences to the total melt calculation. No studies have yet to make observations immediately next to near-vertical ice faces, or measure melt dynamics with the resolution necessary to investigate these dynamical feedbacks. This grant supports the development of a first-of-its-kind network of coordinated underwater acoustic, optical and in-situ unmanned sensors to be deployed at LeConte Glacier, Alaska. Using methods that meld glaciology, oceanography, and robotics, these systems will collect the first geophysical observations of the turbulent boundary layer at a near-vertical glacier face. Specifically, we will directly measure velocity, salinity and temperature through a buoyancy-forced near-vertical boundary layer and relate these to observations of the subsurface ice morphology (e.g., slope, roughness) across several spatial scales. By combining these data with high-resolution realistic simulations, we will characterize the dominant contributions to boundary layer turbulence and explicitly relate these to local melt rates. Our ultimate goal is to determine what parameters need to be measured (e.g., fjord u,T,S) over what time and space scales, as well as what assumptions can be made in order to connect dynamics from the small-scale ice interface to the large-scale ocean and glacier forcing. This grant builds an observational capacity that does not exist at present. Measurements will span a sufficient range of the parameter space (in ocean temperature, velocity variance and ice morphology) for us and others to test existing and advance new melt models that underlie many ice-ocean community models. This award is co-funded by the Arctic Natural Sciences Program and the Physical Oceanography Program. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.