Turbulent Dynamics of Buoyant Melt Plumes Adjacent Near-Vertical Glacier Ice

At marine-terminating glaciers, both buoyant plumes and local currents energize turbulent exchanges that control ice melt. Because of challenges in making centimeter-scale measurements at glaciers, these dynamics at near-vertical ice-ocean boundaries are poorly constrained. Here we present the first observations from instruments robotically bolted to an underwater ice face, and use these to elucidate the interplay between buoyancy and externally forced currents in meltwater plumes. Our observations captured two limiting cases of the flow. When external currents are weak, meltwater buoyancy energizes the turbulence and dominates the near-boundary stress. When external currents strengthen, the plume diffuses far from the boundary and the associated turbulence decreases. As a result, even relatively weak buoyant melt plumes are as effective as moderate shear flows in delivering heat to the ice. These are the first in-situ observations to demonstrate how buoyant melt plumes energize near-boundary turbulence, and why their dynamics are critical in predicting ice melt. Melting glaciers are projected to produce 10s of centimeters of sea level rise over the next few decades. Despite this threat, the fundamental fluid dynamics which drive melt at tidewater glaciers remain poorly characterized. This is primarily attributed to challenges associated with measuring the temperature and velocity of ocean water at the submerged cliffs of actively calving glaciers. To this end, we have developed a robotically deployed instrument that can be bolted to a glacier's face. This instrument is capable of measuring temperature and kinetic energy of ocean waters within a few centimeters of the ice, representing the first measurements of their kind. Our observations demonstrate the ways in which meltwater at ice boundaries can accelerate melt. In particular, the meltwater tends to be less salty (and hence lighter) than the nearby ocean waters (which are salty, warm and heavy), so the meltwater rises along the ice face, creating an energetic, near boundary flow. With our new measurements, we show that these flows are as important as large-scale currents in providing energy to the ice to fuel melt. We anticipate these data will help our community create more accurate models of ice melt needed to predict the advance or retreat of marine ice cliffs of Greenland, Alaska and Antarctica. Robotic observations at a submerged near-vertical iceberg face capture turbulent dynamics of buoyant melt plumes and background currents Buoyant plumes extend 20-50 cm from the boundary and generate broad-spectrum temperature and velocity fluctuations that drive horizontal turbulent transports of heat When ambient horizontal flows are weak, buoyant plumes are the dominant source of boundary layer turbulence that drives heat flux to the ice