At marine-terminating glaciers, the interplay between meltwater buoyancy and local currents control turbulent exchanges. Because of challenges in making centimeter-scale measurements at glaciers, turbulent 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 tug-of-war between meltwater-derived buoyancy and externally-forced currents in controlling boundary-layer dynamics. 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 strengthened, the plume diffused 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.
