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.