In this dissertation, driving mechanisms and energetics of small-scale turbulence in the stratified flow under Arctic sea ice were investigated. Since turbulent mixing drives the oceanic heat flux to the ice, this work contributes to advancing our understanding of Arctic sea ice loss. The work is based mainly on observations obtained from moored instruments, autonomous instrumentation on drifting ice floes, and high-resolution profiling instruments. A particular focus was on the effect of the seasonal meltwater release on turbulence, since the associated buoyancy flux stabilizes the water column, suppressing heat flux to the ice. The analyses comprise of three process studies that cover different scales and dynamical regimes. The first process study is based on turbulence measurements from <1 m to 3 m below the ice, a layer in which depth-resolving turbulence observations are rare, but most relevant, since it is where the meltwater has the strongest impact. Investigating the balance of production and dissipation of turbulent kinetic energy (TKE) it was shown that about two-thirds of produced TKE are dissipated, the remainder was suggested to be attributable to meltwater effects. A skin drag coefficient was estimated. Compared to related literature, the estimate was low, leading to little generation of turbulent stress in the boundary layer. An analysis of the spectral variance of the boundary forcing revealed that oceanic processes on sub-daily time scales can be as important as wind-driven turbulence, further suggesting inertia-gravity waves (IGWs) from tide-topography interactions to drive turbulence in the under-ice boundary layer. The second process study was based on a day-long high-resolution survey of a mesoscale eddy, combined with year-long measurements from a nearby mooring. For this analysis, velocity data were scale-separated in time, revealing that cross-scale interactions between the mesoscale field and upward-propagating IGWs can be a driving mechanism for upper ocean turbulence. In the third process study, characteristics of velocity shear in the seasonal stratification were scrutinized in detail. Both in moored and ice drift-based measurements, the shear magnitude grows and decays intermittently over the course of several consecutive hours, leading to pronounced spikes. Asserting that the enhanced shear leads to enhanced TKE production, one may assume that the spikes drive mixing in the upper ocean, which is supported by anecdotal evidence. The shear spikes were found to be correlated with tidally-driven IGWs for two of three analyzed moorings, which forms a basis for future analyses of the phenomenon. Overall, the results suggest that turbulence in the uppermost meters of the ice-covered water column can be modulated by upwards-propagating waves through several mechanisms. As turbulent mixing in the upper ocean has implications for Arctic sea ice loss and marine ecosystem dynamics, the presented results motivate further research, for which they provide a comprehensive foundation.
Simon F. Reifenberg (Wed,) studied this question.