Entrainment induced by near-inertial drift of sea ice and its impact on under-ice biogeochemical processes in marginal ice zones

Mooring observation of hydrography, hydrodynamics and suspended particles distribution under a drifting sea ice revealed the mixing and entrainment pattern in the upper mixed layer (ML) of the marginal ice zone. The ice floe where the mooring system was installed drifted as near-inertial motion with approximately 12-h cycle. The mixing pattern induced by this near-inertial drift can be divided into two distinct regimes. First, simple entrainment (upward) fluxes from the seasonal pycnocline to sea ice–water boundary are induced by shear across ML and seasonal pycnocline during the period when ice floes drift toward pack ice. The entrainment speed was in the range of 0.25–2 m h−1, which matches well with thickening and thinning of the ML during a near-inertial period. Turbulent wakes on the boundary between sea ice and open water occurred behind the advancing edge of ice. In the second regime, when ice floes drift toward open ocean, the turbulent wakes at the advancing edge of ice are combined with the entrainment caused by near-inertial motion, which results in a complex mixing pattern of both upward and downward fluxes in the ML. The echo intensity observed by the acoustic Doppler current profiler and beam attenuation from transmissometer revealed the elevated concentration of suspended particulate materials in the ML, which can be direct evidence visualizing the mixing pattern. Results suggest that the mixing and entrainment found in our study sustain particulate matters in suspension within upper ML for a few months. This may provide a potential mechanism to sustain abundant organic particulates in the ML and upper pycnocline for months after under-ice bloom. Under strong wind events like storms, the entrainment induced by near-inertial motion may also get enhanced, which causes elevated supply of nutrients from the deeper, permanent pycnocline to the ML.