Spatial characteristics of frazil streaks in the Terra Nova Bay Polynya from high-resolution visible satellite imagery

Coastal polynyas around the Antarctic continent areregions of very strong ocean-atmosphere heat and moisture exchange that areimportant for local and regional weather, sea ice production, and water massformation. Due to extreme atmospheric conditions (very strong offshorewinds, low air temperature, as well as humidity) the surface ocean layer inpolynyas is highly turbulent, with mixing due to combined Langmuir,wind-induced, and buoyancy-driven turbulence. One of the visible signs ofcomplex interactions between the mixed-layer dynamics and the forming seaice are frazil streaks, elongated patches of high ice concentrationseparated by areas of open water. In spite of their ubiquity, observationaland modelling analyses of frazil streaks have been very limited largely dueto the fact that their significance for heat flux and ice production is onlyjust becoming apparent. In this study, the first comprehensive analysis ofthe spatial variability of surface frazil concentration is performed for theTerra Nova Bay Polynya (TNBP). Frazil streaks are identified inhigh-resolution (pixel size 10-15 m) visible satellite imagery, and theirproperties (surface area, width, spacing, and orientation) are linked to themeteorological forcing (wind speed and air temperature). This provides asimple statistical tool for estimating the extent and ice coverage of theregion of high ice production under given meteorological conditions. It isalso shown that the orientation of narrow streaks tends to agree with thewind direction, suggesting the dominating role of the local wind forcing intheir formation. Very wide streaks, in turn, deviate from that pattern, asthey are presumably influenced by several additional factors, includinglocal water circulation and the associated convergence zones. An analysis ofpeak wavelengths and directions determined from the images, compared toanalogous open-water wavelengths computed with a spectral wave model,demonstrates a significant slow-down in the observed wave growth in TNBP.This suggests an important role of frazil streaks in modifying wind-wavegrowth and/or dissipation in polynyas.

Automated summary

Coastal polynyas in the Antarctic continent play a crucial role in heat and moisture exchange, weather patterns, sea ice production, and water mass formation. Frazil streaks, visible manifestations of mixed layer dynamics and sea ice formation, have been poorly studied due to their recent recognition of their significance. This study provides a comprehensive analysis of surface frazil concentration in the Terra Nova Bay Polynya and establishes a statistical tool to estimate ice coverage under different meteorological conditions. The study also highlights the influence of wind forcing on the formation of narrow streaks and the potential effects of local water circulation and convergence zones on wider streaks.