Dominant frazil ice production in the Cape Darnley polynya leading to Antarctic Bottom Water formation

Abstract

Antarctic Bottom Water (AABW) occupies the abyssal layer of the world ocean and contributes to the global overturning circulation. It originates from dense shelf water, which forms from brine rejection during sea ice production. An important region of AABW formation has been identified off the Cape Darnley polynya. However, it remains unclear why and how high ice production leads to AABW formation. Using moored acoustic measurements and a satellite microwave algorithm, we reveal that underwater frazil ice dominates in the polynya. This underwater ice formation prevents heat-insulating surface-cover ice forming, thereby enabling efficient ice production. The high ice production in the nearshore and longer residence times create high-salinity source water for the AABW. Underwater frazil ice occurs as long as strong winds continue and occasionally penetrates depths of at least 80 m. Deep-penetrating frazil ice is particularly prominent in this polynya, while it also occurs in other Antarctic coastal polynyas.

Underwater ice boosts production of the world ocean’s densest waters

“…Frazil ice can form near the surface where the water is in a turbulent state, with the heat exchange between the atmosphere and ocean allowing supercooling to occur, and it was recently noted that supercooled waters are far more widespread across the Southern Ocean than previously thought ( 5 ). The moorings data used by Ohshima et al ( 3 ) reveal repeated occurrences of supercooling in the Cape Darnley polynya, and by monitoring the interior of the ocean acoustically, they observe that frazil ice dominates ice production here. It was noted that strong wind events (gusts exceeding 15 ms −1 ) were critical in causing frazil formation and that it can occur to unexpected depths (80 m or more).…”

“…Understanding where and how this sea ice grows is thus critical. Writing in this issue of Science Advances , Ohshima et al ( 3 ) report unexpected findings concerning the method of sea ice growth in a key AABW production site, with potential implications elsewhere around Antarctica.…”

“…Using satellite observations and data from year-round oceanographic moorings, Ohshima et al ( 3 ) demonstrate the importance of underwater frazil ice to dense water production. Frazil ice is well known to Antarctic oceanographers; it forms as irregularly shaped crystals where the temperature of seawater is reduced below its typical surface freezing point.…”

“…This process could be important at coastal polynyas other than Cape Darnley, where strong winds can drive supercooling and frazil ice formation. Ohshima et al ( 3 ) provide an initial assessment of this using their satellite data, but direct measurements will be needed to verify the level of importance at multiple other locations.…”

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

“…On one 40 branch of that feedback loop, formation and growth of ice crystals and their agglomerates, as well as their transport within the OML, are governed by OML's temperature structure and dynamics: Ekman currents, Stokes drift, buoyancy-induced mixing and Langmuir turbulence (e.g., Belcher et al, 2012;Herman et al, 2020; in the context of OML material transport in general, see Chamecki et al, 2019). As a result, complex, three-dimensional patterns emerge, with ice crystals reaching depths of meters or even tens of meters (Drucker et al, 2003, Ito et al, 2019, Ohshima et al, 2022. At the 45 surface, they tend to accumulate in convergence zones between neighbouring Langmuir cells, forming characteristic elongated streaks of grease ice separating areas of low crystal concentration.…”