Frazil ice growth and production during katabatic wind events in the Ross Sea, Antarctica

Abstract

Abstract. Katabatic winds in coastal polynyas expose the ocean to extreme heat loss, causing intense sea ice production and dense water formation around Antarctica throughout autumn and winter. The advancing sea ice pack, combined with high winds and low temperatures, has limited surface ocean observations of polynyas in winter, thereby impeding new insights into the evolution of these ice factories through the dark austral months. Here, we describe oceanic observations during multiple katabatic wind events during May 2017 in the Terra Nova Bay and Ross Sea polynyas. Wind speeds regularly exceeded 20 m s−1, air temperatures were below −25 ∘C, and the oceanic mixed layer extended to 600 m. During these events, conductivity–temperature–depth (CTD) profiles revealed bulges of warm, salty water directly beneath the ocean surface and extending downwards tens of meters. These profiles reflect latent heat and salt release during unconsolidated frazil ice production, driven by atmospheric heat loss, a process that has rarely if ever been observed outside the laboratory. A simple salt budget suggests these anomalies reflect in situ frazil ice concentration that ranges from 13 to 266×10-3 kg m−3. Contemporaneous estimates of vertical mixing reveal rapid convection in these unstable density profiles and mixing lifetimes from 7 to 12 min. The individual estimates of ice production from the salt budget reveal the intensity of short-term ice production, up to 110 cm d−1 during the windiest events, and a seasonal average of 29 cm d−1. We further found that frazil ice production rates covary with wind speed and with location along the upstream–downstream length of the polynya. These measurements reveal that it is possible to indirectly observe and estimate the process of unconsolidated ice production in polynyas by measuring upper-ocean water column profiles. These vigorous ice production rates suggest frazil ice may be an important component in total polynya ice production.

High-resolution simulations of interactions between surface ocean dynamics and frazil ice

“…8). The larger-scale streaks of frazil, although clearly discernible in all modeling results with wind greater than or equal to 15 m s −1 , have irregular shapes and spacing, and their form constantly evolves in time, which, qualitatively, agrees with observations (see, e.g., photographs and satellite images in Eicken and Lange, 1989;Ciappa and Pietranera, 2013;Hollands and Dierking, 2016;Thompson et al, 2020). In particular, it can be seen in Fig.…”

“…8 clearly show that relationship, as well as the fact that the individual convergence zones tend to be narrow, elongated, and oriented approximately in the wind/wave propagation direction. The model reproduces also the characteristic pattern of Y-shaped junctions formed by those zones, known from observations and earlier modeling studies on Langmuir turbulence (Thorpe, 2004). Remarkably, the maps of ice concentration at the domain scale reveal two overlapping patterns: one related to the locally elevated values of c in individual convergence zones and the other in the form of wider, long streaks oriented obliquely to the wind/wave direction with a typical spacing of hundreds of meters.…”

“…In (a), a "to" convention is used, i.e., S means the current flowing to the south and so on. In muir cells only rarely have regular shapes predicted by the classical theory (Thorpe, 2004). The results of our idealized simulations suggest that, at sufficiently high wind speed, the patterns of frazil concentration at the surface emerge from multi-scale processes, with locally elevated ice concentrations in individual surface convergence zones oriented approximately in the wind/wave direction and larger-scale wide frazil bands oriented obliquely to the wind/wave direction (Figs.…”

“…1), it is known that frazil streaks are common in coastal polynyas of both hemispheres. They have been observed and analyzed, e.g., in Terra Nova Bay (Ciappa and Pietranera, 2013;Hollands and Dierking, 2016;Thompson et al, 2020), Laptev Sea Figure 1. Sentinel-2 L1C image acquired on 10 March 2018 showing frazil streaks forming off the Ross Ice Shelf, Antarctica (image size ca.…”

“…Under highly idealized conditions, the average combined Ekman-Stokes flow can be computed analytically (McWilliams et al, 1997). In realistic, turbulent conditions, Langmuir "cells" rarely form regular, uniformly spaced, and steady patterns but rather have a transient nature and tend to deform and merge with each other forming Y junctions pointing in a downwind direction (Thorpe, 2004;Sullivan and McWillliams, 2010). Although several aspects of the unstable character of Langmuir circulation (or, rather, Langmuir turbulence) remain poorly understood, LES models successfully reproduce those features (McWilliams et al, 1997;Chamecki et al, 2019, and references therein).…”