Observations of platelet ice growth and oceanographic conditions during the winter of 2003 in McMurdo Sound, Antarctica

Abstract

[1] Platelet ice is the name given to ice crystals that nucleate in the ocean and grow either at depth or loosely attached to the ice-water interface. Related to the proximity of ice shelves, it is known to form in supercooled seawater. This study details the conditions for its growth during the austral winter of 2003 in McMurdo Sound, Antarctica. A key finding is that the presence of platelet ice in the sea ice cover is conclusively linked to the time history of the appearance of ice crystals in the water column, monitored using the strength of the backscattered signal from an acoustic Doppler current profiler as a proxy. Generally, these crystals appeared from mid-May as water near the interface became supercooled. This near-surface supercooling appears to be related to a simultaneous, abrupt change in the structure of the upper 250 m of the water column, from one that behaved dynamically and contained both the warmest and coolest measured temperatures to one in which the water was essentially isothermal near its surface freezing temperature. Near-surface ocean temperature was also affected by tidal mixing and by an increase in the thickness and density, over the course of winter, of the surface mixed layer created by salt rejection during ice growth. These processes allowed cold water, trapped by buoyancy in a band at the base of the mixed layer in early winter, to gain access to the ice-water interface by midwinter. This band of cold water probably had its origin beneath the McMurdo Ice Shelf.

Observations of turbulence beneath sea ice in southern McMurdo Sound, Antarctica

“…To date there has been little oceanographic work focusing on turbulence and mixing in the fast ice-shelf interface region. As well as directly influencing biological production (Arrigo et al, 2008), these small-scale processes influence ice formation and growth under sea ice and ice shelves (McPhee and Morison, 2001;Leonard et al, 2006;Stevens et al, 2006;McPhee, 2008;McGuiness et al, 2009). One important pathway for this influence is the effect on appearance and growth of frazil ice crystals both in their role as the initial building block of sea ice and as further contributors once ice thickens .…”

Flow and mixing near a glacier tongue: a pilot study

“…While the tides typically generate maximum flows of ∼0.15-0.20 m s −1 in the Sound-proper (Barry and Dayton, 1988;Leonard et al, 2006;Stevens et al, 2006), at the MSC station we saw flow magnitudes reach 0.40 m s −1 (Fig. 4c and d).…”

“…However, Leonard et al (2006) observed cylinders of platelets 30 cm in diameter growing on near-surface mooring lines after periods as short as three days. This suggests that if, as the observations suggest (Fig.…”

Response of sub-ice platelet layer thickening rate to variations in Ice Shelf Water supercooling in McMurdo Sound, Antarctica

“…7b suggest possible relationships between ASTR and supercooling in MMS, but observations of suspended frazil ice crystal sizes and turbulence within the ISW would be needed to calculate * . To date, there are limited observations of frazil ice in situ, and the majority of the observations that have been taken instruments not specifically designed for ice crystal detection (Leonard et al, 2006). 25…”

“…(SIPL) (Hunkeler et al, 2016;Langhorne et al, 2015;Leonard et al, 2006;Robinson et al, 2014). The SIPL not only harbours some of the highest concentrations of sea ice algae on Earth (Arrigo et al, 2010) but also contributes to the sea ice thickness when the water within the pores of the SIPL freezes to become incorporated platelet ice (Smith et al, 2001).…”