Abstract
ABSTRACTTwo non-invasive optical Schlieren methods have been adapted to visualize brine channel development and convective processes in experimentally grown sea ice obtained when a NaCl aqueous solution is cooled from above in a quasi-two-dimensional Hele–Shaw cell. The two different visualization methods, i.e. traditional and synthetic Schlieren optical imaging, produce high spatial resolution images of transport processes during ice growth, without any external perturbation. These images allow observations of the flow dynamics simultaneously within the ice layer, around the ice/water interface, and in the liquid water layer, revealing connections between the processes occurring within the two phases. Results from these methods show that desalination of the growing ice layer occurs by two concurrent, yet independent, mechanisms: (1) boundary layer convection persisting throughout the ice growth period, with short fingers present just below the ice/water interface, and (2) gravity-driven drainage from the brine channels producing deep penetrating convective streamers, which appear after a given time from the beginning of ice growth. The improved visualization and qualitative characterization of these processes show that Schlieren optical methods have exciting potential applications for future study of convective processes during sea-ice growth.
Highlights
The changing extent and increased seasonality of Arctic sea ice due to anthropogenic climate change will have important effects on feedback mechanisms within the Earth’s climate system (IPCC, 2014)
The two different visualization methods, i.e. traditional and synthetic Schlieren optical imaging, produce high spatial resolution images of transport processes during ice growth, without any external perturbation. These images allow observations of the flow dynamics simultaneously within the ice layer, around the ice/water interface, and in the liquid water layer, revealing connections between the processes occurring within the two phases. Results from these methods show that desalination of the growing ice layer occurs by two concurrent, yet independent, mechanisms: (1) boundary layer convection persisting throughout the ice growth period, with short fingers present just below the ice/water interface, and (2) gravity-driven drainage from the brine channels producing deep penetrating convective streamers, which appear after a given time from the beginning of ice growth
In the adapted Schlieren experiment (Fig. 7, Video 2), when brine channels can be identified within the ice layer, the deeper penetrating brine streamers are seen to originate from these channels, while short fingers seem to be independent of any channels in the ice (e.g. Figs 7c–f; Video 2)
Summary
The changing extent and increased seasonality of Arctic sea ice due to anthropogenic climate change will have important effects on feedback mechanisms within the Earth’s climate system (IPCC, 2014). Is the areal extent of Arctic sea-ice cover diminishing in summer, but the composition of that ice is changing; as the sea ice becomes more seasonal rather than perennial, larger amounts of first-year ice are present compared with multi-year ice (Weeks, 2010; Maslanik and others, 2011) These changes will be important as the rejection of brine from growing and decaying sea ice influences the distribution of salt in the ocean and global thermohaline circulation (Gill, 1973; Aagaard and others, 1981; Morison and others, 1993; Ohshima and others, 2013), while nutrients and dissolved gases are entrained in the moving brine and sea water (Vancoppenolle and others, 2013; Zhou and others, 2013). Previous observations have proposed that desalination of sea ice takes place through a variety of mechanisms during its growth and decay: e.g. salt segregation at the growing ice–ocean interface, brine diffusion, brine expulsion, gravity drainage and flushing (Cox and Weeks, 1975; Weeks and Ackley, 1986; Notz and Worster, 2009; Weeks, 2010 and references therein)
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