Great to see this reinvigorated forum. Melt ponds have a lot of non-intuitive aspects. The article pair below is rather surprising for the late date of discovery within the century of melt pond study. With first year ice becoming ever more abundant and melt occurring perhaps earlier via Arctic Amplification, the intricate processes described below become ever more important.
Brine exclusion during FYI maturation ends up on the ice surface, in interior channels, and as non-buoyant water that sinks, affecting stratification and inhibiting later mixing. The Polarstern scientists are already skimming ice off the surface before lowering gear and seeing connecting currents.
In the North Pole region, with half the area in melt ponds, some measuring as deep as 1.5m, what fraction of the ice volume in December will be completely refrozen melt ponds rather than thermodynamic bottom ice? Some fraction will have drained and so have lower topography which might have consequences for re-formation via trapping in the following melt season.
Arctic melt ponds form when meltwater clogs ice pores
Pond formation mechanism previously unknown
https://unews.utah.edu/melt-ponds/ popular account
In 2014, Golden, along with study first author Chris Polashenski of the U.S. Army Corps of Engineers Cold Regions Research and Engineering Laboratory and colleagues traveled aboard the U.S. Coast Guard cutter Healy to the Chukchi Sea, between Alaska and Siberia, to investigate massive algae blooms below the ice, which had been first observed in 2011. As part of their study they needed to measure the permeability of the ice. Permeability is a measure of how well interconnected voids and channels within a material allow fluid to flow through.
Their first attempt involved drilling a hole in the ice down below the “freeboard level,” or water table, to see how quickly the water filled the hole back in.
“It filled up to the freeboard level in about a second and a half,” Golden says, indicating the ice was too permeable to make a measurement. Next, the team tried to add water to the hole to see how quickly the water level re-equilibrated to the freeboard level. They planned several attempts, and noticed that in the second attempt, the water level fell much more slowly than in the first attempt.
“And then the third time was the charm,” Golden says. The team poured water into the hole and the level didn’t go down at all. “We formed a melt pond!” he says.
Intrigued, the team tested different levels of water salinity in boreholes and used dyes to trace the progress of the water through the ice. The team used red and green food coloring from the Healy’s kitchen. All of their experimentation pointed to a clear mechanism for melt pond formation.
“The freezing point of the fresh meltwater from snow is zero Celsius,” Golden says. “But the ice itself is maybe -1 or -1.5. The freezing point of seawater is -1.8. So basically, you’re getting this infusion of fresh water and there’s enough cold there to clog up the pores. You’re lowering the permeability of the ice by this process of freezing freshwater plugs into the porous microstructure.” With lowered permeability, the meltwater can form a pool on top of the ice.
Percolation blockage: A process that enables melt pond formation on first year Arctic sea ice
Chris Polashenski Kenneth M. Golden Donald K. Perovich et al
16 January 2017
https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/2016JC011994 journal account
Melt pond formation atop Arctic sea ice is a primary control of shortwave energy balance in the Arctic Ocean. During late spring and summer, the ponds determine sea ice albedo and how much solar radiation is transmitted into the upper ocean through the sea ice. The initial formation of ponds requires that melt water be retained above sea level on the ice surface.
Both theory and observations, however, show that first year sea ice is so highly porous prior to the formation of melt ponds that multi-day retention of water above hydraulic equilibrium
should not be possible.
Here we present results of percolation experiments that identify and directly demonstrate a mechanism allowing melt pond formation. The infiltration of fresh water into the pore structure of sea ice is responsible for blocking percolation pathways with ice, sealing the ice against water percolation, and allowing water to pool above sea level.
We demonstrate that this mechanism is dependent on fresh water availability, known to be predominantly from snowmelt, and ice temperature at melt onset. We argue that the blockage process has the potential to exert significant control over inter-annual variability in ice albedo
While optical properties of individual ponds vary, the areal fraction of the surface that the ponds cover is by far the most important aspect of pond formation in determining spatially averaged albedo and solar partitioning.
On first year sea ice, early in the melt season, pond coverage is largely controlled by a hydraulic balance of meltwater inflows and outflows and the bathymetry of the depressions available for this water to pool in. Limited outflow pathways result in an accumulation of meltwater above sea level and large pond coverage on undeformed ice.
.Later in the melt season, pathways for water to pass between the ice surface and ocean become relatively unrestricted, first through the formation of large drainage holes, and later through the onset of permeability through the ice matrix [Polashenski et al., 2012]. After large levels of permeability are established, pond coverage is controlled by the fraction of the ice surface situated below sea level ...[many more pages of detail]