Under-ice melt ponds in the Arctic

Email: n.e.smith@pgr.reading.ac.uk

The Arctic’s climate is one of those most rapidly changing globally, and as such the region has become a poster-child of climate change. Sea ice area is frequently used as an indicator of the rate of change of the system, providing striking visualisations of the rapidity of the change in recent years. The sea ice is also a driver of climate change, with areal cover greatly affecting the planet’s albedo and ice melt cooling and desalinating the Arctic ocean, altering circulation globally.

meltwaterLayerHHopNorskPolarinstitutt

Photo: Haakon Hop, Norsk Polarinstitutt

During the summer months, incoming solar radiation melts the surface layers of the sea ice. This melt water collects in hollows on the surface of the sea ice forming pools called melt ponds. Since the sea ice is porous, water from these ponds can percolate down or flow down through macroscopic flaws in the ice and out of the base of the ice. The melt water is relatively warm and fresh compared to the ocean below, so it floats between the ice and the ocean, gathering in pools beneath the sea ice called under-ice melt ponds. [1]

A couple of types of ice growth have been observed associated with these ponds. Most importantly, a sheet of ice can form at the interface between the pond and the ocean, completely isolating the fresh water from the ocean. As they create the illusion that they are the base of the sea ice, these sheets of ice are commonly referred to as ‘false bottoms’. [2]

We have developed a one-dimensional thermodynamic model of under-ice melt ponds to investigate how they affect their surroundings. We have carried out a number of sensitivity studies using this model, which have lead to some interesting conclusions about how these ponds evolve and affect the ice above them.

For example, the thicker the sea ice above an under-ice melt pond, the longer it takes to freeze due to a shallower temperature gradient above. As a result, more ice is gained due to under-ice melt ponds beneath thicker ice. This could be a positive feedback cycle, since we expect to see thinner ice on average as the Arctic warms, leading to less ice gained due to the ponds beneath it.

eguSens0

We also see that, as well as the outcome observed in the field, in which the false bottom migrates upwards and thickens as it freezes through the pond, it can also ablate under certain conditions. For example, ponds that are relatively salty at the start of the simulation freeze more slowly, and the false bottom ablates before it is able to reach the base of the sea ice.

eguSens1

Our sensitivity studies show that under-ice melt ponds could be responsible for up to 7.9% additional ice thickness at the end of a 50 day simulation. This would equate to up to 3.2% more ice volume across the Arctic dependent on the area of the ice underlain by these pools.

Recently, we have coupled our under-ice melt pond model with a simple, zero-dimensional model of the oceanic mixed layer. Using this coupled model, we see that the false bottom ablates more rapidly than a slab of sea ice, releasing more fresh water into the mixed layer. This strong reduction of salinity causes a shallowing of the mixed layer. We are currently further investigating the effects that the ponds have on the ocean below them.

Under-ice melt ponds and false bottom insulate the sea ice from below and affect the basal fluxes of salt and fresh water into the mixed layer, and thicken the ice above them allowing less radiation to penetrate through from the surface. They are clearly significant to the mass balance of the ice and the ocean below them, yet are not currently accounted for in the sea ice components of climate models. A parameterisation of their effects would be useful to include.

[1] Notz, Dirk, et al. “Impact of underwater‐ice evolution on Arctic summer sea ice.” Journal of Geophysical Research: Oceans 108.C7 (2003).

[2] Martin, Seelye, and Peter Kauffman. “The evolution of under-ice melt ponds, or double diffusion at the freezing point.” Journal of Fluid Mechanics 64.3 (1974): 507-528.

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