Melt ponds over Arctic sea ice

By Daniela Flocco

Melt ponds develop over Arctic sea ice during the melting season from the accumulation of melt water from ice and snow. These have become increasingly important over the last few decades because they have been more prevalent and absorb much more solar energy due to their dark colour compared to the highly reflective white sea ice (Perovich et al., 2002). Where ponds form, the ice beneath becomes thinner due to increased melting. Towards the end of the summer, the air temperature drops and a thin layer of ice forms over melt ponds. The ponds’ melt water trapped in the ice acts as a heat store and does not allow the underlying ice to start thickening until all the pond’s water is frozen. Ponds are up to 1.5 m deep and it can take over two months to freeze their volume of water. Considering that ponds cover up to 50% of the sea ice extent their impact cannot be neglected (Flocco et al., 2015).

Credit: Donald Perovich

A strong negative correlation exists between the change in successive mean winter ice thicknesses and the length of the intervening melt season, suggesting that summer melt processes play a dominant role in determining mean Arctic sea ice thickness for the following winter (Laxon et al., 2003). Another indication of the importance of melt ponds in explaining thinning of sea ice is that melt ponds are present in the Arctic more than in the Antarctic, where the sea ice thinning is less striking.

Ponds are rather irregular in shape but occur at a higher percentage over thin young ice: since the area of young ice is increasing (relatively to the total amount of ice which is instead decreasing), the impact of melt ponds will also become increasingly important. This will lead to a positive feedback effect in which thin ice will start thickening later in winter and will possibly be a preferential area for the formation of melt ponds in the following spring. Furthermore, corresponding to where melt ponds form, specular lenses of fresh water form under the sea ice cover, impacting the freezing point of water at the ice–ocean interface. At the beginning of the season sea ice is impermeable, so once ponds form they can be above sea level. When they start melting the ice, it becomes more permeable and when the ponds are fully developed they are in hydrostatic balance with the ocean so they drain to sea level.

Schemes handling melt ponds have only recently been included in global circulation models and are rather crude: the melt water was assumed to be flushed into the ocean without dwelling on the sea ice. Recent studies have shown that the lack of a melt pond parameterization can give an overestimation of sea ice thickness of up to 40% during summer (Flocco et al 2010, 2012). Model results have shown a good ability to forecast the minimum September ice extent, relating it to the melt pond area calculated by the model in May (Schröder et al 2014). This is one demonstration of how we have used the principles of physics to understand the changes we have observed in the cryosphere.

References

Flocco, D., D. L. Feltham, and A. K. Turner, 2010. Incorporation of a physically based melt pond scheme into the sea ice component of a climate model. J. Geophys. Res., 115, C08012, doi:10.1029/2009JC005568.

Flocco, D., D. Schröder, D. L. Feltham, and E. C. Hunke, 2012. Impact of melt ponds on Arctic sea ice simulations from 1990 to 2007. J. Geophys. Res., doi:10.1029/2012JC008195.

Flocco, D., D. L. Feltham, E. Bailey, and D. Schröder, 2015. The refreezing of melt ponds on Arctic sea ice. J. Geophys. Res. Oceans, 120, 647–659

Laxon, S., N. Peacock and D. Smith, 2003. High interannual variability of sea-ice thickness in the Arctic region. Nature, (425) October 30, 947-950.

Perovich, D.K., W.B. Tucker III, and K.A. Ligett, 2002. Aerial observations of the evolution of ice surface conditions during summer, J. Geophys. Res., 107 (C10), 8048, doi:10.1029/2000JC000449.

Schröder D., D. L. Feltham, D. Flocco, M. Tsamados, 2014. September Arctic sea-ice minimum predicted by spring melt-pond fraction. Nature Clim. Change, DOI: 10.1038/NCLIMATE2203.

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