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Towards a more seasonal Arctic sea ice cover

In a new study published in Ocean Science, Bjerknes Centre scientist Anders Sirevaag and co-authors present results from a 16 day long drift experiment in the central Arctic Ocean in 2008. Compared to the early 1990’s, the measurements from 2008 show that the upper central Arctic contained more freshwater in the summer, but was saltier during winter. This indicates that there has been a transition towards a more seasonal Arctic ice cover, which means that more ice is forming during winter and melting during summer.

 
 Photo: Anders Sirevaag

 

 

Heat Exchange

 


Even though sea ice forms an efficient barrier, which protects the upper Arctic Ocean from exchanging heat with the atmosphere, a significant amount of solar radiation is transmitted through the ice and is warming the upper ocean. Since the upper Arctic Ocean is well protected from the warm water in the deeper layers, it will receive most of its heat from the atmosphere, mainly from solar radiation directly into the water in areas without any ice cover, or through openings in the ice cover such as melt ponds and leads. However, the summer sea ice cover has also shown to be able to transmit a significant amount of solar radiation to the upper ocean.

Figure shows the upper ocean temperatures during the experiment in August 2008. Red colors on top in the first half of the period show how the upper layers got heated by solar radiation through the ice. The figure shows that this heat supply was reduced for the second half of the period.


Solar radiation through thick ice   

The experiment was located on the Swedish icebreaker Oden and was a part of the Arctic Summer Cloud Ocean Study (ASCOS). By making detailed measurements, the researchers could study the evolution of the upper ocean during the transition from melting to freezing season. The measurements show that during summer conditions, solar radiation is transferred through the 1.8 m thick ice and accounts for up to one fourth of the heating in the upper ocean. As the winter season approaches, freezing of the ice surface combined with snowfalls, reduce the transfer of solar radiation by 60% and heat will rather be redistributed internally in the upper ocean.
 

Reference

Sirevaag, A., de la Rosa, S., Fer, I., Nicolaus, M., Tjernström, M., and McPhee, M. G. (2011): Mixing, heat fluxes and heat content evolution of the Arctic Ocean mixed layer , Ocean Science, 7, 335-349, doi:10.5194/os-7-335-2011