In the marginal ice zones, where drifting sea ice often meets relatively warm water, we often find large melting rates and intense exchange of heat between the ocean and the atmosphere. Generally, melting rates increase with increased ocean temperatures. However, earlier laboratory- and model-studies have indicated that these melting rates can be limited by so called double diffusive effects on the interface between ice and ocean. Double diffusion means that heat is transferred much faster than salt from the ocean to the ice. An efficient transition from solid to liquid state of sea ice is dependent on efficient transfer of both heat and salt; hence this will reduce melting rates.
Direct measurements from sea ice
In a newly published study in Geophysical Research Letters, Anders Sirevaag, from the Bjerknes Centre, presents measurements of turbulent exchange of heat and salt in the marginal ice zone north of Svalbard. This is the area where the northernmost branch of the so called Gulf Stream enters the Arctic Ocean and is known as an important area when it comes to exchange of heat between the ocean and the atmosphere. The measurements, obtained wintertime from the drifting sea ice, show that the effect of double diffusion is significant and of the same order as earlier assumed from laboratory- and model-studies.
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| Feltarbeid på isen (foto: Anders Sirevaag, BCCR) |
20 – 30% overestimated ice melting
When estimating melting of sea ice, one usually considers a simplified heat balance at the interface without considering molecular effects. The consequence of this approach is investigated by estimating melting rates both with and without considering double diffusive effects for a range of water temperatures and stress on the ice/ocean interface. The comparison shows that by ignoring double diffusive effects, calculated melting rates can be overestimated with several cm per day compared to the actual melting rate, which can add up to a significant error in these areas over the entire melting season. For the marginal ice zone north of Svalbard, ignoring double diffusive effects will overestimate melting by 20 – 30%.
The study shows the importance of including molecular effects when estimating melting rates of sea ice. These effects are important in areas where the exchange between the ocean and the atmosphere is intense and it is important to include these to estimate the development of the ice cover in these regions in a realistic way. For the majority of the Arctic, ice melting rates are moderate and molecular effects will not significantly influence estimated melting rates.