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Arctic sea ice threatened by formidable “thermal bombs”

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Over the past decade, the retreat of the Arctic sea ice has accelerated considerably. Especially between late spring and early fall. A rhythm that climate models are struggling to transcribe, which suggests that some of the processes at work are not – or are poorly – represented in the simulations …

Following a campaign of very detailed oceanographic observations, researchers believe they have identified one of these mechanisms. Specifically, the measurements revealed how the warm, salty waters of the North Pacific entered the Arctic Basin and caused sea ice to melt below.

Sea ice weakened by basal melt

Indeed, while infiltrating through the bering strait, these dense waters plunge under the cold and slightly salty layer of the surface then isolate themselves in formidable whirlpools that scientists have named thermal bombs. During several months, they thus drift towards the interior of the basin in continuously diffusing their heat towards the surface layers where the ice floes float.

Note that these plumes occur mainly during the summer. One of them was directly observed in September 2018 by the group of researchers as part of the scientific project A SODA (Stratified Oceanic Dynamics of the Arctic) funded byNaval Research Office. A first which allowed a real leap forward in the understanding of the phenomenon.

Vertical section through the plume observed in September 2018. South is to the left and north to the right. The colored field represents the temperature. Also, note the intrusion of hot water through the Bering Strait. Credits: Jennifer A. MacKinnon et al. 2021.

“The rate of acceleration of sea ice melt in the Arctic has been difficult to predict accurately, in part because of all the local and complex feedbacks between ice, ocean and atmosphere,” notes Jennifer MacKinnon, lead author of the paper. “ This work highlights the important role that ocean water plays in warming, as part of these feedbacks. “.

The analysis of the exchanges between the two basins showed that the intrusions of peaceful plumes were more important than one thought. They would also have intensified over the last decade, partly explaining why the melting sea ice accelerated by the Pacific flank during the summer. In addition, as this mechanism was until then little understood and that it involves processes of small scales badly resolved numerically, it is not surprising to note that the models have a difficulty to account for the real evolutions.

Same as before, but for salinity. A high value corresponds to salt water. Here too, we can clearly see the signature of the peaceful plume. Credits: Jennifer A. MacKinnon et al. 2021.

A poorly understood impact on the Arctic ecosystem

In addition to the high heat and salt content of the eddies, the measurements reported unique concentrations of organic matter as well as chemical elements. A growing contribution which necessarily contributes to changing the functioning of the arctic ecosystem. However, the way in which these alterations develop and express themselves remains to this day poorly understood.

It was a privilege for us to collaborate with our American colleagues to collect the biogeochemical measurements made during this field experiment. », Reports Yueng-Djern Lenn, one of the study’s many co-authors. “ These nutrient and isotope data that we collected were useful in tracing the origin of the plume. (…) ”. The results published on April 23 appear in the scientific journal Communications of nature.

The next step will be the development of adapted digital representations, in order to include these important mechanisms in future climate models. ” A better understanding and better modeling ability of the processes described here will help predict the detailed geography as well as the timing of ecosystem evolution and the accelerated retreat of sea ice in the Arctic. Says the paper in its conclusion.

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