Milestone: MS8 Work package in charge: WP2: Lower latitude drivers of Arctic changes Actual achievement date of this milestone: Project month 29 Partner organisation in charge of the milestone and lead authors: Centre National de la Recherche Scientifique (CNRS): Christophe Herbaut, Marie-Noelle Houssais, Anne-Cécile Blaizot Reviewers: Faroe Marine Research Institute (HAV): Karin Margretha H. Larsen, National University of Ireland Maynooth (NUIM): Gerard McCarthy Milestone Type: Report Dissemination level: Public Means of verification of attainment of the milestone: Identification of robust metrics for the mean Atlantic Water temperature, the Atlantic Water section area, the Atlantic Water heat content and the heat and volume transports. Achieved: Yes Abstract: The decline of Arctic sea ice in the recent decades is regionally dependent, with an increasing trend since the mid 2000s in the Eurasian Basin (Close et al., 2015). The increase of the oceanic heat flux has been suggested to be the main driver of this ice loss in the eastern Eurasian Basin, with an estimated contribution of 40‐50 cm in ice growth reduction in the recent years (Polyakov et al., 2017). According to these authors, the decrease of the stability in the halocline and the shoaling of the Atlantic Water (AW) interface, often referred to as the Atlantification of the Eurasian Basin, altogether have provided favorable conditions for increased ocean heat transfer towards the surface. The source of these changes should be found in the region north of Svalbard where the warm AW inflowing through the Fram Strait first encounters the sea ice, being responsible for increased sea ice melt in the recent period. Our goal in the Blue-Action project is to investigate how the heat provided by the Atlantic inflow through Fram Strait is subsequently modified along its pathway in the southern Eurasian Basin and to which extent its distribution controls the amount of heat which is transferred to the surface layer and ultimately to the sea ice. The analysis which is presented in this milestone focuses on the region around Svalbard to evaluate the upstream conditions at the entrance of the Arctic Ocean. It relies on results of a model simulation carried out using a 1/24° ocean‐sea ice model. The results have been compared to in situ observations (collected from moorings and shipborne CTD (Conductivity‐Temperature‐Depth)) in order to identify the metrics which are the more relevant to characterize the Atlantic water layer. The considered metrics are related to the heat content and transports of AW. They are used to assess the model ability to represent the observations.

The Blue-Action project has received funding from the European Union's Horizon 2020 Research and Innovation Programme under Grant Agreement No 727852 www.blue-action.eu