Mechanisms Underlying Recent Arctic Atlantification

Recent warming and reduced sea ice concentrations in the Atlantic sector of the Arctic Ocean are the main signatures of ongoing Arctic “Atlantification.” The mechanisms driving the warming trends are nevertheless still debated, particularly regarding the relative importance of oceanic and atmospheric heat fluxes. Here, heat budgets along main Atlantic water pathways through the Barents Sea and Fram Strait are constructed to investigate the mechanisms of Atlantification during 1993–2014. The largest warming trends occur south of the winter ice edge, with ocean advection as the main driver. Warming in the marginal ice zone is mainly due to low surface heat loss from the 1990s to the mid‐2000s. In the ice‐covered northwestern Barents Sea, ocean advection and air‐sea heat fluxes act in concert to drive a gradual warming of the upper ocean. Despite a weakened stratification, no evidence is found of vertical oceanic temperature fluxes driving this upper‐ocean warming. Plain Language Summary Recent “Atlantification” of the Arctic is characterized by warmer ocean temperatures and a reduced sea ice cover. The Barents Sea is a “hot spot” for these changes, something which has broad socioeconomic and environmental impacts in the region. However, there is, at present, no complete understanding of what is causing the ocean warming. Here, we determine the relative importance of transport of heat by ocean currents (ocean advection) and heat exchanges between the atmosphere and the ocean (air‐sea heat fluxes) in warming the Barents Sea and Fram Strait. In the ice‐free region, ocean advection is found to be the main driver of the warming trend due to increasing inflow temperatures between 1996 and 2006. In the marginal ice zone and the ice‐covered northern Barents Sea, ocean advection and air‐sea heat fluxes are found to be of interchanging importance in driving the warming trend through the 1993–2014 period analyzed. A better understanding of the recent warming trends in the Barents Sea and Fram Strait has implications for how we understand the ocean's role in ongoing and future Arctic climate change. Key Points Atlantification of the Barents Sea and Fram Strait is documented using the ECCOv4‐r3 ocean state estimate Mechanisms driving the warming trends along Atlantic water pathways are regionally dependent and not stationary in time Stratification is weakened in the northern Barents Sea, but increased vertical temperature fluxes are not driving the upper‐ocean warming