Basin have increased winter ventilation in the ocean interior, making this region 46 structurally similar to that of the western Eurasian Basin. The associated enhanced 47 release of oceanic heat has reduced winter sea-ice formation at a rate now comparable to 48 losses from atmospheric thermodynamic forcing, thus explaining the recent reduction in 49 sea-ice cover in the eastern Eurasian Basin. This encroaching "atlantification" of the 50Eurasian Basin represents an essential step toward a new Arctic climate state, with a 51 substantially greater role for Atlantic inflows. 52 53 3 Over the last decade, the Arctic Ocean has experienced dramatic losses of sea-ice loss in 54 the summers, with record-breaking years in 2007 and 2012 for both the Amerasian Basin 55 and the Eurasian Basin (EB). More remarkably, the eastern EB has been nearly ice-free 56 (<10 % ice coverage) at the end of summer since 2011 (Fig. 1). Most sea ice-mass loss 57 results from summer solar heating of the surface mixed layer (SML) through cracks in the 58 ice and open water, and consequent melting of the lower surface of the ice (1-3). Heat 59 advected into the EB interior by Atlantic water (AW) generally has not been considered 60 an important contributor to sea-ice reduction, due to effective insulation of the overlying 61 cold halocline layer (CHL) (4) that separates the cold and fresh SML and pack ice from 62 heat carried by the warm and saline AW. 63There are, however, reasons to believe the role of AW heat in sea-ice reduction is not 64 negligible, and may be increasingly important (5). Nansen (6) warming has slowed slightly since 2008 (Fig. 2c). 74Strong stratification, which is found in most of the Arctic Ocean, prevents vigorous 75 ventilation of the AW. One notable exception is the western Nansen Basin, north and 76 4 northeast of Svalbard, where proximity to the sources of inflowing AW makes possible 77 significant interactions between the SML and the ocean interior (5). Specifically, weakly 78 stratified AW entering the Nansen Basin through Fram Strait is subject to direct 79 ventilation in winter, caused by cooling and haline convection associated with sea ice 80 formation (15). This ventilation leads to the reduction of sea-ice thickness along the 81 continental slope off Svalbard (16, 17). In the past, these conditions have been limited to 82 the western EB, since winter ventilation of AW in the eastern EB was constrained by 83 stronger stratification there. However, newly acquired data show that conditions 84 previously only identified in the western Nansen Basin now can be observed in the 85 eastern EB as well. We call this eastward progression of the western EB conditions the 86 "atlantification" of the EB of the Arctic Ocean. 87 Overview of sea ice state 88The progressive decline in sea ice coverage of the Arctic Ocean during the satellite era, at 89 13.4 % per decade during September (18), has been accompanied by decreases in average 90 sea ice thickness of at least 1.7 m in the central Arctic (19, 20). In the region of t...

Borealization of the Arctic Ocean future suggests that Arctic borealization will continue under scenarios of global warming. Results from this synthesis further our understanding of the Arctic Ocean's complex and sometimes non-intuitive Arctic response to climate forcing by identifying new feedbacks in the atmosphere-ice-ocean system in which borealization plays a key role.

The dramatic reduction in minimum Arctic sea ice extent in recent years has been accompanied by surprising changes in the thermohaline structure of the Arctic Ocean, with potentially important impact on convection in the North Atlantic and the meridional overturning circulation of the world ocean. Extensive aerial hydrographic surveys carried out in March–April, 2008, indicate major shifts in the amount and distribution of fresh‐water content (FWC) when compared with winter climatological values, including substantial freshening on the Pacific side of the Lomonosov Ridge. Measurements in the Canada and Makarov Basins suggest that total FWC there has increased by as much as 8,500 cubic kilometers in the area surveyed, effecting significant changes in the sea‐surface dynamic topography, with an increase of about 75% in steric level difference from the Canada to Eurasian Basins, and a major shift in both surface geostrophic currents and freshwater transport in the Beaufort Gyre.