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.

A 15-year duration record of mooring observations from the eastern (&gt;70°E) Eurasian Basin (EB) of the Arctic Ocean is used to show and quantify the recently increased oceanic heat flux from intermediate-depth (∼150-900 m) warm Atlantic Water (AW) to the surface mixed layer (SML) and sea ice. The upward release of AW heat is regulated by the stability of the overlying halocline, which we show has weakened substantially in recent years. Shoaling of the AW has also contributed, with observations in winter 2017-2018 showing AW at only 80 m depth, just below the wintertime surface mixed layer (SML), the shallowest in our mooring records. The weakening of the halocline for several months at this time implies that AW heat was linked to winter convection associated with brine rejection during sea ice formation. This resulted in a substantial increase of upward oceanic heat flux during the winter season, from an average of 3-4 W/m2 in 2007-2008 to &gt;10 W/m2 in 2016-2018. This seasonal AW heat loss in the eastern EB is equivalent to a more than a two-fold reduction of winter ice growth. These changes imply a positive feedback as reduced sea ice cover permits increased mixing, augmenting the summer-dominated ice-albedo feedback.

This analysis evaluates the thermal state of the intermediate (depth range of 150-900 m) Atlantic Water (AW) of the Arctic Ocean, beginning in the 1950s and with particular focus on the transition from the 1990s to the 2000s and on changes during the 2000s. Using an extensive array of observations, the authors document AW warming trends across various time scales and demonstrate that the 2000s were exceptionally warm, with no analogy since the 1950s or probably in the history of instrumental observations in the Arctic Ocean. Warming in the recent decade was dominated by a warm AW pulse in addition to the underlying trend. Since 1997, the Canadian Basin experienced a faster warming rate compared with the Eurasian Basin. The relative role of the AW warmth in setting the net energy flux and mass balance of the Arctic sea ice is still under debate. Additional carefully orchestrated field experiments are required in order to address this question of ongoing Arctic climate change.

In this study, we propose a new Arctic climate change indicator based on the strength of the Arctic halocline, a porous barrier between the cold and fresh upper ocean and ice and the warm intermediate Atlantic Water of the Arctic Ocean. This indicator provides a measure of the vulnerability of sea ice to upward heat fluxes from the ocean interior, as well as the efficiency of mixing affecting carbon and nutrient exchanges. It utilizes the well-accepted calculation of available potential energy (APE), which integrates anomalies of potential density from the surface downwards through the surface mixed layer to the base of the halocline. Regional APE contrasts are striking and show a strengthening of stratification in the Amerasian Basin (AB) and an overall weakening in the Eurasian Basin (EB). In contrast, Arctic-wide time series of APE is not reflective of these inter-basin contrasts. The use of two time series of APE-AB and EB-as an indicator of Arctic Ocean climate change provides a powerful tool for detecting and monitoring transition of the Arctic Ocean towards a seasonally ice-free Arctic Ocean. This new, straightforward climate indicator can be used to inform both the scientific community and the broader public about changes occurring in the Arctic Ocean interior and their potential impacts on the state of the ice cover, the productivity of marine ecosystems and mid-latitude weather.