Satellite records show a decline in ice extent over more than three decades, with a record minimum in September 2012. Results from the Pan‐Arctic Ice‐Ocean Modelling and Assimilation system (PIOMAS) suggest that the decline in extent has been accompanied by a decline in volume, but this has not been confirmed by data. Using new data from the European Space Agency CryoSat‐2 (CS‐2) mission, validated with in situ data, we generate estimates of ice volume for the winters of 2010/11 and 2011/12. We compare these data with current estimates from PIOMAS and earlier (2003–8) estimates from the National Aeronautics and Space Administration ICESat mission. Between the ICESat and CryoSat‐2 periods, the autumn volume declined by 4291 km3 and the winter volume by 1479 km3. This exceeds the decline in ice volume in the central Arctic from the PIOMAS model of 2644 km3 in the autumn, but is less than the 2091 km3 in winter, between the two time periods.

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...

[1] Particle fluxes measured with time series sediment traps deployed below 2000 m at 68 sites in the world ocean are combined with satellite-derived estimates of export production from the overlying water to assess the factors affecting the transfer of particulate organic matter from surface to deep water. Multiple linear regression is used to derive an algorithm suggesting that the transfer efficiency of organic carbon, defined as the settling flux of organic carbon normalized to export production, increases with the flux of carbonate and decreases with water depth and seasonality. The algorithm predicts >80% of the organic carbon transfer efficiency variability in diverse oceanic regions. The influence of the carbonate flux suggests that the ballasting effect of this biogenic mineral may be an important factor promoting export of organic carbon to the deep sea by increasing the density of settling particles. However, the lack of a similar effect for biogenic opal suggests that factors other than particle density also play a role. The adverse effect of increasing seasonality on the transfer efficiency of carbon to the deep sea is tentatively attributed to greater biodegradability of organic matter exported during bloom events. In high latitude opal-dominated regions with high f-ratios and seasonality, while a higher fraction of net production is exported, a higher fraction of the exported organic matter is remineralized before reaching bathypelagic depths. On the other hand, in warm, low latitude, carbonate-dominated regions with low f-ratios and seasonality, a higher fraction of the exported organic matter sinks to the deep sea.

[1] We investigate basin-scale mechanisms regulating anomalies in freshwater content (FWC) in the Beaufort Gyre (BG) of the Arctic Ocean using historical observations and data collected in [2003][2004][2005][2006][2007]. Specifically, the mean annual cycle and interannual and decadal FWC variability are explored. The major cause of the large FWC in the BG is the process of Ekman pumping (EP) due to the Arctic High anticyclonic circulation centered in the BG. The mean seasonal cycle of liquid FWC is a result of interplay between the mechanical (EP) and thermal (ice transformations) factors and has two peaks. One peak occurs around June-July when the sea ice thickness reaches its minimum (maximum ice melt). The second maximum is observed in November-January when wind curl is strongest (maximum EP) and the salt input from the growing ice has not yet reached its maximum. Interannual changes in FWC during [2003][2004][2005][2006][2007] are characterized by a strong positive trend in the region varying by location with a maximum of approximately 170 cm a À1 in the center of EP influenced region. Decadal FWC variability in the period 1950-2000 is dominated by a significant change in the 1990s forced by an atmospheric circulation regime change. The center of maximum FWC shifted to the southeast and appeared to contract in area relative to the pre-1990s climatology. In spite of the areal reduction, the spatially integrated FWC increased by over 1000 km 3 relative to climatology.

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