Hydrographic data collected from research cruises, bottom-anchored moorings, driftingIce-Tethered Profilers, and satellite altimetry in the Beaufort Gyre region of the Arctic Ocean document an increase of more than 6,400 km 3 of liquid freshwater content from 2003 to 2018: a 40% growth relative to the climatology of the 1970s. This fresh water accumulation is shown to result from persistent anticyclonic atmospheric wind forcing accompanied by sea ice melt, a wind-forced redirection of Mackenzie River discharge from predominantly eastward to westward flow, and a contribution of low salinity waters of Pacific Ocean origin via Bering Strait. Despite significant uncertainties in the different observations, this study has demonstrated the synergistic value of having multiple diverse datasets to obtain a more comprehensive understanding of Beaufort Gyre freshwater content variability. For example, Beaufort Gyre Observational System (BGOS) surveys clearly show the interannual increase in freshwater content, but without satellite or Ice-Tethered Profiler measurements, it is not possible to resolve the seasonal cycle of freshwater content, which in fact is larger than the year-to-year variability, or the more subtle interannual variations. Plain Language AbstractThe Beaufort Gyre centered in the Canada Basin of the Arctic Ocean is the major reservoir of fresh water in the Arctic. The primary focus of this study is on quantifying variability and trends in liquid (water) and solid (sea ice) freshwater content in this region. The Beaufort Gyre Exploration Program was initiated in 2003 to synthesize results of historical data analysis, design and conduct long-term observations, and to provide information for numerical modeling under the umbrella of the FAMOS (Forum for Arctic Observing and Modeling Synthesis) project. The data collected from research cruises, moorings, Ice-Tethered Profiler observations, and satellite altimetry document an increase of more than 6,400 km 3 of liquid freshwater content from 2003 to 2018, a 40% growth relative to the climatology of the 1970s. This fresh water volume is comparable to the fresh water volume released to the sub-arctic seas during the Great Salinity Anomaly episode of the 1970s. Thus, since the 2000s, the stage has been set for another possible release of fresh water to lower latitudes with accompanying climate impacts, including changes to sea ice conditions, ocean circulation, and ecosystems of the Sub-Arctic similar to the influence of the Great Salinity Anomaly observed in the 1970s.

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The warming (solar insolation) and freshening (sea ice melting and riverine water inputs) of the Arctic Ocean during summer increase stratification and suppress the upward mixing of nutrients into the euphotic zone (Codispoti et al., 2013). However, sea ice is now thinner and less compact (Kwok, 2018; Perovich et al., 2020); thus, the Arctic Ocean is more responsive to wind stress (Kwok et al., 2013), which enhances the nutrient supply (Bluhm et al., 2015). Shelf-break upwelling is becoming more prominent in the Arctic as the sea ice edge retreats poleward with ongoing climate change, exposing the shelf break to more direct wind forcing (Arrigo et al., 2014; Carmack & Chapman, 2003; Tremblay et al., 2011). Recently, Lewis et al. (2020) reported that annual net primary production (NPP) increased by 57% over the Arctic Ocean between 1998 and 2018. They also found that increased chlorophyll-a (Chl-a) was responsible for the sustained increase in annual NPP between 2009 and 2018, particularly along the interior shelf break. These results suggest that additional nutrients were supplied from increased vertical mixing near the shelf break into the nutrient-depleted upper euphotic zone (Arrigo & van Dijken, 2015; Lewis et al., 2020) and that the changes in ocean circulation in response to recent sea ice loss and increased wind mixing could significantly influence biological production (Ardyna & Arrigo, 2020). The Arctic Ocean is experiencing radical modifications in its hydrographic properties and in its overall circulation (Ardyna & Arrigo, 2020). For example, Polyakov et al. (2017) reported that the recent increase in Abstract Atlantic-origin cold saline water has previously not been considered an important contributor to the nutrient supply in the Pacific Arctic due to the effective insulation by the overlying Pacific-origin waters that separate the surface mixed layer from the deeper Atlantic Water. Based on hydrographic observations in the northwestern Chukchi Sea from 2015 to 2017, we demonstrate that the intrusion of Atlantic-origin cold saline water into the halocline boundary between Pacific and Atlanticorigin waters in 2017 lifted Pacific-origin nutrients up to the surface layer. We find that the cyclonic atmospheric circulation in 2017 was considerably strengthened, leading to lateral intrusions of two bodies of cold halocline water from the Eurasian marginal seas into the northwestern Chukchi Sea. Our results reveal that the intrusions of cold halocline waters caused unprecedented shoaling of the nutricline and anomalously high surface phytoplankton blooms in typically highly oligotrophic surface waters in the region during summer. Plain Language Summary Nutrient depletion, especially nitrogen, in Arctic surface waters during the summer is common due to biological uptake and intense stratification caused by sea ice melting and riverine water inputs, which restricts the upward mixing of nutrients into the euphotic zone. Although Atlantic-origin cold saline water has previously not been considered an important ...

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