[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.

The spatial pattern of recent ice reduction in the Arctic Ocean is similar to the distribution of warm Pacific Summer Water (PSW) that interflows the upper portion of halocline in the southern Canada Basin. Increases in PSW temperature in the basin are also well‐correlated with the onset of sea‐ice reduction that began in the late 1990s. However, increases in PSW temperature in the basin do not correlate with the temperature of upstream source water in the northeastern Bering Sea, suggesting that there is another mechanism which controls these concurrent changes in ice cover and upper ocean temperature. We propose a feedback mechanism whereby the delayed sea‐ice formation in early winter, which began in 1997/1998, reduced internal ice stresses and thus allowed a more efficient coupling of anticyclonic wind forcing to the upper ocean. This, in turn, increased the flux of warm PSW into the basin and caused the catastrophic changes.

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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.

[1] The 1990-1991 influx of Atlantic water, both anomalously warm and in greater volume than in the past, enveloped the Chukchi Borderland in the western Canada Basin by 2002 and spread across the southeastern Canada Basin by 2007. Warmer, younger (more ventilated), and less dense Fram Strait Branch waters have replaced colder, older, and denser waters, increasing the temperature of the Fram Strait Branch core from a 50-year or more mean of $0.45°C to $0.7°C. Physical and geochemical data collected from 1993 to 2007 show that the two main transport mechanisms are the boundary current and thermohaline intrusions, established by large thermal gradients. The boundary current operates in a cyclonic direction whereas the thermohaline intrusions operate in an anticyclonic direction because of the influence of the Beaufort Gyre. This shows that the Beaufort Gyre's effect on ocean circulation extends into the Fram Strait Branch of the Atlantic layer. The boundary current, a fully pan-Arctic structure, is much slower in the Canada Basin than in basins upstream, with an effective speed of $0.5 cm/s. The effective spreading rate of the thermohaline intrusions, relative to the core, is 0.2 cm/s. Thermohaline intrusions show signs of dissipation near the Northwind Ridge in 2007 suggesting that as temperature gradients between inflowing and resident waters decrease, they will disappear from the Canada Basin.