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Abstract Nutrient‐rich, Pacific‐origin winter water (PWW) is a primary source of nitrate for the western Arctic Ocean, spurring primary production at the base of the food web. This cold water exits the Chukchi Sea shelf via Herald Canyon and Barrow Canyon, which feed the eastward‐flowing Chukchi Shelfbreak Jet (CSJ) and the westward‐flowing Chukchi Slope Current, respectively. Using historical measurements of nitrate, hydrography, and velocity between 2002 and 2023, we characterize the spatial distribution of nitrate along the northern edge of the Chukchi shelf. Our analysis reveals that the nitrate concentration of PWW increases significantly to the west. Documenting the sources of nitrate in Herald Canyon and Barrow Canyon, and following its downstream evolution in the two currents, we demonstrate that the westward increase in nitrate is primarily associated with the CSJ. Our results shed light on the physical basis for seasonal phytoplankton growth and carbon sequestration in the region. 

Abstract Newly ventilated winter water (NVWW) is a cold, salty, nutrient‐rich water mass that is critical for supporting the ecosystem of the western Arctic Ocean and for ventilating the halocline in the Canada Basin. While the formation of NVWW is well‐documented on the Chukchi shelf, there remain fundamental questions regarding its formation on the western Beaufort shelf. In this study, we use hydrographic data from two late‐fall cruises in 2018 and 2022 to investigate the roles of sea ice production and wind‐driven upwelling in the formation of NVWW and the implications for the nutrient content of the water. For each of the shipboard transects, we apply proxies for the extent of the winter water formation and the strength of the associated upwelling, respectively. It is demonstrated that the NVWW attains higher levels of nitrate due to two factors: (a) more active formation of the water associated with enhanced sea ice production and (b) more extensive upwelling of water high in nutrients from the basin to the shelf following an easterly wind event. The latter process would be less common on the wide Chukchi shelf. These findings have significant implications for the regional primary production. 

Abstract The Beaufort Shelf has historically been reported to exhibit limited polynya activity in winter. Yet, recent satellite observations show episodic recurrence of a large polynya west of Mackenzie Canyon, a site of significant shelf‐basin exchange. Here, we investigate satellite‐detected occurrences of this polynya over winters 2003–2025, including their relation to regional winds, ice drift, and ocean conditions. The polynya is observed to open when easterly winds drive rapid ice drift over the shelf, mechanically opening the ice near Qikiqtaruk (Herschel Island). Under strong and persistent forcing, open water extends northwestward, sometimes occupying large portions of the shelf. Its comparison to a 1‐D coastal polynya model suggests that this observed polynya growth could reflect contributions from ocean heating. Fluxes of interior ocean heat to the shelf are confirmed across two winters of mooring observations, which revealed coincident upwelling along the western flank of Mackenzie Canyon as polynyas formed. Warm upwelled waters were advected by a strong shelf current directed along the axis of polynya extension. Transported heat could suppress an estimated of daily ice growth over the shelf, comparable to that otherwise expected from the estimated surface heat losses. Recent years have featured several extreme polynyas, some exceeding 400 km in length. These events are rare and occur under exceptional wind forcing. However, increased ice drift speeds in the last decade coincide with more frequent and extensive openings, suggesting that large polynyas may be becoming a more prominent feature over the shelf as the mobility of the winter ice cover increases. 

Abstract Over the last two decades, ocean warming and rapid loss of sea ice have dramatically changed the Pacific Arctic marine environment^1–3. These changes are predicted to increase harmful algal bloom prevalence and toxicity, as rising temperatures and larger open water areas are more favourable for growth of some toxic algal species⁴. It is well known that algal toxins are transferred through food webs during blooms and can have negative impacts on wildlife and human health^5–7. Yet, there are no long-term quantitative reports on algal toxin presence in Arctic food webs to evaluate increasing exposure risks. In the present study, algal toxins were quantified in bowel samples collected from 205 bowhead whales harvested for subsistence purposes over 19 years. These filter-feeding whales served as integrated food web samplers for algal toxin presence in the Beaufort Sea as it relates to changing environmental conditions over two decades. Algal toxin prevalences and concentrations were significantly correlated with ocean heat flux, open water area, wind velocity and atmospheric pressure. These results provide confirmative oceanic, atmospheric and biological evidence for increasing algal toxin concentrations in Arctic food webs due to warming ocean conditions. This approach elucidates breakthrough mechanistic connections between warming oceans and increasing algal toxin exposure risks to Arctic wildlife, which threatens food security for Native Alaskan communities that have been reliant on marine resources for subsistence for 5,000 years (ref.⁸). 

The Arctic Ocean has experienced significant sea ice loss over recent decades, shifting towards a thinner and more mobile seasonal ice regime. However, the impacts of these transformations on the upper ocean dynamics of the biologically productive Pacific Arctic continental shelves remain underexplored. Here, we quantified the summer upper mixed layer depth and analyzed its interannual to decadal evolution with sea ice and atmospheric forcing, using hydrographic observations and model reanalysis from 1996 to 2021. Before 2006, a shoaling summer mixed layer was associated with sea ice loss and surface warming. After 2007, however, the upper mixed layer reversed to a generally deepening trend due to markedly lengthened open water duration, enhanced wind-induced mixing, and reduced ice meltwater input. Our findings reveal a shift in the primary drivers of upper ocean dynamics, with surface buoyancy flux dominant initially, followed by a shift to wind forcing despite continued sea ice decline. These changes in upper ocean structure and forcing mechanisms may have substantial implications for the marine ecosystem, potentially contributing to unusual fall phytoplankton blooms and intensified ocean acidification observed in the past decade 

Abstract In recent years, blooms of the neurotoxic dinoflagellateAlexandrium catenellahave been documented in Pacific Arctic waters, and the paralytic shellfish toxins (PSTs) that this species produces have been detected throughout the food web. These observations have raised significant concerns about the role that harmful algal blooms (HABs) will play in a rapidly changing Arctic. During a research cruise in summer 2022, a massive bloom ofA. catenellawas detected in real time as it was advected through the Bering Strait region. The bloom was exceptional in both spatial scale and density, extending > 600 km latitudinally, reaching concentrations > 174,000 cells L⁻¹, and producing high‐potency PST congeners. Throughout the event, coastal stakeholders in the region were engaged and a multi‐faceted community response was mobilized. This unprecedented bloom highlighted the urgent need for response capabilities to ensure safe utilization of critical marine resources in a region that has little experience with HABs. 

Abstract A mooring array has been maintained across the West Greenland shelf and slope since 2014 as part of the Overturning in the Subpolar North Atlantic Program (OSNAP). Here, we use the first 8 years of data to investigate the interannual variability of the two overflow water components of the deep western boundary current (DWBC): the Denmark Strait Overflow Water (DSOW) and the Northeast Atlantic Deep Water (NEADW). While the velocity structure has remained similar throughout the record, both water masses have freshened considerably, especially the NEADW salinity core. Using revised density criteria to define these two components, their transports decreased significantly between 2014 and 2022: from 6.2 to 3.8 Sv (1 Sv ≡ 10⁶m³s⁻¹) (−0.33 Sv yr⁻¹) for the DSOW and from 5.4 to 4.1 Sv (−0.19 Sv yr⁻¹) for the NEADW. Since the overflows across the Denmark Strait and the Faroe Bank Channel have remained steady over this period, this points to decreased entrainment downstream of the sills as a possible mechanism for the observed transport reduction south of Greenland. Using shipboard and mooring data from the two sills, and a hydrographic database for the surrounding region, we predict the downstream transport of the two DWBC components via the framework of a streamtube model. The predicted transport explains 94% of the observed DSOW trend and 63% of the observed NEADW trend. This implies that further entrainment of the NEADW must occur during its long pathlength, which would also help explain the fresher-than-predicted NEADW salinity observed at the OSNAP array. 

Abstract Barrow Canyon in the northeast Chukchi Sea is a critical choke point where Pacific‐origin water, heat, and nutrients enter the interior Arctic. While the flow through the canyon has been monitored for more than 20 years, questions remain regarding the dynamics by which the Pacific‐origin water is fluxed offshore, as well as what drives the variability. In 2018, two high‐resolution shipboard surveys of the canyon were carried out—one in summer and one in fall—to investigate the water mass distribution and velocity structure of the outflow. During the summer survey, high percentages of Pacific water (summer water + winter water) were present seaward of the canyon, associated with strong northward outflow from the canyon and a well‐developed westward‐flowing Chukchi Slope Current (CSC). By contrast, high percentages of Pacific water were confined to the canyon proper and outer Chukchi shelf during the late‐fall survey, at which time the canyon outflow and CSC were considerably weaker. These differences can be attributed to differences in wind forcing during the time period of two surveys. A cyclone‐like circulation was present in the canyon during both surveys, which was also evident in the satellite‐derived sea surface height anomaly field. We argue that this feature corresponds to an arrested topographic Rossby wave, generated as the outflow responds to the deepening bathymetry of the canyon. By applying a self‐organizing map analysis using the satellite altimeter data from 2001 to 2020, we demonstrate that such a cyclone‐like structure is a prevailing aspect of the canyon outflow.