Barrow Canyon Research Articles

Assessing the importance of terrestrial organic carbon in the CHUKCHI and Beaufort seas

The western Arctic Ocean is a unique part of the world's oceans, and is an area undergoing significant climate changes. The effects and possible climate feedbacks of these changes are not yet fully understood. Decreased sea ice cover could lead to increased productivity in the water column, or a reduction in total productivity through the loss of sea ice algal contributions. Climate change could also alter the patterns of terrestrial inputs from rivers and coastal erosion.In order to better understand the balance of organic material inputs in the western Arctic Ocean, this report presents carbon and nitrogen elemental and isotopic compositions from 19 surface sediment samples and 7 cores from 4 shelf to basin transects as evidence of the sources of organic matter buried in the sediments of the Chukchi and Alaskan Beaufort Seas. The C:N ratios for surface sediments were between 5.3 and 11.5 (mean 9.0 ± 1.3). The mean %TOC was 1.2 ± 0.3% and mean %TN was 0.16 ± 0.02%. The highest %TOC contents were observed in the Barrow Canyon transect, likely reflecting heightened overlying productivity. The δ13C of this preserved material varied from −22.1 to −16.7‰ (mean −19.4 ± 1.3‰). A trend to depletion in 13C and higher C:N in eastern sites was observed. Among the surface samples, δ15N varied from 4.1 to 7.6‰ (mean 5.7 ± 1.1‰).An evaluation of the inputs of organic matter to the western Arctic sediment was accomplished using δ13C and δ15N values with the novel technique of a Bayesian analysis multi-source mixing model. This model was used to estimate proportional contributions of sea ice algae, water column phytoplankton and terrestrial organic matter. We conclude that water column productivity is the source of between 50 and 70% of the organic carbon buried in this portion of the western Arctic. The remaining 25–35% of carbon is mainly supplied by sea ice algal productivity, with at most 15% of sedimentary carbon derived from terrestrial inputs.

The influence of winter water on phytoplankton blooms in the Chukchi Sea

The flow of nutrient-rich winter water (WW) through the Chukchi Sea plays an important and previously uncharacterized role in sustaining summer phytoplankton blooms. Using hydrographic and biogeochemical data collected as part of the ICESCAPE program (June–July 2010–11), we examined phytoplankton bloom dynamics in relation to the distribution and circulation of WW (defined as water with potential temperature ≤−1.6°C) across the Chukchi shelf. Characterized by high concentrations of nitrate (mean: 12.3±5.13µmolL−1) that typically limits primary production in this region, WW was correlated with extremely high phytoplankton biomass, with mean chlorophyll a concentrations that were 3-fold higher in WW (8.64±9.75µgL−1) than in adjacent warmer water (2.79±5.58µgL−1). Maximum chlorophyll a concentrations (~30µgL−1) were typically positioned at the interface between nutrient-rich WW and shallower, warmer water with more light availability. Comparing satellite-based calculations of open water duration to phytoplankton biomass, nutrient concentrations, and oxygen saturation revealed widespread evidence of under-ice blooms prior to our sampling, with biogeochemical properties indicating that blooms had already terminated in many places where WW was no longer present. Our results suggest that summer phytoplankton blooms are sustained for a longer duration along the pathways of nutrient-rich WW and that biological hotspots in this region (e.g. the mouth of Barrow Canyon) are largely driven by the flow and confluence of these extremely productive pathways of WW that flow across the Chukchi shelf.

Northeasternmost record of a North Pacific fin whale (Balaenoptera physalus) in the Alaskan Chukchi Sea

Fin whales (Balaenoptera physalus) in the North Pacific consistently range as far north as the Bering Sea. While occasional sightings of fin whales occur north of the Bering Strait, they are rarely documented north of 67°N. However, a recent increase in passive acoustic monitoring has shown that fin whales are more prevalent in the Chukchi Sea than previously thought. During a 2012 field survey in the Alaskan Chukchi Sea, fin whale calls were detected on a DiFAR sonobuoy deployed at 71.575°N, 157.823°W, 50 km off Barrow near the mouth of Barrow Canyon. On August 27, 2012, approximately 30 fin whale downsweep calls were detected over an hour and a half, at bearings of approximately 270–290° from the sonobuoy. Since only one sonobuoy was deployed, in situ localization of the calls was not possible. Post hoc range estimates using a combination of modal dispersion techniques, nonlinear time-domain warping, and geoacoustic inversion resulted in a source range estimate of less than 5 km. This location is approximately 280 and 365 km northeast of the previous closest acoustic detection and confirmed visual sighting of a fin whale, respectively. These results represent the farthest northeast record of fin whale calls in the Alaskan Arctic and illustrate the importance of continued passive acoustic monitoring in a rapidly changing environment.

Seasonal spatial patterns in seabird and marine mammal distribution in the eastern Chukchi and western Beaufort seas: Identifying biologically important pelagic areas

The Chukchi and Beaufort seas are undergoing rapid climate change and increased human activity. Conservation efforts for upper trophic level predators such as seabirds and marine mammals require information on species’ distributions and identification of important marine areas. Here we describe broad-scale distributions of seabirds and marine mammals. We examined spatial patterns of relative abundance of seabirds and marine mammals in the eastern Chukchi and western Beaufort seas during summer (15 June–31 August) and fall (1 September–20 November) from 2007 to 2012. We summarized 49,206km of shipboard surveys for seabirds and 183,157km of aerial surveys for marine mammals into a grid of 40-km×40-km cells. We used Getis-Ord Gi∗ hotspot analysis to test for cells with higher relative abundance than expected when compared to all cells within the study area. We identified cells representing single species and taxonomic group hotspots, cells representing hotspots for multiple species, and cells representing hotspots for both seabirds and marine mammals. The locations of hotspots varied among species but often were located near underwater canyons or over continental shelf features and slopes. Hotspots for seabirds, walrus, and gray whales occurred primarily in the Chukchi Sea. Hotspots for bowhead whales and other pinnipeds (i.e., seals) occurred near Barrow Canyon and along the Beaufort Sea shelf and slope. Hotspots for belugas occurred in both the Chukchi and Beaufort seas. There were three hotspots shared by both seabirds and marine mammals in summer: off Wainwright in the eastern Chukchi Sea, south of Hanna Shoal, and at the mouth of Barrow Canyon. In fall, the only identified shared hotspot occurred at the mouth of Barrow Canyon. Shared hotspots are characterized by strong fronts caused by upwelling and currents, and these areas can have high densities of euphausiids in summer and fall. Due to the high relative abundance of animals and diversity of taxa, these sites are clearly important areas of congregation for seabirds and marine mammals that should be prioritized in the development of management and conservation plans.

Water properties, heat and volume fluxes of Pacific water in Barrow Canyon during summer 2010

Over the past few decades, sea ice retreat during summer has been enhanced in the Pacific sector of the Arctic basin, likely due in part to increasing summertime heat flux of Pacific-origin water from the Bering Strait. Barrow Canyon, in the northeast Chukchi Sea, is a major conduit through which the Pacific-origin water enters the Arctic basin. This paper presents results from 6 repeat high-resolution shipboard hydrographic/velocity sections occupied across Barrow Canyon in summer 2010. The different Pacific water masses feeding the canyon – Alaskan coastal water (ACW), summer Bering Sea water (BSW), and Pacific winter water (PWW) – all displayed significant intra-seasonal variability. Net volume transports through the canyon were between 0.96 and 1.70Sv poleward, consisting of 0.41–0.98Sv of warm Pacific water (ACW and BSW) and 0.28–0.65Sv of PWW. The poleward heat flux also varied strongly, ranging from 8.56TW to 24.56TW, mainly due to the change in temperature of the warm Pacific water. Using supplemental mooring data from the core of the warm water, along with wind data from the Pt. Barrow weather station, we derive and assess a proxy for estimating heat flux in the canyon for the summer time period, which is when most of the heat passes northward towards the basin. The average heat flux for 2010 was estimated to be 3.34TW, which is as large as the previous record maximum in 2007. This amount of heat could melt 315,000km2 of 1-meter thick ice, which likely contributed to significant summer sea ice retreat in the Pacific sector of the Arctic Ocean.

Summertime circulation in the eastern Chukchi Sea

The transport of Pacific-origin water across the eastern Chukchi Sea during summer is studied using shipboard hydrographic and Acoustic Doppler Current Profiler data from multiple surveys. The study identifies two major transport pathways in this region. The well-known Alaskan Coastal Current (ACC) flows poleward along the Alaska coast, while a more recently discovered and slower current flows northward through the Central Channel between Herald and Hanna Shoals. The two currents separate past Point Hope and appear to merge again in Barrow Canyon via previously unresolved pathways flowing west to east across the Chukchi Sea. The collective flow that transports Pacific water from Bering Strait to Barrow Canyon is termed the Bering to Barrow Current System (BBCS). The coastal branch of the BBCS, which encompasses the ACC, is a weakly baroclinic flow with a maximum speed in excess of 80cm/s inside Barrow Canyon. The Central Channel branch of the BBCS is considerably weaker, with a maximum flow speed under 20cm/s. A portion of the Pacific water that flows towards Herald Canyon to the west may also contribute to the BBCS, the amount, but the amount is within the margin of error of our data. The difference in the advective time scales of the various branches allows water masses of different seasonal characteristics to be transported simultaneously by the BBCS. Summer water masses occupy most of the southern and central Chukchi Sea while winter water masses occupy Hanna Shoal and shelfbreak regions in the northern Chukchi Sea during summer. The total BBCS volume transport in summer is measured at four different locations in the eastern Chukchi Sea and found to be generally consistent. The overall mean transport is 0.86Sv, compared to the summer Bering Strait transport of 1.08Sv. This study suggests that the BBCS is the main transport pathway delivering Pacific water masses into the western Arctic Basin.

Vertical heat transfer based on direct microstructure measurements in the ice-free Pacific-side Arctic Ocean: the role and impact of the Pacific water intrusion

This study quantifies diapycnal mixing and vertical heat transfer in the Pacific side of the Arctic Ocean, where sea-ice cover has disappeared between July and September in the last few decades. We conducted microstructure measurements in the open water region around the Canada Basin from late summer to fall in 2009 and 2010 using R/V Mirai. In the study domain, the dissipation rate of turbulent kinetic energy, e, is typically as low level as O(10−10) W kg−1, resulting in vertical heat diffusivity of O(10−7) m2 s−1, which is close to the molecular diffusivity of heat, suggesting comparatively little predominance of mechanical turbulent mixing. An exception is the case at the Barrow Canyon, where the strong baroclinic throughflow generates substantial vertical mixing, producing e > O(10−7) W kg−1, because of the shear flow instability. Meanwhile, in the confluence region, where the warm/salty Pacific water and the cold/fresh Arctic basin water encounter, the micro-temperature profiles revealed a localized enhancement in vertical diffusivity of heat, reaching O(10−5) m2 s−1 or greater. In this region, an intrusion of warm Pacific water creates a horizontally interleaved structure, where the double-diffusive mixing facilitates vertical heat transfer between the intruding Pacific water and the surrounding basin waters.

Seasonal to interannual variability of the Pacific water boundary current in the Beaufort Sea

Between 2002 and 2011 a single mooring was maintained at the core of the Pacific water boundary current in the Beaufort Sea, approximately 150km east of Pt. Barrow, Alaska. Using velocity and hydrographic data from six year-long deployments, we examine the variability of the current on seasonal to interannual timescales. The seasonal signal is characterized by enhanced values of volume, heat, and freshwater transport during the summer months associated with the presence of two summertime Pacific water masses, Alaskan Coastal Water and Chukchi Summer Water. Strikingly, over the decade the volume transport of the current has decreased by more than 80%, with comparable reductions in the heat and freshwater transports, despite the fact that the flow through Bering Strait has increased over this time period. The largest changes in the boundary current have occurred in the summer months. Using atmospheric reanalysis fields and weather station data, we demonstrate that an increase in summer easterly winds along the Beaufort slope is the primary cause for the reduction in transport. The stronger winds are due to an intensification of the summer Beaufort High and deepening of the summer Aleutian Low. Using additional mooring and shipboard data in conjunction with satellite fields, we investigate the implications of the reduction in transport of the boundary current. We argue that a significant portion of the mass and heat passing through Bering Strait in recent years has been advected out of Barrow Canyon into the interior Canada Basin – rather than entering the boundary current in the Beaufort Sea – where it is responsible for a significant portion of the increased sea ice melt in the basin.

Spatial patterns in zooplankton communities and stable isotope ratios (δ13C and δ15N) in relation to oceanographic conditions in the sub-Arctic Pacific and western Arctic regions during the summer of 2008

We define the biogeographic status quo of zooplankton communities sampled during the summer of 2008 in the sub-Arctic Pacific and western Arctic regions and examine spatial patterns of stable isotopes in the context of regional oceanography. Eight zooplankton assemblages were identified and corresponded to these regions: Gulf of Alaska, Bering Sea Slope, St-Lawrence Island, Western Bering Strait, Eastern Bering Strait, Barrow Canyon, Beaufort Gyre and Beaufort Shelf. Neocalanus spp., Eucalanus bungii and Metridia pacifica were abundant in the warmer and saltier waters of the Gulf of Alaska and on the Bering Sea Slope, whereas Calanus hyperboreus, Calanus glacialis and Metridia longa were abundant in the cold and fresher Arctic waters on the Beaufort Shelf and in the Beaufort Gyre. Salinity, nitrate and temperature, all of which were strongly correlated with latitude, and water column depth were the main factors influencing variation of zooplankton composition and spatial distribution. The δ15N values were less enriched in the Gulf of Alaska and on the Bering Sea Slope and more enriched in the eastern portion of Bering Strait and in Barrow Canyon. The δ13C values of zooplankton were more depleted in the Gulf of Alaska and in the Beaufort Sea regions and were more enriched in Western Bering Strait. The level of carnivory in the zooplankton community appeared highest in the Beaufort Gyre decreasing on the Beaufort Shelf and lowest in Western Bering Strait and in the Gulf of Alaska. Our results highlight the existence of large intra- and inter-specific differences in zooplankton isotopic signatures in relation to community composition and environmental conditions.

Distribution, abundance, biomass and diversity of benthic infauna in the Northeast Chukchi Sea, Alaska: Relation to environmental variables and marine mammals

In summer 2009 and 2010, as part of Chukchi Sea Offshore Monitoring in Drilling Area – Chemical and Benthos (COMIDA CAB) program, we performed a quantitative assessment of the biomass, abundance, and community structure of benthic infaunal populations of the Northeastern Chukchi Sea. This analysis documented a benthic species inventory of 361 taxa collected from 142 individual van Veen grab samples (0.1m−2) at 52 stations. Infaunal abundance was dominated by Polychaeta, Mollusca, and Crustacea. Large concentrations of bivalves (up to 1235m−2; 920.2gwwm−2) were collected south of Hanna Shoal where flow from two water masses converge and deposit labile carbon to the seafloor, as indicated by low surface sediment C:N ratios. Amphipods (up to 1640m−2; 26.0gwwm−2), and polychaetes (up to 4665m−2; 114.7gwwm−2) were documented from multiple stations west of and within Barrow Canyon. This high productivity was most likely due to the “canyon effect”, where marine and coastal detrital carbon supplies are channeled by the canyon structure, enhancing carbon deposition and flux, which supports rich benthic communities within the canyon and surrounding areas. To examine the relationships between infaunal distributions of all collected taxa with the physical environment, we used a Biota and Environment matching (BIO–ENV) routine. A combination of water depth, bottom-water temperature and salinity, surface sediment total organic nitrogen (TON) and sediment C:N molar ratios correlated closest with infaunal abundance distribution (ρ=0.54), indicating that multiple factors influence the success of benthic communities. BIO–ENV routines produced similar correlation results when performed on targeted walrus prey items (bivalves (ρ=0.50), polychaetes (ρ=0.53), but gray whale prey items (amphipods) were not strongly correlated to any combination of physical environmental factors (ρ=0.24). Distributions of primary prey items for gray whales (amphipods) and walruses (bivalves, gastropods and polychaetes) were compared with gray whale and walrus distribution as described by sightings from the 2009 and 2010 aerial survey component of COMIDA. In general, concentrations of walruses and their prey occurred in a swath located south of Hanna Shoal and on the shoal itself although the large differences in sea-ice distribution between the two study years showed that walrus distributions were closely linked to sea-ice location. Other areas within Barrow Canyon and the shelf west of the canyon showed high concentrations of benthic amphipods that were coincident with gray whale sightings as quantified by COMIDA aerial surveys. Overall, data collected on this project indicate that the Northeast Chukchi Sea supports a highly productive and diverse benthic ecosystem that is of significant importance to higher trophic level megafauna.

Dive Behavior of Eastern Chukchi Beluga Whales (<i>Delphinapterus leucas</i>), 1998–2008

We provide an exploratory description of the dive behavior of 23 beluga whales of the eastern Chukchi Sea stock, tagged with satellite-linked time and depth recorders at Point Lay, Alaska, between 1998 and 2007. Because of differences in how transmitters were parameterized, we analyzed data from tags deployed from 1998 to 2002 (n = 20 tags) and data from tags deployed in 2007 (n = 3 tags) separately. Using cluster analysis, we found three basic dive types in the 1998–2002 dataset. “Shallow” diving behavior was characterized by dives mostly 50 m in depth. “Intermediate” diving behavior was characterized by having one mode near the surface and a second mode near 250 m. “Deep” diving behavior was characterized by having one mode near the surface and a second mode more than 400 m from the surface. The average number of dives per hour ranged from 5.1 (SD = 2.1) to 9.8 (SD = 2.9) across dive types, with the fewest dives per hour in the deep diving category. In general, duration of dives ranged from 1 to 18 minutes; however, dives up to 21 minutes occurred in the deepest diving category. We found little evidence that dive behavior of the belugas in our sample varied by sex or age. In general, belugas dove more deeply in the eastern Beaufort Sea than in the western Beaufort or Chukchi Seas. The depths to which belugas most commonly dive in Barrow Canyon and along the Beaufort shelf break (200–300 m) correspond to the boundary where colder Pacific water overlies warmer Atlantic water, which is probably where Arctic cod (Boreogadus saida) are most dense. Diving depths within the Arctic Basin suggest that belugas are foraging mostly within the warm layer of Atlantic Water (~200–1000 m).

Barrow Canyon volume, heat, and freshwater fluxes revealed by long‐term mooring observations between 2000 and 2008

Barrow Canyon, in the northeast Chukchi Sea, is a major conduit for Pacific Water to enter the interior Arctic basins. Assemblies of annual (September 2000 to August 2008) temperature, salinity, and velocity data acquired from a mooring array in the mouth of Barrow Canyon and high‐resolution hydrographic and velocity transects along the mooring array in 2002, 2010, and 2011 have enabled a direct computation of volume, heat, and freshwater fluxes. Annual mean volume transport through Barrow Canyon was 0.45 Sv, which consisted of 0.44 Sv of Pacific Water and 0.01 Sv of Atlantic Water. Annual mean Pacific Water transport through Barrow Canyon represents 55% of the long‐term mean Pacific Water inflow through the Bering Strait. During summer, more of the Pacific inflow was advected to an eastern path as the Alaskan Coastal Current that flows along the Alaskan coast to Barrow Canyon. The freshwater flux through Barrow Canyon was 904 km3/yr, which is equivalent to 5% of the freshwater content of the Canada Basin. The annual averaged heat flux displayed substantial interannual variability, ranging from 0.93 to 3.02 TW, which could melt 88,000–290,000 km2 of 1 m thick ice. A relationship exists between the measured Barrow Canyon transport and local winds such that under southerly winds the northward flow of water through the canyon increases. Such wind conditions are induced by the weaker sea level pressure contrast between the Arctic and North Pacific, caused by a decrease in the pressure over the Arctic and an increase over the North Pacific.

Distribution and sources of organic matter in surface marine sediments across the North American Arctic margin

As part of the International Polar Year research program, we conducted a survey of surface marine sediments from box cores along a section extending from the Bering Sea to Davis Strait via the Canadian Archipelago. We used bulk elemental and isotopic compositions, together with biomarkers and principal components analysis, to elucidate the distribution of marine and terrestrial organic matter in different regions of the North American Arctic margin. Marked regional contrasts were observed in organic carbon loadings, with the highest values (≥1 mg C m−2 sediment) found in sites along Barrow Canyon and the Chukchi and Bering shelves, all of which were characterized by sediments with low oxygen exposure, as inferred from thin layers (<2 cm) of Mn oxihydroxides. We found strong regional differences in inorganic carbon concentrations, with sites from the Canadian Archipelago and Lancaster Sound displaying elevated values (2–7 wt %) and highly depleted 14C compositions consistent with inputs from bedrock carbonates. Organic carbon:nitrogen ratios, stable carbon isotopes, and terrigenous organic biomarkers (lignin phenols and cutin acids) all indicate marked regional differences in the proportions of marine and terrigenous organic matter present in surface sediments. Regions such as Barrow Canyon and the Mackenzie River shelf were characterized by the highest contributions of land‐derived organic matter, with compositional characteristics that suggested distinct sources and provenance. In contrast, sediments from the Canadian Archipelago and Davis Strait had the smallest contributions of terrigenous organic matter and the lowest organic carbon loadings indicative of a high degree of post‐depositional oxidation.

210Pb and 137Cs in margin sediments of the Arctic Ocean: Controls on boundary scavenging

210Pb and 137Cs were measured in 25 sediment cores collected during the International Polar Year from a transect spanning the North American Arctic margin, from the North Bering Sea to Baffin Bay/Davis Strait. Profiles and inventories of the radioisotopes were used to determine sediment mixing and accumulation at each site and to assess the intensity of scavenging and burial. Sediment accumulation rates derived from 210Pb and validated using 137Cs are between ≤0.04 and 0.23 g cm−2 yr−1. 210Pb cannot be used to derive sedimentation rates for vigorously biomixed sediments from the North Bering‐Chukchi shelf. Elevated 137Cs activities and inventories in recently deposited sediments imply delayed inputs of particle‐associated 137Cs to the sediments, likely transported from the watershed to the coast and subsequently redistributed to shelf/slope sediments. Inventories of 210Pbex in all cores meet or exceed the estimated supply of 210Pbex from atmospheric deposition and decay of 226Ra in the water column. This implies that in contrast to the deep Arctic Ocean basin, there is a sufficient supply of suspended particulates along the North American Arctic margin to scavenge the supply of 210Pbex. 210Pbex inventories in sediments are up to 21‐fold greater than the in situ supply at some sites. Large inventories of 210Pbex in sediments along the North Bering‐Chukchi shelf result primarily from focusing, while those along the north Chukchi slope (Barrow Canyon) and in Baffin Bay/Davis Strait reflect strong boundary scavenging, likely supported by lateral exchanges with deep/interior Atlantic‐origin waters.

Correlation of a strong Alaska Coastal Current with the presence of beluga whales Delphinapterus leucas near Barrow, Alaska

Oceanographic features and physical processes in the ocean can create regions where prey, and therefore predators, may accumulate. Beluga whales Delphinapterus leucas are the most numerous cetacean in the Arctic. In the Alaskan Beaufort Sea, they prefer continental slope habitat in summer and autumn, presumably because such areas provide enhanced foraging opportunities. Passive acoustic detections of beluga whale calls, current velocity measurements, historical wind records, and 29 yr of beluga whale observations from aerial surveys were used to explore the hypothesis that the foraging success of beluga whales in Barrow Canyon and along the western Beaufort Sea slope is enhanced when the Alaska Coastal Current (ACC) is well- developed and flows east-northeastward and is diminished when the flow of the ACC and its shelf break extension are reversed. Aerial sightings of beluga whales, average observed beluga whale group size, and hours with whale vocalizations were more common when the ACC was well- developed and flowed east-northeastward. When the ACC flow is strong, it is separated from Arctic basin waters by a well-defined front that promotes aggregation of prey species. We specu- late that the greater numbers of animals per group sighted and hours with recorded vocalizations may be indicative of enhanced foraging opportunities for beluga whales.

Linkages among halocline variability, shelf-basin interaction, and wind regimes in the Beaufort Sea demonstrated in pan-Arctic Ocean modeling framework

To address the mechanisms controlling halocline variability in the Beaufort Sea, the relationship between halocline shoaling/deepening and surface wind fields on seasonal to decadal timescales was investigated in a numerical experiment. Results from a pan-Arctic coupled sea ice-ocean model demonstrate reasonable performances for interannual and decadal variations in summer sea ice extent in the entire Arctic and in freshwater content in the Canada Basin. Shelf-basin interaction associated with Pacific summer and winter transport depends on basin-scale wind patterns and can have a significant influence on halocline variability in the southern Beaufort Sea. The eastward transport of fresh Pacific summer water along the northern Alaskan coast and Ekman downwelling north of the shelf break are commonly enhanced by cyclonic wind in the Canada Basin. On the other hand, basin-wide anti-cyclonic wind induces Ekman upwelling and blocks the eastward current in the Beaufort shelf-break region. Halocline shoaling/deepening due to shelf-water transport and surface Ekman forcing consequently occur in the same direction. North of the Barrow Canyon mouth, the springtime down-canyon transport of Pacific winter water, which forms by sea ice production in the Alaskan coastal polynya, thickens the halocline layer. The model result indicates that the penetration of Pacific winter water prevents the local upwelling of underlying basin water to the surface layer, especially in basin-scale anti-cyclonic wind periods.

Western Arctic primary productivity regulated by shelf-break warm eddies

The response of phytoplankton to the Beaufort shelf-break eddies in the western Arctic Ocean is examined using the eddy-resolving coupled sea ice–ocean model including a lower-trophic marine ecosystem formulation. The regional model driven by the reanalysis 2003 atmospheric forcing from March to November captures the major spatial and temporal features of phytoplankton bloom following summertime sea ice retreat in the shallow Chukchi shelf and Barrow Canyon. The shelf-break warm eddies spawned north of the Barrow Canyon initially transport the Chukchi shelf water with high primary productivity toward the Canada Basin interior. In the eddy-developing period, the anti-cyclonic rotational flow along the outer edge of each eddy moving offshore occasionally traps the shelf water. The primary production inside the warm eddies is maintained by internal dynamics in the eddy-maturity period. In particular, the surface central area of an anti-cyclonic eddy acquires adequate light, nutrient, and warm environment for photosynthetic activity partly attributed to turbulent mixing with underlying nutrient-rich water. The simulated biogeochemical properties with the dominance of small-size phytoplankton inside the warm eddies are consistent with the observational findings in the western Arctic Ocean. It is also suggested that the light limitation before autumn sea ice freezing shuts down the primary production in the shelf-break eddies in spite of nutrient recovery. These results indicate that the time lag between the phytoplankton bloom in the shelf region following the summertime sea ice retreat and the eddy generation along the Beaufort shelf break is an important index to determine biological regimes in the Canada Basin.

Interannual variability of Pacific Winter Water inflow through Barrow Canyon from 2000 to 2006

We examined the interannual variability of Pacific Winter Water (PWW), both upstream in the northeastern Chukchi Sea and Barrow Canyon using mooring observations from 2000 to 2006, and downstream in the Canada Basin using hydrographic data acquired in 2002–2006. The interannual variation of PWW salinity is governed by two factors: (1) variability in the salinity of Pacific Water that flows northward through Bering Strait in winter; and (2) the input of salt associated with sea ice formation during winter in an intermittent coastal polynya located along the Alaskan coast between Cape Lisburne and Point Barrow. During the winters of 2000/2001 and 2001/2002 an increased transport of cold and saline PWW (S > 33.5) to the basin via Barrow Canyon was observed. In 2000/2001 enhanced ice formation in the polynya contributed to the increased salinity of PWW, whereas in 2001/2002 the salinity of water entering through the Bering Strait was higher, and this resulted in more saline PWW being delivered to the basin. In the following four winters (2002/2003, 2003/2004, 2004/2005 and 2005/2006) the transport of cold and saline PWW in winter to the basin was less than that in the two preceding winters. In three of these four winters (2003/2004 being the exception) the coastal polynya was less active, thus reducing the input of salt due to brine enrichment. In the winter of 2003/2004, however, warmer water within the polynya region constrained ice formation and thus less cold and saline PWW was produced, despite the fact that the coastal polynya was active and frequently open.

Turbulent Kinetic Energy Dissipation in Barrow Canyon

Abstract Pacific Water flows across the shallow Chukchi Sea before reaching the Arctic Ocean, where it is a source of heat, freshwater, nutrients, and carbon. A substantial portion of Pacific Water is routed through Barrow Canyon, located in the northeast corner of the Chukchi. Barrow Canyon is a region of complex geometry and forcing where a variety of water masses have been observed to coexist. These factors contribute to a dynamic physical environment, with the potential for significant water mass transformation. The measurements of turbulent kinetic energy dissipation presented here indicate diapycnal mixing is important in the upper canyon. Elevated dissipation rates were observed near the pycnocline, effectively mixing winter and summer water masses, as well as within the bottom boundary layer. The slopes of shear/stratification layers, combined with analysis of rotary spectra, suggest that near-inertial wave activity may be important in modulating dissipation near the bottom. Because the canyon is known to be a hotspot of productivity with an active benthic community, mixing may be an important factor in maintenance of the biological environment.

Wind‐driven modification of the Alaskan coastal current

Across‐shelf transects over the eastern flank of Barrow Canyon were obtained in August 2005 with an autonomous underwater vehicle (AUV). Here, the shelf topography creates a “choke” point in which a substantial portion of Pacific inflow from the Bering Strait is concentrated within 30 km of the coast, providing an ideal setup for monitoring the flow with the AUV. Four transects, extending ∼10 km offshore of Barrow, Alaska, inshore of the ∼80 m isobath, were used in conjunction with a process‐oriented numerical model to diagnose the wind‐driven modification of the Alaskan coastal current. Poleward transports of 0.12 Sv were consistent among all sections, although the transport‐weighted temperature was about 1°C colder in the transect obtained during peak winds. An idealized numerical model reproduces the observed hydrographic structure and across‐shelf circulation reasonably well in that (1) winds were not sufficient to reverse the poleward flow, (2) upwelling was most pronounced in the nearshore, and (3) the onshore return flow occurred throughout the interior as opposed to the bottom boundary layer. The across‐shelf circulation provides a possible mechanism for a meltwater intrusion observed on the offshore side of the AUV transect made during peak winds. Also of interest is that the observed anticyclonic shear was much stronger (∣∂u/∂y∣ > f) than previously measured in the region.