Water Mass Evolution and Circulation of the Northeastern Chukchi Sea in Summer: Implications for Nutrient Distributions

Abstract

Synoptic and historical shipboard data, spanning the period 1981–2017, are used to investigate the seasonal evolution of water masses on the northeastern Chukchi shelf and quantify the circulation patterns and their impact on nutrient distributions. We find that Alaskan coastal water extends to Barrow Canyon along the coastal pathway, with peak presence in September, while the Pacific Winter Water (WW) continually drains off the shelf through the summer. The depth‐averaged circulation under light winds is characterized by a strong Alaskan Coastal Current (ACC) and northward flow through Central Channel. A portion of the Central Channel flow recirculates anticyclonically to join the ACC, while the remainder progresses northeastward to Hanna Shoal where it bifurcates around both sides of the shoal. All of the branches converge southeast of the shoal and eventually join the ACC. The wind‐forced response has two regimes: In the coastal region the circulation depends on wind direction, while on the interior shelf the circulation is sensitive to wind stress curl. In the most common wind‐forced state—northeasterly winds and anticyclonic wind stress curl—the ACC reverses, the Central Channel flow penetrates farther north, and there is mass exchange between the interior and coastal regions. In September and October, the region southeast of Hanna Shoal is characterized by elevated amounts of WW, a shallower pycnocline, and higher concentrations of nitrate. Sustained late‐season phytoplankton growth spurred by this pooling of nutrients could result in enhanced vertical export of carbon to the seafloor, contributing to the maintenance of benthic hotspots in this region.

Characteristics and Transformation of Pacific Winter Water on the Chukchi Sea Shelf in Late Spring

“…A second example is the slightly tighter recirculation limb centered at 71.5°N. This southeastward migration of waters has been previously observed and thus appears to be a persistent feature Lin et al, 2019). The notion that part of the western pathway joins the central pathway is also supported by the SUBICE measurements (note the flow to the northeast between 72 and 72.5°N in the northwest part of the study domain).…”

Observational and Modeling Evidence of Seasonal Trends in Sediment‐Derived Material Inputs to the Chukchi Sea

“…The highest tracer concentrations over the shelf were observed southeast of Hanna Shoal. This result is consistent with observations of high nitrate concentrations (Lin et al, 2019;Lowry et al, 2015) and enhanced benthic biomass and biological productivity (Grebmeier et al, 2015) in this region of the Chukchi shelf. The timing of peak tracer concentrations over the shelf also showed a latitudinal trend: the highest tracer concentrations were observed in the winter over the southern shelf and in the spring over the central shelf.…”

Mean and Seasonal Circulation of the Eastern Chukchi Sea From Moored Timeseries in 2013–2014

“…This is consistent with previous studies demonstrating that the alongcoast wind threshold for upwelling (and reversed flow) in Barrow Canyon is 5-6 m/s (Fang et al, 2017;Pisareva et al, 2019;Weingartner et al, 1998 Interestingly, the flow at moorings C2, Bu, and C6 is also reversed, even though these moorings are a fair distance from the coast. In the case of C2 and Bu, this is in line with the results of Lin et al (2019) who found that the flow was directed to the west in this same vicinity when the coastal current was reversed. However, it is not obvious why the flow at C6 reverses in response to the wind.…”