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

Abstract

Data from a late spring survey of the northeast Chukchi Sea are used to investigate various aspects of newly ventilated winter water (NVWW). More than 96% of the water sampled on the shelf was NVWW, the saltiest (densest) of which tended to be in the main flow pathways on the shelf. Nearly all of the hydrographic profiles on the shelf displayed a two‐layer structure, with a surface mixed layer and bottom boundary layer separated by a weak density interface (on the order of 0.02 kg/m3). Using a polynya model to drive a one‐dimensional mixing model, it was demonstrated that, on average, the profiles would become completely homogenized within 14–25 hr when subjected to the March and April heat fluxes. A subset of the profiles would become homogenized when subjected to the May heat fluxes. Since the study domain contained numerous leads within the pack ice—many of them refreezing—and since some of the measured profiles were vertically uniform in density, this suggests that NVWW is formed throughout the Chukchi shelf via convection within small openings in the ice. This is consistent with the result that the salinity signals of the NVWW along the central shelf pathway cannot be explained solely by advection from Bering Strait or via modification within large polynyas. The local convection would be expected to stir nutrients into the water column from the sediments, which explains the high nitrate concentrations observed throughout the shelf. This provides a favorable initial condition for phytoplankton growth on the Chukchi shelf.

Late Spring Nitrate Distributions Beneath the Ice‐Covered Northeastern Chukchi Shelf

“…Convective overturning was likely taking place during our survey within small leads in the pack ice, as demonstrated by Pacini et al (2016) who used a polynya model in conjunction with a 1-D mixing model to show that the water column on the interior shelf would overturn in a matter of hours subject to the observed air-sea forcing during the first part of our survey. This implies that the convective mixing would stir regenerated nutrients from the seafloor into the upper water column.…”

“…Sea ice concentrations were determined by 2-hourly ship observations and passive microwave satellite data at each station during our study and were all approximately 100% (Figure 1). The surface melt ponds that are common in this area in summer (Arrigo et al, Journal of Geophysical Research: Biogeosciences 10.1002/2017JG003881 2014) were absent during our study due to persistently subfreezing air temperatures (see also Pacini et al, 2016).…”

Shelf‐Basin Interactions and Water Mass Residence Times in the Western Arctic Ocean: Insights Provided by Radium Isotopes

“…Chukchi winter water (WW) forms as a result of brine rejection from late fall to early spring (Muench et al, 1988;Weingartner et al, 1998;Woodgate et al, 2005). This water mass is cold, saline, and becomes enriched in sediment-derived materials, including regenerated nutrients and trace metals, through water column convection induced by brine rejection (Arrigo et al, 2017;Lowry et al, 2015;Pacini et al, 2016;Pickart et al, 2016;Vieira et al, 2018). In the summer, solar heating of this water mass transforms it into remnant winter water (RWW; Gong & Pickart, 2016).…”

Under‐Ice Phytoplankton Blooms Inhibited by Spring Convective Mixing in Refreezing Leads

“…The model demonstrates that under-ice blooms can form even beneath snow-covered sea ice in the absence of mixing but not in more deeply mixed waters beneath sea ice with refreezing leads. Future estimates of primary production should account for these phytoplankton dynamics in ice-covered waters.On shallow shelves such as the Chukchi Sea and the Bering shelf, convective mixing can completely overturn the water column and lead to exchange with remineralized benthic nutrients, resulting in a relatively uniform and extremely nutrient-rich water mass (Lowry et al, 2015;Pacini et al, 2016;Pickart et al, 2016; Key Points:Spring phytoplankton blooms were present beneath fully consolidated sea ice with snow Under-ice phytoplankton biomass was inversely correlated with lead fraction Convection in refreezing leads enhances mixing and inhibits under-ice bloom development…”

“…On shallow shelves such as the Chukchi Sea and the Bering shelf, convective mixing can completely overturn the water column and lead to exchange with remineralized benthic nutrients, resulting in a relatively uniform and extremely nutrient-rich water mass (Lowry et al, 2015;Pacini et al, 2016;Pickart et al, 2016; Key Points:…”