Characteristics and Transformation of Pacific Winter Water on the Chukchi Sea Shelf in Late Spring. Issue 10 (24th October 2019)

• Record Type: Journal Article
• Authors: Pacini, Astrid; Moore, G. W. K.; Pickart, Robert S.; Nobre, Carolina; Bahr, Frank; Våge, Kjetil; Arrigo, Kevin R.
• Is Part Of: Journal of geophysical research. Volume 124:Issue 10(2019)
• Page Start: 7153
• Page End: 7177
• Publication Date: 2019-10-24
• Subjects: brine rejection -- Chukchi Sea -- convection -- winter water; Oceanography -- Periodicals; 551.4605
• DOI: 10.1029/2019JC015261
• Languages: English
• ISSNs: 2169-9275

Abstract

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/m 3 ). 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. Plain Language Summary: Shipboard data from a rare late spring cruise to the Chukchi Sea, in May–June 2014, are analyzed in order to understand the physical conditions of the waters on the shelf. More than 95% of the water on the shelf was very cold, high‐nutrient winter water. The vertical structure of the water column (in temperature, salinity, and density) consisted of two layers, a well‐mixed surface layer and a bottom boundary layer, separated by a weak density interface. When ice is formed at the surface, brine is rejected into the water column, densifying the surface water. This leads to convective overturning as dense water sinks and mixes with the subsurface water. If enough brine is rejected, the density interface between the mixed layers can be eroded and the water column mixes to the bottom, resulting in a uniform density profile. Nutrients are then stirred into the water column from the sediments, which preconditions the water for phytoplankton growth. Using realistic atmospheric conditions, together with a polynya model and ocean mixing model, it was demonstrated that this process was likely happening within small openings in the sea ice throughout the domain. This represents a new paradigm for forming winter water on the Chukchi shelf. Key Points: Newly ventilated Pacific‐origin winter water, with high nutrient content, is prevalent across the Chukchi Sea shelf in late spring. A polynya model, used with a 1D mixing model, demonstrates that brine rejection from refreezing leads quickly overturns the water column. Salinity signals and ice concentration data support the notion that winter water is modified within small leads throughout the shelf.