The Western Eurasian Basin Halocline in 2017: Insights From Autonomous NO Measurements and the Mercator Physical System

• Published in: Journal of geophysical research. Oceans 2020-07, Vol.125 (7), p.n/a
• Main Authors: Bertosio, Cécilia, Provost, Christine, Sennéchael, Nathalie, Artana, Camila, Athanase, Marylou, Boles, Elisabeth, Lellouche, Jean‐Michel, Garric, Gilles
• Format: Article
• Language: English
• Subjects: Arctic Eurasian Basin; Arctic Oscillation; biogeochemistry; Buoys; Computer simulation; Continental slope; Convection; Dissolved oxygen; Drift; Geophysics; Halocline; Horizon; Isohalines; Mixed layer; NO parameter; Ocean, Atmosphere; Oceans; ocean‐sea ice operational model; Oxygen; Parameter estimation; Profiles; Salinity; Salinity effects; Sciences of the Universe; Sea ice; Sea ice models; Slopes; Spatial distribution; Tracers; Water; Water masses; Winter
• ISSN: 2169-9275; 2169-9291
• DOI: 10.1029/2020JC016204

We present the first sensor‐based profiles of the quasi‐conservative NO parameter obtained with an autonomous ice‐tethered buoy in the Arctic Ocean. Data documented the halocline in the Transpolar Drift and Nansen Basin in 2017. A NO minimum was found in the Nansen Basin on a σ‐horizon of 27.8 kg·m−3 corresponding to the lower halocline, while a lower NO minimum of 380 μM straddled the 27.4 σ‐horizon and marked the cold halocline in the Transpolar Drift. Back trajectories of water parcels encountered along the buoy drift were computed using the Mercator physical system. They suggested that waters within the NO minimum at 27.4 kg·m−3 could be traced back to the East Siberian Sea continental. These trajectories conformed with the prevailing positive phase of the Arctic Oscillation. The base of the lower halocline, at the 27.85 σ‐horizon, corresponded to the density attained in the deepest winter mixed layer north of Svalbard and cyclonically slowly advected from the slope into the central Nansen Basin. The 27.85 σ‐horizon is associated with an absolute salinity of 34.9 g·kg−1, a significantly more saline level than the 34.3 psu isohaline commonly used to identify the base of the lower halocline. This denser and more saline level is in accordance with the deeper winter mixed layers observed on the slopes of Nansen Basin in the last 10 years. A combination of simulations and NO parameter estimates provided valuable insights into the structure, source, and strength of the Arctic halocline. Plain Language Summary Dissolved oxygen and nitrate data, measured for the first time by an autonomous ice‐tethered profiler in the Arctic Ocean, were combined to compute the NO parameter, a tracer useful for differentiating oceanic water masses. Together with ocean and sea ice model simulations, the spatial distribution of this NO parameter provided valuable insights into the structure and sources of the water in the central Arctic. The halocline, a layer near the surface where salinity increases rapidly with depth, isolates sea ice from the heat stored in the salty Atlantic water below. Waters in the halocline could be traced back to two different sources: the East Siberian Sea continental slope and the slope north of Svalbard where deep winter convection is important. Additionally, our analyses show that previous definitions of the bounds of the halocline layer no longer apply, likely because of increasing influence of saltier Atlantic water near the surface. Key Points Fi