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

Abstract

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.

Atlantic Water Modification North of Svalbard in the Mercator Physical System From 2007 to 2020

“…Pfirman et al, 1994; Rudels et al, 2004; Steele et al, 1995). Recently, using the NO geochemical tracer, Bertosio et al (2020) showed that the base of the lower halocline which used to be associated with SA ~ 34.46 g/kg (34.3 psu) was now found on a saltier level of 34.9 g/kg in the Eurasian Basin. This denser level, corresponding to isopycnal 27.85 kg/m 3 , is in agreement with the deeper winter mixed layers reproduced by PSY4 in Nansen Basin: during the extreme winters 2012/13 and 2018/17, deep convection reached the 27.95 kg/m 3 isopycnal.…”

“…Exploratory 2‐D Lagrangian trajectories using PSY4 velocities suggested advective pathways from the continental slope to the center of the Nansen Basin in the 30–150 m layer (i.e. halocline waters), with time scales larger than 3 years (Bertosio et al, 2020). Full 3‐D Lagrangian tracking would extend this precursory work and improve our understanding of the fate of the convected water and its impact on the halocline water properties.…”

“…Exploratory 2-D Lagrangian trajectories using PSY4 velocities suggested advective pathways from the continental slope to the center of the Nansen Basin in the 30-150 m layer (i.e. halocline waters), with time scales larger than 3 years (Bertosio et al, 2020). Full 3-D Lagrangian…”

Insights Into Water Mass Origins in the Central Arctic Ocean From In‐Situ Dissolved Organic Matter Fluorescence

“…This was supplemented with water chemistry measurements, primarily inorganic nutrients, alkalinity, barium and stable oxygen isotopes (Bauch et al., 1995; Guay & Falkner, 1997; Jones & Anderson, 1986). The increasingly widespread deployment of additional biogeochemical sensors (oxygen and a selection of optical probes) on profiling instruments offers higher resolution measurements and an opportunity to further explore processes involved with HL formation, and source fractionation of freshwater distribution (Athanase et al., 2019; Bertosio et al., 2020; Boles et al., 2020; Dmitrenko et al., 2019; Laney et al., 2014). A fraction of the organic compounds present in DOM fluoresce and this can be used as a proxy for dissolved organic carbon in the Arctic (Amon et al., 2003; Gonçalves‐Araujo et al., 2016; Guay et al., 1999).…”

Atlantic Water Modification North of Svalbard in the Mercator Physical System From 2007 to 2020 

“…Pfirman et al, 1994;Rudels et al, 2004;Steele et al, 1995). Recently, using the NO geochemical tracer, Bertosio et al (2020) showed that the base of the lower halocline which used to be associated with SA~34.46 g/kg (34.3 psu) was now found on a saltier level of 34.9 g/kg in the Eurasian Basin. This denser level, corresponding to isopycnal 27.85 kg/m 3 , is in agreement with the deeper winter mixed layers reproduced by PSY4 in Nansen Basin: during the extreme winters 2012/13 and 2018/17, deep convection reached the 27.95 kg/m 3 isopycnal.…”