The processes driving the seasonal variability of the mixed layer salinity in the Arctic Ocean are investigated using a simulation performed with regional ocean – sea ice model at high resolution. While the seasonal variations of the mixed layer depth remain small, in particular under the perennial sea ice (O(30m)), the mean salinity of the mixed layer varies largely, with a seasonal cycle as high as 3 pss. On the shelves, where the sea ice is seasonal, the mixed layer is much fresher but exhibits a seasonal cycle with a similar amplitude. Overall, the seasonal variability of the mixed layer salinity results largely from a 1D vertical balance between the freshwater flux at the surface arising from the sea ice melt and freezing processes, and vertical mixing and entrainment occurring at the base of the mixed layer. The largest variations are found in summer, when the mixed layer is the thinnest. Over the shelves, this simple 1D balance is complexified due to the role of advection and river runoff that can locally affect the mixed layer depth and salinity. Interestingly, the largest variations are found less than 100km on each side of the sea ice edge, where all the processes affecting the mixed layer are amplified. This suggests the need to better observed and understand the ocean-sea ice-atmosphere exchanges in these regions.

Sea Surface Salinity (SSS) is an increasingly-used Essential Ocean and Climate Variable. The SMOS, Aquarius, and SMAP satellite missions all provide SSS measurements, with very different instrumental features leading to specific measurement characteristics. The Climate Change Initiative Salinity project (CCI+SSS) aims to produce a SSS Climate Data Record (CDR) that addresses well-established user needs based on those satellite measurements. To generate a homogeneous CDR, instrumental differences are carefully adjusted based on in-depth analysis of the measurements themselves, together with some limited use of independent reference data. An optimal interpolation in the time domain without temporal relaxation to reference data or spatial smoothing is applied. This allows preserving the original datasets variability. SSS CCI fields are well-suited for monitoring weekly to interannual signals, at spatial scales ranging from 50 km to the basin scale. They display large year-to-year seasonal variations over the 2010-2019 decade, sometimes by more than +/-0.4 over large regions. The robust standard deviation of the monthly CCI SSS minus in situ Argo salinities is 0.15 globally, while it is at least 0.20 with individual satellite SSS fields. r2 is 0.97, similar or better than with original datasets. The correlation with independent ship thermosalinographs SSS further highlights the CCI dataset excellent performance, especially near land areas. During the SMOS-Aquarius period, when the representativity uncertainties are the largest, r2 is 0.84 with CCI while it is 0.48 with the Aquarius original dataset. SSS CCI data are freely available and will be updated and extended as more satellite data become available.

We investigate the Chukchi and the Beaufort seas, where salty and warm Pacific Water flows in from the Bering Strait and interacts with the sea ice, contributing to its summer melt. For the first time, thanks to in-situ measurements recorded by two saildrones deployed during summer 2019 and to refined sea ice filtering in satellite L-Band radiometric data, we demonstrate the ability of satellite Sea Surface Salinity (SSS) observed by SMOS and SMAP to capture SSS freshening induced by sea ice melt, referred to as meltwater lenses (MWL). The largest MWL observed by the saildrones during this period occupied a large part of the Chukchi shelf, with a SSS freshening reaching -5 pss. it persisted for up to one month, to this MWL, induced low SSS pattern which restricted the transfer of air-sea momentum to the upper, as illustrated by measured wind speed and vertical profiles of currents. Combined with satellite-based Sea Surface Temperature, satellite SSS provides a monitoring of the different water masses encountered in the region during summer 2019. Using sea ice concentration and estimated Ekman transport, we analyse the spatial variability of sea surface properties after the sea ice edge retreat over the Chukchi and the Beaufort seas. The two MWL captured by both, the saildrones and the satellite measurements, result from different dynamics. Over the Beaufort Sea, the MWL evolution follows the meridional sea ice retreat, whereas in the Chukchi Sea, a large persisting MWL is generated by advection of a sea ice filament.

Future changes in subduction are suspected to be critical for the ocean deoxygenation pre- dicted by climate models over the 21 st century. However, the drivers of global oxygen subduction have not been fully described or quantified. Here, we address the physical mech- anisms responsible for the oxygen transport across the late winter mixed layer base and their relation with water-mass formation. Up to 70% of the global oxygen uptake takes place during Mode Water subduction mostly in the Southern Ocean and the North At- lantic. This oxygen subduction is driven by the combination of strong currents with large mixed-layer-depth gradients at localized hot-spots and by the wind-driven vertical velocity within the Subtropical gyres. Although oxygen diffusion, often neglected, is uncertain, it is likely to be important for the global oxygenation. The physical mass flux dominates the total oxygen subduction while the oxygen solubility plays a minor role in its modulation.