SMOS and SMAP combined SSS L3 maps over the Chukchi and the Beaufort seas during summer 2019
SSS weekly fields are derived from SMOS and SMAP measurements from June to September 2019.
SMOS SSS is from a modified version of the CEC-LOCEAN L3 Arctic, distributed by the “Centre Aval de Traitement des Données SMOS” (CATDS; Supply et al., 2020).
In this new version, the SMOS level 2 SSS are from the ESA Climate Change Initiative (CCI) v3.2 reprocessing described in Perrot et al. (2021). In comparison with level 2 SSS used in Supply et al. (2020):
- SSS is computed with an updated dielectric constant parametrization (Boutin et al., 2020).
- SMOS vicarious calibration, the so-called Ocean Target Transformation, is derived using Argo optimal interpolated SSS (Gaillard et al., 2016) instead of a climatology.
- SST and wind speed used as priors in the SSS retrieval are taken from European Centre for Medium-Range Weather Forecasts (ECMWF) ERA5 instead of ECMWF forecasted fields.
- Updated sea ice filtering derived from the difference between SMOS retrieved pseudo dielectric constant (Acard parameter) and the one expected from retrieved SSS and SST: instead of being applied only at level 3, sea ice filtering is applied both at level 2 (swath product) and at level 3 (weekly averaged product).
SSS is derived with a correction of SST-induced bias using Remote Sensing Systems (RSS) SST.
SMOS SSS are averaged over 9 days on a 25km Equal-Area Scalable Earth (EASE) 2.0 grid adapted to polar areas. These fields have an effective spatial resolution close to 50 km, corresponding to the original resolution of SMOS SSS (no spatial interpolation is applied from level 2 to level 3).
Given the difficulty to find a reference to adjust absolute SSS values in the Arctic Ocean, this product does not contain any Land Sea Contamination (LSC) correction; nevertheless, in the averaging process, SMOS SSS is weighted by the Chi2 of the retrieval, and we expect it to be degraded on SMOS dwell lines largely contaminated by LSC.
SMAP SSS is from the Jet Propulsion Laboratory (JPL; Fore et al., 2020) version 4.3 8-day averaged SSS provided on a 0.25° regular grid, with a spatial interpolation from level 2 to level 3. In this product that uses a LSC correction, the effective spatial resolution of SSS is close to 60 km. The use of JPL SMAP SSS instead of SMAP SSS distributed by RSS is motivated by a less restrictive ice mask in the polar regions. Between 19 June 19th and July 23rd 2019, SMAP was in safe mode and did not provide SSS estimates.
SMOS, SMAP and combined SSS are provided with an uncertainty estimated by the SSS retrieval algorithm.
Additional sea ice filtering applied to SMOS and SMAP SSS: only SSS estimates with an uncertainty lower than 0.6 pss are considered in the following.
SSS estimates from both satellites (SMAP interpolated in the 25 km EASE 2.0 grid) are intercalibrated using summer 2019 saildrones measurements (Saildrone (2020), Vazquez-Cuervo et al., 2021) and finally combined.
Video: illustration of SSS evolution (SMOS, SMAP and combined SSS) during summer sea ice retreat between June and September 2019 (Ocean and Sea Ice Satellite Application Facility (OSI-SAF) Sea Ice Concentration (SIC) derived from AMSR-2 measurements, provided by the Danish and the Norwegian Meteorological Institute).
Sea Surface Salinity, SMOS, SMAP, Arctic Ocean, Meltwater lenses
Soil Moisture and Ocean Salinity (SMOS) satellite mission and Soil Moisture Active and Passive mission (SMAP).