SMOS and AQUARIUS SSS in and Around the SPURS/STRASSE Experiment
Hernandez, Olga1; Boutin, Jacqueline1; Reverdin, Gilles1; Martin, Nicolas1; Reul, Nicolas2

In the frame of the "Salinity Processes in the Upper Ocean Regional Study"(SPURS) experiments, the STRASSE (Subtropical Atlantic surface salinity experiment, PI G. Reverdin) campaign took place in August-September 2012, centered in 26°N, 35.5°W. It provides a very high resolution monitoring of the salinity variability in the high salinity region of the subtropical north Atlantic. In addition to this survey, Sea Surface Salinity (SSS) monitoring in the vicinity of this region is performed every month by ships of opportunity (Toucan and Colibri) which cross the maximum SSS region. In this presentation, we take advantage of this wide in situ SSS monitoring for validating the SMOS (European Space Agency (ESA) real time version 5 and Centre Aval de Traitement des Données SMOS (CATDS) expertise center version 2) and AQUARIUS (level 3 version 1.3.9) SSS.

Our comparisons show that the absolute calibration of the satellite SSS remains imperfect in 2012, with particularly large biases on SMOS SSS. In the ESA v5 reprocessing in 2010 and 2011, the SMOS SSS biases have been partly mitigated by an empirical bias correction (the so-called 'Ocean Target Transformation') computed in the eastern southern Pacific (5S-45S) every two weeks with reasonnable results. On another hand, in the 2012 real time processing, the OTT correction was applied only every month and with several weeks delay leading to large observed SSS biases. In order to get rid of them, we test two additionnal bias corrections derived from the mean SMOS-climatology SSS difference either in the OTT region or in the northern subtropical latitudes.

After removal of large scale biases, we concentrate on the spatio-temporal SSS variability at scales larger than 100km derived from SMOS and AQUARIUS data, and at the capability of SMOS to sense SSS variability at scales shorter than 100km. Figure 1 below illiustrates high resolution SSS acquired during the SPURS/STRASSE campaign and SMOS and AQUARIUS SSS in the surrounding area.

Fig. 1: SSS measured during the SPURS/STRASSE campaign in August-September 2012 (left) and derived from satellite data in August 2012 after adjusting a mean SSS bias over the region (black square indicates SPURS/STRASSE region): SMOS v5 data (middle) and Aquarius v13.9 (right). Note different color scale.

Using transects of Toucan-Colibri ships over this region, we find that once a correction derived from mean bias computed in the OTT region is applied on SMOS SSS, the rmse of SMOS SSS (averaged over 18 days and 100 km around ship measurements) minus ship SSS is less than 0.3. An example in May 2012 is shown on Fig 2 (rmse=0.25).

Fig 2: Comparison between SMOS ESA v5 SSS (bue: ascending passes; red: descending passes; purple: all passes) and Toucan ship SSS (green) in May 2012. A mean bias correction derived from biases observed on SMOS SSS in the OTT region has been applied on SMOS SSS. Left: SSS as a function of time (from North to South); Right: SSS differences.