Surface salinity drifters for SMOS validation
Morisset, S. ; Reverdin, G. ; Boutin, J. ; Martin, N. ; Yin, X. ; Gaillard, F. ; Blouch, P. ; Rolland, J. ; Font, J. ; Salvador, J.
The ESA/SMOS (European Space Agency/Soil Moisture and Ocean Salinity) satellite mission provides new measurements of Sea Surface Salinity (SSS) using L-band radiometry. After correcting SMOS brightness temperatures from systematic biases, SMOS sea surface salinity (SSS) reproduces quite well large scale expected SSS variations [Font et al., 2012]. At L-band frequency, the skin depth is 1 centimetre while most in situ SSS measurements are taken at a few meters depth. A preliminary study based on ARGO vercal profiles [Henocq et al., 2010] indicated that vertical salinity differences between 1m and 10m depth higher than 0.1 psu are observed in the 3 oceans, mainly between 0° and 15°N, coinciding with the average position of the Inter Tropical Convergence Zones characterized by high precipitation rates. In order to better document the variability of salinity near the sea surface, which is currently not often measured by other in situ observations (current Argo uppermost data is between 5 and 10m depth, whereas TAO-type measurements are near 1 or 2m depth), surface drifters have been equipped with Conductivity-Temperature (C-T) cells near a depth of 50 cm, which proved reliable for mid-latitude deployments [Reverdin et al., 2007]. Since then, two manufacturers of SVP (Surface Velocity Program) drifters, Metocean and the Pacific Gyre have instrumented SVP drifters with sensors measuring conductivity at 30-50cm depth. In addition, new light floats named SURPLAS have been built at LOCEAN laboratory to measure conductivity at 15cm depth for a duration of a few weeks to a few months. SURPLAS floats have been tied to SVP drifters allowing the study of the SSS and SST (Sea Surface Temperature) stratification between 15cm and 50cm depth. In addition, ICM/CSIC has built slightly larger drifters with C-T cells also near 50 cm depth, but without an anti-fouling protection of the cell. The sampling characteristics of the different drifters are slightly different. The SURPLAS drifter provides a value (average over 8”) every 15 minutes of T (Temperature) and S (Salinity); the Pacificgyre SVP-BS drifter, a value every 30 minutes (average over 5 minutes), the Metocean SVP-BS drifter, a value every hour (average of 7 values over 10 minutes), and the ICM/CISC provide values at the time of Argos transmissions (not averaged). Most of the drifters and floats transmit through Argos, although Metocean drifters since 2009 mostly transmit data (and a 3-hourly gps position) through iridium communication. Since 2007, we deployed 37 Metocean SVP-BS drifters, 29 Pacificgyre drifters, 21 ICM/CSIC drifters and 17 surplas floats. In 2010 and 2011, simultaneous to the first two years of SMOS measurements, 68 SVP drifters (49 Metocean and PacificGyre and 19 ICM drifters) and 13 SURPLAS floats have been deployed by the French and Spanish teams involved in the SMOS Cal/Val projects mostly in the North Atlantic, in the Bay of Biscay, in the equatorial and subtropical South Atlantic and in the western tropical and equatorial Pacific Ocean. Altogether in 2010-2011, they recorded measurements during 13500 days (Metocean+PacificGyre 8821, Surplas 472, ICM 4266). In this paper, we will first comment on the data return of these drifters, on our efforts to quality control and correct the data. Then, we will summarize results on tropical SSS freshening events linked to rain events as recorded at various depths by autonomous drifters and as deduced from the SMOS radiometer measurements.
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