Abstract
Abstract Salinity measurements from 119 surface drifters in 2007–12 were assessed; 80% [Surface Velocity Program with a barometer with a salinity sensor (SVP-BS)] and 75% [SVP with salinity (SVP-S)] of the salinity data were found to be usable, after editing out some spikes. Sudden salinity jumps are found in drifter salinity records that are not always associated with temperature jumps, in particular in the wet tropics. A method is proposed to decide whether and how to correct those jumps, and the uncertainty in the correction applied. Northeast of South America, in a region influenced by the Amazon plume and fresh coastal water, drifter salinity is very variable, but a comparison with data from the Soil Moisture and Ocean Salinity satellite suggests that this variability is usually reasonable. The drifter salinity accuracy is then explored based on comparisons with data from Argo floats and from thermosalinographs (TSGs) of ships of opportunity. SVP-S/SVP-BS drifter records do not usually present significant biases within the first 6 months, but afterward biases sometimes need to be corrected (altogether, 16% of the SVP-BS records). Biases start earlier after 3 months for drifters not protected by antifouling paint. For the few drifters for which large corrections were applied to portions of the record, the accuracy cannot be proven to be better than 0.1 psu, and it cannot be proven to be better than 0.5 psu for data in the largest variability area off northeast South America. Elsewhere, after excluding portions of the records with suspicious salinity jumps or when large corrections were applied, the comparisons rule out average biases in individual drifter salinity record larger than 0.02 psu (midlatitudes) and 0.05 psu (tropics).
Highlights
Near-surface salinity is largely determined by the global hydrological cycle as well as by the oceanographic circulation and vertical mixing processes (Schmitt 2008)
After applying geophysical corrections on Soil Moisture and Ocean Salinity (SMOS) brightness temperatures and removing data contaminated by radio frequency interference (RFI) or land effects, SMOS sea surface salinity (SSS) reproduces expected variations at large scales (Font et al 2013; Boutin et al 2012b; Reul et al 2012; Banks et al 2012) as well as rainfall-related signals (Boutin et al 2012a), but its absolute accuracy as well as the accuracy of Aquarius retrievals are still subject to large uncertainties on the order of 0.3 psu that require sets of validation data
Metocean Surface Velocity Program (SVP)-BS drifters, 40 Pacific Gyre (PG) SVP-BS drifters, 17 ICM/Consejo Superior de Investigaciones Cientıficas (CSIC) drifters, 17 Surplas floats, and 25 PG SVP with salinity (SVP-S) drifters were deployed
Summary
Near-surface salinity is largely determined by the global hydrological cycle as well as by the oceanographic circulation and vertical mixing processes (Schmitt 2008). To better document the variability of salinity near the sea surface, which is currently not often measured by other in situ observations, Surface Velocity Program (SVP) drifters have been equipped in the last 10 years with Sea-Bird SBE37 with serial interface (SI) conductivity and temperature (C–T) sensors near a depth of 50 cm. The data of these drifters proved reliable and correctable for midlatitude deployments in the eastern Atlantic Ocean (Reverdin et al 2007). We will investigate the overall accuracy of the dataset from independent data
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