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Adjustement on water samples

Correction of TSG measurements

 

Over the duration of a ship trip, fouling may develop and produce a drift in the TSG measurements, especially on ships of opportunity after months at sea without any cleaning of the tank and conductivity cell. Also sudden shifts can occur after calls in dirty harbor calls. The drift is generally negative (underestimation of salinity), of the order of minus 0.1 pss/month in warm waters, while abrupt shifts, also negative, can be as large as 1 pss.

 

It is thus highly recommended to collect water samples or perform CTD measurements on a daily basis for reference. Otherwise, near surface data from Argo floats collocated with the ship trajectory can be used, although it lowers precision. Then, corrections based on the differences between these external data and TSG data can be applied to the TSG time series, at least to parts that were previously flagged Good or Probably Good. It is also possible to requalify Probably Bad TSG data as Probably Good after correction has been applied.

 

Simple corrections based on least-square fitting to the differences between external and TSG data, after exclusion of outliers, are recommended. A linear adjustement is adapted in case of drifts, a constant bias adjustement when there is no obvious drift. Discontinuity in the correction should be avoided outside of harbor calls where shifts are most likely to occur. An error related to the dispersion of differences should be associated to the correction : either the uncertainty of a linear fit, or the standard deviation of differences in case of bias. Even in cases when no external data are available, a bias and error can be roughly estimated by comparison with the climatologal SSS.

 

 

References

Softwares used to perform quality control of thermosalinometer data:
Published articles on the method
  1. Alory G., T. Delcroix, P. Téchiné, D. Diverrès, D. Varillon, S. Cravatte, Y. Gouriou, J. Grelet, S. Jacquin, E. Kestenare, C. Maes, R. Morrow, J. Perrier, G. Reverdin, and F. Roubaud, 2015. The French contribution to the Voluntary Observing Ships network of Sea Surface Salinity. Deep Sea Res., 105, 1-18, http://doi:10.1016/j.DSR.2015.08.005
  2. Gaillard Fabienne, Diverres Denis, Jacquin Stéphane, Gouriou Yves, Grelet Jacques, Le Menn Marc, Tassel Joelle, Reverdin Gilles (2015). Sea surface temperature and salinity from French research vessels, 2001–2013. Scientific Data, 2(150054), 1-9. Publisher's official version : http://doi.org/10.1038/sdata.2015.54.