A merchant ship thermo-salinograph network in the Pacific Ocean

The need for better knowledge of sea surface salinity (SSS) and sea surface temperature (SST) distribution in the tropical oceans was brought to light during the 10 years of the TOGA programme (1985–1994). In order to improve on the original “meteorological bucket” method of sampling, we developed a network of merchant ships among those operating regular routes through the tropical Pacific, and equipped them with thermo-salinographs. Accurate positioning of the ships was obtained using a satellite positioning system. High frequency sampling (every 15 s) and recording of the median values for every 5 min resulted in quite precise monitoring of the SST and SSS, and in a description of the salinity fronts along the routes followed by the ships. Using this equipment, we were able to detect the influence of local rainfall on salinity and temperature and the diurnal cycle of sea surface temperature. The accuracy of salinity measurements (0.02) was better than that of the old sampling technique by one order of magnitude. Temperature measurement by the thermo-salinograph was very stable but yielded results that are 0.2–0.3°C higher than sea surface temperature. To ensure reliability of data, the equipment was carefully checked every time the ships called at Noumea (i.e. every 2–3 months), and re-calibration every year or two is essential.