The salinity field of the ocean is a natural integrator of the surface
water flux. Thus, it can provide exceptional insight into the maritime component of the global water cycle. Since 87% of global evaporation and 78% of global precipitation are estimated to occur over the oceans, any improvement in understanding the water cycle and how it may change with climate must involve the oceans. Indeed, as the thermohaline circulation has a particularly sensitive response to the water cycle, adequate understanding of the climate system itself depends crucially on improved salinity measurements. However, salinity has always been one of the more difficult variables to measure, and the lack of data has stifled progress in developing an accurate picture of the oceanic water cycle.
Recently, progress has been made in the measurement of salinity from
autonomous profiling floats. Initiated by R. Davis at the Scripps
Institution of Oceanography, several groups are now making quality
salinity measurements from profiling floats. These drift at depth for
periods of 1-4 weeks, then rise to the surface and report position and a profile of temperature and salinity via satellite. This process is
repeated for ~ 100 profiles, yielding a very low cost per profile. The ARGO proposal of Roemmich et al seeks to maintain an array of ~3000 profiling floats around the globe with an average spacing of around 300 km. This will provide a basic monitoring array for the global salinity field. The 1500 m depth of the profiles, and the steadiness of the deep temperature-salinity relationship, provides an important means of evaluating sensor performance. Thus, variability in upper ocean salinities can be monitored reliably. Recent performance of such floats is reviewed. It is suggested that a program which integrates salinity data from floats with the products of thermosalinograph records from research vessels and volunteer observing ships would make a significant contribution to our knowledge of the upper ocean salinity field, and provide essential "sea truth" for proposed remote sensing techniques. As an integrator of evaporation minus precipitation, salinity can provide a more sensitive indication of the water flux than is achievable by differencing these two large and uncertain variables. Thus, global salinity measurements must have high priority in future climate research efforts.