Measuring the salinity of the Clyde Sea from remotely sensed ocean colour

Abstract This paper describes a novel method of deriving surface salinity from remotely sensed ocean colour. The method is based on two important observations of optical properties in regions of freshwater influence (ROFI). The first is the strong effect that a form of dissolved organic matter (yellow substance) has on ocean colour when present in relatively high concentrations. The second is the close relationship between salinity and yellow substance originating from fresh water runoff. In this paper these relationships are demonstrated for the Clyde Sea, Scotland, and applied to SeaWiFS imagery for the derivation of yellow substance and surface salinity. The empirical relationships demonstrated in this study allow the satisfactory prediction of yellow substance and salinity in the Clyde Sea from remotely sensed ocean colour. The r.m.s. difference between the observed and predicted parameters are 0.19 m −1 and 1.1 for yellow substance and salinity, respectively, over a range of salinity from 16 to 34. Salinity maps created from the satellite-retrieved ocean colour identify features in the surface salinity distribution that to date have only been observed by in situ instrumentation. For example, the salinity distribution derived in this study is suggestive of an anticlockwise circulation in the Clyde Basin, driven both by the input of fresh water from the River Clyde and the intrusion of more saline water from the North Channel. The movement of deep water in the Clyde Sea has been well documented (Proc. R. Soc. Edinb. 90B (1986) 67), however, the residual circulation of surface water remains uncertain. The ability to obtain synoptic views of salinity such as those presented in this paper therefore provides great potential in furthering the understanding of Shelf Sea and coastal dynamics.