Processes Affecting the Salinity of the Irish Sea

This paper examines, quantitatively as far as possible, the effects of rainfall, evaporation, run-off from the coasts, turbulent mixing and the mean current on the distribution of the mean salinity in the Irish Sea and its mean seasonal variation. An attempt is made to apply Knudsen's method to determine the mean flow of water through the Irish Sea, dividing it into three successive regions. The results are inconsistent unless it is assumed that the effects of transport of salt by longitudinal mixing are comparable with the effects of advection by the mean flow. By applying Knudsen's method to the central part of the Sea only, the mean flow across the Dublin-Holyhead section is estimated to be 0.35 cm./sec. (III km./year), only about one-third of the previously accepted value. The coefficient of eddy diffusion in the direction of the current, Kx, is estimated to be between 5 × 104 cm.2/sec. in the southern entrance. In the central part of the Irish Sea, Ky (transverse to the current) is estimated to be 0.36 × 106 cm.2/sec. The mean seasonal variation of salinity near the coasts is shown to be closely related to the variation in the influx of fresh water from rivers. It seems probable that the greater excess of rainfall over evaporation in summer is the main cause of the low salinity found above the thermocline in those areas in the southern entrance where temperature and salinity layering occur in summer. From the form of the solution obtained for an idealized type problem in diffusion and advection, general conclusions are drawn as to the effects on salinity of seasonal variations in run-off, excess of rainfall over evaporation, salinity of the incoming water and the rate of flow.