Processes Affecting the Salinity of the Irish Sea

Abstract

Summary 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.

Pelagic dispersal of Norway lobster Nephrops norvegicus larvae examined using an advection-diffusion-mortality model

“…There are relatively few observations of mean currents in the western Irish Sea so much of the evidence is indirect. Salt and caesium-l37 budgets for the Irish Sea suggest a basln-averaged northward mean flow of only 0.012 m S-' (Bowden 1950, Wilson 1974. Furthermore, during late spring and summer (May onwards) a pronounced tidal mixing front is established in the western Irish Sea (Sirnpson & Hunter 1974) and water above the mud patch region become thermally stratified.…”

“…Larvae are released from a mud patch of dimensions L, = L , = 100 km and are subject to a natural mortality m = 0.04 d-l. Turbulent diffusion on the European continental shelf is relatively large because of the highly energetic tidal regime there and a reasonable estimate for K in the Irish Sea is 500 m2 S-' (Bowden 1950, Prandle 1984). In the model, K has been varied from 50 m2 S-' to a n extremely large value of 2000 m2 S-'.…”

Transport of Sellafield-Derived 14C from the Irish Sea through the North Channel

“…Ambient 14 C background activity from natural production and nuclear weapons testing fallout will differ between the four biogeochemical fractions and must be subtracted from the measured activities to obtain 14 C activities due to the Sellafield discharges. As the main source term for Irish Sea water is the North Atlantic (Bowden 1950), the DIC background was estimated from measurements on near-shore water samples and seaweed from Burtonport, on the west coast of Ireland (Figure 1). Burtonport is a suitable sampling site as the water exiting the Irish Sea, through the North Channel, is tightly entrained along the west coast of Scotland and studies have shown that there is negligible activity from Sellafield discharges to the west of Malin Head (McKinley et al 1981;Jefferies et al 1973).…”

“…Removal of water and associated radionuclides from the Irish Sea occurs via these small residual currents through the North Channel (Ramster and Hill 1969;Howarth 1982). The water flux through the North Channel is highly variable and previous studies have shown a range of fluxes varying between 1.72 and 7.76 km 3 day −1 (Bowden 1950;McKay and Baxter 1985;Jefferies et al 1982;Howarth 1982;Brown and Gmitrowicz 1995;Knight and Howarth 1999;Young et al 2000). There is also a counter current running in from the Atlantic Ocean along the Irish Coast although it is small in comparison to the total flux (Brown and Gmitrowicz 1995).…”

Behaviour influences larval dispersal in shelf sea gyres: Nephrops norvegicus in the Irish Sea

“…A recent modelling investigation found evidence to suggest that there is a long-term trend in the dynamics of the gyre due to changes in atmospheric conditions, resulting in a stronger, but less retentive gyre (Olbert et al 2011). The long-term mean flow through the Irish Sea is considered to be weak (< 0.01 m s −1 ) and northwards (Bowden 1950, Wilson 1974.…”