Density currents in the Irish Sea

Abstract

Summary. Density currents in the Irish Sea are evaluated using a three-dimensional numerical model. Regimes of current are derived from two horizontal σt distributions representative of 1972 September 17—October 6 and 1971 September 6–20, respectively. Most consideration is given to the circulation of the eastern Irish Sea — defined as that sea area lying to the east of a line joining Anglesey to the Isle of Man, including Liverpool Bay. Here, it is found that water enters near the sea bottom moving generally eastwards towards the coast of Northwest England and, after upwelling, leaves near the sea surface moving in directions between west and north. The vertically integrated flow then constitutes a clockwise gyre, giving support, for the first time from hydrodynamics, to a residual circulation earlier inferred from biological observations. As part of this gyre, there is a south-going stream along the English coast. Comparing the density currents with wind-induced currents (also derived from the numerical model) it is demonstrated that the latter predominate for wind speeds above 5–10 m/s. Results obtained indicate that a common sequence of winds, changing in direction from southeast round to west, induces an anticlockwise circulation in the eastern Irish Sea in agreement with a residual circulation pattern earlier deduced from current measurements and drifter experiments. The density currents form a north-going flow of water through the Irish Sea which bifurcates around the Isle of Man, most of it passing to the west of the Isle. Southerly-type winds also maintain a northwards flow through the Sea, most of this passing to the east of the Isle. It is shown that the magnitude of the computed north-going density flow is significantly dependent on the nature of the numerical model's open boundary conditions — these being applied across the North Channel (in the north) and across St George's Channel (in the south). The pattern of density current in the eastern Irish Sea, and particularly in Liverpool Bay, appears to be relatively insensitive to alterations in these conditions.