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This report describes the physical oceanography of the Irish Sea from St George's Channel in the south to the North Channel and the Clyde Sea in the north. The report covers the topography and depth of the sea floor, the tides, the impact of storms, stratification and its consequences and the mean circulation, all with a view to mean and extreme conditions.

The predominant dynamical process is the semi-diurnal tides, with areas of both high and low energy embraced. Superimposed on this is the full range of water column structure – areas that are well mixed throughout the year; areas that stratify thermally in summer; areas of freshwater influence from river discharges, leading both to nearshore density gradients and stratification; frontal regions between the well-mixed and stratified regions.

The movement of the water and the amount of mixing prescribe physical effects, such as forces on structures and the movement and dispersion of contaminants, but also significantly influence biogeochemical processes including sediment erosion / deposition and movement, particularly of suspended sediment, benthic exchanges and primary productivity (via stratification, nutrient exchanges and light levels).

This report was prepared by Dr M. J. Howarth of the Proudman Oceanographic Laboratory.

SEA7 Technical Report - Hydrography

SEA7 lies across the northern end of the Atlantic Meriodional Overturning Circulation and contains a number of major oceanic currents carrying surface water northward across the whole region and returning cold Artic water around the foot of some of its slopes.

In general both ocean current and depth averaged tidal velocities are small, of order 5 cm s-1. Maximum speeds are about 15 to 20 cm s-1 in some local regions such as the Rockall Bank and parts of the European shelf edge.

Mesoscale eddies, internal tides and internal waves can enhance these background current velocities quite significantly, particularly in the Rockall Trough where large non-linear internal waves have been observed.

The mean wind speed over the region is about 7.7 m s-1 in summer rising to about 11.2 m s-1 in winter. The most frequent surface wave has, on average, a height of 2.4 m and a period of 8.5 s, although in winter this height increases to about 3m and in summer it decreases to about 1.5 m.

The continental slope represents a transition between the oceanic and shelf systems, and a persistent northward slope current with speeds in the range of 15 to 30 cm s-1, centred approximately over the 500 m isobath, is the physical manifestation of this transition.

To the east of the slope current in water shallower than 200 m we enter the Malin and Hebrides Shelf Seas. Here, though the net flow is still wind driven and northward (with a typical speed of 5 cm s 1), the tides dominate the flow fields with current velocities up to 4.3 m s-1 in some of the channels.

The principal tidal components are the semi-diurnal (twice daily) tides although in some limited regions the diurnal (daily) tides are significant.

The physical structure of the shelf seas is largely determined by a balance between the stratifying influences of solar radiation and fresh water run-off from the land, and the mixing influences of the strong tidally and wind driven flows, themselves shaped by the intricate and irregular bathymetry and coastline of the SEA7 region.

This report was prepared by M E Inall and T J Sherwin of SAMS Research Services Ltd.