SEA7 Hydrography SAMS Research Services Ltd SEA7 Technical Report - Hydrography M.E. Inall and T.J. Sherwin SAMS Research Services Ltd Dunstaffnage Marine Laboratory Oban PA37 1QA A Marine Physics report for the Department of Trade and Industry SAMS Report No 251 November 2006 SEA7 Hydrography SAMS Research Services Ltd SEA7 Technical Report - Hydrography M.E. Inall and T.J. Sherwin This document was produced as part of the UK Department of Trade and Industry's offshore energy Strategic Environmental Assessment programme. The SEA programme is funded and managed by the DTI and coordinated on their behalf by Geotek Ltd and Hartley Anderson Ltd. © Crown Copyright, all rights reserved SEA7 Hydrography SAMS Research Services Ltd Executive Summary SEA7 is the largest of the SEA regions with an overall area of 3.6×105 km2, a volume of 4.6×105 km3 and a mean depth of 1270 m. It covers the Scottish west coast shelf, Rockall Trough and Rockall Plateau and at its furthermost point, 900 miles from the Scottish mainland at 23º 52’.26 W, extends well into the Iceland Basin. The oceanic part is a remote and, in winter, hostile region. The Iceland Basin is deepest part (3220 m) but in general is not well charted; the Rockall Plateau contains a large isolated shelf sea region (the Rockall Bank), while the Rockall Trough has a number of isolated seamounts. 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 3 m 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. SEA7 Hydrography SAMS Research Services Ltd Table of Contents 1 Introduction........................................................................................................1 2 The SEA7 Oceanic Region ................................................................................2 2.1 Bathymetry.................................................................................................2 2.1.1 The Iceland Basin..............................................................................2 2.1.2 The Rockall Plateau ...........................................................................2 2.1.3 The Rockall Bank ..............................................................................3 2.1.4 The Rockall Trough ...........................................................................3 2.2 The tides.....................................................................................................4 2.2.1 Tidal elevation...................................................................................4 2.2.2 Barotropic tidal currents ....................................................................5 2.3 Circulation and water masses.....................................................................5 2.3.1 Patterns of water movement from drifter tracks ................................6 2.3.2 Water mass types ...............................................................................7 2.3.3 The hydrography of the Iceland Basin...............................................7 2.3.4 The Rockall Plateau ...........................................................................9 2.3.5 The hydrography of the Rockall Trough ...........................................9 2.3.6 Mesoscale activity in the Rockall Trough .......................................11 2.3.7 The European Slope Current............................................................11 2.4 Topographically related non-linear processes .........................................13 2.4.1 Internal tides and non-linear internal waves ....................................13 2.4.2 Cascades...........................................................................................14 2.5 Wind and waves.......................................................................................14 2.5.1 Wind speeds.....................................................................................15 2.5.2 Wave climate ...................................................................................15 3 The SEA7 Shelf Region...................................................................................17 3.1 Introduction: in from the ocean and onto the shelf..................................17 3.2 Bathymetry...............................................................................................17 3.2.1 Background......................................................................................17 3.3 Circulation................................................................................................19 3.3.1 Background......................................................................................19 3.4 Water masses: physical and chemical characteristics..............................21 3.4.1 Temperature and salinity..................................................................21 3.4.2 Dissolved metals and nutrients ........................................................22 3.5 Tides.........................................................................................................23 3.5.1 Tidal currents...................................................................................24 3.5.2 Extreme tidal currents......................................................................24 3.5.3 Extraordinary diurnal currents: shelf waves ....................................25 3.5.4 Distribution with depth: mixing and shear stresses .........................25 3.6 Wind and surface waves ..........................................................................26 3.6.1 Background......................................................................................26 3.7 Storm surges.............................................................................................26 4 Conclusion .......................................................................................................28 5 References........................................................................................................29 SEA7 Hydrography SAMS Research Services Ltd List of Tables 1.1 The dimensions of the SEA7 region......................................................................1 2.1 Periods of the main tidal constituents....................................................................4 2.2 Definitions of water masses...................................................................................7 2.3 Transport of water in the Scottish slope current..................................................12 2.4 Mean wind speeds................................................................................................15 List of Figures 2.1 The SEA7 oceanic region ....................................................................................34 2.2 M2 elevation and phase .......................................................................................35 2.3 K1 elevation and phase........................................................................................36 2.4 M2 major and minor axes ....................................................................................37 2.5 K1 major and minor axes.....................................................................................38 2.6 S2 and O1 major axes ..........................................................................................39 2.7 Circulation of the surface waters of the North Atlantic.......................................40 2.8 Annual mean θ an S along 57.5º N......................................................................41 2.9 Annual mean σt and dissolved oxygen along 57.5º N .........................................42 2.10 Annual mean profiles in the Iceland Basin..........................................................43