UK Met Office NW Shelf ocean model system

The UK Met Office runs a suite of operational ocean forecasting systems for the global ocean to the UK Shelf Seas. The coastal seas models forecast ocean currents driven by the complex combination of winds, tides, and contrasts in ocean density, as well as temperature, salinity and other parameters. The coastal seas models cover the whole European continental shelf including the waters around the UK and Ireland.

The Medium-Resolution Continental Shelf (MRCS) model is a coupled hydrodynamic-ecosystem model, nested in a series of one-way nests to the Met Office global ocean model. The hydrodynamics are supplied by the Proudman Oceanographic Laboratory Coastal Ocean Modeling System (POLCOMS) and the ecosystem component is supplied by the European Regional Seas Ecosystem Model (ERSEM), developed at Plymouth Marine Laboratory (PML). It is the operational product for the Northwest Shelf Sea.

The physical model used in POLCOMS is based on the Proudman Oceanographic Laboratory three-dimensional baroclinic model POL3DB and has been developed to incorporate features suitable for the modelling of baroclinic processes on the shelf, at the shelf-slope and in ocean regions to allow long term coupled ocean-shelf simulations. Versions of this model have been running operationally at the UK Met Office since 2000, though its development can be traced back to the mid 1980s, as shown in the bibliography.

The simulation of seasonal currents, water quality parameters and plankton variability in shelf seas requires a physical model which goes beyond the well established tide and surge models to include the effects of horizontal and vertical density variations. This is necessary since the former introduces seasonal transports not seen in constant density models, while the latter controls the vertical fluxes crucial to biological production. Moreover thermal fronts affect both the horizontal and vertical transport of tracers. To this end a three-dimensional model with temperature and salinity treated as prognostic variables has been developed: the Proudman Oceanographic Laboratory Three-dimensional Baroclinic B-grid model (POL3DB). For a detailed description of the POLCOMS model, visit http://cobs.pol.ac.uk/modl/metfcst/POLCOMS_DOCUMENTATION/polcoms.html.

Since the model is designed to cover sea areas which include both shelf and deep-sea regions it includes a number of techniques pertinent to the treatment of deep water: although the model equations remain in sigma-coordinates in the vertical, the spacing of these coordinate surfaces on the finite difference grid is allowed to vary in the horizontal according to the s-coordinate transform of Song and Haidvogel (1994); the pressure gradient calculations are made by interpolation onto horizontal planes through the points where velocities are defined. A term for the variation of compressibility with temperature and salinity is included in the model equation of state for sea water. Horizontal diffusion is included as an option, but it is not needed for model stability.

The ecosystem component of the model is supplied by the European Regional Seas Ecosystem Model, initially developed collaboratively within the EU it has since been further developed at Plymouth Marine Laboratory. It was conceived as a generic model and when coupled to a qualitatively correct physical model it is designed to be capable of correctly simulating the spatial pattern of ecological fluxes throughout the seasonal cycle and across eutrophic to oligotrophic gradients. There are perhaps three reasons that allow ERSEM this flexibility. Firstly it includes detailed representations of the benthic system, which are vital for the correct treatment of shelf seas. Secondly it decouples carbon and nutrient dynamics which gives a far better approximation to how nutrient limitation acts on cells. Thirdly it can simulate both the “classical” large cell production / grazing dynamics and the small cell microbial loop, thereby representing the continuum of trophic pathways evident in marine systems.

The model is located on north-west European continental shelf, ranging from 12° W to 13° E and 48° N to 62° N, as shown in the figure above. It has a “moveable open boundary” which follows the 200 m depth contour around most of domain. The resolution is approximately 6 km in the horizontal and there are 18 S-coordinate levels in the vertical.

Baretta, J.W., Ebenhöh W., Ruardij P. (1995) The European regional Seas Ecosystem Model, a complex marine ecosystem model. Netherlands Journal of Sea Research 33, 233-246.

Blackford, J. C., Allen, J. I. and Gilbert, F. J. (2004). Ecosystem dynamics at six contrasting sites: A generic modelling study, Journal of Marine Systems, 52, 191-215.

Holt, J.T., James, I.D. (2001). An s-coordinate density evolving model of the North West European Continental Shelf. Part 1 Model description and density structure. J. Geophys. Res. 106(C7) 14015-14035.

Siddorn, J.R., Allen, J.I., Blackford, J.C., Gilbert,F.J., Holt, J.T., Holt, M.W., Osborne, J.P., Proctor, R.,Mills, D.K. (2006). Modelling the hydrodynamics and ecosystem of the North-West European continental shelf for operational oceanography. Journal of Marine Systems, doi:10.1016/j.jmarsys.2006.08.001.

SPATIAL RESOLUTION: 1/10° lat x 1/15o lon

GRID TYPE or GEOGRAPHICAL PROJECTION: regular latitude longitude grid

VERTICAL COVERAGE:The products are interpolated onto standard vertical levels (in m) [0, 3, 10,15, 20, 30, 50, 75, 100, 125, 150, 200, 250, 300, 400, 500, 600]

Forecast length: 5 days

Analysis length: 1 day

Update frequency: daily

• Horizontal velocity of the water column (currents)
• Salinity of the water column
• Temperature of the water column
• Sea level
• Suspended particulate matter (two size classes)

• Phosphate Concentration
• Silicate Concentration
• Nitrate Concentration
• Dissolved Oxygen Concentration
• Chlorophyll a
• Picoplankton biomass
• Flagellate biomass
• Dinoflagellate biomass
• Diatom biomass
• Zooplankton biomass

To access these products, please email: servicedesk@metoffice.gov.uk