Physics Sub-Project

Objectives

To identify the various processes controlling fluxes of water and particles at the European ocean margins.

To determine by in situ measurements and remote sensing the water mass structure, circulation and exchange characteristics along contrasting shelf edges for subsequent assimilation into physical models.

To develop prognostic models involving the physically controlled advective and diffusive transport in varied shelf-edge contexts.

Processes

An assessment was made of the physical processes active at the European continental margin. Several contributed typical currents of the order of 0.1 m/s or cross-slope transport of the order of 1 m²/s. Some phenomena were observed to be very much stronger. For example, currents such as the Gulf Stream at the western ocean margin, meanders and eddies on strong currents, or canyons in the shelf break were identified as potential causes of large local exchange. In areas where the shelf width is wide, tidal currents can be strong. In the OMEX I physics sub-project, the scales of processes were related to factors such as shelf width, wind strength, etc.

In situ measurements

Currents were measured by mooring instruments off south-west Portugal, over the Goban Spur and off north-west Ireland. Drogued buoys and floats were released and tracked west of Portugal and northwards from the Goban Spur. The vertical distribution of temperature, salinity and other water properties was measured in all these locations by lowering instrument packages from research vessels.

West of Portugal, these measurements together with surface temperature "maps" from satellites, showed that water originally on the continental slope east of Cape St. Vincent left the slope near Lisbon as an eddy with a diameter of between 30 and 60 km that moved westwards at an average speed of 0.05 to 0.07 m/s.  In this way water and its contents were transferred from slope to ocean.

Models

A three-dimensional prognostic model for currents, temperature and salinity was developed to provide detailed modelling of ocean margins. It used a special grid to resolve the upper layers and the steep slope. Aspects of the numerical calculation were also specially designed for these contexts.

Realistic winds provided by a meteorological model were used to force the physical oceanographic model to produce temperature and current distribution maps.  Associated routines computed fluxes and tracked particles. A sub-model of the Iberian area with a resolution of 4 km driven by locally measured winds was also developed to provide more detailed information on phenomena such as upwelling and filament formation that are associated with this region of the European continental margin.