Topographic and eddy effects in a primitive equation model of the Iceland-Faeroes front

Abstract

The position of the Iceland-Faeroes Front is closely related to the submarine ridge connecting Iceland with the Faeroes. The ridge and the front inhibit mixing of the warmer salty North Atlantic water with the cooler fresher water to the north. Excursion of the front from the ridge, and instabilities along the front play an important role in the mixing of North Atlantic with Arctic water masses. These phenomena are studied using an idealised fine-resolution multilayer version of the Cox primitive equation model, which uses a horizontal grid spacing of 5 km and 15 vertical levels with depths not greater than 75 m. The model carries only temperature; salinity is disregarded. The model is 360 km square and has cyclic boundary conditions on the “east” and “west” boundaries perpendicular to the front, and open boundary conditions to the north and south. The model is used to investigate the growth of baroclinic instabilities on an initial idealised front having temperature contrasts and gradients appropriate to the Iceland-Faeroes Front. The effect of a topographic ridge on these instabilities is extensively investigated; the ridge can result in bottom trapping and magnification of instabilities, as seen in observations, or can suppress the surface signature of the instability, depending on how the front intersects the ridge. This simple model is also used to examine eddy propagation tendencies associated with eddy-front interactions. Eddies are formed by instabilities on the front, and motion away from and parallel to the frint is investigated as a function of eddy structure and strength, and eddy-front separation. These results are compared both with previous analytical and numerical work published in the literature, and with a sequence of infrared images of the area obtained by satellite. The effects of using differing frictional representations, namely harmonic and biharmonic diffusion, are also investigated. In particular, the effect on sub-mesoscale motions of these different frictional formulations is examined in terms of underwater sound propagation through frontal regions.