Simulations of water transport over the Iceland–Scotland Ridge improved by local grid refinement around the Faroe Islands

Abstract. In the region between Iceland and Scotland, the warm water currents towards north and the cold water currents towards south are crossing a submarine ridge. Due to this complex bathymetry it is challenging for large scale ocean models to simulate the water flow realistically. High-resolution models perform better but are computationally more expensive. In this study, we investigate whether refining the model grid locally between Iceland and Scotland can improve the simulation of both the warm Atlantic water inflow and the cold deep overflow. The model grid is refined from 10 km to 3.3 km by using a two-way nest in a regional setup of the NEMO-TOP ocean model. A dye passive tracer is used to clarify the course of cold overflow water masses. Comparing the simulated water transports with in-situ observations around the Faroe Islands illustrates the benefit of the nest: 1) With the nest, more Atlantic water flows over the Iceland–Faroe Ridge and less through the Faroe–Shetland Channel, hence correcting the under-/overestimation seen in the coarse resolution model. 2) Overflow of deep water in the Faroe Bank Channel is increased by the nest to 2.14 Sv, which resembles well the observed value of 2.22 Sv. Additionally, the dye passive tracer simulation reveals that the nest enables the overflow water masses to decent to greater depths. These results indicate that the use of a local grid refinement by a two-way nest in a confined region is a promising and cost-efficient approach for improving the simulation of large scale water transports in the Atlantic.