Formation and dynamics of a coherent coastal freshwater influenced system

On the Northwest European Shelf rivers provide freshwater to the coastal seas. This coastal freshwater can be misrepresented in ocean models without effective coastal resolution. This leaves an unanswered question; is freshwater retained around Scotland and what affects its variability? Here, we deploy and run an unstructured model with enhanced coastal resolution to simulate the Northwest European Shelf from 1993 to 2019, the Scottish Shelf Water-Reanalysis Service (SSW-RS) long-time run. The unstructured nature of the model grid means it more accurately captures a “bubble” of Coastal Water than a 7 km structured grid model (the Atlantic Margin Model 7 km). Surface salinity in the SSW-RS shows salinity fronts within 80 km of the coast around west and north Scotland that disintegrates east of Orkney. There are periods characterized by high coastal salinity when freshwater is more actively advected away from the coast. Empirical orthogonal function statistical analysis shows the first two modes in surface salinity account for 66% of the variance. The first mode correlates with North Atlantic Oscillation and the salinity driven velocity variability which change the salinity through advection and diffusion. The second mode correlates with Ekman transport variability where the north of Scotland acts as a wedge causing bipolar dynamics either side. Freshwater is trapped in the west, while saline water from the north reduces the freshwater pathway to the North Sea. This is important for salinity distribution, stratification in the North Sea, marine habitats and frontal transport.