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. The outflow of riverwater into the sea around Scotland is important for marine habitats, nutrient concentrations and the transport of pollution or planktonic larvae. The response of the sea to the sum of all river outflow is often missed in simulations of the ocean and leaves questions about how much water is retained near the coast and when it can escape. In our ocean simulation the Scottish Shelf Water-Reanalysis Service long run, we tackle this problem with high coastal resolution and an extensive river data set. Our simulation shows an improvement in salinity over a coarser resolution model of the same region around Scotland. Generally fresh, river-sourced water is retained by a front within 80 km of the coast around Scotland to the west of Orkney but this pattern is disrupted to the east of Orkney. Statistics show the majority of the variability in the freshwater around Scotland goes through stages at the time period of years when is it captured close to the coast and stages when it is more able to move away from the coast. This is found to correlate with density driven currents and the wind speed which uses the north of Scotland as a wedge for the water, causing bipolar dynamics either side. Our simulation accurately resolves coastal freshwater due to its variable resolution A coastal salinity front is found west of Orkney with periodic release of its freshwater east of Orkney The movement of freshwater into and away from the coastal region is influenced by salinity driven currents and wind