Influence of Coastal Wind Jet on Waves in Fjords: A SWAN+WRF Model

Wind and wave fields undergo drastic changes in fjords due to complex coastline and steep topography of the surrounding mountainous terrain. Highly variable wind amplification during funnelling through mountain gaps has been studied by meteorologists, whereas the influence of complex bathymetry and/or orography around fjords on wind and circulation and ecosystem has been investigated by oceanographers. However, the interaction between wind and waves is not well understood, especially in large fjords, where the interplay of local and remote wind effects on waves becomes crucial for wave dynamics in fjords. Recently, several studies have examined wind and wave funnelling in fjords; however, there is still a lack of understanding of the underlying processes in relatively large fjords. This paper provides the first comprehensive comparison study on the highly variable wind amplification caused by the mountains and islands funnelling effects, and its impact on wave propagation and transformation in multiple large sea lochs (fjords) in West Scotland. To investigate the effects of wind jet formation on the wave field inside Scottish fjords, we used the Simulating WAves Nearshore (SWAN) model driven by the Weather Research and Forecasting (WRF) model and ECMWF ERA5 reanalysis data. During Storm Eva in winter 2015, the maximum differences in the predicted wind speed and wave height between WRF-SWAN and ECMWF-SWAN, with 2 km and ∼27 km resolution, are 8 m·s -1 and 1 m inside Loch Linnhe, 4 m·s -1 and 0.5 m in Loch Hourn, and 1 m·s -1 and 0.5 m inside Sound of Jura, respectively. These differences arise primarily from the higher resolution of the WRF model, which better captures the wind field variability channelled by the fjord’s orography. As a result, wave exposure is more variable under the WRF forces. The sea states within the fjords vary according to their size, geometry, orientation, and prevailing wind directions. Thus, in the wide and long Loch Linnhe fjord system, which is exposed to prevailing onshore south-westerlies wind, wave heights increase towards the inner fjords in response to local wind forcing. In contrast, both wind and waves decrease in the Sound of Jura, as its more north-south orientation deviates from the prevailing south-westerly winds. Therefore, local wind dominates the wave conditions within the fjords, while the interaction between remote and local wind-generated waves becomes more significant further offshore. Spectral energy shift from low- to high-frequency waves is predicted by both WRF-SWAN and ECMWF-SWAN, but the WRF-SWAN yields a higher wave energy density nearshore. The present study provides new insights into complex, coupled wind-wave dynamics and their potential ecological impacts on fjords and surrounding marine environments.