Wind‐Driven Currents in a “Wide” Narrow Channel, With Application to Douglas Channel, BC

This paper applies a structured grid, 3‐Dimensional Regional Ocean Modeling System to examine wind‐driven currents of an idealized stratified channel, representative of Douglas Channel, British Columbia, Canada, where the increased marine activities require an improved understanding of the physical oceanography. The surface along‐channel elevation slope resulting from the wind stress is strongly affected by the surface stratification and can serve as a proxy for gauging surface stratification in operational systems. In the case of strong surface stratification, due to rotational effects an apparently narrow (width ≪ length) channel can be dynamically wide with pronounced cross‐channel variation. The thickness of the surface wind‐driven layer is scaled using the thermal wind relation, which provides a scaling factor to estimate the thickness of the surface layer. This scaling factor is not restricted to the wind‐driven flow and could be expanded to the surface mean estuarine outflow layers. Plain Language Summary Douglas Channel is the main waterway in the Kitimat fjord system in the northern British Columbia coast. In this paper, we use an idealized numerical model to study the wind‐driven responses in the channel. We find that the surface slope caused by the wind can be used as a proxy to measure how fast density changes from the surface to the deeper water. Model results show that cross‐channel flow differences exist indicating that if the density changes rapidly in the upper layer, narrow channels like Douglas Channel should be considered as wide for us to not neglect cross‐channel variance. A scaling factor is proposed to explain the thickness of the surface layer depth and this could be expanded to estimate mean estuarine outflow layers. Key Points Physically narrow channels can be dynamically wide in the case of strong surface stratification Thermal wind provides a scaling argument for determining the thickness of the surface wind‐driven layer Outflowing surface layer can be thicker than the highly stratified surface layer