A numerical study of rectification driven by a tidal jet

Abstract

The barotropic tide in fjords is often assumed to play a negligible role in driving the circulation over timescales greater than the tide itself, the residual circulation. However, this is not necessarily true when the tide interacts with the fjords’ topography. Of particular interest are narrow channels, where the tide can excite non-linear flow features such as flow separation eddies and establish a tidal jet. The jet and associated eddies are known to influence transport through the channel and dominate local dispersion. We investigate how the jet can influence the residual circulation in Kvænangen, a Northern Norwegian fjord. We model the flow dynamics using an unstructured grid ocean model, solving the shallow water equations. The model results show that the jet can drive residual currents around the fjords basin if the jet forces a vorticity flux across closed bathymetric contours. We demonstrate that the rectification is sensitive to how the channel is represented in the ocean model. With a wide channel, the fjord-scale residual circulation diminishes. Our work thus highlights the importance of accurately resolving the actual geometry of the coastline, since small-scale flow phenomena generated in narrow channels can force a far-field circulation response and consequently influence the drift of tracers in the fjord.