Tidally‐Modulated Turbidity and Density Fluctuations in a Rotationally‐Modified Gravity Current.

Rotationally‐modified gravity currents are known to occur on the ocean bottom in many locations around the world. Although their time scales are long enough that the Coriolis force is important in their dynamics, tidal forcing may also play a role in intermediate‐term variability by periodically enhancing or diminishing the flow and bottom friction, although the importance and implications of this variability has not previously been studied. Here we investigate hourly‐to‐tidal variations in the turbidity and density of intermittent gravity currents associated with deep‐water renewal events in the Strait of Georgia, Canada. We find that features such as the height and magnitude of the gravity current's nose, as well as bottom stress, undergo tidal variations, and that sediment resuspension largely occurs at the times of greatest stress at peak flood, with highest turbidities occurring in the hours following that peak. Larger particles soon settle again, but smaller ones can remain suspended. In addition, there is a significant tidally‐driven lateral advection of the gravity current, which results in density fluctuations in observations at a fixed location. Plain Language Summary: The ocean bottom is not flat, and in many cases heavy water moves from one ocean basin to another by flowing over sills and downhill into the next basin. These "gravity currents" are affected by both the earth's rotation, causing them to turn rightwards (in the northern hemisphere) into the slope, and by tidal currents in the ocean above. Here we find that tides speed up and slow down a gravity current in the Strait of Georgia, Canada, and that in turn means that the current can pick up particles off the bottom only at certain times in the tidal cycle when currents at the bottom are strongest. The tides also move the gravity current sideways, which makes it difficult to separate out the effects of tides within the current and their effect on the location of the current. Key Points: A rotationally‐modified gravity current in the Strait of Georgia, Canada, has strong tidal variations in its vertical structureSignificant sediment resuspension occurs at and just after the peak flood tides when tidal flow is added to the gravity currentThere is also a significant lateral advection of the current, associated with large‐scale tidal currents [ABSTRACT FROM AUTHOR]