A Predictably Intermittent Rotationally Modified Gravity Current in the Strait of Georgia

Abstract

Abstract The Strait of Georgia is a large and deep fjordlike basin on the northeastern Pacific coast whose bottom waters are dramatically renewed by a series of intermittent gravity currents in summer. Here, we analyze a dataset that includes moored observations from 2008 to 2021 and shipborne measurements from a 2018 field program to describe the vertical and cross-channel structure of these gravity currents. We show that the timing of these currents for more than a decade is well predicted by proxy measurements for both tidal mixing strength in the Haro Strait/Boundary Pass region and coastal upwelling on the west coast of Vancouver Island. Renewals occur as an ∼30-m-thick turbid layer extending along the right-hand slope of a broad V-shaped valley that forms the southern end of the strait. Currents are primarily along-isobath at speeds of up to 20 cm s−1 with a small downhill component. A diagnostic analytical model with a depth-dependent eddy viscosity is fitted to the observations and confirms a clockwise rotation of current vectors with height, partly driven by boundary layer dynamics over a scale of a few meters and partly driven by Coriolis forces in the near-bottom linear density gradient. Bottom drag and (small) entrainment parameters are similar to those found in other oceanic situations, and the current is “laminar” with respect to large-scale instabilities (with Froude number ≈1 and Ekman number ≈0.01), although subject to turbulence at small scales (Reynolds number of ∼106). The predictability and reliability of this accessible rotationally modified gravity current suggests that it is an ideal geophysical laboratory for future studies of such features.