A study of intermediate water circulation in the Strait of Georgia using tracer-based, Eulerian, and Lagrangian methods

Abstract

The intermediate circulation of the Strait of Georgia, British Columbia, Canada, plays a key role in dispersing contaminants throughout the Salish Sea, yet little is known about its dynamics. Here, we use hydrographic observations and hindcast fields from a regional 3D model to approach the intermediate circulation from three perspectives. Firstly, we derive and model a “seasonality” tracer from temperature observations to age the water, estimate mixing, and infer circulation. Secondly, we analyze modeled velocity fields to create mean current maps and examine the advective and diffusive components of the mean flow field. Lastly, we calculate Lagrangian trajectories to derive Transit Time Distributions and Lagrangian statistics. In combination, these analyses provide an overview of the mean intermediate circulation that can be summarized as follows: subducting water in Haro Strait ventilates the intermediate water primarily via an up-strait boundary current that flows along the eastern shores of the southernmost basin in 1–2 months. This inflowing water is either incorporated into the interior of the basin, recirculated southwards, or transported into the northernmost basin, mixing steadily with adjacent water masses during its transit. A second, shallower ventilating jet emanates southwards from Discovery Passage, locally modifying the Haro Strait inflow signal. Outside of these well-defined advective features, diffusive transport dominates in the majority of the region. The intermediate renewal signal fully ventilates the region in 100–140 days, which serves as a benchmark for contaminant dispersal timescale estimates.

Stark Physical and Biogeochemical Differences and Implications for Ecosystem Stressors in the Northeast Pacific Coastal Ocean

“…Here, a small seasonal increase in nutrient concentrations (Figure 7b) and a decrease in temperature (Figure 3d) attain their maximum and minimum values respectively in July, with no accompanying increase in deep density (Figure 7a) or salinity (Figure 3b) (these reach a maximum in October/November). These differences in seasonal cycle magnitude and the timing of the arrival of upwelled water in QCSt/JS versus the SoG/DI regions may be related to the modification that occurs en route to the southern Strait of Georgia (LeBlond et al., 1991; Mackas & Harrison, 1997; Stevens et al., 2021), as discussed in Section 4.1.…”

“…By contrast, open‐ocean water enters the SoG via the Strait of Juan de Fuca to the south of Vancouver Island, undergoing significant modification as it passes over the sills in Haro Strait en route (Johannessen et al., 2014; LeBlond et al., 1991; Mackas & Harrison, 1997) and further steady mixing during its transit to the northern end of the SoG (Stevens et al., 2021). The transit time from the open ocean to the northern SoG is expected to be on the order of months, with transit times for intermediate water from Haro Strait to the northern SoG expected to be 4–5 months (Stevens et al., 2021). The residence time for deep SoG waters is estimated to be a year or more (MacCready et al., 2021; Pawlowicz et al., 2007; Sutton et al., 2013).…”

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

“…Under this is an estuarine inflow. At intermediate depths from 50 to 200 m there is a continual inflow into the Strait from both the northern and southern passages, and a strong recirculation (LeBlond et al., 1991; Stevens et al., 2021). Below 200 m the Strait has a deep water which remains stagnant through the winter, with decreasing concentrations of dissolved oxygen and decreasing density, but is replaced several times over the summer by a series of intermittent renewal events that also increase bottom density.…”

Seasonal and Interannual Salish Sea Inflow Origins Using Lagrangian Tracking

“…Lagrangian ocean analysis tracks free moving entities, real (e.g., Paris et al., 2013; Pawlowicz et al., 2019) or virtual (e.g., Brasseale et al., 2019; Stevens et al., 2021), to estimate ocean pathways by applying the Lagrangian lens of fluid dynamics (Bennett, 2006). In a virtual sense, this method tracks an ensemble of simulated water parcels to see their path and how their compositions change along this path based on the time varying velocity fields of an ocean model (Van Sebille et al., 2018); often leading to complex and unpredictable paths (LaCasce, 2008).…”