Estuarine Circulation, Mixing, and Residence Times in the Salish Sea

Abstract

The exchange flow is widely recognized as a defining property of tidally averaged estuarine circulation. The persistent inflow of deep, salty water and outflow of shallow, somewhat fresher water is often many times greater in volume flux than all the rivers entering a system. As a result, the exchange flow controls residence times and biogeochemical gradients. Physically this circulation is a result of the density contrast between ocean and river water, combined with mixing and advection from tidal currents (Geyer & MacCready, 2014; MacCready & Geyer, 2010). The theory of estuarine exchange flow was developed and tested for shallow, straight coastal plain estuaries such as the Hudson (Hansen & Rattray, 1965; Ralston et al., 2008) but is less well-developed for more complex systems. In this paper we focus on the circulation of one of these complex systems-the Salish Sea, a large, interconnected system of very deep basins and shallow straits forced by Abstract A realistic numerical model is used to study the circulation and mixing of the Salish Sea, a large, complex estuarine system on the United States and Canadian west coast. The Salish Sea is biologically productive and supports many important fisheries but is threatened by recurrent hypoxia and ocean acidification, so a clear understanding of its circulation patterns and residence times is of value. The estuarine exchange flow is quantified at 39 sections over 3 years (2017-2019) using the Total Exchange Flow method. Vertical mixing in the 37 segments between sections is quantified as opposing vertical transports: the efflux and reflux. Efflux refers to the rate at which deep, landward-flowing water is mixed up to become part of the shallow, seaward-flowing layer. Similarly, reflux refers to the rate at which upper layer water is mixed down to form part of the landward inflow. These horizontal and vertical transports are used to create a box model to explore residence times in a number of different sub-volumes, seasons, and years. Residence times from the box model are generally found to be longer than those based on simpler calculations of flushing time. The longer residence times are partly due to reflux, and partly due to incomplete tracer homogenization in sub-volumes. The methods presented here are broadly applicable to other estuaries. Plain Language Summary The Salish Sea is a large estuarine system that includes the cities of Vancouver on the Strait of Georgia and Seattle on Puget Sound. Despite the many rivers flowing into the Salish Sea, the water in the system is mostly ocean water, and there is rapid exchange with the ocean. This exchange is important because it brings in most of the nutrients that feed the ecosystem, and it flushes the system relatively rapidly, leading to generally good water quality. Nonetheless, there are places and times in the Salish Sea that experience problems like hypoxia and fish kills. The goal of this work is to clearly describe the patterns of circulation and mixing throughout the Salish Sea so that we may understand th...

Propagation of the 2014–2016 Northeast Pacific Marine Heatwave Through the Salish Sea

“…Sustained warming of Salish Sea waters was due to the combined effect of higher atmospheric heat flux (+ 19% relative to reference 45.2 W/m 2 ) and conveyance of warmer waters from the shelf via strong estuarine exchange flow into the Salish Sea. The magnitude of the estuarine flow is estimated in the range of 100-150 × 10 5 m 3 /s (Sutherland et al, 2011;Khangaonkar et al, 2017Khangaonkar et al, , 2018Olson et al, 2020), MacCready et al, 2021). It is a function of mean water depth (∝ H 3 ) and depth-averaged density (salinity) gradient (∝ ∂ S ∂X ) as presented analytically by Hansen and Rattray (1965), Dyer (1973), andMacCready (2004), MacCready (2007) for partially mixed estuaries, by Rattray (1967) for fjords, and by Khangaonkar et al (2011) for Salish Sea subbasins.…”

Toward a conceptual framework for managing and conserving marine habitats: A case study of kelp forests in the Salish Sea

“…Situated in a temperate rainforest and composed of deep fjords and large glacial‐fed estuaries, the oceanography of the Salish Sea is distinct from many of the other regions represented in our global temperate literature search. The estuarine environment is unusual for kelp, with periodic or seasonal changes in salinity, temperature, turbidity, and other water column parameters that are often much larger than observed in open coast environments where most kelps are found (MacCready et al, 2021 ). Research has shown that kelps can exhibit population‐level differences in response to environmental stress (Buschmann et al, 2004 ; Flukes et al, 2015 ; Hollarsmith et al, 2020 ; King et al, 2019 ), and recent population genetic work on bull kelp in the Salish Sea revealed distinct genetic clusters that aligned with oceanographic currents, geographic and benthic features, and environmental variables (Gierke, 2019 ).…”

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

“…However, these views of the circulation are often qualitative since speeds are not well known, and in certain cases a geostrophically controlled circulation could be directed along isopleths rather than across them. Quantitative tracer-based approaches include the ''box-model''-based Knudsen relations for estuarine flow (Hansen and Rattray 1965;MacCready et al 2018) or more complex variations thereof, applied locally by Pawlowicz (2001), Pawlowicz (2014), andMacCready et al (2021); or optimal water mass analyses (Tomczak 1981)-this latter approach was previously used along the thalweg of the Salish Sea as a whole by Masson (2006).…”

“…properties-which can be separated from the mean flux via a Reynolds decomposition and termed ''eddy flux'' (Davis 1991)-depends on details of the time-varying current field, and is important to the transport of tracers in energetic regions of the ocean (Wunsch 1999). Numerical investigations have been attempted for the Salish Sea circulation on a number of occasions, but have generally concentrated on circulation aspects closer to the Pacific entrance (Marinone and Pond 1996;Masson and Cummins 2004;MacCready et al 2021).…”