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 three 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 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.