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Abstract The Salish Sea is a large, fjordal estuarine system opening onto the northeast Pacific Ocean. It develops a strong estuarine exchange flow that draws in nutrients from the ocean and flushes the system on timescales of several months. It is difficult to apply existing dynamical theories of estuarine circulation there because of the extreme bathymetric complexity. A realistic numerical model of the system was manipulated to have stronger and weaker tides to explore the sensitivity of the exchange flow to tides. This sensitivity was explored over two timescales: annual means and the spring‐neap. Two theories for the estuarine exchange flow are: (a) “gravitational circulation” where exchange is driven by the baroclinic pressure gradient due to along‐channel salinity variation, and (b) “tidal pumping” where tidal advection combined with flow separation forces the exchange. Past observations suggested gravitational circulation was of leading importance in the Salish Sea. We find here that the exchange flow increases with stronger tides, particularly in annual averages, suggesting it is controlled by tidal pumping. However, the landward salt transport due to the exchange flow decreases with stronger tides because greater mixing decreases the salinity difference between incoming and outgoing water. These results may be characteristic of estuarine systems that have rough topography and strong tides.
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Estuarine Circulation, Mixing, and Residence Times in the Salish Sea
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.
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- PAR ID:
- 10449982
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Journal of Geophysical Research: Oceans
- Volume:
- 126
- Issue:
- 2
- ISSN:
- 2169-9275
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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