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  1. 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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  2. Abstract A salinity variance framework is used to study mixing in the Salish Sea, a large fjordal estuary. Output from a realistic numerical model is used to create salinity variance budgets for individual basins within the Salish Sea for 2017–19. The salinity variance budgets are used to quantify the mixing in each basin and estimate the numerical mixing, which is found to contribute about one-third of the total mixing in the model. Whidbey Basin has the most intense mixing, due to its shallow depth and large river flow. Unlike in most other estuarine systems previously studied using the salinity variance method, mixing in the Salish Sea is controlled by the river flow and does not exhibit a pronounced spring–neap cycle. A “mixedness” analysis is used to determine when mixed water is expelled from the estuary. The river flow is correlated with mixed water removal, but the coupling is not as tight as with the mixing. Because the mixing is so highly correlated with the river flow, the long-term average approximation M = Q r s out s in can be used to predict the mixing in the Salish Sea and Puget Sound with good accuracy, even without any temporal averaging. Over a 3-yr average, the mixing in Puget Sound is directly related to the exchange flow salt transport. 
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  3. Abstract. For more than a century, estuarine exchange flow has been quantified by meansof the Knudsen relations which connect bulk quantities such as inflow andoutflow volume fluxes and salinities. These relations are closely linked toestuarine mixing. The recently developed Total Exchange Flow (TEF) analysis framework, which usessalinity coordinates to calculate these bulk quantities, allows an exactformulation of the Knudsen relations in realistic cases. There are howevernumerical issues, since the original method does not converge to the TEF bulkvalues for an increasing number of salinity classes. In the present study,this problem is investigated and the method of dividing salinities,described by MacCready et al. (2018), is mathematically introduced. Achallenging yet compact analytical scenario for a well-mixed estuarineexchange flow is investigated for both methods, showing the properconvergence of the dividing salinity method. Furthermore, the dividingsalinity method is applied to model results of the Baltic Sea to demonstratethe analysis of realistic exchange flows and exchange flows with more thantwo layers.

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