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Abstract Estuarine exchange flow regulates aspects of estuarine biogeochemical processes; however, other tracer‐specific factors can also play an important role. Here, we analyze realistic simulations from a coupled physical‐biological model to quantify volume‐integrated budgets of heat, total nitrogen (TN), and dissolved oxygen (DO) in the Salish Sea and its sub‐basins. Our goal is to evaluate the role of exchange flow in shaping tracer budgets, extending beyond the traditionally emphasized salt budget in estuaries. The three budgets reveal that exchange flow is a consistently important term with a clear annual cycle, but its relative role differs across tracers. For heat, exchange flow‐driven cooling is primarily offset by atmospheric heating, with the two reaching opposing seasonal extremes in summer. For TN, seasonal variability is dominated by exchange flow, whereas the annual mean is dominated by inputs from rivers and wastewater outfalls, and a loss due to benthic denitrification. The DO budget is the most complex: sinks from exchange flow export and respiration are balanced by sources from photosynthesis and air‐sea transfer. Across all three budgets, the sign of the inflow‐outflow tracer concentration differences determines whether exchange flow imports or exports tracers. These concentration differences, which are strongly influenced by coastal wind conditions, set the distinct seasonality of the exchange flow budget terms, while variations in the exchange flow volume transport play a minor role. Our budget quantification approach, based on archived model output, can be extended to other tracers such as carbon and other estuaries for long‐term budget studies.
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Tracer Budgets on Lagrangian Trajectories
The Lagrangian particle method is widely used to understand scalar tracer concentration fields in models of the atmosphere and oceans. Simulating virtual particles provides an alternative description of advection to the Eulerian representation in models and aids in identifying pathways, timescales, and connectivity. Atmospheric and oceanic models solve advection‐diffusion‐reaction equations to simulate tracers, in which only the advective component is captured by traditional Lagrangian approaches. In this work, we report a novel method that closes tracer budgets on Lagrangian trajectories in a manner consistent with Eulerian budgets in finite‐volume models. The scalar tracer concentrations on grid cell walls are derived from the model advection scheme and then interpolated inside grid boxes along streamlines. The divergence of the diffusive flux and reaction terms are interpolated based on velocity and tracer concentration, ensuring the tracer budget closes in terms of both trajectory and volume integrals. Compared to the Eulerian budget analysis, which considers a fixed volume, our method quantifies the tracer evolution within a volume that moves along with the flow. We demonstrate the method using a case study of Southern Ocean biogeochemistry. Another case study involves analyzing the heat budget of the 2011 Western Australian marine heat wave. The method bridges the gap between Eulerian budget and Lagrangian particle analyses by representing the advective processes with particle movements and interpolating the diffusive and reactive processes onto trajectories in a way consistent with the finite‐volume description.
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- PAR ID:
- 10672031
- Publisher / Repository:
- American Geophysical Union
- Date Published:
- Journal Name:
- Journal of Advances in Modeling Earth Systems
- Volume:
- 17
- Issue:
- 9
- ISSN:
- 1942-2466
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript
- Journal Article:
- https://doi.org/10.1029/2024MS004848
