| 07 Aug 2025
Impact of water mass dynamical and property variability on the inflow of a semi-enclosed sea
Abstract. The biogeochemistry of the Salish Sea is strongly connected to its Pacific Ocean inflow through Juan de Fuca Strait (JdF), which varies seasonally and interannually in both volume and property flux. Long-term trends in warming, acidification, and deoxygenation are a concern in the region, and inflow variability influences the flux of tracers potentially contributing to these threats in the Salish Sea. Using ten years (2014–2023, inclusive) of Lagrangian particle tracking from JdF, we quantified the contributions of distinct Pacific water masses to interannual variability in JdF inflow and its biogeochemical properties. We decompose variability in salinity, temperature, dissolved oxygen, nitrate, and carbonate system tracers into components arising from changes in water mass transport (dynamical variability) and changes in source properties (property variability). Observations in the region provide insight into water mass processes not resolved by the model, including denitrification and trace metal supply. Deep water masses dominate total inflow volume and drive variability in nitrate flux through changes in transport. Shallow water masses, particularly south shelf water, exhibit greater interannual variability and disproportionately affect temperature, oxygen, and [TA–DIC], driving change through both dynamical and property variability. This study highlights the combined roles of circulation and source water properties in shaping biogeochemical variability in a semi-enclosed sea, and how these roles differ between biogeochemical tracers. It provides a framework for attributing flux changes to specific source waters and physical and biogeochemical drivers, with implications for forecasting coastal ocean change under future climate scenarios.
In this manuscripts, the authors present a detailed analysis of the role of changes in advection and source water properties in driving variability in water properties within the Salish Sea. Their analysis is based on Lagrangian experiments performed from outputs of an ocean model, and informed by observational data. The authors present interesting results whereby the processes governing variability differ across tracers, and discuss the implications for future changes. The manuscript is convincing, well-structured and well-written. I believe it will be ready for publication following minor revisions.
General comments
1.1 The authors currently present the results primarily in terms of years and seasons. I suggest that the authors also provide some of the results in terms of density or depth, since we can expect different water masses to contribute differently to different depth-levels. This perspective could for instance help to assess local impacts, since different areas/depths of the Salish Sea are probably fed by specific density-levels. For example, the authors could provide figures similar to Fig. 3a and 7 but plotted against density. These discussion of these results could be linked to densities presented at L260 onward.
1.2 Related to this point, it would be informative to show whether the density of water entering the JdF line varies interannually and seasonally, and show how this variability relates to changes in upwelling/downwelling or shifts in water source properties.
2. I invite the authors to clarify how observational data was used in this study, and how the inter-annual variations in the source water properties were computed. If I understand correctly, these inter-annual variations were derived from the model, but this should be stated explicitly in the method section, along with details on the procedure of how they were calculated (e.g. temporal averaging, selection of grid cells, etc.). Stating clearly how the observations will be used in section 3.3 should help alleviate the ambiguity.
In addition, I find it unclear what observations were used to produce Fig. 5., i.e. if the averaging was performed on data close to the source lines or within regions, and if so how the spatial variability within those regions was considered. One can expect water properties to differ substantially from the westernmost point to the easternmost dark-blue point on the subpanel of Fig. 1.
3.1 I have a concern regarding terminology. The authors refer to the different sources as “water masses” (e.g. L290). However, those sources are defined based on the properties at fixed lines, rather than by distinct water masses with coherent properties. The authors discuss the similarity between some of the source waters at L290-295. I recommend revising the terminology throughout the manuscript to “water sources” instead of “water masses”.
3.2 Related to this, I suggest that the authors comment on the similarity of source waters noted at L290-295 and discuss the meaning and implication of this similarity.
4. While the manuscript presents a separation into the upwelling vs downwelling season, the analysis stemming from this separation could be further developed. An attribution of the drivers per season, similar to what is presented in Fig. 7, would help discuss the impact of varying length of the upwelling and downwelling season (Fig. 2) on water properties.
Specific comments
This is really only a suggestion, but I found the title not very engaging, and would suggest finding a more engaging title. Maybe putting forward the variability in water properties, with something like “Water property variability in a semi-enclosed sea dominated by dynamics, modulated by properties”.
Throughout the manuscript, best practice would be to provide the most recent citation first when referring to multiple papers.
L64-66: I suggest supporting this statement with a reference or with a figure.
L81: For uniformity, I suggest including the range of values for oxygen as well.
L118: I believe we should read “Resolution gradually decreases” and not “increases”.
L169: Providing an histogram of the time it takes for particles to cross the different boundaries would help convince the reader that 100 days is sufficient, in addition to providing useful information about the dynamics of the region. Such a figure could go in the supplementary material.
In addition, how does the advection time compares with the duration of upwelling and downwelling events, and should a delay between the source water property definition and measurement at JdF be considered.
L171: Given the chaotic nature of particle trajectories, which will look different for different experiments due to numerical error, how will using three separate runs per analysis affect the results? For example, one specific parcel might not provide exactly the same oxygen concentration at its source location if the experiment was run multiple times, and could in this case lead to associate oxygen and nitrate concentrations from different sources. Running multiple experiments for the same tracer would allow to diagnose the uncertainty associated with this method. I suggest presenting the results from such tests.
L176: Please provide the proportion of tidally-pumped parcels.
In addition, are there parcels moving inland from the initialization line, and what is their proportion? These would not be the same as lost particles between the initialization line and sources lines, but could rather come from recirculation or surface currents? I understand that the current method, whereby parcels are saved when they reach a line, might not allow to answer this question, but tests with a smaller number of particles could.
L191-195: Since this can be seen in the figures and tables and does not affect the method, I suggest removing.
L210-212: When reading the first time, it is not very clear how this classification will be used. I suggest more explicitly stating that these are the definitions used for the source water masses. Moreover, please specify the maximum depth along the north boundary, to confirm that all waters sourced there can be defined as shallow waters.
Figure S3: Panels a-o do not highlight a clear separation. I suggest focusing on the S-property plots.
Fig. 1 :
a. In the subpanel, I suggest using a paler color for the 2000 m, since it can be confused with the coast for people not used to looking at this region.
b. I believe the initialization line is red, not brown.
L221: “grey bars in Fig. 2b, Bakun...”
Eq. 2: I would like to invite the authors to clarify Eq. 2. If we think of fields P and J as being separable into a mean and a yearly anomaly term (P = Pbase + Pyear), we would have four terms, including a crossed PbaseJbase term.
L254: Fig. 3a does not show the looped parcels. The authors should point to Fig. S6 or add the looped particles to Fig. 3.
L260: What is the full density range of the water at JdF? Having an idea of the vertical structure of the section would help visualizing what dynamics we are looking at.
Fig. 3: I suggest adding the percentages on Fig. 3a if possible, to relate the result more easily with those of Fig. 7.
Fig. 4: I suggest showing the total volume flux. Visualizing the seasonal variability of the total volume flux would help seeing how the different source waters contribute to the total variability.
Fig. 5: This is really just a suggestion, but, since the objective of this figure is to compare the water properties of different water sources, I suggest inverting the axis of the figure (hence show different water sources in x and variables in y) to ease the comparison.
L335: For 2018, it appears that property variability plays a larger role for all properties, not only TA-DIC. Maybe this is worth discussing.
Fig. 6: I suggest adding the standard deviation on the seasonal means on the right panels, to provide information on how the intra-seasonal variability compared with the separation between seasons.
L440: Near the surface, N* can be affected not only by denitrification, but also by nitrogen deposition (e.g. https://doi.org/10.5194/bg-5-1199-2008) and mixing. I suggest that the authors consider whether and how this could affect their interpretation of the N* results.
L500-504: I suggest to move this to method section, to clarify how the data was used and how the water source properties were defined.
L507: I suggest to mention this earlier, in section 3.2.