Review of “Controls on dense water formation along the path of the North Atlantic subpolar gyre” by Tooth et al.

The authors present a Lagrangian analysis of the dense water formation in the North Atlantic subpolar gyre. To advect the Lagrangian particles, the authors use three-dimensional ocean velocities from a 1/12° NEMO-based, forced ocean model. The particles are initialized along OSNAP East and run forward in time to diagnose their trajectories around the subpolar gyre. The authors find that the primary control on variability in the formation of dense water is the strength of the subpolar circulation, rather than variability in the water mass transformation along the particles’ trajectories.

I found the paper to be fascinating and well-presented. The text is dense but understandable and the figures clearly demonstrate the points the authors make in the text (and vice versa). I have a few minor edits below, but I recommend the paper be accepted pending minor revisions.

Suggestions:

• Why is the focus of this paper on dense water formation rather than the AMOC explicitly? According to the findings from OSNAP, there is no connection between dense water formation and dense water export (i.e., AMOC), see Zou and Lozier (2016).
• The authors should explain how their results impact the idea that temperature or salinity anomalies propagate on a steady ocean circulation (e.g. Sutton and Allen, 1997; Årthun et al., 2017), rather than a varying ocean circulation creates temperature and salinity anomalies (e.g. Foukal and Lozier, 2016; Desbruyères and Chafik, 2021).
• Fig. 1: the model streamfunction is also broader than observations, which indicates that the upper limb waters are lighter than observed and the water mass transformation in the subpolar North Atlantic is larger than observed. Furthermore, there is considerably more formation of very dense waters (sigma>27.75 kg/m³), which implies that this model suffers from the well-known issue of too strong convection in the Labrador Sea (e.g., Menary et al., 2020). This issue should be discussed in the conclusions as a limitation of the study.
• Line 100: OSNAP imposes a -1.6 Sv flow through OSNAP West and a +1.6 Sv flow through OSNAP East (Lozier et al., 2019). Did the authors consider the effect of this northward flow across OSNAP East as well?
• Fig. 2: How does the strength of the ‘SPG pathway’ compare to a Eulerian measure of SPG strength, such as from OSNAP?
• Fig. 3: This is a beautiful figure – please overlay isopycnals on panels b and d to look at baroclinicity in the water column. The strength of the baroclinicity in different parts of the region could explain why some water masses make it over the Greenland-Scotland Ridge and some are retained in the subpolar basin.
• Line 365: Hakkinen and Rhines (2004) used an EOF of SSH to derive their ‘gyre index’, not a SSH gradient as indicated in the text here. See Foukal and Lozier (2017) for a discussion of the ‘gyre index’ in comparison to a SSH gradient metric. See also Chafik and Lozier (2025) for further discussion of why the gyre index is not a good metric of subtropical-to-subpolar connectivity.
• Line 384-386: This paragraph should include the context that this relationship occurs in the model they are analyzing, and may not apply to the real ocean. The authors should consider adding this caveat to other parts of their paper as well.
• Figure 1 appears before its first mention (line 90).

References:

Årthun, M., Eldevik, T., Viste, E. et al. Skillful prediction of northern climate provided by the ocean. Nat Commun 8, 15875 (2017). https://doi.org/10.1038/ncomms15875

Chafik, L., & Lozier, M. S. (2025). When simplification leads to ambiguity: A look at two ocean metrics for the subpolar North Atlantic. Geophysical Research Letters, 52, e2024GL112496. https://doi.org/10.1029/2024GL112496

Desbruyères, D., Chafik, L. & Maze, G. A shift in the ocean circulation has warmed the subpolar North Atlantic Ocean since 2016. Commun Earth Environ 2, 48 (2021). https://doi.org/10.1038/s43247-021-00120-y

Foukal, N., Lozier, M. No inter-gyre pathway for sea-surface temperature anomalies in the North Atlantic. Nat Commun 7, 11333 (2016). https://doi.org/10.1038/ncomms11333

Foukal, N. P., and M. Susan Lozier (2017), Assessing variability in the size and strength of the North Atlantic subpolar gyre, J. Geophys. Res. Oceans, 122, 6295–6308, doi:10.1002/2017JC012798.

Häkkinen, S., and Rhines, P. B. Decline of Subpolar North Atlantic Circulation During the 1990s.Science304,555-559(2004).DOI:10.1126/science.1094917

Lozier, M.S. and 37 co-authors (2019). A sea change in our view of overturning in the subpolar North Atlantic.Science363,516-521.DOI:10.1126/science.aau6592

Menary, M. B., Jackson, L. C., and Lozier, M. S. (2020). Reconciling the relationship between the AMOC and Labrador Sea in OSNAP observations and climate models, https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2020GL089793

Sutton, R., Allen, M. Decadal predictability of North Atlantic sea surface temperature and climate. Nature 388, 563–567 (1997). https://doi.org/10.1038/41523