Please see the attached document CommentsToAuthors.pdf.

Evans et al., examined overturning and water mass transformation in the region north of the OSNAP regions, from observations, reanalysis, and model. They break down the transformation in density, temperature, salinity, and temperature-salinity spaces. This type of detailed work is useful for our understanding of the water mass transformation of the region.

There are some drawbacks, however. The most important one is that there is a large difference among three products, and it is not clear how to understand them. 1. The time frame difference MAY contribute to part of the difference (and examine the result over some same full-year period would be helpful). 2. The nature of the data-assimilated products. For example, although one can simply call the residual as contribution due to mixing, it is however not necessarily due to the diapycnal mixing that is prescribed in the model. Some of the “mixing” contribution is due to the data-assimilation (which may be viewed as a “forcing” term that takes place in the full water column when there is difference between model and data profile). I am not sure what is the best way to address this issue.

Details

Title suggests “… in the subpolar North Atlantic” whereas the work clearly includes the Nordic Sea.

L6. “Complementary roles of air-sea buoyancy and mixing in setting …” because the mixing term is diagnosed as the difference between (the overturning and buoyancy forcing), it is probably more appropriate to say that the air-sea buoyancy flux alone does not account for all the transformation and mixing term can be quite significant.

L12. Not sure why single out “climate models” here, all models need that. Also, I think one other key result, other than the importance of mixing which has been previously documented in numerical model, the difference among different products is large. And some discussion on this difference would be helpful.

L22. I guess “Bower et al. (2019)” here is referring to the spreading of NADW, but the way it is included here is a bit strange as it reads like a reference for “slightly dense water masses formed in the Irminger and Lab Seas…” even if it is for the NADW spreading, it should be noted that Bower et al. (2019) emphasized Lagrangian view, you may want to add some reference in Eulerian view as well (Rhein et al., 2015-JGR)

Rhein, M., D. Kieke, and R. Steinfeldt (2015), Advection of North Atlantic Deep Water from the Labrador Sea to the southern hemisphere, J. Geophys. Res. Oceans, 120, 2471–2487, doi:10.1002/2014JC010605

L25. There are many papers in the subpolar region, and I honestly do not expect one to include all. But I think some are important under the topic of “water mass transformation in the subpolar North Atlantic” and should be included. For example, Brambilla et al. 2008; Marsh 2000; Grist et al., 2014.

Brambilla, E., L. D. Talley, and P. E. Robbins, 2008: Subpolar mode water in the northeastern Atlantic: 2. Origin and transformation. J. Geophys. Res., 113, C04026, https://doi.org/ 10.1029/2006JC004063.

Marsh, R., 2000: Recent variability of the North Atlantic thermohaline circulation inferred from surface heat and freshwater fluxes. J. Climate, 13, 3239–3260, https://doi.org/ 10.1175/1520-0442(2000)013,3239:RVOTNA.2.0.CO;2.

Grist, J. P., S. A. Josey, R. Marsh, Y. O. Kwon, R. J. Bingham, and A. T. Blaker, 2014: The surface-forced overturning of the North Atlantic: Estimates from modern era atmospheric reanalysis datasets. J. Climate, 27, 3596–3618, https://doi.org/ 10.1175/JCLI-D-13-00070.1.

The terminology is a bit strange (to me), and I assume you have followed previous study. Your total transformation is really the tendency or the drift (dv/dt), which should be close to zero if one thinks the ocean is close to a steady state. To me, the sum of overturning and the tendency should be the total transformation, which then break down into the transformation due to air-sea fluxes and due to mixing.

Table 1. It would be useful to list the time frame considered. I assume it is the OSNAP observational period for observations, and it was mentioned that 1992-2018 is used for ECCOv4r4. But I did not find the time information for Reanalysis. Time frame difference can contribute the difference between the three products. For example, the total transformation (tendency term) is large in observations in part because it includes some seasonal variability (as OSNAP period is short and not full year), whereas this term is much smaller in ECCOv4r4 (as it is long-term and full year).

L168. It is probably inappropriate to call ECCOv4r4 model based, ECCO is a state estimate which assimilates a lot of data just like reanalysis.

L197 and Figure 1. It is unclear the exact region “the subpolar North Atlantic and Nordic Seas” referred to. I was mentioned north of OSNAP, so it also includes the northwestern Lab Sea, the Buffin Bay (i.e., north of the OSNAP-west). On the eastern side, where is the northern boundary of Nordic Seas? Does it include the entire Arctic Ocean?

L210. Figure 1 include three panels and there seems quite some similarity and difference, none of them was mentioned in the manuscript.

L285. 35.17 g/kg I guess?

Diathermal transformation in Figure 3, why the air-sea flux term differs so much, especially in ECCO there is large transformation close to -2C, is that due to ice?  In general, working on temperature and salinity space is quite dangerous for large area, because water masses with very different density and source may end up at similar temperature/salinity.  And it is quite difficult to understand the difference between Figures 2-4 (do you change signs? That the overturning is on positive side in Figure 2 but on negative side in Figures 3-4)