General Comments

This paper investigated the dynamics  North Brazil Current (NBC) rings and computed volume and heat transports of the surface and subsurface NBC rings. Their computations relied on the recent in situ observations from the EUREC4A-OA field experiment and satellite altimetry. Compared to previous studies, their computations  utilized the vertical structure of the eddies provided by the unparalleled vertical and spatial resolution of the EUREC4A-OA field experiment. Previous studies emphasized surface rings as the dominant force of water transport in this region. They estimated that the subsurface brazil current rings transport water mass between 0.4 Sv and 9.7 Sv, and the surface rings transport about 1.5 Sv. Their estimates cast doubt on previous estimates,  but their estimates of subsurface ring transport still has large uncertainty. The drift velocity and transported volume can depend on factors like surrounding flow, topography etc, which I think the paper should remind the reader of those factors.

Heat transport of the surface and subsurface rings were estimated to be lower than previous estimates.  I find Methods and assumptions are clearly explained, and their computations support their conclusions.

Specific Comments

1. The paragraph starting from line 344 does not have enough data to support the statements. Is it appropriate to include the paragraph? 

    

2.  The section on Cyclonic eddy seems to be isolated in the paper. I don't see the connection of this section with the rest of the paper. 

3.  In section 4.2, eddy boundaries are determined using $|\Delta EPV_z|/|EPV_x|$. However, section 3.2 line 215 says eddy boundaries are identified using a chosen isoline of $\zeta$. I get confused which method is used for the calculations in section 4.2 and what are the purposes of the two methods of identifying eddy boundaries.

4. On line 397, eddy boundaries are determined using $|\Delta EPV_z|/|EPV_x| = 30 $. How sensitive is the volume estimate to this criteria?

5. The drifting velocity of $NBC_{sub}2 $ is used to estimate transported volume. What about also using the drift velocity of $NBC_{surf}1$ to make a lower estimate of transported volume of surface rings? It seems that the transported volume of both surface and subsurface rings can have large variability due to factors like background flow, topography, seasonality etc.

Technical Corrections

1.  In Figure 12 panels (c) and (d), the x-axis extents are different from panel (a) and (b). I think it will be better to have the same x-axis limits if you have the data.

2.  In figure 9, it seems that WNACW and ENACW are shaded, but not mentioned in the caption.

    

3.   Line 303 and 332 mention "Section 1", but it's not clear which section they are referring.  Similarly,   "Section 4" on line 341 is also confusing. 

4.   Line 225 should delete word "used".

5.  In equation (7), $\Delta z$ is the layer depth. I find it confusing it to express it as $z_{sup} - z_{inf}$, which is the depth of the whole eddy.

6.  Add space on line 421 between  "km" and "captures".

7.  Add space on line 440 between "section" and "3.3".

The manuscript entitled “Mesoscale Dynamics and Transport in the North Brazil Current as revealed by the EUREC4A-OA experiment” by Barabinot et al. uses a series of in situ observations (CTD, uCTD, ADCP, MVP, Argo floats) and satellite measurements to characterize the 3-D structure of surface and subsurface North Brazil Current rings (size, Rossby number, depth, T-S properties) and estimate the associated mass and heat transport that plays an important role in the interhemispheric water exchange and, consequently, to the AMOC. The manuscript is very well written and exposed logically; the results are clear and important, particularly when it comes to quantifying subsurface eddy transport once these eddies are not detectable from altimetry. Also, the authors have done a great job with the literature review and putting their results into context by comparing them with previous studies. I am suggesting some (minor) additional work to improve the clarity of the manuscript. Thus, I recommend the publication of this manuscript after minor revisions. 

I suggest adding a figure (Figure 1) of the region of interest with some of the key currents and the NBC rings for broader context (either a schematic or satellite ADT map). This will help the readers to (1) visualize the region of interest and the dynamics associated and (2) put your results into a broader context in terms of interhemispheric water exchange and link to the AMOC.

l. 1. NBC rings are not a mechanism but features. I suggest writing ‘The North Brazil Current (NBC) rings are key features…’.

l.2. Better written as ‘…South Atlantic and North Atlantic Ocean’ .

l.3. Water masses are associated with T-S properties, so to me, the 'properties of water masses' doesn’t make much sense. I suggest writing ‘…by these structures and the water masses they advect.’

l. 10  is a bit confusing. Suggestion: ‘We estimate that the heat transport by surface and subsurface NBC rings is 5.8 TW and 0.3 TW, respectively, which is significantly lower than previous findings.’

l. 12. And -> to? ‘for South Atlantic Waters across the equator to the Tropical North Atlantic.’

l. 16-19. ‘This retroflection’ -> it is not clear that there is a retroflection based on the first line. Also, NBC rings are *formed* by NBC shedding; they do not shed *into* NBC. I suggest rewriting the first paragraph. Adding Figure 1 (broad scope) would help as well. 

l. 73. ‘Further research is needed…’ this sentence makes it sound that the current manuscript doesn’t cover this, which is not the case. I suggest rephrasing to indicate that it’s a gap and/or that you are addressing this in the current manuscript. 

l. 81. Typo in the L’Hegaret et al. 2020 citation.

l. 122 and l. 131. Add somewhere in this section that the first baroclinic Rossby radius of deformation in the equatorial region is ~ >150 km, which means that the horizontal resolution of the instruments is high enough to resolve mesoscale eddies/NBC rings (assuming you need 4 to 6 grid points to solve a feature).

l. 137. Make it clear that the changes in sign of the velocity happen in the horizontal direction 

l. 220. Across -> along depth

l. 225 Typo. Remove ‘used’.

l. 228. I suggest adding at the end: ‘…, as well as the depth variation in the eddy radius.’             

l. 233. Seawater specific heat capacity

l. 272. ‘regarding that the’ -> ‘according to the’

Figure 2 and Figure 4 are very nice!

l. 299-300. What makes the authors believe that the surface and subsurface eddies are not part of the same eddy structure? Is the small tilting enough to claim that these are individual eddies? Just curious, but I think including this in the text is relevant.

Figure 10. Great figure! Very instructive.

l. 333 typo ‘.’ After NBC_sub2

Figure 4. What is being called NBC_surf2 eddy is actually a recirculation feature. Although this feature shows as a closed SSH contour and would probably be detected in other eddy methods, this is not strictly speaking an eddy. It is just a recirculation within the retroflection (or an about-to-form eddy, but there it is still attached to the current, thus it will not drift away as an eddy would). I suggest discussing this. 

l. 501 Typo. remove ‘only transport’.

l. 505 The large transport in the surface rings compared to the subsurface ones is due to the higher temperatures but also to the larger translation velocities associated with the surface eddies. 

l. 421 space missing between ‘km’ and ‘capures’.