Review of “The 2023 Marine Heatwave In The North Atlantic Tropical ocean” by Loubet et al.

This work presents an analysis of the 2023 marine heatwaves that happened in the North Atlantic Ocean in 2023, an unprecedented year in terms of temperature anomaly and extreme values. The work is timely and interesting, although I have the impression it lacks some depth in the discussion of the results presented, which may be imposed by the lengths of the works submitted to this issue. This makes the manuscript superficial at times, and we have the impression the authors rush over some of the topics.

My major comment would be for the authors to include more detailed information (including a figure) on the spatio-temporal distribution of the mixed layer depth, which is an important parameter in the sub-surface propagation of marine heatwaves and its role depends on the region.

Hereafter some detailed comments, in order of appearance:

- Abstract: It is mentioned that the MHW peaks in June, but in the short summary after the abstract it says May.

- Line 51. I have seen the claim that subsurface MHWs are less studied multiple times, but the field of MHW is being studied by a large amount of scientists, and subsurface MHWs have received a lot of attention as well. The authors themselves provide a good list of references about subsurface MHWs. I would rephrase this or remove this sentence.

- Line 56: “but require the system…” I don’t understand the “but” here, it looks like you are presenting reanalysis as negative because they require all these things (data assimilation, high spatial and temporal resolution…) but you are going to use a reanalysis, so why presenting it as if it was a hindrance?. Many reanalysis assimilate data, and have the necessary temporal coverage Also, in order to study open waters MHW, the requirement of high spatial resolution is not that crucial, and most reanalyses would have a sufficient resolution. In fact it would be nice to know why your choice of reanalysis (GLORYS 12V1 from CMEMS) is the adequate with respect to others.

- Line 94: some symbols are missing in the formula so I cannot assess its correctness

- Figure 1. The lines used in panels b are too similar in colour (I printed the paper to read it and I had to revert to screen for the images, but even so I found them too similar). Also, on panel c, you choose to colour events by mean activity, but the events display an increasing Activity with increasing duration and intensity (x and y axes), so first come all greys, then the greens, etc, so not much newer information is shown. Maybe colouring by decade would give us information of which years are anomalous in the progressive increasing trend? For example, 1998 would stand as a anomalous year for its decade.

Figure 2: too many colour in panel b, I cannot really see anything… I would colour by season form example, or at least use the same palette for each season (shades of blue for winter, shades of orange in fall… But I think one colour per season should be more than enough.

Line 72: “Evolution of MHW Intensity and Extent” add “at depth” to the title?

Line 78: May (not mai)

Figure 3. Again the colour here are too similar and in panel a we cannot really see the difference between NASE and CARB. In panels b, c and d, consider using the same colours for the same depth (i.e. 40 m, continuous orange for mean intensity, dashed orange for spatial extent (not extend as in the legend). Also in the caption these lines are referred to as black, but they are grey.

Line 85. The sentence starting “It suggests…” is quite complex, please separate it in two or reformulate.

In this section (starting in line 73) is where the MLD starts to be mentioned and referred to, to discuss the results. I think  a figure showing the depth of the mixed layer at the different regions discussed is missing, for different times of year. It would be much easier to understand some of the results shown.

Line 117 (the pdf is cut and all numbers after 100 appear with only 2 digits, so line 17 in fact): “is always higher than shallower depth”. I don’t understand this sentence.

Figure 4. Caption shows “copyright” symbol instead of (c). First mention to panel (f) should be panel (d). In panel d there is a vertical line at about 80degrees west, what is causing this? I was surprised to see that scales seem to be finer and shorter at 156 m than at the surface, what is causing this? I guess this is the eddies mentioned in the paper that transfer heat to other latitudes, but why are these eddies forming at this depth (and not at others).