Eyouni et al. present a nice overview of the various, complex mechanisms driving water mass formation in the northern Red Sea from a glider study occurring between January 31 to April 18, 2019. The authors do a nice job detailing many of the physical mechanisms from observations and model data as well as providing historical context to inform these findings. However, if the authors want to tie these physical findings to biogeochemical impacts in the region, further analysis and discussion is needed. I would recommend major revision but foresee this paper developing into a nice, comprehensive study. Below are my major and minor comments.

Major Comments:

1. On lines 432-434, the authors write what I interpret to be a key argument of their study: “Regardless of the details of the mechanism, subduction is a process that needs to be considered in the physical and biogeochemical dynamics of the northern Red Sea.” If the authors want to stress the biogeochemical significance of (all of) these physical processes, there needs to be more discussion (and actual chlorophyll and oxygen concentrations!) of the biogeochemical dynamics. The authors do not provide a biogeochemical background for the region. What limits primary production here, nutrients? light? A high salinity gradient? Does this region exhibit substantial or minimal primary production? What about carbon export? Mixed layer depth is discussed at length (great!), but the euphotic zone should be discussed in greater detail and included on figures, particularly if they relate to subduction.

Additionally, bbp should be included in the analysis. I outline below a number of places where actual concentrations should be provided and further explanation of the author’s proposed biogeochemical impacts or changes is needed. I completely agree with the authors that glider profiles show important biogeochemical changes resulting from physical processes, but more discussion is needed to make the argument coherent enough for a general audience.

1. The various controls on water mass formation presented here are fascinating and I think readers would really benefit from a schematic showcasing these physical processes (the eddy transport of water, subduction, etc.).
2. Could dive-averaged current (DAC; Frajka-Williams et al., 2011) be calculated from the glider data? It would make a nice addition when discussing the source of waters.

Minor Comments:

Line 91: was backscatter (bbp) measured? Presumably it was measured on the Wetlabs and could be included in the paper?

Line 94: what is the average depth of bottom here?

Line 111: can the authors please clarify whether they did divide glider chlorophyll by 2 in accordance with Roesler?

Line 111: was any sort of quenching correction applied to the chlorophyll data?

Figure 3: errors in the way panels d and h were printed/copied

Figure 3: could you please add a North arrow to one panel for orientation?

Figure 3: in the caption can you please clarify where sea level anomaly and geostrophic velocity data are coming from? ESA correct?

Lines 245-246: I think part of this sentence is missing? “. . .while the depth of 500 m has been selected [to represent the near bottom] because. . .”

Figure 4: in the caption be consist with how you refer to panel labels (“A” vs. “a”)

Line 278-279: can you please include average values or a range of chlorophyll and oxygen to give the reader an idea of how “elevated” they were? The oxygen is not particularly easy to evaluate based on the figure

Line 279-280: can you please be explicit about why you presume the high chl and dissolved oxygen waters originated closer to the surface and were subducted downward along the isopycnal. I agree with you, but to someone without a strong biogeochemical background, it may not be 100% obvious as to why these waters had to be subducted and could not have generated elevated chlorophyll and DO at depth

Line 281-282: perhaps this definition should come before the first use of the word or incorporated into the sentence: “. . . at 20 km offshore, suggestive of subduction (i.e., transfer of fluid. . .)”

Line 304-305: can you expand on the bolus of highly oxygenated waters? Were the chlorophyll concentrations high during this period as well? What are the average TS of the Gulfs of Aqaba or Suez?

Figures 5 and 6: are your chlorophyll concentrations mg L-1 or µg L-1? I’m assuming this is a typo and should be corrected to either mg m-3 or µg L-1

Line 321-322: can you comment as to why the uplift of low DO and low CHL waters is potentially biogeochemically important? 100 m is still quite deep. Are you hypothesizing the waters will be uplifted further, into the euphotic zone, thus enabling phytoplankton to engage in primary production? Or are you hypothesizing more biomass rich waters could contribute to the grazing or remineralization structure of those more near shore waters?

Lines 350-353: this is currently a very short paragraph. Can you expand on this or potentially combine it with the following paragraph?

Lines 364-365: “this negative phase is consistent with the period when the circulation was anticyclonic”. To me, this seems like a noteworthy finding. Perhaps emphasize it more?

Lines 397-399: The statement beginning “Krokos et al.” could be tied to the authors’ PWP finding better.

Line 401: Typo: “The”

Line 420: “Water mass subduction is a component of the formation of Red Sea Outflow Water during winter and a contributor to the vertical carbon flux from the euphotic layer to the interior of the Red Sea”. I think this is the first time carbon is really discussed. While I have no doubt this is true, if the authors want to stress this as part of why better understanding the formation of RSOW is important, there needs to be carbon data (or bbp700 or bbp460 which again, I assume was measured via the wetlabs). Chlorophyll and oxygen ≠ carbon

Line 428: what does “a measurable fraction of total chlorophyll” mean?

Figure 11: axes labels are hard to read and look squashed. A-d labels look abnormally large relative to other figures. I appreciate the inclusion of panel d but I don’t think it is discussed anywhere in the paper? Overall figure (export) quality could be improved.

Table 2: I would suggest introducing Table 2 earlier, before the conclusion

The proposed work deals with the formation of intermediate waters in the northern Red Sea (RSOW) during winter (2019), analysing physical and biogeochemical mechanisms through glider observations, satellite data and models. The authors explore the role of cyclonic vortices, atmospheric forcing in ventilation and nutrient transport. The work highlights key processes such as cooling, convective mixing and mesoscale interaction in ocean dynamics. The paper presents interesting new features and, in general, is well written with a high quality of figures that allows for a quick and thorough understanding of the work. Despite this, some doubts remain about the description and motivation for the use of some datasets, and some minor considerations about the writing.

1) Recheck the introduction of acronyms, they are often not introduced properly, even if you add the citation, the acronym must be described explicitly. E.g. RSOW, AE etc, possibly with the acronym next to the extended form.

2) The introduction is detailed with respect to some physical aspects of the studied area, but the topic of the work could be described more accurately, the state of the art of this topic with a quick overview of the motivations.

3) Please emphasise the motivations of the study also in the conclusions.

4) Suggest putting the results obtained into more context with previous studies on this area.

5) Why did you choose to use the MERRA dataset, even though there are better performing datasets with a higher resolution (e.g. ERA5)? I suggest justifying your choices and showing that MERRA-2 is the best choice, particularly for the basin in question. It would be interesting to show a very brief comparison MERRA-2 vs ERA5 vs FNL-GDAS (and/or analysis data from global models, such as IFS or GFS).

6) Regarding the figures, I suggest enlarging the fonts and optimising the spaces between the panels