This manuscript presents the latest edition of the IAP OHC data set, including a comprehensive description of methodological advancements and evaluation. The paper is informative for developers of similar data sets as well as users and will be a useful reference for the community. It is generally well written, but I have a number of comments and suggestions for clarification and improvement. In addition, the section on the sea level budget appears half-baked to me. That section requires more substantial revision and elaboration, or it could possibly be removed given the paper is already quite long.

Specific comments:

L44: The authors postulate consistency of IAP OHC data with EEI. Given the only moderate correlation and the only modest visual agreement in Fig. 17 the authors should specify based on which metric they conclude “consistency”.

L328-331: I understand that even post 2005 monthly data are actually based on 3-month windows. This is important to note more explicitly. It also has implications for the variance of the time series as presented in table 2 – in fact this method will reduce the monthly variance compared to other data sets which might represent truly monthly data. Regarding time- and depth varying windows: Could this be illustrated with a time-depth Hovmoeller diagram displaying the employed window length? Also, was the impact of time-varying window length on signals assessed with synthetic data?

L343: Does that mean the influence radius changes with depth? Is this a physically based choice or is this pragmatic owing to data availability?

L345-346: “real forcings” is perhaps a bit overconfident. Better say something like “reconstructed”.

L360: Please explain E_i and M. If “E” is instrumental error, is it meant to represent a bias (which could simply be subtracted) or random error?

Table 1: Instead of saying doi: “YES” I suggest to simply state the doi.

L416-423: As a reader I would like to see a number of how strong the effect of the VC on OHC trends is (especially as a number from earlier works is provided).

Fig. 6: Here and in other instances I suggest to bring in the Lyman and Johnson (2023; LJ2023) data (https://doi.org/10.1175/JTECH-D-22-0058.1) as they state generally improved quality over IAPv3 data. Good agreement with the LJ2023 data (which are derived in a different fashion than the IAP data) would strengthen the confidence in state of the art OHC data sets.

Fig. 7: IAPv4 clearly looks more plausible than v3, but it is still only qualitative. Is there a way to really validate the data, e.g., with data from ice-tethered profilers?

L575-584: Given that cooling SST trends in the eastern equatorial and south-eastern Pacific have received quite some attention recently, I suggest to discuss more potential causes of the non-existing cooling trend in that region in the IAP data.

Fig. 11: This is a good figure, but in addition I would like to see a figure including other OHC data sets (similar to Fig. 9 for SST).

Table 2: The annual results suggest a variance ratio between IAPv4 and CERES of >2, while Lyman and Johnson (2023) get a ratio of 1.3 for their data. This should be mentioned. A reason for this might be the fact that LJ (2023) seem to actually apply a stronger than annual smoothing (their annual OHC variation is obtained by differencing subsequent annual means) to their data, but reading your lines 792-793 it appears you are doing the same for this comparison? Needs to be clarified.

Table 2: Why is no CERES trend provided?

Fig. 13: Please comment why the eastern Pacific cooling signal in upper 300m OHC is so much more prominent than in SST?

L695: This is only true for the tropics, not for higher latitudes.

L730ff: Please add “Pacific” everywhere (also when stating correlations) to be clear you are not discussing full zonal averages

L759: add “based on our data”

L768: to me “EEI” is a rate of change, not an accumulated value. So maybe better to add “accumulated” before “EEI”

L772-773: validation method (1) does not appear very meaningful to me, as the integrated CERES value depends on the one-time global adjustment for the EBAF product. Changing this adjustment to match the IAPv4 average OHC increase (as apparently done by the authors) does enforce the agreement seen in Fig. 16. I am unconvinced this is a meaningful approach and recommend to only keep method (2).

L795: “consistency” in which sense? E.g., is the correlation significant?

Fig. 17: The CERES series does not look de-trended.

L815ff a: The methods are not clear to me. In my understanding, only steric sea level can be derived from IAPv4 data. It would be useful to note how the conversion from T/S profiles to sea level is performed? A reference would be useful.

L815ff b: It is confusing in table 3 that for all terms there is an IAPv4 entry, although everything but steric stems from other sources. Specifically, I understand that GMSL as well as “sum of contribution” (which itself should be explained better) are taken from Frederikse et al., but the values still differ. What is the reason for this? Is it only because of the different approach for trend computation? Can the authors provide the sensitivity to the trend estimation method based on IAP data?

L815: 1991 or 1993?

L852-854: This is an important result, and it seems to suggest that the stronger warming in recent years as indicated by IAPv4 is more realistic. This should be stated more clearly. Also, here it would be useful to make a link to Fig. 11 or a potential new OHC figure including other OHC products.

L916: Is there a reference for the sampling bias of CERES on monthly time scales?

Typos/edits:

L41: suggest replacing “first” with “uppermost”

L107: “support the follow-on studies on climate assessments” – This does not read very smooth. Please rewrite.

L137: “this paper” à perhaps better say “the presented product”

L222: is it a warm or cold bias?

L232: “systematic biases” is redundant: either say “biases” or “systematic errors”

L243: here and in several other instances the references have the parentheses wrongly placed. Please revise.

L270: I am not sure that “adjustive” exists

L282: It is unclear what you mean by “such a choice”

L303-304: This alone is not necessarily a problem. But I assume salinity data are not always available?

L435: I assume “monthly” climatology?

L462: please spell out “CERES-EBAF” once.

L663: Please add a reference to the subsection where this is explained

L937: What is meant by “T/OC”? Do you mean T/OHC?

The manuscript presents the description of the technical methods employed for the  creation of the temperature and ocean heat content estimate IAPv4 and a basic assessment of the product in comparison to some other products. Additionally independent data such as sea level change or meridional ocean heat transport are employed to verify the product.

IAPv4 is an update with respect to its predecessor IAPv3 and a great deal of the manuscript is dedicated to the changes and impact between these different products as the reader would expect to see.

The manuscript is well written and lacks only few information. Detailed comments and suggestions are as follows:

L 55 I assume "based on gridded products" is more appropriate

L 78 Maybe a newer citation to point at the current product.

L 124-125 Would it be possible to have for Fig.1a something like observed number of grid cells/months in addition to the casts? Fig.1a suggest the dominating importance of GLD while they provide very high resolution (in time and space) data which your product is not really be able to benefit from so much.

L 137 Which are the sources. That may be interesting to know for users that are looking for data.

Fig.1 Define GLD

L 332-336 Often when too small influence radii are used, the anomalies may become zero and reconstructions fall back to climatology. This can be seen  for instance in the earlier years of the EN3 objectively analysed fields. Do you have mechanisms to prevent this from happening, or are zero anomalies being accepted in case of lack of data. How frequent would that happen?

L 345-350 Unclear what flow-dependent means and how the constraint with observations work, more information is needed here. How do you diagnose which type of flow is present when applying the flow dependent covariances or is this basically just done according to the location?

 L360 What is E and i?

 Fig 4 "Variance" probably should read standard deviation since the unit is deg C

L 498 What is the relevance of the different land-sea distribution.  Maybe you want to point to the amplitude?

L 518 Check the description: IAPv3 is black in the legend above

L 525-527 Not clear why IAPv4 is considered less physical than IAPv3, there are clearly non-physical features in IAPv3 appearing as rays emerging from the pole

L 522 "Anomaly" maximum "change" is from Sep to Dec.

L 536-537 Why January and July? Maximum MLD is expected to be later in the year: around March and August. Deep MLD in the Labrador Sea is surprising shallow.

L 546 "Norwegian Sea", but the maximum appears to be southeast of Iceland which is in the Iceland Basin

L 552-555 de Boyer Mont.gut et al., pointed out limitations of the delta T criteria. I think it is useful to acknowledged that these limitations also apply for the MLD estimate here.

L 582 to the south of

L 609 Interanual variations are also different

L 644 Which depth range is used?

L 679-680 Given the extend of that pattern I would rather call this a negative PDO phase related to the fact that  a long warm phase ended in 1999 and since then it is mixed with somewhat more cold phases. Maybe bring this together with your following remarks about PDO

L 687-688 They describe an intensification in the South but a spin-down in the North Pacific

L699-701 It would be good to briefly outline how the OHC enters the estimate of the MHT, maybe also give an idea how important OHC is in comparison to Fs

L 734-735 What does it mean released from 20S-5N  to 5S-20S?  I assume  "released" means to the atmosphere, otherwise it would be better to write redistributed., or do you argue is that the redistribution involves release and re-absorption?

L 771-774 Why is 90% EEI used in Fig.16? What does this discrepancy mean?

Table 3 What is the difference between GMSL and sum of components, how is the IAPv4 GMSL computed if not as a sum of components?

L 925 Why in particular warm eddies as opposed to cold eddies?

Summary: Regarding methods to improve the estimate. Could you comment on the interpolation of the anomalies on isopycnal surfaces rather than depth levels, this could facilitate larger radii and better gap filling without the danger of making the solution overly smooth.