The manuscript describes the recent advances in the global ocean and sea-ice operational forecasting system by Met Office, widely known as FOAM. Improvements concern both the model and data assimilation (DA) scheme, however the changes in DA scheme are responsible for most of the improvements in terms of RMSE and BIAS metrics, as the Authors shown. Multiple one-year-long experiments are performed to compare the skills between the new and old system using different set up and two different model resolutions: 1/4° and 1/12°  (DA is performed always at 1/4°). The largest improvement is seen when only surface data are assimilated, the old system presents un-realistic drifts in OHC and AMOC that are resolved in the new system, demonstrating the possibility to use the new system during the pre-argo era for reanalysis purposes.

FOAM is a well-known system that is largely used for different ocean-related applications. The manuscript is well written, results are robust and critically presented. Beside minor corrections, I can support the manuscript for publication however I am concerned about the fact GMD strongly recommends the dissemination of the code. In the present case, the main improvements come from the DA code that is not free. Ocean Science can be a good alternative.

 MINOR COMMENT

GENERAL

There is a sort of confusing nomenclature throughout the manuscript that I encourage the Authors to clarify for readability purposes. To my understanding, GO6/GOIS9 are generally used to point previous and new system while ORCA12 /ORCA025 are used to identify the resolution. 

• In the Figures  GO6 /GOI9 are used to differentiate the experiments, while the resolution (ORCA12 or ORCA025) is written in the caption. This is however confusing since ORCA12 parametrisation is different between GO6 and GOI9. I can suggest expanding the table 2 by introducing an different name for each experiment, something like GOIS9e12, GOIS9e025 etc.
• The use of ORCA12 DA is to be avoid since DA is performed at ¼° to my understanding. If two different DA set up at 1/4 ° are used, a description can be added to table 2.
• "FOAM" is used an attribute to both the system or the resolution, i.e. FOAM GO6 or FOAM ORCA12, etc., my impression is that you can safely remove "FOAM" everywhere.

SPECIFIC

Line 25: “In the absence of profile DA [..]”

 The sentence is difficult to follow and “profile DA” is not clear. Please rephrase it you can start with something like “Limited to the assimilation of surface data only [..]”

Line 96-101

I do not understand if those improvements are applied only to 1/12 ° or also to ¼°

Figure F3 is blurry please replace it.

Line 193 and Figure 2:

Those results seem not conclusive, is there a reason why statistics are calculated only between Jan-May 2019 without covering the full year? Do you expect the results to be the same by including more statistics?

Line 220

Is the minimization in GOIS9 always converging within 120 iterations? If not, can the Authors comment whether it is a noise problem, caused by the use of a first-order minimizer, or else.

Fig4

Similarly to my previous doubt, I am not sure the results in Fig4 are conclusive. There is a small difference among experiments and the statistics consider only   4 months. These  results can be related to the specific position of few argo that do not  fully span the depts and  the area. Can the Authors add the number of observation, extend the statistics or provide some study on the significativity?

Line 288

How many cores per node? Which is the frequency of the cpu ?

Lines 310-315

Can the Authors specify whether there is some preprocessing on the drifter velocities? are the drifters without drogues, used?

325  “4.2 Impacts on the […] with all observation types assimilated”

Which is the full set of observation assimilated? It is written that it is similar to the operational system, but the latter may vary in time. For example it is not clearly whether the satellite SST is assimilated or not together with the drifter SST. Is  the new rejection algorithm impacting the number of insitu profile assimilated significantly?

418: “Additionally, the same SI3 settings from […]“

If I understood correctly the sea-ice DA is not changed passing from the old to the new system. Is it so? Is the same parametrization used for ORCA12 and ORCA025 in Si3?. What about CICE (old system)? Was there a different parametrization for ORCA12 and ORCA025?

421 “Potential impacts […] pre-Argo reanalysis ”

I have not understood whether satellite sst are assimilated in the pre-argo era configuration.

422-426

The Authors missed to comment the importance of a good initial condition in pre-argo era. Especially in the southern hemisphere, the model state can be far from true state and the assimilation of few SLA measurements can exacerbate any hidden biases. Initial condition in 2019 are instead well spin up and the absence of drastic changes in OHC can partially come from that. Please add a comment on this.

430 : “GOSI9-NoTSProf run […]  SLA RMSDs in comparison to its original […]”

I was expecting GOSI9-NoTSProf to be closer to SLA obs since the system is assimilating mainly SLA data. Why the RSME is similar to the one that ingest insitu data too?

493-497

Which is the method used to estimate the AMOC? Are the Authors using the RAPID decomposition? Probably this is the cause of the difference. I noticed that the assimilation seems to degrade the correlation with RAPID timeseries, do you have any reason why?   

I am recommending this article be published with some minor revisions.  A detailed review of the paper is attached.  Summarizing:  The manuscript is well written and rigorous in its scientific analysis.  I have a few recommendations to include a review of important aspects of the NEMOVAR system, in particular the balance relationships applied, so as to better understand the effects and reasons for the system update.  I also recommend some revision to the discussion of particular components of the update so as to better demonstrate their effectiveness.