Dear Author,

The work is flowing and clear in the exposition. There are some graphical inaccuracies and greater attention to local processes such as in the Mediterranean and Black Sea.

Please, correct the UNSCEAR quote which is incorrect in almost all of the text.

line 56 the dissolved ¹³⁷ Cs

line 490 Papucci

line 1850 Papucci, C., Salvi, S., Lorenzelli R.

line 1857 Papucci

please, define tap in a more explicit way: apparent half-life

please, define the start of half year because layering could affect the mixed layer

Correct the follow figure:

fig 1 (a-b) unreadable labels and coordinates

fig 2 label, mediterranean

fig 5 b unreadable coordinates

fig 6 b unreadable coordinates

fig 7 (a-j) unreadable label

fig 8 (a-j) unreadable coordinates

fig 10 (a b) unreadable coordinates

fig 12 (a-j) unreadable labels and coordinates

fig 18 (b) unreadable coordinates

fig 19 (a-i) unreadable label

fig 20 (a-i) unreadable coordinates



line 480

the value that is almost equal to the value before Chernobyl accident demonstrates the importance of increased release from the rivers flowing through the area affected by the accident: in fact, in Mediterranean Sea, where this input is negligible, the value in 2002 were less than half pre-Chernobyl accident.

Thank you

General comments

The theme of this work is valuable in that its result is expected to be helpful to understand the long-term spatiotemporal variation of Cs137 in the surface water of global ocean associated with the global fallout, Chernobyl and Fukushima NPP accidents, and discharges from reprocessing facilities. The manuscript is reasonably well written though some improvements are obviously needed. Furthermore, some questions about mass balance and inflow/outflow Cs137 amount need to be explained in detail which are, reviewer think, the most essential information in this study.

The manuscript in present form is not sufficient enough to recommend the publication in ESSD. Major revision is recommended to provide the opportunity of strengthening the manuscript.

Detailed specific comments

Abstract:

Line 10, Pg.1:

Reviewer understands that authors used mainly HAMGlobal2021 data and partly MARIS data, but authors mentioned only HAMGlobal2021 data It is better to describe this matter correctly.

Lines 11-17, Pg.1:

Author used data from 1956-2021 data but no statement before 1970 is found in the abstract. It is desirable to include comments before 1970. 

Introduction:

Line 30, Lines 35-36, Line 46, Pg.2:

Authors repeatedly elaborated that the Cs137 originates from large-scale atmospheric weapons tests. Writing improvements are needed.

Lines 54-55, Pg.2:

Elaborate in more detail what the local fallouts represent.

Lines 80-83, Pg. 3:

-Reviewer is confused with authors statement such that the atmospheric deposition of 137Cs into the ocean was estimated to be 11.7-14.8 PBq (Aoyama et al., 2016b) and that the 137Cs inventory into the North Pacific Ocean was estimated to be 15.2-18.3 PBq (Aoyama et al., 2016b, Inomata et al., 2016; Tsubono et al., 2016). Clear description is required.

-Regarding the statement “directly discharged liquid 137Cs from the F1NPS was estimated to be 3.6 ± 0.7 PBq (Tsumune et al., 2012, 2013)”, comments that there are various estimates in literature need to be added.

Lines 148-151, Pg. 5:

-It is not clear what the statement “latitudinal and longitudinal distributions, and the locations of global fallout, reprocessing plants, and nuclear power plants” represents. “latitudinal and longitudinal distributions of global fallout, and the locations of reprocessing plants, and nuclear power plants”?? 

-Spatial distribution of Cs127 concentrations is uniform? In each box?

-Horizontal and vertical transport of ocean water is almost same? In each box? It is needed to elaborate in detail the basis of this assumption somewhere in this manuscript. There are several questions on the assumption. What is the assumption in shallow marginal seas? Are the horizontal and vertical transports of ocean water time-invariant or time-varying? If each box is further divided into two, the assumption such that horizontal and vertical transports of ocean water are almost same is still valid?

Lines 148-160, Pg. 5:

Elaborate why the box configuration is changed.

1. Data and methods:

Lines 193-194, Pg. 10:

Authors state that the currents and major source of Cs137 in the surface water has a 0.5-year time interval. It is not clear what “the currents” and the major source? Describe the detailed information. One more thing, are the 5-year interval data instantaneous values or some mean values?

Lines 217, Pg. 11:

 is used without definition and without physical meaning (Eq. (3) should appear in advance)

Lines 221-224, Pg. 10:

-What is the definition of   ?

-What is the definition of Tpo and its physical meaning?

Lines 279-280, Pg. 13:

Authors state that the maximum monthly mixed layer depth was used because Cs137 is easily transported to the subsurface under deeper mixed layer. Hard to understand why easier subsurface transport is necessary. How about using mean monthly mixed layer depth?

Lines 293, Pg. 13:

Authors described “horizontal” transport as “outflow to the downstream box” transport. Reviewer thinks it is incorrect because there can be inflow-related transport. “horizontal (net outflow to the downstream box) transport” needs to be used.

Lines 296-302, Pg. 13:

-Subscripts in C i, box and C0, box in (8) are a little bit confusing. Better to use notations with two subscripts (including box number I and time).

-Describe how the “initial year” is defined.

-To reviewer’s knowledge, the transport is composed of advective and diffusive fluxes.  Explain which data were used for the fluxes. If only advective flux was used, elaborate which current (u,v and w) data were used. Furthermore, authors mentioned that distinguishing the horizontal and downward transports were difficult. Explain why. No w velocity?

1. Results

Line 308, Pg. 13:

Authors state that correlation coefficient is between 0.51 and 1.0. It appears that the range is very large. Elaborate why. And clarify the correlation coefficient between what?

Lines 561-562, Pg. 28:

Describe why the model results are considerably different from the estimate by Aoyama

et al(2006) and this study.

Lines 630-631, Pg. 30:

Better to use 0.0 PBq instead of 0 PBq.

Lines 770, 785, Pg. 37:

Hard to see the max. value of scale bars.

Lines 923-924, Pg. 48:

The mixed layer depth can have strong seasonal variability in some marginal seas where

vertically well mixed in winter while stratified in summer. Elaborate how authors deal with

Lines 984-986, Pg. 51:

Authors state that the 137Cs discharged into the Irish Sea.1 was 985 transported into the

Irish Sea.2, followed by transport to the northern North Atlantic Ocean.1, North Sea, and

Barents Sea and coast of Norway. Reviewer is interested in its transport direction in Irish

Sea. To reviewer’s knowledge, simulation by Prandle (1983) showed that Cs137 moved

to the north channel of Irish Sea. How about in this study? Discuss this matter.

1. Discussion

Line 1259, Pg. 67:

Hard to understand the expression “outflowed 137Cs is larger than the flow of 137Cs”.

Improvement in writing is required.

Lines 1356-1357, Pg. 67:

Regarding the statement “although the contribution of directly discharged 137Cs from the F1NPS might be included in the values in the western North Pacific Ocean”, comment when it happened. 2011? 2012? That is, how long does it take the directly released Cs137 moves from Fukushima NPP to the southern sea region of Kuroshio extension?

Lines 1399-1357, Pg. 75:

The expression “Cs137 is transported via advection” is not correct. “via advection” needs to be changed to “via advection and diffusion”. Authors appear to neglect diffusion, right?

Lines 1406-1408, Pg. 75:

Authors mentioned inflow/outflow, upstream box and downstream box. Reviewer hardly understand how such information can be obtained. Up to now authors had never mentioned current data. Clarify this point.

Line 1428, Pg. 75:

Authors mention “considering the current system”. What is the source of the current system?

Lines 1011-1714, Pg. 94:

What is represented by “1~2 years” in Fig. 22 shown in North Pacific?