| 19 Mar 2026
Assessing the Impact of Freshwater Fluxes from Major Rivers on the Atlantic Ocean
Abstract. This study evaluates the impact of freshwater fluxes on the Atlantic Ocean. The river discharge has been estimated at the outlet of 18 major rivers flowing into the Atlantic by solving the water mass balance equation at the river basin scale. In this approach, water storage changes are evaluated with satellite gravimetry measurements. In contrast, atmospheric fluxes (i.e., precipitation, evapotranspiration) are assessed with atmospheric reanalyses, in situ measurements from a global network of rain gauges, or a global hydrological model. The river discharge estimated with the water mass balance is consistent with independent river gauge measurements across all South American rivers, in particular the Amazon, where the annual and monthly climatology can be estimated with an error of less than 5 % when compared with in situ measurements. Larger discrepancies are observed for other basins, likely due to uncertainties in the precipitation and evapotranspiration fluxes. When climatological estimates of the river discharge are replaced by the water mass balance approach in ocean model simulations, a decrease in the sea surface salinity bias is observed at the outlet of the Amazon and across the whole Atlantic. The water mass balance approach can bring new observational constraints on freshwater fluxes flowing from the continent to the ocean, but is not universally reliable due to potential biases in atmospheric flux estimates. When combined with in situ measurements or hydrological model predictions, the water mass balance approach can enhance freshwater flux quantification, leading to improved ocean model simulations near major river outlets. In particular, the enhanced river runoff dataset significantly reduces salinity biases, with associated changes in reduced vertical stratification, enhanced upper ocean circulation, and meridional transports.
Review of Kern et al. ‘Assessing the Impact of Freshwater Fluxes from Major Rivers on the
Atlantic Ocean’
The manuscript describes assessing the water mass balance (WMB) method for estimating river discharge into the Atlantic basin, in which change terrestrial water storage calculated from satellite. River discharge from variations on this method are compared with in situ observations. The impact of the different estimates of river discharge on ocean model simulations of salinity and currents are further investigated. The manuscript is well-written, and both the potential and the limitations of the method are reasonably communicated. The manuscript should be a useful contribution to the literature. There are a couple of areas where the content could be expanded. Firstly, the authors remark how the method is sensitive to the reanalyses used - it would good to get an idea of the spread possible with a range of reanalyses. I don’t suggest repeating the analysis multiple times, but it might be possible to get a picture of the spread by calculating one diagnostic (say net evaporation over the amazon) for different reanalyses. Secondly, it would be good for the authors to unpack the confusing finding that WMB1 is better than WMB2 at improving the representation of river discharge but worse at improving the representation of ocean salinity.
Further specific and technical comments.
Title: Suggest a title more like ‘Quantifying freshwater fluxes from Major rivers and assessing their impact on the Atlantic Ocean’
Abstract
Line 10. Suggest ‘riverine freshwater fluxes’.
Methods
Line 84. A little justification/ quantification required here. Which rivers were excluded? What was the discharge of the largest of these? What was the total annual discharge excluded (say as a percentage of the Amazon)?
Seven rivers without in-situ observations. What uncertainties does this bring to the result. Is there enough data to estimate variability of in those rivers?
Line 167-170. Consistency of fluxes. Are the ERA5 fluxes self-consistent? Specifically, globally does precipitation=evaporation on a reasonable time-scale? (I enquire because we know that most reanalyses are not close to producing a globally balanced net ocean heat flux).
Tsujino et al (2018) use a catchment basin river routing model driven by JRA reanalysis. Should n’t the fact the fact that this is driven by reanalysis mean the approach is not too dissimilar to the authors and that there might not be much quantitative difference?
Line 188. ‘Several experiments’. My understanding is the authors conducted 6 experiments (4 experiments and 2 ‘control’ simulations). Could the authors list these in a table, giving the control simulations an abbreviation?
Lines 200-202. I find this unclear. Firstly NEMO is an ocean model, so it does not have river basins as such, merely river outlets. It makes sense to regrid or reassign the outlets on to the NEMO grid, but I don’t understand regridding the basin. Secondly, I cannot understand the phrase ‘the local spread of runoff with the coastlines of pertinence of each river drainage basin, proportional in area to the monthly climatology’. Could the authors, break this down into smaller more easily comprehensible chunks?
Line 213. The correlation is high because of the strong seasonal cycle. The authors should correlate the time series with the seasonal cycle removed.
Lines 217 onwards. Do you the authors understand the reason for the weaker agreement in the annual cycle they describe in WB2?
Lines 230- 234. Suggest this paragraph is replaced by something like. ‘Linear trends of the lines in Fig 3a were calculated and found to be statistically insignificant.’
Line 254. ‘and highlight’ suggest ‘but highlight’.
Line 296. ‘overestimated’. Just say it is larger in WMB2 by this amount. It cannot be described as an overestimate as WMB1 is not ground truth.
Line 330. ‘overestimated’ suggest ‘overestimated relative to the in-situ observations.’
Table 2/ 3 could the authors clarify the difference between Table 2 and Table 3.
Line 345. ‘Fig. 6’ should be ‘Fig. 5’, I think.
Line 368. ‘Figure 7’ should be ‘Fig. 6’ ,
If I understand correctly Fig. 6a shows the MAE of A1 relative to EN4 minus the MAE of ref relative to EN4. I think this could be stated with greater clarity in the caption
Line 373. I suggest the authors add ‘not shown’. i.e. ‘The REF experiment…West African coast (no shown). Alternatively, the authors could show The MAE of REF relative to EN4 in another panel.
Line 376. ‘Key Regions’. Because the MAE of Ref relative to EN4 is not shown we don’t know what the key regions are.
Line 384. Suggest ‘into the ocean model’
Please comment on the larger impact of the WMB method around the Gulf of St. Lawrence and Congo.
Also on Table 3 it would be of great interest to add the Dai and Trenberth Values as 2 columns to Table 3 if possible and comment on them. This is highly relevant since the authors compare WB forced ocean simulations with those forced by Dai and Trenberth.
Figure 7 and elsewhere. The authors do not include any caveats about the EN4 dataset. The data set is far from perfect and the key input from Argo floats are absent in the shelf regions around the coasts.
Line 422. Fig. 8 ERA5 WMB1 is better than WMB2 at improving river discharge but worse (or no differerent) at simulating ocean salinity. How can this be explained?