|This is a review of “Freshwater in the Arctic Ocean 2010-2019” by Solomon and others. The authors review relevant literature and make their own computations of freshwater content (FWC) for the Arctic Ocean from hydrography and remote sensing (altimetry and GRACE). They argue that Arctic Ocean freshwater content has stabilized since 2010. The authors have responded to prior review comments and the revised paper is a substantial improvement over the earlier version. However, the authors themselves raise questions about adequacy of the their data for reaching solid conclusions. As they point out, inadequate spatial coverage is a likely reason freshwater content from their in situ observations do not agree with their remote sensing after 2010. Further, although they are using altimetry-derived sea surface heights from DTU that they argue uses a retracker suitable for detecting sea surface height in sea ice regions, they have no remote sensing-derived freshwater content in major important regions: north of 82°N and the whole Fram Strait quadrant of the Arctic Ocean. Therefore, it isn’t clear from the data analysis what freshwater content has done in the 2010s. Consequently, as it stands the utility of the paper is its timely review of research. Additional references suggested below or a closer look at existing ones might help improve the data analysis and provide additional insights into changing freshwater content.|
L130 and Captions Figure Labels Fig. 2a - Units? Freshwater content in meters? What are the offsets for the different lines?
L132-137 - This is an important point. Fig. 2b shows the increase in FWC in the Beaufort Sea is compensated at least to some degree by FWC loss on the Russian- Eurasian side of the Arctic Ocean. This is characteristic of the cyclonic mode of circulation [Morison et al., 2021; Morison et al., 2012; Sokolov, 1962]. The wide variability in FWC change among the ORAs on the Russian side is likely due the paucity of observations there in recent years. Morison et al., (2021) speculate that the in situ observations have had an increasing spatial bias toward the Beaufort Sea. It would be a great contribution if this paper could address this by somehow illustrating the locations observations that went into the ORAs.
L166-169 - Yes, but sea ice was one of the main motivations for launching CryoSat2, and there are efforts in addition to the ones used in this study that discuss dynamic ocean topography from CryoSat-2 for the ice-covered seas, e.g., Kwok and Morison, (2016, 2017); Armitage et al.( 2018a, 2018b).
L170-179 - Why is there no DTU satellite data north of 82°N or in the Fram Strait quadrant? The various versions of the DTU mean sea surface (MSS) don’t have gaps this large. These are critical regions, particularly the Eastern Arctic, where we think in situ observations may be lacking.
L180-189 - It seems like a comparison with the cyclonic mode of Morison et al. (2021) would be appropriate here. For 2010-2019, their results suggest a brief initial shift out of the cyclonic mode in response to a record low winter AO index in 2010 that tended to return freshwater to the Russian side of the Arctic Ocean. This was followed by a return to the cyclonic mode for most of the decade tending to shift freshwater once again out of the East longitudes and into the Beaufort Sea. The mix of circulation regimes would make it hard to draw a general conclusion about FWC change in the 2010s, especially not having regular repeat in situ observations well distributed across the Arctic Ocean.
L199-201 - Yes, I agree “a good part of the difference after 2009 may be due to the contribution by the rest of the basin outside the Beaufort Gyre.”
In situ observations outside the Beaufort Gyre have become increasingly rare.
In a review it would be impactful to illustrate this with a figure showing the locations of the hydrographic stations used in the analysis.
For the original research part of this paper, a good way to combine remote sensing and in situ observations is to first compare them say for FWC (or for altimetry derived DOT and in situ derived dynamic heights [Morison et al., 2018; Morison et al., 2012]) at the locations and times of the in situ observations. Then after making reasonable corrections, apply the remote sensing FWC to the whole Arctic Ocean and for other times with some confidence that these remote sensing results represent a good proxy for in situ observations [Morison et al., 2018; Morison et al., 2012]. The authors could do this by combining the data in Table 2 and the DTU-derived FWC. It would also be illuminating and relatively easy to compare DTU-derived FWC to the FWC of the ORAs and see how the regions of good and bad comparison relate to the distribution of in situ observations.
Table 2 - Why are there no NABOS data? Except for perhaps the Polarstern data, there isn't much data at all along the Russian margins of the Basin where Rabe et al. (2011) and Morison et al (2012) found freshwater decrease offsetting the Beaufort Gyre increases in the 2000s. Again, it would be helpful to show charts of the data locations to illustrate if there is a geographic sampling bias.
L260 - Figure 4 is an analysis by the authors, not Thompson and Wallace, correct? In Figure 4, the positive correlation after 2010 is clear but the negative correlation before is not so obvious.
L383 - The Hill et al. references is missing.
L385 - The glaciers drain into the shelf regions around Greenland and are carried south, so their contribution to Arctic Basin is undoubtedly very small. It╒s hard to see the point in considering Greenland melt in the freshwater balance of the Arctic Ocean, even more so considering the complete lack of FWC data shown for the region around Northern Greenland.
L424 and Redistribution of Arctic Freshwater section generally - The results of Morison et al. (2021) provide a longer-term perspective on the effect on freshwater distribution of the cyclonic mode of circulation epitomized in the changes of the early 90s (Morison et al., 2000) and 2007-2008 (Morison et al., 2012). The cyclonic mode is characterized as the first EOF of ocean surface height variability (dynamic heights, 1959-1989 and satellite DOT, 2004-2019). In the cyclonic phase, surface depression, reduced freshwater content, and cyclonic circulation are imposed on the whole Russian side of the Arctic Basin. This offsets gains in freshwater in the Beaufort Gyre. The cyclonic mode is related to the AO index with order 1-year latency, and has become more prominent under enhanced winter AO since 1990.
L464-466 - Dewey et al. (2018) indicate the Ekman pumping and ice drag feedback mechanism stabilize ice and ocean velocity at time scales of less than a week while eddy propagation feedback has time scales measured in years.
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Armitage, T. W. K., R. Kwok, A. F. Thompson, and G. Cunningham (2018b), Dynamic topography and sea level anomalies of the southern ocean: Variability and teleconnections, J. Geophys. Res., 123(1), 613╨630.
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Morison, J. H., R. Kwok, C. Peralta-Ferriz, M. Alkire, I. Rigor, R. Andersen, and M. Steele (2012), Changing Arctic Ocean freshwater pathways, Nature, 481(7379), 66-70, doi:http://www.nature.com/nature/journal/v481/n7379/abs/nature10705.html - supplementary-information.
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