the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 16 Feb 2024
Continued warming of deep waters in Fram Strait
Abstract. Fram Strait is the only deep gateway between the Arctic and the rest of the World Ocean, and is thus a key region to understand how the deep Arctic will evolve. However, studies and data regarding the deep ocean are scarce, making it difficult to understand its role in the climate system. Here we analyse oceanographic data from the deep ocean from two long-term mooring locations (F11 and HG-FEVI) in Fram Strait between 2010–2023, to investigate long-term changes in the hydrographic properties. We compile hydrographic profile data since the 1980’s for additional context in the upstream basins: the Greenland Sea, and the Eurasian Basin. At mooring F11 in western Fram Strait, we find a clear seasonality, with increased Greenland Sea Deep Water (GSDW) presence during summer, and increased Eurasian Basin Deep Water (EBDW) presence during winter. Evaluating long-term changes, we find a modest temperature increase of ∼ 0.1 °C for EBDW since the 1980’s. For GSDW, south of Fram Strait, we find a strong temperature increase of ∼ 0.4–0.5 °C for the same period. The different warming rates have led to GSDW becoming warmer than EBDW since ∼ 2017/2018. This means that the Greenland Sea is no longer a heat sink for the Arctic Ocean at depth, but a heat source. It is therefore possible that EBDW temperatures will increase faster in the future.
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RC1: 'Review of “Continued warming of deep waters in Fram Strait” by Karam et al', Anonymous Referee #1, 19 Mar 2024
Review of “Continued warming of deep waters in Fram Strait” by Karam et al
Worldwide there is a great interest in the evolution of the temperature in the oceans over the instrumental record. However, the deep ocean is sparsely sampled and rarely considered in such estimates. This is especially the case for the deep Arctic Ocean where there are only a very small number of such studies. Related to the fact that bottom reaching convection in the Greenland Sea stopped in the 1980s, the deep Greenland Sea has been warming much faster than other parts of the deep ocean. In addition to CTD profile data, the submitted manuscript uses mooring data from two moorings in the deep Fram Strait, one located in the east and one in the west. It identifies a regime shift around 2018 associated with the fact that the Greenland Sea is no longer colder than the deep Arctic Ocean to the north.
The manuscript is well written with clear figures. It uses appropriate and interesting analyses. The scope is fully within the journal Ocean Science. I find no major issues, but I have a few suggestions for minor improvements. I would therefore suggest minor revision after which the manuscript should be accepted.
Detailed comments:
In the abstract, you just mention "deep ocean", specify that you talk about 2500m, XXm above the bottom, and at sill depth between the 2 basins, also L301. It would be useful to sharpen exactly what you mean by “deep ocean” in this context and which layers (above and below) you do not consider in your analysis. It is fine that you don’t consider them, but it would be good not to pretend that you cover the whole ocean below the mixed layer. e.g. Langehaug&Falck, unlike you, consider “intermediate and deep waters”.
L16 all these references deal with the Greenland Sea which often is considered Nordic Seas rather than Arctic Ocean. At least clarify what your definition of Arctic Ocean is then.
L72 The north-south extent of the Fram Strait box is actually quite a bit larger than around the sill across the strait. Maybe comment on the meridional extent of the sill separating the basins.
L76 to the exact choice of
L82 typo: -0.7 --> -0.8degC
L91 moorings F10...
L107 conductivity which translates to XX for salinity
Tab 1 4.03degE -- 4.333degE. I don't think the range of the location in the zonal direction was that large. The mooring was always close to 4.33degE = 4deg20minE
L117 Are those in the bottom boundary layer? By comparison to the treatment of the temperature/salinity data, the info on velocity data is comparably brief. Do the links to data sets that you give for temperature/salinity also contain the velocity data?
Fig 2 what is the -1degC 35.05 excursion of GSDW in 2000?
Might note that (deep!) Argo is only 6 profiles in GSDW in one year, thus it does not really contribute to your conclusions.
You could show the locations of the profiles in Fig. 1 as dots in the basins
L204 Fig 5b
Fig 6 caption Clarify whether the direction of the velocity impacts whether it is +1 or -1. This might be especially useful when in Fig 7 caption you just state that it is the same.
Eqn 2: EKE is 1/2 times what you show in eqn 2. This is correct in Fig 8 caption, though it is not clear why you need the info in the figure caption and the main text.
L248 associated with
L256 "thus likely plays" maybe replace by "may play". The causal relationship why one should be more important in the net than the other is not clear to me.
L260 However... grammar!
L269 increase
L284 What about vertical mixing unrelated to convection?
L311 "tragically" not sure that is the best word. Maybe tone down a bit.
L312 understanding the
L464 data set from 2010 contains data until 2015?
Citation: https://doi.org/10.5194/egusphere-2024-458-RC1 -
RC2: 'Review of “Continued warming of deep waters in Fram Strait” by Salar Karam et al.', Anonymous Referee #2, 07 Apr 2024
General comments:
This study uses a combination of historical CTD profiles (1980-2023) and CTD and velocity data from two moorings (2010-2022) to investigate long-term changes in the deep (~2500m depth) water masses in Fram Strait and its surroundings (the Greenland Sea and the Eurasian Basin of the Arctic Ocean). The deep Arctic Ocean is sparsely sampled and Fram Strait is the only gateway for exchange of deep water between the Arctic Ocean and the rest of the World Ocean. Hence, this study provides an important contribution to understanding how the deep Arctic Ocean has evolved and may change in the future. In particular, the results show a strong warming of the deep water in the Greenland Sea, which since ~2018 has become warmer than the deep water in the Arctic Ocean and may provide as a deep heat source. The manuscript is well structured and written, has clear figures, with appropriate analyses that support the conclusions. I only have some minor comments/questions that should be addressed before the manuscript is published.
Specific comments and technical corrections:
L3: Define “deep ocean”
L5: Suggest replacing “upstream basins” with “surrounding basins” or similar, as the basins can also be “downstream” in terms of the flow.
L22: Rapid warming of GSDW or less GSDW in the mixture? Related to my comment on the water mass definition (L76-77) below.
L31: Do you mean mixing in Fram Strait or exchange across the strait? I think exchange is a better word here.
L36: “… GSDW now being a lighter …”: I’m missing a reference here. Is this based on Somavilla et al., 2013? Then the word “now” is not appropriate. Rather specify when the transition happened.
L51: “knowledge of …, since 2012/2014, is lacking.”: why do you include two different years?
L53-61: I found the start of this paragraph (L53-57) a bit difficult to follow because the main goal and focus of the paper are not mentioned in the introduction yet. It would be good to clarify this in the beginning of this paragraph (instead of later in L58-59).
L73: Is there a typo in the east-west extension of the GS box in the text? It says 12W-12E (same as the FS box), but on the map it doesn’t look like the GS box goes to 12E (rather 0E, which makes more sense too, in terms of excluding the NS).
L76: exact choice “of” depth levels
L76-77: I agree that it is suitable to use a property-independent definition to look at deep water changes in specific regions. However, I think it would be good to clarify that the deep waters you look at contain (are mixtures of) water masses with different origin. GSDW is often referred to in the literature as a class of water that was formed in the Greenland Sea prior to the 1980s. Your GSDW definition likely contains an increasing amount of other water masses such as EBDW and GSAIW. You do discuss this later on in the discussion but it would be good to include one or two sentences here to avoid confusion.
L80-83: Please be consistent with the number of decimals you present for the salinity values (and temperature values). This is also the case in other parts of the manuscript (e.g., section 2.5 and the first paragraph of section 3.1).
L81: … which “corresponds to a” change of … in Practical Salinity … (same for temp.)
L95-98: Does this mean that you exclude deployments during the 2010-2022 period or are the deployments with Aanderaa RCMs before 2010 (which is why you start in 2010)? This is not clear. Also, the Aanderaa RCMs are dataloggers/platforms with point current meters and the option to install different types/versions of temperature sensors (etc.) with different specs. Which temperature sensor (product number) has the accuracy you refer to of +/-0.05degC?
Table 1: “Not all data is always available during the deployments.” I think this needs some elaboration. Is it because of gaps in the deployments, sensor failures, or because you excluded deployments with RCMs or other reasons?
L121: “… compare our results to previous studies, …”: which results? I’m missing one introductory sentence about why you are doing the cross-correlation analysis (like the first sentence you have in section 2.4, L139-140).
L124: Remove one “the” in “We define the the upper and lower bounds…”.
L124-126: “We define the upper and lower bounds by …, to get estimates of the upper and lower bounds of water mass properties.”: The last part of this sentence is not needed/is a repeat of the start (ie. remove the part after “to get estimates ….”).
L152-153: “The results were mostly robust to the choice of year (not shown), ..”: It is difficult to know what “mostly robust” means. Could you be more quantitative or give an example?
L157: …, which have “an” initial accuracy …
L175-176 and Fig 2: What does the density change correspond to in terms of potential density anomaly referenced to the surface? (particularly for the DW in the Greenland Sea).
L176-177: It is not clear from the data in fig.2d that the GSDW temp is higher than EBDW after ~2015. That is, I would just state that they have converged here. Evidence of larger GSDW temp comes primarily from the mooring data in Fram Strait and the cross-correlation analyses later.
L217: either “flow switches” or “flows switch”
Fig. 4 caption: … as defined “by the regime shift analysis” in Sec. 2.4.
L226: Remove “in fact” before “EBDW-dominated”.
L230-231: If I understand it correctly it is primarily the seasonal cycle in velocity combined with the different rates of warming of GSDW and EBDW that leads to the stronger seasonality in temperature. What about seasonality in temperature in the upstream water masses?
Equation 2: multiply by 1/2
L242: … direction of the “deep” flow in Fram Strait …
L248: … associated “with” sampling …
L282: “However, the possibility remains that GSAIW is replacing GSDW.” Could you plot typical properties of GSAIW to check this? What about geothermal heating? Could this be another source of the increased temperature at 2500m?
L312: … play for understanding the changing Arctic. Remove “our” and “on” before and after “understanding”, respectively.
Citation: https://doi.org/10.5194/egusphere-2024-458-RC2
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RC2: 'Review of “Continued warming of deep waters in Fram Strait” by Salar Karam et al.', Anonymous Referee #2, 07 Apr 2024
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