the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 19 Jul 2024
Impacts of North American forest cover changes on the North Atlantic ocean circulation
Abstract. Atmosphere-ocean heat fluxes in the North Atlantic Labrador Sea region are a key driver of deep water formation and the Atlantic Meridional Overturning Circulation (AMOC). Previous research has shown that anthropogenic warming leads to reduced ocean heat loss and thereby reduced deep mixing in the North Atlantic. This results in AMOC decline and causes regional cooling of sea surface temperatures (SSTs) which has been referred to as the North Atlantic warming hole (NAWH). Similar responses of the AMOC and the formation of a NAWH have been found for changes in wind stress and fresh water forcing in the North Atlantic. Moreover, recent research has also revealed such an AMOC and North Atlantic SST response in global-scale forestation experiments and a reversed response in deforestation experiments. Here, we test the hypothesis that forest cover changes in particular over North America are an important driver of this response in the downstream North Atlantic ocean. To this end, we perform simulations using the fully coupled Earth system model CESM2 where pre-industrial vegetation-sustaining areas over North America are either completely forested (forestNA) or turned into grasslands (grassNA), and compare it to the control scenario without any forest cover changes. Our results show that North American forestation and deforestation induce a North Atlantic warming and cooling hole, respectively. Furthermore, the response is qualitatively similar to previously published results based on global extreme land cover change scenarios. Forest cover changes mainly impact the ocean through modulating land surface albedo and, subsequently, air temperatures. Around 80 % of the ocean heat loss in the Labrador Sea occurs within comparably short-lived cold air outbreaks (CAOs) during which the atmosphere is colder than the underlying ocean. A warmer atmosphere in forestNA compared to the control scenario results in fewer CAOs over the ocean and thereby reduced ocean heat loss, with the opposite being true for grassNA. The induced SST responses further decrease CAO frequency in forestNA and increase it in grassNA. Lagrangian backward trajectories starting from CAOs over the Labrador Sea confirm that their source regions include (de-)forested areas. A closer inspection of the ocean circulation reveals that the subpolar gyre circulation is more sensitive to ocean density changes driven by heat fluxes than to changes in wind forcing modulated by land surface roughness. In forestNA, sea ice growth and the corresponding further reduction of ocean-to-atmosphere heat fluxes forms an additional positive feedback loop. Conversely, a buoyancy flux decomposition shows that freshwater forcing only plays a minor role for the ocean density response in both scenarios. Overall, this study shows that forest cover changes over North America alter the frequency of CAOs over the North Atlantic and, as a consequence, the circulation of the North Atlantic. This highlights the relevance of CAOs for the formation of North Atlantic SST anomalies.
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RC1: 'Comment on egusphere-2024-2087', Anonymous Referee #1, 28 Aug 2024
The manuscript determines important impacts of North American forest cover changes on downstream North Atlantic ocean. Responses of ocean circulation and sea surface temperatures (SSTs) are investigated and the associated mechanisms are analyzed based on idealised forestation and deforestation simulation experiments with CESM2 under pre-industrial climate condition. The important role of short-lived cold air outbreaks (CAOs) in SSTs anomalies and ocean circulation changes induced by forest cover changes is highlighted.  In the manuscript, the experiment simulations are reasonable, and results are well-presented. What’s more, findings are of considerable interest. I recommend minor revisions of the manuscript. The detailed comments are as follows:
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General comments:
1.The manuscript is entitled by “Impacts of North American forest cover changes on the North Atlantic ocean circulation”, while “the objective of this study is to illuminate the processes involved in the formation of the NAWH downstream of large-scale forestation and deforestation across North America” is described in the introduction. Changes in ocean circulation and SST anomalies (eg, NAWH) are paid more attention throughout the manuscript. Even the latter seems to be mentioned more frequently. There are some confusions, 1) Does the manuscript focus on ocean circulation or NAWH, or both? It would be better if this was made clear. The corresponding parts in the introduction, results, and conclusions also need to be revised accordingly to highlight the focus of the manuscript. 2) Although “The emergence of NAWH has been linked to changes in ocean circulation, in particular the AMOC” is mentioned in the introduction, what is the relationship between changes AMOC and the formation of NAWH, and how do the two influence each other, especially in this manuscript?
2. In the abstract, the introduction to the study is too long, and the significance of this study may be also missing. Besides, it would be better to make emphasis more prominent and conclusions more clear. Please rephase the abstract to better present the significance and findings of the study.
3. It would be better to give an overview of the purpose and significance of this study in the introduction, which may greatly enhance the interest of the manuscript.
4. In the results, many variables are described and discussed, such as mixed layer depth, temperature, salinity, CAOs, etc. Although correlations between some variables are mentioned, such as “changes in MLD overlap well with the integrated ocean-to-atmosphere heat flux associated with strong CAOs”, “the negative SST anomaly overlaps with a strong MLD anomaly sea ice wind salinity”, etc., what is the specific causal relationship, and what is the logic chain of variables mentioned in 3.2? How much did each discussed factor contribute to the change in AMOC or the formation of NAWH? It would be better to present a logical diagram to briefly show the influence mechanism of each factor and its contribution, which would make findings more clear.
5. This manuscript focuses on the response of the ocean. Simulations with forest vegetation cover changes only run for 300 years. Has the climate system, especially ocean, reached equilibrium?
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Specific comments:
1.Line 42, “was been shown” should be “was shown”.
2.Line 179, “from from” should be “from”. Besides, a closing parenthesis is missing in this sentence, please complete it.
3.Line 195, “allows” should be “allow”.
4. Some variables in 2.2 and 2.3 lack unit descriptions, please complete them.
5. It would be better to note turbulent heat fluxes as THF in the description of Figure 4.
Citation: https://doi.org/10.5194/egusphere-2024-2087-RC1 - AC1: 'Reply on RC1', Victoria Bauer, 28 Nov 2024
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RC2: 'Comment on egusphere-2024-2087', Anonymous Referee #2, 14 Oct 2024
Summary
The authors of this study investigate the Atlantic response to (admittedly extreme) forestation and deforestation scenarios applied only to North America. Warming in the forestation experiments drives a reduction in the strength of AMOC, while cooling over land in the deforestation experiments (grassNA) drives an increase in its strength. Consideration is also given to the formation of a North Atlantic Warming Hole, its location, and sign. The importance of cold air outbreak events is found to be pivotal. Overall, the study is well structured and of broad interest. I suggest minor revisions, detailed below.
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Major comments
- It’s not clear that there’s been any kind of statical significance testing conducted to determine differences in forestNA and grassNA relative to control, but that would lend much more credibility to the findings of the study. It would be particularly useful in contour plots such as in Figures 1 and 2.
- In general, comparison is made to both observations and to simulations with global land cover changes but neither is plotted. As possible, it would be helpful to show the global-change results and/or observations, so that interested readers can find all the relevant comparisons in a single paper. For example:
- Lines 239-246: For comparison to observations as here, it would help to show the observations themselves; maybe in Fig 1a,c as a contour?
- Lines 247-248 “Compared to global forestation and deforestation…”: Can there be some added figure element to show the response in the global runs of Portmann et al. 2022, rather than asking readers to have both documents handy? Maybe overlay the global signals as contours, or add more panels?
Minor Comments
- Line 44 “…found to also induce anomalies in the ocean”: Could be more specific than just “anomalies” – temperature anomalies, circulation, etc?
- Line 59 “w.r.t.”: Please spell out “with respect to” rather than adding acronyms.
- Lines 61-62 “Hereby, it is important to note that recent studies have found considerable model dependency of the climate response to vegetation changes (De Hertog et al., 2024, 2023).”: Please expand; this seems like an important point.
- Lines 99-100: The second research question posed could reasonably be removed, or should be rephrased. The others are scientific questions, the second is contextualizing results (which should be an assumed piece of the work)
- Line 121: “Vegetation-sustaining areas” – would be helpful to elaborate on what qualifies as a vegetation sustaining area. Are there water or nutrient limitations on where the forests or grasslands can exist? Are urban areas converted as well?
- Line 130 “…and the north western USA”: Should probably include mention of the eastern half of the US
- Line 176-177 “The resulting CAO masks include for example the center of a CAO of 9 K in the (4, 8] category…” : Please explain? Shouldn’t a 9 K CAO fall into the (8,12] category?
- Line 178: “Summing up the heat fluxes…” : Are these summed in time or space (or both)?
- Line 208 “Sverdrup balance is not fully accurate in the subpolar North Atlantic” – could the authors give a sense of how large of a difference might be expected between the balance and observations? Is this something like a 10% difference or a 100% difference?
- Line 223 “…over parts of Asia.”: Could be more specific; where in Asia? This is somewhat hard to see on the map.
- Line 223 “The impact of forestation on the Northern Hemisphere surface temperature is confined to North American land.” : This seems to directly contradict what was just discussed (cooling over N Atlantic). Please clarify/rephrase.
- Lines 250-253 “Potentially, this very dry region…”: Is there no way to confirm the ability of the vegetation to survive? If the forests are dying out, does the model simulate no vegetation there, which seems to be the implication?
- Lines 253-254 “However, the impact of these changes is local and it is reasonable to assume that this increase in wind speed has only a weak downstream influence on the North Atlantic”: Is there evidence to back up the assertion that the impact is local and not impactful for the North Atlantic?
- Fig 2 discussion: It might be helpful to show the control, forestNA and grassNA depth profiles themselves, not just the differences. Otherwise, it would be helpful to the reader to add more description to the paragraph starting on Line 269 – for example, how can one see that the overturning has shallowed in forestNA from 2a? How does the positive anomaly at 60N suggest a shift equatorward?
- Discussion of Fig 2c: I don’t see any note regarding the impact of the relatively fast and extreme increase in AMOC maximum for the grassNA simulation; this seems like a big shock to the system. Is that initial shock expected? In general, discussion around this panel should be expanded.
- Line 310 “…SST fingerprint in response AMOC changes”: Missing a word somewhere?
- Lines 337-338 “…cold temperatures in grassNA on land lead to increases in sea ice along the coasts (Fig. 3f)…”: Sea ice fraction doesn’t seem to increase markedly in the Davis Strait along the coasts, so I’m not sure how it limits the MLD response. Can this be described more to help the reader see what the authors are suggesting?
- This also seems to be contradicted in the next few sentences (“Sea ice anomalies are however small…”), perhaps suggesting a more detailed discussion is needed.
- Line 345 “The same SST and MLD patterns as in…”: This comparison to Gervais et al. (2018) is somewhat hard to follow and to see the benefit/relevancy of. Suggest reworking this.
- Lines 353-355 “Moreover, the North Atlantic is anomalously fresh and cold…”: It seems that a hypothesis is both offered and refuted here, which makes it confusing for the reader to know what to take away. Perhaps phrase this section differently to highlight the relevant points.
- Fig 4 caption: Define “THF” somewhere, as it’s used in subplot titles.
- Fig 5: I’m not sure what the green/brown shading adds to the interpretation of the figure; suggest removing it for clarity (would be easier to focus on the rainbow contours)
- Lines 448-449: “…suggests that forest cover change does not only directly influence the air parcels upstream but does so by also influencing the surrounding regions.”: I don’t see the evidence for this claim.
- Lines 522-524: “Here, annual averages are chosen over DJFMAM…”: Would be helpful to confirm that differences between annual and winter-half year averages are small. Or explain what some of the differences are, if not.
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Citation: https://doi.org/10.5194/egusphere-2024-2087-RC2 - AC2: 'Reply on RC2', Victoria Bauer, 28 Nov 2024
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