the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 25 Mar 2024
Surface buoyancy control of millennial-scale variations of the Atlantic meridional ocean circulation
Abstract. Dansgaard-Oeschger (DO) events are a pervasive feature of glacial climates. It is widely accepted that the associated changes in climate, which are most pronounced in the North Atlantic region, are caused by abrupt changes in the strength and/or latitude reach of the Atlantic meridional overturning circulation (AMOC), possibly originating from spontaneous transitions in the ocean-sea-ice-atmosphere system. Here we use an Earth System Model that produces DO-like events to show that the climate conditions under which millennial-scale AMOC variations occur are controlled by the surface ocean buoyancy flux. In particular, we find that the present day-like convection pattern with deep water formation in the Labrador and Nordic Seas becomes unstable when the buoyancy flux integrated over the northern North Atlantic turns from negative to positive. It is in the proximity of this point that the model produces transitions between different convection patterns associated with strong and weak AMOC states. The buoyancy flux depends on the surface freshwater and heat fluxes and on sea surface temperature through the temperature dependence of the thermal expansion coefficient of seawater. We find that larger ice sheets tend to stabilize convection by decreasing the net freshwater flux while CO2-induced cooling decreases buoyancy loss and destabilizes convection. These results help to explain the conditions under which DO events appear, and are a step towards an improved understanding of the mechanisms of abrupt climate changes.
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Status: open (until 20 May 2024)
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RC1: 'Comment on egusphere-2024-819', Anonymous Referee #1, 26 Apr 2024
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Review of Willeit et al: Surface buoyancy control of millennial-scale variations of the Atlantic meridional ocean circulation
Willeit and colleagues present a large ensemble of CLIMBER-X simulations with various combinations of continental ice sheet configurations and atmospheric CO2 concentrations. This unprecedented ensemble allows them to analyse the physical conditions that determine the forcing range in which CLIMBER-X produces DO-like, millennial-scale climate variability. They find that this "sweet spot" is controlled by the sign of the surface buoyancy flux north of 55N. Millennial-scale transitions between week and strong AMOC states occur when the buoyancy flux north of 55N is about to switch sign. A strong/present day like AMOC occurs when the buoyancy flux is negative and deep water formation takes place in the Labador and Nordic Seas. When the buoyancy flux switches sign, this modern-like deep water formation pattern becomes unsustainable. The conditions under which this sign switch occurs are controlled by the boundary conditions. LGM-like ice sheets tend to enhance buoyancy loss, while low CO2 concentrations tend to decrease it. The balance of the two effects seems to be well captured by CLIMBER-X as the strongest DO-like variability occurs at realistic MIS3-like boundary conditions.
Some of the conclusions are not exactly new, e.g. the cancellation of the effects of ice sheet size and CO2 concentration. However the range and combination of covered boundary conditions is unprecedented, and the results are very relevant for the DO- and wider CP community and thus definitely worthy of publication. It is also very much appreciated that the authors define a metric that could be used to compare the physical conditions that control the "sweet spot" across models. Before publication, I would ask the authors to provide more context in some parts and to address a few issues as outlined in my comments below.
**Major Comments**:
1. Introduction/Discussion: Please provide more context on what has already been suggested in terms of physical control on the sweet spot. At least Galbraith & de Lavergne (2019) and Klockmann et al (2018) provided some suggestions, e.g. the overall volume of Antarctic Bottom Water (AABW) present in the deep ocean, the density difference between AABW and North Atlantic Deep Water, presence of deep water formation in the Nordic Seas. Also spell out more directly how the additional CLIMBER-X simulations can help in pin-pointing the physical control across models. Because the physical control might also be model dependent.2. I agree that the buoyancy flux analysis in this paper and the one in Klockmann et al (2018) cannot be compared directly one to one but at least a qualitative comparison should be possible and would actually strengthen the authors arguments even further. This could e.g. take place in the Discussion section.
Overall, the mode transitions in the experiments with PI ice sheets in Klockman et al also occur when net buoyancy flux over their NAtl&LabSea region changes from buoyancy loss to buoyancy gain (Klockmann et al use density instead of buoyancy, so the sign is flipped). In their Nordic Seas region, the buoyancy flux is close to zero for the CO2 range where the transition takes place, so the Nordic Seas would not change the sign. This qualitative agreement makes the suggested metric M in the discussion of this manuscript even stronger.
One interesting difference can be seen in the effect of ice sheets on the thermal component. In Klockmann et al, the stronger net buoyancy loss with glacial ice sheets is due to increased heat loss over the deep convection sites, while in the present study, it is due to the reduced freshwater input. I do not have an immediate hypothesis where this difference might arise from. Perhaps it is simply due to the different areas of integration.3. What is the role of sea ice in the buoyancy flux? Is the effect of freezing/brine release and melting included in the freshwater and heat budgets? Sea ice typically plays a big role in feedback loops regarding convection patterns. Even though it can be difficult to determine whether sea-ice is driving the change in the convection patterns or responding to it, it is still worth to be included more explicitly in the analysis.
4. It might be insightful to show the buoyancy flux also for the equilibrium simulations, e.g. in a similar style as Fig. 3 with buoyancy flux as the colour coding. That would help in linking the results from the transient and equilibrium simulations.
**Minor Comments**
l.3 "latitudinal reach" or "northward extent" instead of "latitude reach"?l.38-40: see major comment 1
l.45: what is the climate-only setup? Are there other setups?
l.75: How sensitive is the model to the area where the noise is applied? Why is it applied only locally and not globally?
l.115: please also state the temperature changes over Greenland in the simulations and in the reconstructions. What does it mean if Greenland change is not capture well but the Iberian margin yes?
l.123-124: "The heat transport [...]" What do you base this sentence on? Is it based on previous studies (if yes, please cite)? Or do you infer it from your results (if yes, please elaborate shortly)?
Fig.7: Please correct the caption. The interstadial sea-ice extent is drawn in dark teal and not grey
Fig.8: In the experiment description and in Fig.9 you mention a total of six noise amplitudes. Here you show only four. Why are 0.0625 and 0.125 not shown? Or did you not cover the full CO2 range for these amplitudes? If so please mention this in the experiment description.
Fig.9: Which CO2 concentration was used in the respective simulations displayed here?
l.152: Please briefly state, how do you define stable here (and elsewhere in the manuscript). Also, how realistic are the deep convection patterns in CLIMBER-X given the very coarse resolution?
l.154: "two modes" instead of "two stable modes". The "stable" in the latter half of the sentence ("are stable under the same CO2") is sufficient.l.161/Fig.10: what about the smaller oscillations that occur around 160ppm with interglacial ice sheets and around 240ppm with mid-glacial ice sheets? In these cases, the buoyancy flux does not change sign.
l.161/273: Is the Arctic Ocean included in the integral of the buoyancy flux?
Fig.11: What is the averaging period shown here? What does the grey circle around the North pole indicate?
l.169-188: This part is difficult to read with the many "increases" and "decreases". Try to spell out more specifically whether the listed factors induce a buoyancy loss or gain. It can become difficult to correctly interpret increase and decrease if a property (such as M) can have different signs with small or large absolute values.
Fig.13 and related text: This figure is discussed very briefly, approximately with one and a half sentence. It might be worth to spend a few more words on this figure and to also make the connections between the left half and the right half clearer. Especially because the information in the right half has already been shown in Fig. 10 and 12. Also the relation between hosing and noisy freshwater forcing could be explained some more.
l.194-201: Compare diapycnal diffusivity results to previous work, e.g. diapycnal diffusivity seems to have played a key role in generating the DO oscillations under LGM conditions in Peltier&Vettoretti (2014).
Citation: https://doi.org/10.5194/egusphere-2024-819-RC1
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