the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 24 Jan 2025
Collapse of Deep-Sea Circulation during an Eocene Hyperthermal Hothouse – A DeepMIP Study with CESM1.2
Abstract. During the Paleocene-Eocene Thermal Maximum (PETM, ~56 Ma), a rapid injection of greenhouse gases (with isotopically depleted carbon) into the atmosphere led to a ~5 °C global temperature rise, ocean acidification, and perturbation of marine and terrestrial ecosystems. In this study, we carried out a series of DeepMIP climate sensitivity experiments t using the Community Earth System Model CESM1.2 to evaluate how changes in the radiative forcing could have contributed to Eocene hyperthermal events. An atmospheric change from 3xCO2 relative to pre-industrial levels (PAL) equivalent during the latest Paleocene to 6xCO2 PAL in response to a carbon input pulse of 1680 PgC resulted in equatorial warming to 36.9 °C consistent with proxy estimates. The lower equator-to-pole temperature gradient in this 6xCO2 PAL scenario as compared to the pre-industrial experiment with 1x CO2 PAL is due to the lack of an ice sheet, the increase in greenhouse gases, and a lower cloud optical depth. The climate simulations suggest an intensified hydrological cycle with higher precipitation in the tropics, particularly over the Indian Eocene continent, and in mid-latitude. In contrast, mega-droughts are prominent in the subtropics, particularly in Africa and South America. Topographic effects such as the closure of the Drake Passage and the more southern location of Australia as well as a lower-than-present meridional temperature gradient contribute to a much weaker surface ocean circulation near the Antarctic continent, as compared to the current pronounced Antarctic Circumpolar Current. In response to the increase in greenhouse gas forcing to 6xCO2 PAL, deep water formation in the Southern Ocean nearly collapsed and changed from a southern-dominated deep-sea ventilation to a weak deep water formation in the North Atlantic Ocean and further to a polar collapse of deep water formation and a shallow haline-mode ventilation in the subtropics at 12xCO2 PAL. Bipolar convective overturning in the Pacific Ocean is not supported and remains uncertain, but southern component water mass formation in the Pacific Ocean has been simulated with 1x CO2 PAL. Increased stratification and reduced solubility of dissolved oxygen caused by warming may have contributed to lower abyssal dissolved oxygen concentrations and thus stresses on the marine ecosystem. However, decreased upwelling and productivity may have decreased the apparent oxygen utilization and thus could have increased the oxygen concentration in the twilight zone.
Competing interests: At least one of the (co-)authors is a member of the editorial board of Climate of the Past. Arne Winguth have a conflict.
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this preprint. The responsibility to include appropriate place names lies with the authors.- Preprint
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RC1: 'Comment on egusphere-2024-4209', Michiel Baatsen, 28 Feb 2025
I read this manuscript with great interest, as I think this type of research is very helpful towards understanding the characteristics and performance of the climate models we use, as well as the properties of past warm climates. Being familiar with the contribution of these simulations to DeepMIP, I was excited to learn more about the results, but the overall clarity and quality of this manuscript was underwhelming (maybe partly due to my expectations).
Generally, the manuscript needs a thorough check on writing and typos while the overall figure quality and clarity can be improved considerably.
The simulations and results shown are both interesting and relevant, but the overall design and use of figures greatly limits their proper assessment.
The general configuration of figures is rather inconsistent, especially in terms of aspect ratio, font sizes, and colour/line specifications. Content-wise, I also feel like the figures do not support the storyline and results well.
In addition, there is extremely limited information on the simulations and experimental setup, again making it hard to understand the outcomes.Perhaps most strikingly, my expectations based on the title are not met for the majority of the manuscript. After a rather superficial presentation of general features regarding temperature/precipitation patterns under the different forcings, there is just 1 section on deep ocean circulation which is extremely limited in terms of the actual results that are presented. Furthermore, the initialisation and equilibration of the model is crucial for a proper assessment. While this does not necessarily imply that any of the conclusions are wrong, this aspect deserves much more care and consideration.
I am in favour of publishing these results for the above reasoning, but also think that substantial improvements to the manuscript are needed.
Please see my further (perhaps superfluous) comments added below. -
RC2: 'Comment on egusphere-2024-4209', Anonymous Referee #2, 11 Mar 2025
This is a review of “Collapse of deep-sea circulation during an Eocene Hyperthermal Hothouse – A DeepMIP study with CESM1.2” by Winguth and co-authors.
The manuscript is well written and easy to read.
I wonder though what is the real added value of this manuscript. The model results are well described but are mostly a catalogue of known model results and the discussion to the now relatively large literature on the model results of the 1st phase of DeepMIP is very limited. For example, though precipitations occupy a central place in the text and the figures (Figs. 3, 4, 9, 10 and 12), there is no mention or discussion of the paper by Cramwinckel et al. (2023) that precisely focusses on the hydrological cycle in DeepMIP model results, or of the paper by Williams et al. (2022) on African hydroclimate.
More importantly, given the title of the manuscript, the case for a collapse of the deep-sea circulation in the simulations described is sloppy at best. The title is misleading because there is no exploration of the mechanisms that make the deep-sea circulation collapse beyond stating that the MOC is less intense. The absence of diagnostics makes it impossible to judge whether the ocean is in “near-equilibrium” (as stated) and fails to provide evidence for the so-called subtropical haline mode that is said to exist at 12x CO2.
In my opinion, the manuscript should be significantly revised either to provide clear arguments and diagnostics in favour of the collapse of the deep-sea circulation and/or to clearly demonstrate how the results presented contribute further — than what the different manuscripts by Jiang Zhu and colleagues have demonstrated (using the same model with an updated atmosphere), and more generally than what the community has learned from DeepMIP phase 1 — to the understanding of the Early Eocene warmth and/or the PETM.
Major issues.
There is no way to evaluate whether the ocean is in “near-equilibrium state”. In particular, I seriously doubt that the high CO2 simulations are really close to equilibrium but there are no time series of temperature in the manuscript that could be used to check if this is the case in the intermediate and deep ocean. For instance, the CESM1.2 DeepMIP simulations of Zhu and colleagues at 6x and 9x CO2 in the supplementary materials of Zhang et al. (2022) show that after 2000 years of integration the global mean ocean is definitely warming, and even more so in the intermediate/deep ocean if we assume that the upper ocean is close to equilibrium. I guess the same is happening in your high CO2 simulations.
That the PETM warming generates a transient collapse of the overturning makes perfect sense but out-of-equilibrium snapshot experiments producing a sluggish circulation are in my opinion only poorly supporting this. For instance, with transient simulations, Alexander et al. (2015) nicely show that the PETM MOC collapses during the first few millennia and slowly reinforces until its intensity exceeds the initial pre-perturbation value (Fig. 5 of their supplementary materials). The simulations of Kirtland-Turner et al. (2024) also show this, although on a much smaller scale because the overturning only weakly slows down (their Fig. 3b).
Section 1.2
The implementation of DeepMIP conditions should be better explained. It is described for aerosols but not for the other forcings mentioned l. 65-67. For instance:
- what are the differences in implementation between this manuscript and the simulations of Zhu and colleagues reported in DeepMIP (e.g. Lunt et al. 2021)? Notably, Zhu et al. implemented a specific marginal sea balancing scheme for the Arctic Ocean to conserve salinity in their DeepMIP simulations (Lunt et al. 2021, section 2.2.1). Is this also the case here? It might be useful to provide salinity time series as well.
- Herold et al. give a river runoff direction map. I would have thought you would use it directly (it was made for the CCSM/CESM model) but your sentence suggests otherwise.
- Eccentricity = 0.06 is not preindustrial. Is this an error?
- how was tidal dissipation implemented?
- the preindustrial solar constant used is 1361 W.m-2 but Winguth et al. (2010) uses a PETM solar constant of 1362 W.m-2. Was the reference solar constant updated?
- it looks like there is a missing minus sign in the initial temperature profile. And should it be 5000 rather than 6000?
Minor points.
l. 15-16. This sentence is correct but not easy to follow. Please reformulate.
l. 26, 189, 293. What do you mean by subtropical haline shallow mode exactly? The deepest MLD in the Northern Hemisphere in the 12x CO2 simulation is found westward of Greenland, as in the other simulations at lower CO2 (Fig. 6).
l. 55-56. Perhaps use some of the diagnostics shown in Zhang et al. (2022) seeing as this paper analyzes the Early Eocene ocean circulation in DeepMIP models.
l. 75. Easiest said that the Eocene ice sheet boundary condition is no ice.
l. 94-96. Remove. That the Eocene aerosol forcing has negligible effect has already been said, plus the simulation PB_PR is actually not shown anywhere.
l. 137. Unclear why the authors states that the simulated MOC is consistent with previous studies. To the least, Goldner et al. (2014) and Toumoulin et al. (2020) do not simulate bipolar deep water formation and Eslworth et al. (2017) may do so (though it is hard to state for sure based on the figures) but only with a deep Drake Passage.
Data availability. Standard good practice today has it that the data should be open-access. Please provide the outputs the replicate the results.
Figure 10b is not used in the text.
Figure 13. At which depth is shown the ideal age? Also, the arrows are hard to catch.
References
Alexander, K. J. et al. (2015). Sudden spreading of corrosive bottom water during the Palaeocene–Eocene Thermal Maximum. Nature Geoscience, 8(6), 458-461.
Cramwinckel, M. J., et al. (2023). Global and zonal‐mean hydrological response to early Eocene warmth. Paleoceanography and Paleoclimatology, 38(6), e2022PA004542.
Kirtland-Turner, S., et al. (2024). Sensitivity of ocean circulation to warming during the Early Eocene greenhouse. Proceedings of the National Academy of Sciences, 121(24), e2311980121.
Williams, C. J., et al. (2022). African hydroclimate during the early Eocene from the DeepMIP simulations. Paleoceanography and Paleoclimatology, 37(5), e2022PA004419.
Zhang, Y., et al. (2022). Early Eocene ocean meridional overturning circulation: The roles of atmospheric forcing and strait geometry. Paleoceanography and Paleoclimatology, 37(3), e2021PA004329.
Citation: https://doi.org/10.5194/egusphere-2024-4209-RC2
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