the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
The first 250 years of the Heinrich 11 iceberg discharge: Last Interglacial HadGEM3-GC3.1 simulations for CMIP6-PMIP4
- 1British Antarctic Survey, Cambridge, UK
- 2Department of Engineering and Physical Sciences, University of Leeds, UK
- 3Department of Meteorology, University of Reading, UK
- 4Met Office, Exeter, UK
- 1British Antarctic Survey, Cambridge, UK
- 2Department of Engineering and Physical Sciences, University of Leeds, UK
- 3Department of Meteorology, University of Reading, UK
- 4Met Office, Exeter, UK
Abstract. The Heinrich 11 event is simulated using the HadGEM3 model during the Last Interglacial period. We apply 0.2 Sv of meltwater forcing across the North Atlantic during a 250 years long simulation. We find that the strength of the Atlantic Meridional Overturning Circulation is reduced by 60 % after 150 years of meltwater forcing, with an associated decrease of 0.2 to 0.4 PW in meridional ocean heat transport at all latitudes. The changes in ocean heat transport affect surface temperatures. The largest increase in the meridional surface temperature gradient occurs between 40–50 N. This increase is associated with a strengthening of 20 % in 850 hPa winds. The stream jet intensification in the Northern Hemisphere in return alters the temperature structure of the ocean heat through an increased gyre circulation, and associated heat transport (+0.1–0.2 PW), at the mid-latitudes, and a decreased gyre ocean heat transport (−0.2 PW) at high-latitudes. The changes in meridional temperature and pressure gradients cause the Intertropical Convergence Zone (ITCZ) to move southward, leading to stronger westerlies and a more positive Southern Annual Mode (SAM) in the Southern Hemisphere. The positive SAM influences sea ice formation leading to an increase in Antarctic sea ice. Our coupled-model simulation framework shows that the classical "thermal bipolar see-saw'' has previously undiscovered consequences in both Hemispheres: these include Northern Hemisphere gyre heat transport and wind changes; alongside an increase in Antarctic sea ice during the first 250 years of meltwater forcing.
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Maria Vittoria Guarino et al.
Status: final response (author comments only)
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RC1: 'Comment on cp-2021-187', Anonymous Referee #1, 06 Mar 2022
This study presents a water hosing simulation using the HadGEM3 to study the dynamical processes that are relevant for the H11 event. The authors document changes in the atmosphere, ocean, and the coupled system. One particularly interesting finding is the sea ice expansion in the Southern Ocean in response to the AMOC weakening, which the author attribute to the atmospheric processes, i.e., shifts of jet stream and changes in the Southern Annual Mode (SAM).
The manuscript is well written. The use of state-of-the-art model, the HadGEM3, is beneficial for the community to study the latest CMIP6-class models. Analyses from the atmosphere, ocean, and the coupled perspectives are also very useful. However, some of the authors’ conclusions are not well-supported by the current analyses and therefore lack well-established casual relationships in the authors’ simulations. Please see my comments below. These comments should be addressed before the manuscript can be published in Climate of the Past.
Major comments
- The authors need to tone down the rhetoric regarding their work being the “the first time that the classical “thermal bipolar seesaw” associated with the H11 event has undiscovered consequences in both Hemispheres”. AMOC and the associated climate change have been studied for several decades using water hosing experiments by the climate dynamics community (e.g., Stouffer et al., 2006). The feedbacks from ocean transport and air-sea interactions that are relevant for the AMOC have also been studied extensively (e.g., Buckley and Marshall, 2016; Stocker et al., 2001). Moreover, how the North Atlantic processes can impact the remote regions has also been studied in much detail, including the impact on the tropics and subtropics (e.g., Zhang and Delworth, 2005), global atmospheric teleconnections (e.g., Wu et al., 2013), in a paleoclimate (e.g., Kageyama et al., 2013), and on the Southern Ocean in the context of climate variability in the present-day climate (e.g., Zhang et al., 2017). Given the rich literature on this topic, I think it is unjustified to claim the authors’ results are the “first” and show “previously undiscovered consequences in both Hemispheres”, unless the authors can show that the AMOC’s influences are uniquely tied to the LIG climate and have different dynamics from previous studies.
- The sea ice increase over the Southern Ocean in the authors’ simulations is interesting and somewhat surprising; more analysis on this is warranted. Is the sea ice increase occurring throughout the year or only in specific seasons? Is the time evolution of the sea ice cover consistent with a SAM-driven mechanism? Can you also show time series of the sea ice expansion? Causal relationship needs to be better established in the authors simulations. Please show the details of the dynamical and thermodynamical processes (Line 242) that drive the sea ice expansion. Are the results model dependent, given the model-dependent responses, e.g., documented in Kageyama et al. (2013)?
- Where does the deep-water formation take place in the PI and the LIG simulations using HadGEM3? Can the model simulate deep convection over the GIN Seas that has been suggested by observations (e.g., Holte et al., 2017; Lozier et al., 2019)? How are the sites of deep-water formation linked to the heat transport by the AMOC (Figure 9e)? Is it possible that the small heat transport by the AMOC at 50N northward (Line 220–223 & Line 359–361) is caused by the model bias in the deep-water formation over the GIN seas.
- A comment related to the above, please consider adding plots of the meridional overturning stream function and the winter maximum mixed-layer depth in the PI, LIG, and H11 simulations. Please also add the mixed-layer depth from observation (Holte et al., 2017) for comparison.
Minor comments:
- Line 79: change “N20” to “N2O”
- Line 109–110: the criteria of p-value should be 0.05 (instead of 0.5), right?
- Figure 2 and related discussion: how is the North Atlantic and Southern Hemisphere defined spatially? Does the North Atlantic include the Greenland, Iceland, and Norwegian Seas?
- Line 179–182: Please add a mechanistic understanding of the shift of the polar jet in the Southern Hemisphere.
- Figure 5: I do not see “error bars” in the plots.
- Figure 12: is the sea ice concentration annual mean, or for the winter?
References:
Buckley, M. W., & Marshall, J. (2016). Observations, inferences, and mechanisms of the Atlantic Meridional Overturning Circulation: A review. Reviews of Geophysics, 54(1), 5–63. https://doi.org/https://doi.org/10.1002/2015RG000493
Holte, J., Talley, L. D., Gilson, J., & Roemmich, D. (2017). An Argo mixed layer climatology and database. Geophysical Research Letters, 44(11), 5618–5626. https://doi.org/https://doi.org/10.1002/2017GL073426
Kageyama, M., Merkel, U., Otto-Bliesner, B. L., Prange, M., Abe-Ouchi, A., Lohmann, G., Ohgaito, R., Roche, D. M., Singarayer, J., Swingedouw, D., & Zhang, X. (2013). Climatic impacts of fresh water hosing under last glacial Maximum conditions: A multi-model study. Climate of the Past, 9(2), 935–953. https://doi.org/10.5194/cp-9-935-2013
Lozier, M. S., Li, F., Bacon, S., Bahr, F., Bower, A. S., Cunningham, S. A., ... & Zhao, J. (2019). A sea change in our view of overturning in the subpolar North Atlantic. Science, 363(6426), 516-521. https://doi.org/10.1126/science.aau6592
Stocker, T. F., Knutti, R., & Plattner, G.-K. (2001). The future of the thermohaline circulation - A perspective. In The oceans and rapid climate change: Past, Present and Future. Geophysical Monograph 126 (Vol. 126, pp. 277–293). AGU. https://doi.org/10.1029/GM126p0277
Wu, L., Li, C., Yang, C., & Xie, S.-P. (2008). Global Teleconnections in Response to a Shutdown of the Atlantic Meridional Overturning Circulation. Journal of Climate, 21(12), 3002–3019. https://doi.org/10.1175/2007JCLI1858.1
Zhang, L., Delworth, T. L., & Zeng, F. (2017). The impact of multidecadal Atlantic meridional overturning circulation variations on the Southern Ocean. Climate Dynamics, 48(5), 2065–2085. https://doi.org/10.1007/s00382-016-3190-8
Zhang, R., & Delworth, T. L. (2005). Simulated tropical response to a substantial weakening of the Atlantic thermohaline circulation. Journal of Climate, 18(12), 1853–1860. https://doi.org/10.1175/JCLI3460.1
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RC2: 'Comment on cp-2021-187', Juan Muglia, 08 Mar 2022
Guarino et al. produced a manuscript on simulations of Heinrich 11, a period of land ice melting during the Last Deglaciation. Because of computational constrains, the authors only show results of a 250 y simulation, a limiting factor for a study of global climate that includes the deep ocean. This limitation is somewhat mentioned in the title of the manuscript, which explicitly states the length of the simulations.
CommentsIn general, 350 y, which is the time that the LIG simulation was run, is not enough time to equilibrate the physics of the deep ocean. See for example Marzocchi and Jansen (2017, GRL), where they have to run CCSM4 with LGM boundary conditions for several thousand years to achieve physical equilibrium in the deep ocean. The authors should give evidence that their LIG simulation is in equilibrium, or explain why such requirement is not needed in this study.
Concerning the 250 y H11 simulation, it is necessary to distinguish between the 250 y of a computer simulation and the first 250 y of H11 in climate history. If the authors want to claim that their 250 y simulation represents the actual first 250 y of H11 they should give evidence of the correct representation of the timing of physical processes in the atmosphere and ocean. If this can't be done, then I would switch the title to "A 250 yea simulation of the Heinrich 11 iceberg discharge: Last
Interglacial HadGEM3-GC3.1 simulations for CMIP6-PMIP4", and all suggestions in the manuscript that this is an actual simulation of the first 250 y of H11 should be changed accordingly.In several places of the manuscript the authors mention that a longer simulation cannot be performed due to computational constrains. However, conjectures are made about the evolution of some processes beyond the length of the H11 experiment (e.g., lines 255 and 295). Could a 500 y H11 experiment be performed before the publication of this paper? I think it would benefit the science shown, increase the relevancy of this work, and would be beneficial for the science community. In its current state, the analysis is limited to a transient 250 y run, and not all processes governing the H11 climate have manifested (according to the authors). In addition, little evidence is shown that the 250 y presented in the manuscript is a correct representation of what happened during the onset of H11, and not just an abstract modeling study (see my comment about correct representation of timing).
Lines 15-20: The authors should better describe the LIG and H11 climate periods. What was the length of LIG, and of H11? Did H11 occur at the onset of the LIG, or somewhere in the middle? This is important in order to put the reader in context of the climate period we are discussing.
Lines 16-24: "warming of the Southern Ocean during this time is attributed to a slowdown of the Atlantic Meridional Overturning Circulation (AMOC), which has been suggested as a mechanism to explain the 2-3°C Southern Ocean warming found in Southern Ocean and Antarctic climate records" Are the papers mentioned to justify this statement modeling-based or data-based? A brief description of them would be important to understand the scientific basis of this work.
Line 79: N2O not N20.
Line 125: The relation between heat transport and AMOC in interesting. However, a more complete analysis should include salt fluxes as well. How do they change between the LIG and H11 simulations? Do they have any relevancy for AMOC? How are they affected by the North Atlanitc hosing?
Line 195: Could you show the changes in meridional heat transport for the other ocean basins? (in the supplement is OK) Although this work focuses on the Atlantic, effects in other ocean basins may be relevant for readers.
Line 208: In Figure 10 I don't see any subtropical gyre intensification for H11.
Lines 249-255: Could the absence of a dipole in the Antarctic sea ice response to a positive SAM be due to issues with the sea ice model, like for example lack of resolution or limitations in the parametrization of processes? Please expand in the paper. Could you show the sea ice in winter months in the supplementary material?
Maria Vittoria Guarino et al.
Maria Vittoria Guarino et al.
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