the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Surface mass balance and climate of the Last Glacial Maximum northern hemisphere ice sheets: simulations with CESM2.1
Sarah Louise Bradley
Raymond Sellevold
Michele Petrini
Sotiria Georgiou
Jiang Zhu
Bette L. Otto-Bliesner
Marcus Lofverstrom
Abstract. The Last Glacial Maximum (LGM, from ~26 to 20 ka BP) was the most recent period with large ice sheets in Eurasia and North America. At that time, global temperatures were 5–7 °C colder than today, and sea level ~125 m lower. LGM simulations are useful to understand Earth System dynamics including climate-ice sheet interactions, and to evaluate and improve the models representing those. Here, we present two simulations of the Northern Hemisphere ice sheet climate and surface mass balance with the CESM2.1(CAM5) with prescribed ice sheets for two time periods that bracket the LGM period: 26 ka and 21 ka BP. CESM2.1 includes explicit simulation of snow/firn compaction, albedo, refreezing, and direct coupling of ice sheet surface energy fluxes with the atmosphere. The simulated mean snowfall accumulation is lowest for the Greenland and Barents-Kara Sea Ice Sheets (GrIS, BKIS) and highest for British and Irish (BIIS) and Icelandic (IcIS) ice sheets. Melt rates are negligible for the dry BKIS and GrIS, and relatively large for the BIIS, NAISC, SIS and IcIS, and are reduced by almost a third in the colder 26 ka BP climate compared with 21 ka BP. The surface mass balance (SMB) is positive for the GrIS, BKIS, SIS and IcIS during the LGM (26 ka and 21 ka BP), and negative for the NAISC and BIIS. Relatively wide ablation areas are simulated along the southern (terrestrial), Pacific and Atlantic margins of the NAISC, across all of the BIIS surface, and along the terrestrial southern margin of the SIS. For 26 ka BP climate the integrated SMB substantially increases for the NAISC and BIIS, but it does not reverse the negative sign. Summer incoming solar radiation at the surface is largest over the high interior of the NAISC and GrIS, and minimum over the BIIS and southern margin of NAISC. Summer net radiation is maximum over the ablation areas and minimum where the albedo is highest, namely in the interior of the GrIS, northern NAISC and all of the BKIS. Summer sensible and latent heat fluxes are highest over the ablation areas, positively contributing to melt energy. Refreezing is largest along the equilibrium line for all ice sheets, and prevents 40–50 % of meltwater entering the ocean. Our SMB results are in qualitative agreement with the climatic variability across the different northern hemisphere ice sheets. The large, simulated melt for the NAISC suggests potential biases in the climate simulation, ice sheet reconstruction, and/or highly non-equilibrated climate and ice sheet at the LGM time.
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Sarah Louise Bradley et al.
Status: open (until 10 Oct 2023)
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RC1: 'Comment on cp-2023-62', Anonymous Referee #1, 31 Aug 2023
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Review of
Surface mass balance and climate of the Last Glacial Maximum northern hemisphere ice sheets: simulations with CESM2.1
by Sarah L. Bradley and othersSummary
This study describes the climate and surface mass balance of NH ice sheets during/surrounding the last glacial maximum, using two long runs with a state-of-the-art earth system model, but with static ice sheets. The paper is original, and well written, albeit with numerous small errors (the list below is not exhaustive), but the length and structure/organization of the paper are sub-optimal, and the figure quality must be improved notably its uniformity.
Major commentsMy main concern is that the paper is a bit of a hodgepodge of topics. The many ice sheets and variables, in combination with two different time slices, make for a huge number of permutations of results to describe. Moreover, the results are mainly described in a qualitative fashion, which reduces the impact of the study. The authors use a plethora of figures, which negatively impacts the readability of the paper. At the same time, I acknowledge that this is not easy to resolve, but I kindly ask the authors to critically assess whether the volume of text and figures could be reduced and or the structure changes to make it more accessible.
Along the same lines, the clarity of the figures and their uniformity must be improved. See some of the remarks below (not exhaustive).
l. 24: For this run, did the ice sheets in the simulation feed freshwater into the oceans? If so, how much and how was it spatially distributed?
The comparison with Kapsch et al. (2021) would be more valuable if (spatially integrated) quantities were quantitatively compared.
Minor and textual commentsl. 2: Please use 'higher' and 'lower' temperatures rather than 'warmer' and 'colder/cooler' temperatures throughout; I realize it is a rear-guard battle but isn’t that the reviewer's prerogative?
l. 5: These acronyms will not be familiar to all readers.
l. 7: "snowfall accumulation "; The term 'snowfall' is commonly reserved for the solid precipitation fraction and 'accumulation' for snowfall minus sublimation. So, to avoid confusion, preferably use 'snow accumulation'.
l. 9: NAISC and SIS are not defined. NAISC is only defined in line 44.
l. 18: " along the equilibrium line " I suppose you mean "in the lower accumulation zone".
l. 25: contributor -> contributors
l. 38: typo " resolutionclimate "
l. 42: Suggest: "See Fig. 1 in Ivanovic etc."
l. 42: solar insolation signal -> insolation
l. 76 typo: "simulates realistically simulates"
l. 92: surface air temperature -> 2 m air temperature
l. 127: correspond ->. corresponds
l. 114: In order of preference: NOT surface temperature BUT 2 m temperature OR near-surface air temperature OR surface air temperature
l. 121: significance. Consider reducing the number of decimals, i.e., "6.8C" instead of "6.84C"
l. 135: " the former of which coincides with highest standard deviations from the model ensemble, up to 9◦C. " Unclear, please clarify/reformulate.
l. 139: " 4◦C and 2◦C 140 cooling at GRIP and VOSTOK ice core sites, respectively " It is not intuitively clear whether you refer to the observed (ice core-based) temperature changes, or model changes at those ice core sites. If the latter, do they agree with the ice core-derived values?
l. 190: " a colder atmosphere can hold less water " This is wrong terminology. Even in the absence of other gases, the saturated water vapor pressure would be the same.
l. 205: " The interior of the Laurentide " Please continue to use the acronyms previously introduced.
l. 265: strength weakens -> weakens
l. 293: a extremely -> an extremely
l. 293: " Two ice sheets have a extremely negative SMB: NAISC and BIIS". This may be true for BIIS but not for NAIS, which is only weakly negative.
l. 297: " High refreezing rates are simulated along the equilibrium line altitude " See previous comment.
l. 298: "inverse sublimation ". Unclear, do you mean deposition. Or is evaporation/deposition included?
l. 306: " rapid retreat would occur. " Rapid retreat would likely occur.
l. 363: that -> than
Fig.1: These frames are somewhat cluttered; consider removing isotherm contours (dashed/solid) and their labels and only retain shades. Increase font in (d) to match other panels.
Fig. 3: The look and feel of these panels are rather different from those in Fig. 1. Please make the format of the figures more uniform and critically assess them for errors or omissions.
Fig. 9: Please increase legend label spacing.
Fig. 5: Please include a legend of red and blue lines in windows a) and c).
Fig. A1: Using similar color scales for ice elevation and ocean depth is confusing for this Arctic application.
Citation: https://doi.org/10.5194/cp-2023-62-RC1 -
RC2: 'Comment on cp-2023-62', Sam Sherriff-Tadano, 16 Sep 2023
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Summary
The authors conducted simulations of the global climate and Surface Mass Balances (SMBs) over ice sheets at the Last Glacial Maximum (LGM) using the recently developed CESM2.1. They first assessed the model's performance in simulating climate variables such as Surface Air Temperature (SAT), atmospheric circulation, Sea Surface Temperatures (SST), sea ice extent, and the Atlantic Meridional Overturning Circulation (AMOC). Subsequently, they analyzed the model's performance of the SMBs.
I find this study intriguing, especially regarding the analysis of the Surface Mass Balance (SMB) and its decomposition into different components. Additionally, the comparisons of SMBs across six different ice sheets are novel. The model also successfully reproduces global climate and oceanic conditions, including the Atlantic Meridional Overturning Circulation (AMOC). However, in Section 3, I occasionally struggled to follow the discussion because some of the analysis results were presented without a clear explanation of their relevance to the SMB field. Overall, I think the study meets the interest of the readers of the “Climate of the Past” and the presented results are new. Therefore, I would recommend publication after some minor/intermediate revision. The following summaries my suggestions for modifying the manuscript.
General Comment
1. Additional analysis on the cloud radiative effect (long wave) and its relation to SMB
A recent study by Gandy et al. (2023) discussed the role of clouds on the SMB over the southern margin of the North American ice sheet. They showed that many of the PMIP models show small cloud cover over the southern margin, which caused an intense downward shortwave radiation at the surface in that region. In this study, the authors have shown that the relatively large cloud cover at the southern margin plays a role in reflecting more sun light. On the other hand, they showed a relatively strong downward long-wave radiation, which could be relative to the existence of clouds or moisture. I would suggest the authors to conduct further analysis on the role of clouds (especially on the long wave side) and add a discussion on its effect on SMB. Effects of clouds on the SMB is highly uncertain in general, and also thought to be important in the future Greenland (Gregory et al. 2020). Additional analysis on this point would make this study more interesting.
2. Implication to the evolution of British Ice sheet during the LGM
Gandy et al. (2018) showed that a warming of climate after 26ka is required to initiate a deglaciation of the British Ice sheet in their ice sheet model simulations. However, the sure about the cause of the warming remained unclear. This study nicely showed that the differences in orbital parameters between 21ka and 26ka can cause a warming and a decrease in SMB at 21ka over the British ice sheet. Adding a discussion on this point citing Gandy et al. (2018) would be beneficial and would also increase the impact of this paper.
3. Discussion on the cause of the large negative SMB at the southern margin of North American ice sheet
One of the advantage of this study is the use of a higher horizontal resolution in the AGCM compared to previous studies. First, I thought that this would improve the representation of the so-called stationary wave effect, which cools down the southern margin and increases the SMB. However, even with the higher resolution used in this model, it still simulates a very large negative SMB like other models. Please add a detailed discussion on what could be the potential cause of this negative SMB, e.g. the downscaling method, representations of clouds, biases in the climate model side and so on.
4. Readability of Section 3.
I would suggest to move subsections 3.2 and 3.3 to the beginning of the section and combine with the analysis of global temperature. In this way, the authors can start the result section from general analysis (larger scale temperature, sea ice & AMOC), followed by detailed analysis on the atmospheric circulation, which then connects well to the SMB section. Hopefully this would increase the readability of the manuscript.
Specific comments
L19: It was a bit hard to interpret this sentence. In particular, what do you mean by climate variability?
L36: Perhaps, you may also cite a recent study by UKESM (Smith et al. 2021) here.
L76: Typo, two “stimulates”
L76: It might be written in a different paper, but could you clarify whether the precipitation field is downscaled or not, please?
L80-85: Could you add a sentence explaining the reason for these modifications? Is it just from a practical reason or is there any science behind them?
L107-109: Please add a sentence on what exactly was modified/adjusted here.
L130: “lower surface elevation”. It this a typo? I would expect a warming when there’s a lower surface elevation due to the lapse rate effect.
L135: Do you have a short explanation on why there’s a hemispherically asymmetric temperature differences between LG-21ka and Osman data?
L145-: I was confused with this paragraph. My understanding was that the differences in the orbital configuration between LG-21ka and PI do not cause large changes in the radiation budjet, but differences in the ice sheet configuration and cloud radiative effect do. In this regard L146 “due to different orbital conditions” doesn’t read right to me. Please restructure the pargraph also following my General comment 1.
L165: On which height/pressure level is the geopotential height field analyzed?
L259: The simulated sea ice field is compared with that of Paul et al. (2021). However, as far as I know, the reconstructions of sea ice area over the Northern Hemisphere during the Last Glacial Maximum (LGM) are not well-constrained. I'll leave it to the authors, but it might be useful to point this out.
Reference
Gandy, N. et al.: Marine ice sheet instability and ice shelf buttressing of the Minch Ice Stream, northwest Scotland. Cryosphere 12, 3635-3651. https://doi.org:10.5194/tc-12-3635-2018, 2018.
Gandy, N. et al.: De-tuning albedo parameters in a coupled Climate Ice Sheet model to simulate the North American Ice Sheet at the Last Glacial Maximum. Journal of Geophysical Research: Earth Surface, 128, e2023JF007250. https://doi.org/10.1029/2023JF007250, 2023.
Smith, R. S. et al.: Coupling the U.K. Earth System model to dynamic models of the Greenland and Antarctic ice sheets. Journal of Advances in Modeling Earth Systems, 13, e2021MS002520. https://doi.org/10.1029/2021MS002520, 2021.
Citation: https://doi.org/10.5194/cp-2023-62-RC2 -
RC3: 'Comment on cp-2023-62', Anonymous Referee #3, 17 Sep 2023
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With prescribed Northern Hemisphere (NH) ice sheet geometry, this study explores climate fields and ice sheet surface mass balance (SMB) in two CESM2.1 simulations of the Last Glacial Maximum, for two time periods -- 26 ka and 21 ka -- respectively. It presents a detailed decomposition of ice sheet SMB budgets for each major NH ice sheet, which is useful for understanding the asynchronous evolution of NH ice sheets. The paper's quality, in terms of both its science and writing, generally meets the standards of Climate of the Past. It could be considered for publication after some minor modifications/clarifications.
One of the main conclusions of this study is that "The large simulated melt for the NAISC suggests potential biases in the climate simulation, ice sheet reconstruction, and/or highly non-equilibrated climate and ice sheet at the LGM time". These are all plausible. But little discussion has been devoted to the simulated melt for the BIIS, which is 3-4 times higher than that for the NAISC, and the simulated BIIS SMB is much more negative. This is likely due to biases in the CESM2.1 simulations. Reconstructions of the BIIS suggest it did not change much or grew slightly between 26 ka and 23 ka, so the strongly negative SMB simulated in this study seems unrealistic. Compared with GLOMAP, JJA SST was too warm in the North Atlantic in both simulations (LG-26ka and LG-21ka), possibly resulting in the excessive melt for the BIIS. The paper would benefit from a more thorough discussion of the large simulated melt for both the NAISC and the BIIS.
Specific comments:l.9: It should be mentioned here that the acronym "NAISC" refers to the "North American ice sheet complex".
l. 37: The previous version CESM2.0 had a very high Equilibrium Climate Sensitivity (ECS). What is the ECS for CESM2.1?
l.130: I think "lower surface elevation" (in LG-21ka compared to LGM-Zhu), based on the lapse-rate argument, should result in a warming not cooling over the ice sheets. It was mentioned previously that ICE6G in LGM-Zhu has a smaller GrIS, so should this be "higher surface elevation"?
l. 145-150: The difference in TOA SW_in between LG-21ka and PI is discussed here, but not shown in Figure 2. I would suggest adding a panel for TOA SW_in in Figure 2, which is useful for understanding the higher surface SW_in in summer months over high latitudes.
l.199: "... move, on average, more along the margin ..." doesn't read right.
l.225: It is not obvious that "In the Southern Hemisphere, differences with GLOMAP are smaller". As seen in Figure 5 (d), differences in sea ice extent equally substantial for the SH, especially in Febuarary.
l.241: A typo after "shallower".
l.298 "snowfall deposition": Should it be "frost deposition"?
l.325-330: I think more discussion is needed to understand the features of SHF and LHF in Figure 9 (g, h). Both SHF and LHF are defined so that they are positive downwards, but it is intriguing that SHF is negative in the interior of NAISC -- meaning the ice surface heats the near-surface air in summer. I also have difficulty understanding why the LHF is positive along the southern margins of the ice sheets.
l.329: Please explain what is "ground heat flux (GHF)" and what physical processes are related to this term?
l.369-370: I agree that a natural follow-up work is to simulate ice sheet flow in an ice sheet model driven by the simulated SMB of this study. Perhaps the authors could mention here what they plan to do with the simulated ice sheet flow. From my perspective, it could provide an estimation of dynamic ice loss from these ice sheets, and (together with SMB) give a full account of ice sheet mass balance.
Table 2: Please specify the latitudinal range of the "Tropical" region (for calculation of Tropical precipitation an Tropical SST)?
Figure 4 (a) subtitle: Missing "water" after "Precipitable".
Figure 8, 10: What are the dashed lines in panel (a)?
Fig. C1: Based on the downward drifts of Global Surface Temperature, it seems the model was not fully equilibrated after running for 500 or 600 years. I understand that millennia may be needed for the ocean model to fully equilibrate, which requires much more computational resource. The authors may discuss whether this non-equilibrium would result in a substantial bias in the simulated AMOC.
Fig. C4 (a, b, c): There is a polynya-like feature between Greenland and Baffin Island/Labrador. Is that due to the intensive vertical mixing or deep water formation?
Citation: https://doi.org/10.5194/cp-2023-62-RC3
Sarah Louise Bradley et al.
Sarah Louise Bradley et al.
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