the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 18 Sep 2023
Multi-annual variability of a new proxy-constrained modeled AMOC from 1450–1780 CE
Abstract. The ongoing discussion about the Atlantic Meridional Overturning Circulation (AMOC) slowdown over the 21st century requires an understanding of preindustrial AMOC variability. Here, we present a new AMOC reconstruction during the Little Ice Age from 1450–1780 CE, generated with a novel nudging technique. The technique uses a 10-member ensemble of ocean-model simulations nudged to proxy-reconstructed sea surface temperature. The new reconstruction improves existing knowledge of the AMOC variability and underlying processes, showing that the AMOC weak phases under stable atmospheric CO2 conditions are mainly driven by a 4-to-7-year lagged effect of surface heat flux associated with the North Atlantic Oscillation (NAO). However, the strong phases are a response to instantaneous surface wind stress. Analyses of our AMOC reconstruction reaffirm previous findings about the mechanisms of AMOC variability and its link to the NAO. In addition, we show that processes leading to the phases of weak and strong AMOC are non-symmetric. Overall, the nudging technique enables us to better constrain past AMOC variability.
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Status: closed (peer review stopped)
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CC1: 'Comment on cp-2023-67', Paola Moffa-Sánchez, 29 Sep 2023
Publisher’s note: the content of this comment was removed on 5 October 2023 since the comment was posted by mistake.
Citation: https://doi.org/10.5194/cp-2023-67-CC1 -
RC1: 'Comment on cp-2023-67', Paola Moffa-Sánchez, 02 Oct 2023
Samakinwa et al., use an ensemble of ocean model simulations which they nudge to proxy-based sea surface temperatures between 1450-1780, a period commonly referred to as the Little Ice Age (LIA), to study variability in the Atlantic Meridional Overturning Circulation. The authors then select the high and low AMOC years to investigate the processes behind them and find that they are mostly responding to different North Atlantic Oscillation patterns.
The paper’s topic is relevant to the scope of CP. It applies novel techniques (nudging) used before (e.g. Ortega et al. 2017) but not widely. The article is well-structured and contains good quality figures. However, at times the explanations are a big vague or unclear. The results section seems thorough although I am not a modeller and I therefore do not feel comfortable commenting on the content of the methods used or their analysis presented. The results support the interpretations but in my view no substantial new conclusions are reached.
-The introduction is very general and often vague. It can be improved by describing the AMOC, what it is, how it is defined, ways of reconstructing this in the past (currently second paragraph is not very accurate). For example, D13C and d18O of benthic foraminifera can tell us something about deep branches of the AMOC but more d13C than d18O. Also, Bond et al., 2001 and Oppo et al., 2003 have very low resolution during the period the manuscript focuses on. Bivalves and coralline algae do not hold proxies like alkenones and planktonic foraminifera as these are from marine sediment cores. And, the records are not favoured the modern era. Almost the opposite we have very few records overlapping the instrumental records and extending back in time.
-The setting and context for the study contains concepts and sentences are not accurate. The justification for looking into this period stems from the stability of the atmospheric CO2. However, this argument can be applied to the last 6000 years. It is also unclear what the study is trying to achieve - What is the objective of this study? Studying the natural variability of the AMOC? If so, then looking at the LIA may not be the best as it was a period with decreasing summer insolation in the northern high latitudes, with increased production of sea-ice/glacial advances…etc. This justification would be easy for the reader to establish the motivation of the work beyond the stability of CO2.
-In addition, a lot of the variability across this period has been linked to external forcing (volcanism and solar variability) and hence this stable CO2 alone may not be a good representation of the variability across this period.
-The authors used Neukom et al., 2019 as their proxy-based data but perhaps a caveat needs to be added in the discussion that this reconstruction does not contain any marine records.
-The results of the AMOC reconstruction seem pretty flat (variability is very much less than the internnual AMOC variability please see Cunningham et al., 2007). These results are then compared to Thibodeau et al. (2018) , an oxygen isotope record from a bay off Canada which in reality tells us something about a combination of changes in salinity and temperature which the authors link to the AMOC although from a physics process perspective it is hard to understand why this should be the case. There are plenty of other records to compare it to. The authors need to make a justification for why they just selected that one. I think the most important thing is that this curve has been flattened too (see y-axis scale in Fig1) so it appears like a flat line which looks like the pretty stable AMOC. There are lots of others records that could be linked to the AMOC and I´m unsure on why the authors chosen just chose Thibodeau et al., 2018 and Rahmstorf et al., 2015.
The results show good correspondence between the NAO and the AMOC with a delay during these years. Note that this delay is not really discussed – is this something we would expect? This a well established process observed in the modern ocean in the instrumental records (see papers by I. Yashayev on the Labrador Sea Water formation and the response to the NAO). So I am a little bit unsure of what the implications of these findings are and the novelty of the results. I think the discussion section should be used to discuss the new findings and their implications in the context of previous work.
Other comments:
L13. AMOC should be defined and described briefly.
l.18/L25. Remember AMOC strength from Paleo-Proxy is hard or almost impossible at these time-scales we just need to use other things.
L.19. Delete analysing past climate could help.
l.21 stream function of the ocean… is incomplete
L.36. This sentence should highlight what factors can interact with SST and also reference the work on this.
L70, Can you please describe why these ocean indices are used?
L.104. Need reference for AMOC definition.
L.283. This sentence is unclear
Citation: https://doi.org/10.5194/cp-2023-67-RC1 -
CC2: 'Comment on cp-2023-67', Jürg Luterbacher, 13 Oct 2023
The authors may also consider the following publication and references therein:
Wang, J., Yang, B., Ljungqvist, F.C., Luterbacher, J., Osborn, T.J., Briffa, K.R., and Zorita, E., 2017: Internal and external forcing of multidecadal Atlantic climate variability over the past 1200 years. Nature Geoscience, 10, 512-517
Citation: https://doi.org/10.5194/cp-2023-67-CC2 -
RC2: 'Comment on cp-2023-67', Anonymous Referee #2, 15 Nov 2023
The comment was uploaded in the form of a supplement: https://cp.copernicus.org/preprints/cp-2023-67/cp-2023-67-RC2-supplement.pdf
Status: closed (peer review stopped)
-
CC1: 'Comment on cp-2023-67', Paola Moffa-Sánchez, 29 Sep 2023
Publisher’s note: the content of this comment was removed on 5 October 2023 since the comment was posted by mistake.
Citation: https://doi.org/10.5194/cp-2023-67-CC1 -
RC1: 'Comment on cp-2023-67', Paola Moffa-Sánchez, 02 Oct 2023
Samakinwa et al., use an ensemble of ocean model simulations which they nudge to proxy-based sea surface temperatures between 1450-1780, a period commonly referred to as the Little Ice Age (LIA), to study variability in the Atlantic Meridional Overturning Circulation. The authors then select the high and low AMOC years to investigate the processes behind them and find that they are mostly responding to different North Atlantic Oscillation patterns.
The paper’s topic is relevant to the scope of CP. It applies novel techniques (nudging) used before (e.g. Ortega et al. 2017) but not widely. The article is well-structured and contains good quality figures. However, at times the explanations are a big vague or unclear. The results section seems thorough although I am not a modeller and I therefore do not feel comfortable commenting on the content of the methods used or their analysis presented. The results support the interpretations but in my view no substantial new conclusions are reached.
-The introduction is very general and often vague. It can be improved by describing the AMOC, what it is, how it is defined, ways of reconstructing this in the past (currently second paragraph is not very accurate). For example, D13C and d18O of benthic foraminifera can tell us something about deep branches of the AMOC but more d13C than d18O. Also, Bond et al., 2001 and Oppo et al., 2003 have very low resolution during the period the manuscript focuses on. Bivalves and coralline algae do not hold proxies like alkenones and planktonic foraminifera as these are from marine sediment cores. And, the records are not favoured the modern era. Almost the opposite we have very few records overlapping the instrumental records and extending back in time.
-The setting and context for the study contains concepts and sentences are not accurate. The justification for looking into this period stems from the stability of the atmospheric CO2. However, this argument can be applied to the last 6000 years. It is also unclear what the study is trying to achieve - What is the objective of this study? Studying the natural variability of the AMOC? If so, then looking at the LIA may not be the best as it was a period with decreasing summer insolation in the northern high latitudes, with increased production of sea-ice/glacial advances…etc. This justification would be easy for the reader to establish the motivation of the work beyond the stability of CO2.
-In addition, a lot of the variability across this period has been linked to external forcing (volcanism and solar variability) and hence this stable CO2 alone may not be a good representation of the variability across this period.
-The authors used Neukom et al., 2019 as their proxy-based data but perhaps a caveat needs to be added in the discussion that this reconstruction does not contain any marine records.
-The results of the AMOC reconstruction seem pretty flat (variability is very much less than the internnual AMOC variability please see Cunningham et al., 2007). These results are then compared to Thibodeau et al. (2018) , an oxygen isotope record from a bay off Canada which in reality tells us something about a combination of changes in salinity and temperature which the authors link to the AMOC although from a physics process perspective it is hard to understand why this should be the case. There are plenty of other records to compare it to. The authors need to make a justification for why they just selected that one. I think the most important thing is that this curve has been flattened too (see y-axis scale in Fig1) so it appears like a flat line which looks like the pretty stable AMOC. There are lots of others records that could be linked to the AMOC and I´m unsure on why the authors chosen just chose Thibodeau et al., 2018 and Rahmstorf et al., 2015.
The results show good correspondence between the NAO and the AMOC with a delay during these years. Note that this delay is not really discussed – is this something we would expect? This a well established process observed in the modern ocean in the instrumental records (see papers by I. Yashayev on the Labrador Sea Water formation and the response to the NAO). So I am a little bit unsure of what the implications of these findings are and the novelty of the results. I think the discussion section should be used to discuss the new findings and their implications in the context of previous work.
Other comments:
L13. AMOC should be defined and described briefly.
l.18/L25. Remember AMOC strength from Paleo-Proxy is hard or almost impossible at these time-scales we just need to use other things.
L.19. Delete analysing past climate could help.
l.21 stream function of the ocean… is incomplete
L.36. This sentence should highlight what factors can interact with SST and also reference the work on this.
L70, Can you please describe why these ocean indices are used?
L.104. Need reference for AMOC definition.
L.283. This sentence is unclear
Citation: https://doi.org/10.5194/cp-2023-67-RC1 -
CC2: 'Comment on cp-2023-67', Jürg Luterbacher, 13 Oct 2023
The authors may also consider the following publication and references therein:
Wang, J., Yang, B., Ljungqvist, F.C., Luterbacher, J., Osborn, T.J., Briffa, K.R., and Zorita, E., 2017: Internal and external forcing of multidecadal Atlantic climate variability over the past 1200 years. Nature Geoscience, 10, 512-517
Citation: https://doi.org/10.5194/cp-2023-67-CC2 -
RC2: 'Comment on cp-2023-67', Anonymous Referee #2, 15 Nov 2023
The comment was uploaded in the form of a supplement: https://cp.copernicus.org/preprints/cp-2023-67/cp-2023-67-RC2-supplement.pdf
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