Caesar, L

Cahill, N

McCarthy, GD

Rahmstorf, S

Thornalley, DJR

English

UCL Discovery

  Title: Consistency of paleo-proxies for the AMOC 1 Authors:   L. Caesar1,2*, G. D. McCarthy1, D. J. R. Thornalley3, N. Cahill4, S. Rahmstorf2,5  2 Affiliations: 3 1Irish Climate Analysis and Research Units (ICARUS), Department of Geography, Maynooth 4 University, Maynooth, Ireland. 5 2Potsdam Institute for Climate Impact Research (PIK), Member of the Leibniz Association, P.O. 6 Box 60 12 03, D-14412 Potsdam, Germany. 7 3Department of Geography, University College London, London, UK. 8 4Department of Mathematics and Statistics, Maynooth University, Kildare, Ireland . 9 5Institute of Physics and Astronomy, University of Potsdam, Potsdam, Germany.  10 We thank the authors for their comment and welcome the opportunity to respond. Kilbourne et 11 al. (2021) make the argument that if more proxies from the North Atlantic had been considered, 12 the conclusion reached in Caesar et al. 1 would have been different. While it is certainly true that 13 a comprehensive set of paleoceanographic proxy data available in the North Atlantic region 14 shows a complex picture of the evolution of the North Atlantic over the past two millennia 2, 15 most of these proxies are not strongly linked to the strength of the AMOC.  16 The main objective of Caesar et al. 1 was to compare a range of previously published 17 reconstructions of features that have been linked to the AMOC, which, due to the different 18 proxies used, represent different subsystems/processes related to the AMOC. Our goal was to 19 check the consistency between these reconstructions and to assess the statistical significance of 20 the main changes found. 21 Kilbourne et al. argue that the AMOC proxies shown in Caesar et al. 1 have substantial caveats 22 that were not discussed and that not all available proxy records potentially related to AMOC 23 variability have been considered. Indeed, the fact that each individual AMOC proxy has 24 influencing factors other than the AMOC (as is the case with all proxy data) was the main 25 motivation for this compilation of multiple proxies. The advantage is that since the proxies come 26 from different locations and with different sources of uncertainties, they have different AMOC-27 unrelated influencing factors and any common behaviour is therefore likely AMOC-related. 28 Kilbourne et al. list a number of possible influences on each proxy in addition to the AMOC, 29 including the general global warming signal, changes in atmospheric conditions, the global 30 nitrogen cycle, and shifts in the location of the AMOC’s deep currents (figure 1). Like Kilbourne 31 et al. we see those factors as a source of uncertainty for the individual proxy, but we argue that 32 the diverse nature of these additional influencing factors is evidence that the common evolution 33 found in the different proxies is not driven by these factors (as it is unlikely that they all changed 34 contemporaneously) but by the driver that is common to all proxies: the AMOC.  35 A more detailed look at the individual proxies supports our conclusion that the combination of 36 these proxies is indeed suitable in reconstructing the long-term evolution of the AMOC. For 37 example, the temperature AMOC index by Rahmstorf et al. 3 which is based on land data shows 38 the same signal as the marine proxy by Thornalley et al. 4 which is based on the analysis of 39 planktic foraminifera. The advantage of foraminiferal-based subpolar temperature proxies is that 40 they typically record subsurface temperatures, and are therefore less subject to seasonal biases 4 41 as it is known that the AMOC signal on SST is most pronounced during the cold season 5. 42 To help counteract the problem of a possible non-stationarity of the deep western boundary 43 current that would affect the signal seen in the sortable silt data, Thornalley et al. 4 examined two 44 sites at different depths (1,718m and 2,009m), 41 km apart from each other, and found that the 45 signal of a slowdown is present in both.  46 Other types of proxy linked to the deep branch of the AMOC are those related to the strength of 47 the Nordic Sea Overflows, which have not been considered in Caesar et al. 1. There are two 48 reasons for this: 1. We focused on the large-scale overturning in the open Atlantic (e.g. as 49 measured by the RAPID array) and not individual branches feeding into it. 2. The link between 50 AMOC strength and the volume transport at one individual deep water formation site is tenuous 51 because the deep branches of the AMOC are fed by multiple sources in the Labrador Sea, 52 Irminger Sea and Nordic Seas. This is reflected, for example, in the fact that the recent 2004-53 2012 weakening of the AMOC, which was mainly due to a slowing of the southward deep flow 54 (below 3000m) 6, is not linked to a reduction in the Nordic Overflows 7. Similarly, while AMOC 55 observations at the RAPID array show strong multi-year variability, overflows across the 56 Greenland-Scotland Ridge do not. To include the overflow data in a meaningful way, one would 57 need a robust reconstruction that can only be provided by a depth transect of cores e.g. 8, but no 58 such suite of records that cover the industrial era has yet been published.   59 Similarly, we did not include any proxy records from the North Iceland shelf 9 (in contrast to 60 what is indicated in Figure 1 by Kilbourne et al.) as these are not obviously linked to the open 61 Atlantic AMOC, and more prone to local changes and atmospheric influences 10. Likewise, Keil 62 et al. 11 have shown that decreasing northward heat transport in the AMOC is linked to 63 increasing northward heat transport into the Nordic Seas. 64 The last point which Kilbourne et al. criticize is that the reconciliation of the proxy data with 65 AMOC observations and reconstructions within the instrumental era given by Caesar et al. 1 is 66 not convincing. We therefore expand this comparison using three indicators for the recent 67 evolution of the AMOC that are independent of the long-term proxy data: The direct AMOC data 68 measured by the RAPID array, a recent reconstruction that combines an empirical model with 69 historical hydrographic data 12 and a reconstruction based on satellite altimetry and cable 70 measurements 13. These data show decadal-scale variability with an increase in strength from the 71 90s until about 2005 that has also been seen in other AMOC reconstructions, followed by a 72 strong decline from about 2004-2012 6 and a slight recovery afterwards. The same evolution is 73 seen in the long-term proxies that extend into the 21st Century (figure 2), increasing our 74 confidence that they are suitable to reconstruct the AMOC strength. 75 Concluding, we agree with Kilbourne et al. that a more thorough review of all the North Atlantic 76 proxies that could possibly be related to the AMOC should be done, with an analysis of the 77 robustness of their link to the AMOC and a detailed analysis of the similarities and differences. 78 But the goal of Caesar et al. 1 was to provide a concise overview of recently published proxy 79 records that have been linked to AMOC strength in the past and to analyze their consistency. 80 And while we cannot rule out that there is an AMOC proxy that does not show the signal of a 81 slowdown over the course of the last century, it remains remarkable that the large variety of 82 proxies presented in Caesar et al. 1 show a very similar behaviour over the last 150 years: the 83 beginning of a first slowdown in the second half of the 19th Century, the start of a second, more 84 rapid one in the mid-20th Century and even consistent variability seen in recent decades. While 85 some climate models have shown this amount of AMOC weakening in response to global 86 warming, the majority shows a lesser or even no weakening up to the present 14 (possibly related 87 to the misrepresentation of relevant drivers like the Greenland meltwater flux). This wide range 88 of model results is a matter of concern which needs to be addressed . 89 References 90 1. Caesar L, McCarthy GD, Thornalley DJR, Cahill N, Rahmstorf S. Current Atlantic 91 Meridional Overturning Circulation weakest in last millennium. Nature Geoscience. 92 2021/03/01 2021;14(3):118-120. doi:10.1038/s41561-021-00699-z 93 2. Moffa-Sánchez P, Moreno-Chamarro E, Reynolds DJ, et al. Variability in the Northern 94 North Atlantic and Arctic Oceans Across the Last Two Millennia: A Review. 95 https://doi.org/10.1029/2018PA003508. Paleoceanography and Paleoclimatology. 96 2019/08/01 2019;34(8):1399-1436. doi:https://doi.org/10.1029/2018PA003508 97 3. Rahmstorf S, Box JE, Feulner G, et al. Exceptional twentieth-century slowdown in 98 Atlantic Ocean overturning circulation. Nature Climate Change. 2015/05/01 99 2015;5(5):475-480. doi:10.1038/nclimate2554 100 4. Thornalley DJR, Oppo DW, Ortega P, et al. Anomalously weak Labrador Sea convection 101 and Atlantic overturning during the past 150 years. Nature. 2018/04/01 102 2018;556(7700):227-230. doi:10.1038/s41586-018-0007-4 103 5. Caesar L, Rahmstorf S, Robinson A, Feulner G, Saba V. Observed fingerprint  of a 104 weakening Atlantic Ocean overturning circulation. Nature. Apr 2018;556(7700):191-196. 105 doi:10.1038/s41586-018-0006-5 106 6. Smeed DA, McCarthy GD, Cunningham SA, et al. Observed decline of the Atlantic 107 meridional overturning circulation 2004–2012. Ocean Sci. 2014;10(1):29-38. 108 doi:10.5194/os-10-29-2014 109 7. Østerhus S, Woodgate R, Valdimarsson H, et al. Arctic Mediterranean exchanges: a 110 consistent volume budget and trends in transports from two decades of observations. 111 Ocean Sci. 2019;15(2):379-399. doi:10.5194/os-15-379-2019 112 8. Thornalley DJR, Blaschek M, Davies FJ, et al. Long-term variations in Iceland–Scotland 113 overflow strength during the Holocene. Clim Past. 2013;9(5):2073-2084. doi:10.5194/cp-114 9-2073-2013 115 9. Wanamaker AD, Butler PG, Scourse JD, et al. Surface changes in the North Atlantic 116 meridional overturning circulation during the last millennium. Nature Communications. 117 2012/06/12 2012;3(1):899. doi:10.1038/ncomms1901 118 10. Lohmann G, Schöne BR. Climate signatures on decadal to interdecadal time scales as 119 obtained from mollusk shells (Arctica islandica) from Iceland. Palaeogeography, 120 Palaeoclimatology, Palaeoecology. 2013/03/01/ 2013;373:152-162. 121 doi:https://doi.org/10.1016/j.palaeo.2012.08.006 122 11. Keil P, Mauritsen T, Jungclaus J, Hedemann C, Olonscheck D, Ghosh R. Multiple drivers 123 of the North Atlantic warming hole. Nature Climate Change. 2020/07/01 124 2020;10(7):667-671. doi:10.1038/s41558-020-0819-8 125 12. Worthington EL, Moat BI, Smeed DA, Mecking JV, Marsh R, McCarthy GD. A 30-year 126 reconstruction of the Atlantic meridional overturning circulation shows no decline. 127 Ocean Sci. 2021;17(1):285-299. doi:10.5194/os-17-285-2021 128 13. Frajka-Williams E. Estimating the Atlantic overturning at 26 ° N using satellite altimetry 129 and cable measurements. Geophysical Research Letters. 2015;42(9):3458-3464. 130 doi:10.1002/2015gl063220 131 14. Weijer W, Cheng W, Garuba OA, Hu A, Nadiga BT. CMIP6 Models Predict Significant 132 21st Century Decline of the Atlantic Meridional Overturning Circulation. Geophysical 133 Research Letters. 2020/06/28 2020;47(12):e2019GL086075. 134 doi:https://doi.org/10.1029/2019GL086075 135 15. Osman MB, Das SB, Trusel LD, et al. Industrial-era decline in subarctic Atlantic 136 productivity. Nature. May 2019;569(7757):551-+. doi:10.1038/s41586-019-1181-8 137  138 Acknowledgments 139 L. Caesar, N. Cahill and G. D. McCarthy are supported by the A4 project. A4 (Grant-Aid 140 Agreement No. PBA/CC/18/01) is carried out with the support of the Marine Institute under the 141 Marine Research Programme funded by the Irish Government. D. J. R. Thornalley is supported 142 by UK NERC grant NE/S009736/1. 143 Competing Interests statement 144 The authors declare no competing interests. 145 Figure captions 146 Fig. 1 Overview of the different proxy data and the major climate system forcing that they may 147 be sensitive to. This shows that while some proxies could be sensitive to a certain external 148 forcing, e.g., the general global warming signal (especially those related to upper ocean 149 temperatures and heat content), other proxies are unlikely to be influenced by it. As there is no 150 individual forcing other than the AMOC that influences all proxies, it is likely that their common 151 trend is indeed caused by changes in the AMOC. 152  Fig. 2 Comparison of direct observational AMOC data 6 and two recent reconstructions 12,13 (all 153 at 26N) to both the SST-based AMOC index (SPG region, 6 yr lag caused by the lagged change 154 in AMOC-related heat transport) as well as two paleo-proxies used in Caesar et al. 1 that extend 155 into the 21st century: the sortable-silt data 4 (DWBC, 6 yr lead, reflecting the modelled lead 4 of 156 the DWBC over AMOC) and the marine productivity data (SPG region, 6 yr lag) 15. As the 157 hydrographic-based reconstruction is irregularly spaced in time all data is plotted as a 4-year 158 running mean.  159 Global warmingMarine subsurfacetemperature proxyInstrumental SST proxyLand and ocean based temperature proxyδ15N in coralsNitrogen source proxyδ18O in foraminiferabottom temperature proxyInstrumental ocean heat content dataSortable Silt dataMarine productivity proxy Global warmingPosition DWBCAtmospheric processesChange global N-CycleTemperature changes within the individual water massesAMOC-independentSPG changesAMOC strengthProxy dataPossible forcing1950 1960 1970 1980 1990 2000 2010 202014151617181920RAPID AMOC (Sv) (ref. 7)1516171819Frajka-W. AMOC (Sv) (ref. 14)−2024Worthington AMOC (Sv) (ref. 13 )−1012Caesar AMOC (Sv) (ref. 6)30.030.531.031.532.056JPC ss (μm) (ref. 5 )1950 1960 1970 1980 1990 2000 2010 2020−2.0−1.5−1.0−0.5 SPG marine productivity  (standard variance) (ref. 16)

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Reply to: Atlantic circulation change still uncertain

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