North Atlantic temperature control on deoxygenation in the northern tropical Pacific

Domain: earth_science
Published: 2024-09-10
Editorial outcome: major_revision

Abstract
Ocean oxygen content is decreasing with global change. A major challenge for modelling future declines in oxygen concentration is our lack of knowledge of the natural variability associated with marine oxygen inventory on interannual and multidecadal timescales. Here, we present 10 annually resolved 200 year-long records of denitrification, a marker of deoxygenation, from a varved sedimentary archive in the North Pacific oxygen minimum zone covering key periods over the last glacial–interglacial cycle. Spectral analyses on these records reveal strong signals at periodicities typical of today’s Atlantic multidecadal oscillation. Modern subsurface circulation reanalyses regressed on the positive Atlantic and Pacific Climatic Oscillation indices further confirm that North Atlantic temperature patterns are the main control on the subsurface zonal circulation and therefore the most likely dominant driver of oxygen variability in the tropical Pacific. With currently increasing temperatures in the Northern Hemisphere high latitudes and North Atlantic, we suggest deoxygenation will intensify in the region.

Introduction
Global warming, through its impact on ocean stratification and O2 solubility, is driving the decline in marine oxygen inventory both at depth and in the subsurface with potentially catastrophic consequences for marine biological production, ecosystem and habitat sustainability1,2. Total oxygen inventory has already declined by 2% since 1960 and deoxygenation is predicted to accelerate over the rest of the century3. One of the effects of decreased oxygenation in the ocean is the rapid expansion of oxygen minimum zones (OMZs) with implications for marine N2O production, marine fixed-nitrogen inventory, biological productivity, as well as the ability of the ocean to sequester carbon away from the atmosphere through the biological carbon pump mechanism4. The combined effect of these processes would represent positive feedback on global warming by enhancing greenhouse gas efflux from the ocean4. The North and Tropical (North and South) Pacific accounts for up to 40% of the existing and forecasted oxygen loss and hosts the largest OMZs in the ocean3. Multidecadal climatic variability such as the Pacific decadal oscillation (PDO) or the Atlantic multidecadal oscillation (AMO) have been shown to modulate the declining oxygen concentration trends over recent decades in the tropical Pacific and Atlantic Oceans, via temperature or ventilation effects, respectively1,3,5. The decrease in oxygen and increase in denitrification (enabled by the lack of oxygen) in the North Pacific OMZs since the late 1970s has been linked to a shift from a negative to a positive phase of the PDO (Fig. 1)6–8. Intriguingly, similar trends of historical O2 anomalies in the ETNP and the AMO index are also observed over the last 60 years (Fig. 1). However, these instrumental records of marine oxygen and climatology are short, only dating back to the 1960s, making it hard to ground truth the apparent relationship between these multidecadal climate oscillations and marine deoxygenation. Crucially, the frequency, sign and amplitude of these climatic oscillations are expected to evolve under warming conditions9, making it essential to understand their impact on marine oxygen for future prediction. In this regard, our lack of knowledge of the natural variability in marine oxygen inventory in the pre-industrial past remains one of the main challenges in predicting future declines in oxygen concentration2. This can be resolved by looking at past archives of marine oxygenation changes.

In OMZs, past changes in deoxygenation can be reconstructed by measuring nitrogen isotopes in the sediments10,11. Nitrogen isotopic signal in these areas has been shown to record the intensity of denitrification, a process that occurs in the absence of oxygen. In this study, we reconstruct denitrification changes, as a natural marker of deoxygenation, in an annually varved sedimentary records from the Gulf of California (GoC), Eastern Tropical North Pacific (ETNP), covering key periods over the last Glacial–interglacial cycle (55 Ka BP). This annually resolved denitrification record reveals the global and regional mechanisms and climatic controls on oxygen variability in the largest OMZ at subdecadal to multidecadal timescales.

The GoC is located at the core of the North Pacific OMZ. The lack of oxygen in the subsurface and intermediate water column (200–800 m) is created by high seasonal productivity and organic matter export through the water column coupled with old ventilation ages of the water within and below the thermocline. The resulting anoxia (O2 < 0.2 ml l−1 12) drives high level of denitrification in the basin year round13. In OMZs, denitrification is the main process governing fixed-nitrogen (mainly nitrate) loss14. Like most biogeochemical processes, it involves isotopic fractionation and therefore its intensity can be measured using relative nitrogen isotope abundance (δ15N)6,14. In the GoC, seasonal alternation between terrigenous (riverine) and biogenic inputs to the seabed, anoxic bottom waters at seafloor lead to the rapid deposition and preservation of annually varved sedimentary sequences. Such conditions produce a unique opportunity to track past changes in deoxygenation on annual timescales.

We previously presented a continuous 55,000 year-long record of denitrification in a composite sediment core (MD02-2515 piston and corresponding MD02-2517 CASQ cores from the same site, 27°29.01 N; 112°04.46 W; 881 m water depth) in the Guaymas basin at centennial timescales (Fig. 2)15,16. We now present nitrogen isotope (δ15N) data from 10 annually subsampled, continuously varved sections, each spanning about 200 years, in order to reconstruct interannual to multidecadal variability in marine hypoxia in the ETNP. These annually resolved sections represent key climatic periods from the last glacial period to the recent pre-industrial epoch.

Although the GoC is a semi-enclosed basin, existing denitrification records spanning the last 150 years show that the signal recorded in the basin is representative of the Northern Pacific OMZ6. While temporal resolution in the published 150 year-long records varies between 3 and 1 year, our varved records provide annual resolution [S1], enabling spectral analyses to be performed to document the natural variability in deoxygenation in the ETNP not only in the recent past but under various climate states (glacial period, millennial scale events and the Holocene) throughout the late Pleistocene (Fig. 2).

Strongly reoccurring climate oscillations cause physical and biogeochemical fluctuations in the North and Tropical Pacific at subdecadal to multidecadal times scales17–19. These climatic oscillations have well-documented impacts on surface and subsurface circulation, mixing, nutrientsupply and biological productivity in the ETNP where our core is located and have the potential to modulate the oxygen budget in the basin and in the OMZs13,20–22. El Niño Southern Oscillation (ENSO) involves zonal changes in sea surface temperature (SST) and currents across the equatorial and tropical Pacific at subdecadal times scales (2–8 years)18. The PDO is a low-frequency (50 years and 20–25 years23) climate oscillation centred over the mid-latitudes in the North Pacific and affects North Pacific SSTs and mixing. The PDO is not a single mode of climatic oscillation and is described increasingly as the sum of processes and climatic oscillations that operate across the Pacific, such as ENSO and the North Pacific Gyre Oscillation (NPGO)23. The NPGO has a periodicity of about 10 years and is reflected by changes in the intensity of the central and eastern branches of the North Pacific gyre circulations19. Recent studies increasingly identify the links between temperature anomalies in the North Atlantic (AMO) and climatic oscillations, atmospheric perturbations and changes in ocean circulation in distant regions (e.g., Asian monsoon)24–26 in the tropics and sub-tropics. Crucially, anomalously warm temperatures in the North Atlantic are found to lead to an northward shift of the ITCZ and a positive wind stress curl anomaly in the subtropical Pacific. This impacts the shallow overturning circulation in the tropical Pacific24, with possible impacts on the PDO and NPGO. The AMO varies with periodicities of about 70–80 years (with a 35-year harmonic)27,28. While we acknowledge ongoing debates on the nature (internal variability versus externally forced) and stability of these climatic oscillations over time, our results show the recurrence and persistence of modes of variability similar in frequencies to modern AMO, PDO, NPGO and ENSO over the last 55,000 BP29.

Results
We carried out time-series analyses in the frequency domain using spectral analyses with several windowing settings and wavelet transform (see section “Methods”) on each of the annually resolved denitrification records. In the first instance, we used band-pass filtering to remove the periodicities over 70 years, hence compromising the detection of the longest periodicities at which PDO and AMO are known to operate today. A subsequent analysis was carried out on non-filtered records to identify the potential occurrence and power of the longest AMO and PDO periodicities.

Pacific climate oscillations in the deoxygenation records
All filtered records share the same dominant periodicities of 25 years followed by a periodicity of 10 years (±2 years) and minor periodicities between 3 and 8 years. On average, the power was 2.6 (±0.6) times greater for the 25 years periodicity compared to the 3–8 years periodicities and 1.8 (±0.8) time greater than that of the 10–12-year periodicity. In other words, anoxia-related denitrification changes in the ETNP responded mainly to multidecadal oscillations at periodicities similar to the shorter periodicity displayed by today’s PDO climate variability (48%) than it did to shorter climate oscillations such as ENSO-like (3–8 years, 18%) and NPGO-like (10–12 years, 34%) variabilities. For all filtered 200 year-long records (n = 10), the main mode of variability is the PDO-like multidecadal oscillation (20–25 years) while ENSO-type periodicities are consistently the weakest mode of variability expressed by the deoxygenation proxy (Supplementary Table 1).

AMO-like signal in the North Pacific deoxygenation records
The spectral analyses performed on non-filtered records reveal that periodicities centred around 83 and 31–35 years are both seen in the Holocene (sections #III and #2) and Bølling-Allerød (sections #9 and #10) sections (n = 4, Fig. 3). These periodicity “doublets” are found consistently across the warm climatic periods and are identical to the ones recorded in Holocene varves from Icelandic lakes28. Model Intercomparison (CMIP5) of AMO historical simulations (last 150 years) also shows periodicities between 70–90 years and around 30 years to be typical of AMO variability27 (Fig. 3). During the Dansgaard–Oeshger events of Isotopic Stage 3, AMO-like periodicities are also observed in three of the four records (records #46, #47 and #50) but show a wider range of duration (60–100 years and 31–37 years) than those observed during the Holocene and BA (Supplementary Table 1). For all these records, the amplitudes (power) of AMO-like periodicities vary within a range similar to that of the PDO-like periods extracted from the corresponding filtered and unfiltered records (see above), indicating that AMO-like variability can explain a significant part of the oxygen changes in the basin.

The AMO-like periodicity “doublets” appear less clearly, however, during the Heinrich Event 1 (#17) and the Last Glacial Maximum (LGM, #26) records when different modes of multidecadal variability are found (41 years and <125 years, respectively). This suggests that during glacial stages and stadials, when North Atlantic climate was much colder and the Atlantic meridional circulation slowed down dramatically or even collapsed30, the AMO might have been fundamentally different to what it is during warm climatic phases, both in nature and frequency. In addition, cold climatic periods of the past such as the LGM and Heinrich 1 were characterised by a contraction of the main OMZs and arguably punctuated by transient oxygenated conditions in parts of the OMZs11. The annually resolved denitrification records #26 and to a lesser extent #17 show higher amplitude variations in δ15N than the other annually resolved records and the occurrence of lighter values, atypical of denitrifying conditions (Fig. 2). This suggests that at these times, denitrification was not always the primary driver of δ15N changes in the GoC because oxygenated conditions prevailed, contrary to today’s situation13 and the warm millennial scale events that characterise the other annual records. The significant negative correlation between denitrification and biological productivity for section #26 confirms the assumption that δ15N during cold periods denotes changes in nitrate utilisation and/or availability rather than denitrification under low oxygen conditions31 (Supplementary Fig. 4).

Methods
Biogeochemical analyses
The age model for Core MD02-2515/17 is detailed in ref. 16. Ten intervals with continuously laminated sediments were selected for detailed analysis. Overlapping sediment slabs measuring 150 × 35 × 10 mm were cut from the sediment using a specially designed tool (“cookie-cutter”). Photographs and/or X-radiographs of the slabs were matched to the core photographs and the varve-counts (Supplementary Fig. 1). Each interval represents a duration between 160 and 200 years, which corresponds to 0.3–0.5 m of sediment thickness. All slabs were subsampled at annual resolution, and analysed for biogenic opal, nitrogen and organic carbon content as well as nitrogen isotope composition.

About 10–20 mg of freeze-dried, ground bulk sediment was used for elemental and isotopic analyses. The determination of the opal content (%) by the molybdate-blue spectrophotometry was adapted from Mortlock and Froelich44, whereby biogenic silica is dissolved using 0.1 M sodium hydroxide and reacts with molybdate to produce a blue colour. The absorbance at 812 nm is then measured by spectrophotometry and calibrated using silica standards. The isotopic composition of sedimentary nitrogen (δ15N, ‰) was measured by continuous flow-isotope ratio mass spectrometry (CF-IRMS) using a CE instrument NA2500 elemental analyser directly coupled to a VG Isotech Prism mass spectrometer at the University of Edinburgh, School of Geosciences. The N2 and CO2 gases released from the sample are separated chromatographically following flash combustion and transferred to the mass spectrometer using a carrier gas (He). Masses 28, 29 and 30 are monitored for N isotopic measurements. The ratios of 15N–14N in the sample were calculated and reported as δ15N relative to atmospheric N2. Analytical precision based on multiple analyses of a laboratory bulk sediment standard was ±0.2% (1 sigma).

Spectral analyses
Spectral analyses on the annually resolved records of both N isotopes and biogenic silica were performed using the software Analyseries 2.0.845 to extract the major time frequencies present in the records. Two sets of analyses were performed on all records: a set of analyses on the detrended record (periodicity > 70 years were removed by applying band-pass filtering) and a set of analyses was done on the original, non-detrended records. The spectral analyses performed with Analyseries followed two classical methods, Blackman Tukey (B Tukey) and Maximum Entropy (MEM). The “input” dataset for each spectral analysis was “resampled” by applying cubic spline simple interpolation to obtain an even time resolution (every year). The linear trend of the series was removed and pre-whitening was applied. The B Tukey method (based on standard Fourier transform) was used to obtain the linear power spectrum and to identify the main linear oscillations. To determine the statistical significance of the frequencies obtained (i.e., not noise signals), F-tests were performed using a Multitaper method (MTM). Only frequencies with >90% confidence level were accepted. Band-pass filtering at the 4–6 main frequencies identified for each record was then performed. The reconstituted record calculated by adding the results of the band-pass filtering was compared to the original record in order to verify that no important feature of the record was missed by the serial analyses and to quantify the fraction of the records explained by short (ENSO-like) or long (PDO- or AMO-like) periodicities. The records were then examined with wavelet analysis to determine the non stationary periodicities of the timeseries. In short, wavelet analysis allows the extraction of the various periodicities that dominate the records at any given time throughout the time series. The wavelet analysis used the Morlet method. The main periodicities extracted by both spectral and wavelet analyses were in good agreements for all records.

Climatological and Ocean data reanalyses
The AMO index is defined as the area averaged annual-mean SST over the North Atlantic (80° W–0° E, 0°–60° N) derived from the Hadley Centre Sea Ice and SST dataset (HadISST) dataset version 1 at 0.5° × 0.5° resolution. The PDO index (http://research.jisao.washington.edu/pdo/PDO.latest) is derived as the leading principal component of monthly SST anomalies in the North Pacific Ocean, poleward of 20° N. The monthly mean global average SST anomalies are removed to separate this pattern of variability from any global warming signal that may be present in the data. Ocean data reanalyses regressed on the positive AMO and PDO indices of (sea surface) temperature and zonal and meridional current velocity in the Pacific Ocean were performed using HadISST dataset version 1 at 0.5° × 0.5° resolution and Simple Ocean Data Assimilation (SODA) dataset version v2p2p4 at 0.5° × 0.5° resolution, respectively.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "REVIEW OF MANUSCRIPT NUMBER “NCOMMS-23-62151”\nIn the manuscript titled “North Atlantic Temperature Control on Deoxygenation in the Northern\nTropical Pacific” Pichevin and coauthors present a high-resolution reconstruction of denitrification\nof the Eastern Tropical Pacific (ETNP). The authors present an annually-resolved 55 kyr-long δ15N\nrecord based on ten core sections. After performing spectral analyses on their datasets, the\nauthors were able to find clear relationships between their proxy values and multi-decadal climate\noscillations.\nThe main strength of the work is the quality and resolution of the data series and the value of the\nspectral analyses performed. The authors found that multi-decadal signals in the form of PDO- and\nAMO-like periodicities dominate the records with only minor influence from NPGO and ENSO.\nWhile this could be true, the lesser influence of shorter-scale variabilities could also be related to\nthe age model of the record. Additionally, the GC has its own local productivity dynamics, which\ncould dominate the proxy records at the annual scale. Despite the latter, I was surprised to see not\nonly PDO but also AMO play a role in the ETNP oxygenation, which is truly valuable information to\ncontinue moving the field forward.\nIn its present state, the manuscript exhibits certain weaknesses. The text is hard to read and feels a\nbit too raw and technical for the audience, so it could use some reorganization. For the scope of the\njournal, I would have expected the authors to focus more on the broader implications rather than\nthe technical aspects of the models and analyzed cores. To a lesser degree, I see other minor\nissues with wording and structure, which will be easily solved after rephrasing and other refining\nefforts.\nI consider the manuscript to be very promising, but it will need a bit more work before it can be\npublished in “Nature Communications”. Hereby, I suggest several ideas of how the manuscript\ncould be improved. I also upload a marked PDF highlighting some minor details.\nI. GENERAL COMMENTS:\nA. What are the noteworthy results?\n- A truly high-resolution denitrification record of the past 50kyr was generated for the Eastern\nTropical North Pacific using the δ15N proxy.\n- Assuming that δ15N is able to show deoxygenation trends, clear responses to climate oscillations\nwere found for the ETNP OMZ after performing the spectral time series analysis.\n- The most promising and noteworthy results are finding statistical evidence for climatic\nperiodicities such as PDO in the preindustrial past.\nB. Will the work be of significance to the field and related fields? How does it compare to the\nestablished literature? If the work is not original, please provide relevant references.\n- Although the manuscript aims to address crucial mechanistic questions, there is some\nuncertainty about its novelty in relation to the journal's scope. Nevertheless, I believe that this\nstudy holds significant relevance and would contribute substantially to the scientific community if\npublished here or elsewhere.\n- While I personally agree that many locations of the Gulf of California (GC) are excellent indicators\nof past ETNP variability, I would not generalize. I would still treat the GC as semi-restricted, which is\naccurately mentioned by the authors. It is also important to acknowledge that not every researcher\nfamiliar with modern GC dynamics agrees that bottom sediments can even accurately register\noxygenation changes. Many oceanographers argue that there are mesoscale dynamics that play a\nsignificant role in transporting nutrients and controlling the productivity (and deoxygenation) of GC\nbasins. This could be the reason why the NPGO and ENSO are not showing up as expected in the\nspectral analysis.\n- The relationship between ETNP oxygenation and multi-decadal variability such as PDO is not new.\nOther authors have noted this before in the GC and the Mexican Pacific using geochemical (Tems et\nal., 2016; Choumiline, et al., 2019) and foraminiferal proxies (Almaraz-Ruiz et al., 2024). That being\nsaid, I am excited to finally see statistical evidence for PDO and potentially even AMO-like variability\nin the ETNP oxygenation records.\nC. Does the work support the conclusions and claims, or is additional evidence needed?\n- An important part of the paper relies on time-resolved sedimentary records, so the validity of the\nage models is of the essence. However, despite referring to another paper, the authors do not\nmention much about how the age models were constructed (besides some mention in the\nsupplement) and how can we ensure the annual resolution that is promised in this manuscript.\nI think it is important to explain this important part in more depth, as many interpretations depend on\nthis.\nD. Are there any flaws in the data analysis, interpretation and conclusions? Do these prohibit\npublication or require revision?\n- Overall, I consider the data analysis, interpretation and conclusions to be sound for the scientific\nquestions that want to be answered by the authors. In my opinion, the numerical and statistical\napproaches used in this manuscript are appropriate and are well explained in the Methods section.\n- I am not surprised that the authors do not find many sub-decadal periodicities during Heinrich\nevents, which could be due to the fact that well-oxygenated waters rarely preserve a clear\noxygenation record. Most geochemical proxies fail to preserve a signal during well-oxygenated\ntimes such as the LGM. Moreover, there are processes in the water column found for sediment trap\nmaterial of the GC that show some authigenic proxy enrichments and isotopic fractionation)\nassociated with reducing microenvironments within marine snow aggregates (Choumiline et al.,\n2019). These additional particulate fluxes could contribute to the interannual noise of the\nsedimentary records.\nE. Is the methodology sound? Does the work meet the expected standards in your field?\n- Overall, I consider the methodological approach to be adequate for the scientific questions that\nwant to be answered by the authors. The obtained and used data appears to be reliable. My only\nconcern would be the ability of generating a true annually-resolved proxy record, which seems hard\nto be achieved by varve counting given the presence of turbidites and recurring dust storms,\ntropical cyclones and earthquakes in the area that can lead to under or overestimation of the\nlaminations. I also understand that there are no other alternatives at this temporal resolution.\nF. Is there enough detail provided in the methods for the work to be reproduced?\n- I think the detail provided in the methods is sufficient.\nII. MINOR COMMENTS\n- As a paper that has as one of its main focuses the past changes of deoxygenation, it is important\nto introduce what is meant by anoxia, hypoxia, suboxia, etc. Different authors refer to these values\nvery differently. An attempt was made by Canfield and Thamdrup, 2009 to suggest avoiding the use\nof vague terms like suboxic, so please refer to this paper. Typically using the dissolved oxygen\nvalues proposed by Hofmann et al., 2011 and mentioned in the Moffitt et al., 2015 paper would be a\ngood idea. However, since we have to rely on proxies, it is nearly impossible to reconstruct exact\nvalues or even enough to affirm that a basin was either completely anoxic or oxic (see line 169). I\nsuggest referring to periods of the pre-instrumental past using terms like “more oxygenated” or\n“less oxygenated”.\n- The manuscript often repeats “10 annually resolved” denitrification records. I don’t think there is a\nneed to repeat that there are 10 records. I think just saying something along the lines of “our\nannually resolved records” or “annually resolved sections” would suffice. Speaking of these\nrecords, it might be a better idea to rename them from #46, #47 to A, B, C, D; 1, 2, 3, or any other\ncode that would help the reader.\n- Please rewrite the sentence that starts at line 152 and explain what you mean by “warm events of\nstage 3”? What stage 3 are we talking about? For instance, Marine Isotope Stage 3 comes to mind\nwhen reading the text.\n- Please be consistent when you refer to figures and materials from the supplement. For instance,\nyou have: “Fig 1”, “fig. 5”, “Figure 5”, “Fig.6A” and “Sup. mat 3”, “Sup. Mat 3”, “Sup. Mat. S3\",\n“Supplementary material 2”. Please pick one style and unify it throughout the text.\n- Please be consistent with the use of uppercase or lowercase terms. For instance, on line 29 you\nrefer to “Oxygen Minimum Zones” by capitalizing each letter. However, in other parts of the text\nsuch as line 35, 54, and 168 you use lowercase “oxygen minimum zones”. Please be consistent with\nthis and other terms. Same applies with the use of Biogenic Silica which also appears as “biogenic\nSilica” or “Biogenic silica”.\n- Several minor things in the text are not polished such as campaign names (e.g. “2002 MONA\nIMAGE campaign” instead of “MD126/IMAGES VIII MONA”) or the brands of the analytical\ninstruments (e.g. “CE instrument NA2500” instead of “CE Instruments NA2500”. Please go over the\ntext and try to hunt for these little mistakes. Some of these issues are on the following lines: 380,\n425,\n- Some parts of the results would better go to the introduction. For instance, the first section that\nstarts on line 126 with “We carried out time series…” would better go to the bottom of the\n“Introduction” section.\n- Please avoid starting sentences with element symbols or acronyms. For instance, in line 381 “N2\nand CO2 gases” would have to be “The N2 and CO2 gases”. Similarly, this happens throughout the\ntext in line: 276, and potentially elsewhere.\n- Some sentences are a bit confusing, incomplete, or could use improvements. Please kindly\nrewrite them. These start at the following lines: 170, 335, 377, and 411.\n- The supplemental materials need work and organization. The figures and table are not captioned\n(under the figures and above the table).\n- I suggest word-proofing the paper using Grammarly or similar software. It will help polish and find\nminor grammar issues.\nIII. COMMENTS ON FIGURES:\n- The figures need work. In my opinion, there are too many of them with some not showing relevant\ninformation. The figures have different font styles and sizes, which also does not help to get a sense\nof a final publishable product. It would be better to unify all of the styling differences using the\ngraphic design software of your choice. For instance, Figure 1 is composed of three figures that\nhave different font styles and sizes. Also, it would be better to refer to the subfigures separately and\nlabel them as Figure 1a, Figure 1b, instead of having the reader figure out which one is which.\n- In Figure 1, the picture of the varved section from Core MD02-2515/17 does not really add any\nvaluable information. I suggest removing it completely by moving from the main text down to the\nsupplement.\n- Figure 6 needs to be redrawn. In its current state, it is composed of four figures showing a variety\nof records glued together. It would be better to unify the font size for the X-axes (years) and Y-axes\n(data values). Even removing the repetitive X-axes on all the records and only leaving one in the\nlower part would help the figure tremendously. Please also adjust the Y-axes to the same vertical\nlines (please refer to the marked manuscript PDF).\n- It would be nice to see the δ15N values for all the sections in the supplement (similarly to the\nprofile shown in Figure 2).\nIV. ADDITIONAL REFERENCES:\nHere are the references mentioned during this review:\nAlmaraz-Ruiz, L., Machain-Castillo, M.L., Sifeddine, A., Ruiz-Fernández, A.C., Sanchez-Cabeza, J.-\nA., Rodríguez-Ramírez, A., Mendez-Millan, M. and Caquineau, S., 2024. Changes of bottom water\noxygenation during the last half millennium in the Gulf of Tehuantepec (Eastern Tropical North\nPacific): A multiproxy approach. Palaeogeography, Palaeoclimatology, Palaeoecology, 636: 111994.\nCanfield, D.E. and Thamdrup, B., 2009. Towards a consistent classification scheme for\ngeochemical environments, or, why we wish the term 'suboxic' would go away. Geobiology, 7(4):\n385-92.\nHofmann, A.F., Peltzer, E.T., Walz, P.M. and Brewer, P.G., 2011. Hypoxia by degrees: Establishing\ndefinitions for a changing ocean. Deep Sea Research Part I: Oceanographic Research Papers,\n58(12): 1212-1226.\nMoffitt, S.E., Moffitt, R.A., Sauthoff, W., Davis, C.W., Hewett, K. and Hill, T.M., 2015.\nPaleoceanographic Insights on Recent Oxygen Minimum Zone Expansion: Lessons for Modern\nOceanography. PLOS ONE.\nTems, C.E., Berelson, W.M., Thunell, R., Tappa, E., Xu, X., Khider, D., Lund, S., González-Yajimovich,\nO. and Hamann, Y., 2016. Decadal to centennial fluctuations in the intensity of the eastern tropical\nNorth Pacific oxygen minimum zone during the last 1200 years. Paleoceanography, 31(8): 1138-\n1151.\nI sincerely hope that my insights will be helpful to the authors and wish them good luck. I really\nenjoyed reviewing this important paper and would like to see it improved and ultimately be\npublished.", "reviewer_2": "North Atlantic Temperature Control on deoxygenation in the Northern Tropical Pacific\nPichevin et al.\nThe thematic of the study is very relevant to the marine community. The Oxygen Minimum Zone\n(OMZ)increase is a cause of concern and has a large impact on marine life and biogeochemical\ncycles. The impact of climate oscillations, such as the Atlantic Multidecal Oscillation (AMO) or the\nPacific Decadal Oscillation (PDO), is still not clear compared to climate change. Based on a 55 ky\nsediment record and the analysis of the SODA 1950-2010 ocean reanalysis, the authors conclude\nthat the Atlantic Multidecadal Oscillation plays a major role in controlling the Pacific Ocean tropical\ncirculation and hence the OMZ extension.\nIn its current form, the study is divided into two parts, which are mostly independent. One focuses\non the discussion of a 55 ky record of denitrification in the gulf of California. The other on the\nmechanisms relating AMO and oxygen levels based on a reanalyis of the last 6 decades (SODA). I\nhave major concerns in both parts :\n1- The frequency of AMO / PDO in the present day is about 50-80 years and 20-30 years as stated by\nthe authors. It is very speculative to assume that this frequency does not change in the past 55.000\nyears. For instance, based on tree rings, a previous study has shown a long multi-centenial PDO\nnegative phase during the last millenium (McDonald and Case, 2005). Even if a “PDO-like”\nfrequency is found in sediment record in one location, it does not mean that the PDO magnitude /\ngeographical pattern (and then implications for oceanic circulation and OMZs) is similar.\nDemonstrating the existence of a AMO-like / PDO-like oscillation during the last 55 ky with a very\nconstrained frequency range would be a major finding. It would however require the analysis of\nseveral sediments cores at different locations over the Pacific basin and temperature proxies in\ncomplement to denitrification.\n2- The authors argue that the AMO has a dominant impact on the equatorial circulation of the\nPacific Ocean. I agree that the AMO may have a significant (even dominant) impact more\nspecifically in the western part of the Pacific Ocean, as shown by Sun et al., (2017).\nSun, C., Kucharski, F., Li, J. et al. Western tropical Pacific multidecadal variability forced by the\nAtlantic multidecadal oscillation. Nat Commun 8, 15998 (2017)\nHowever many studies have demonstrated a major role of the PDO in the forcing of the equatorial\nPacific circulation (which includes the shallow overturning circulation and the subtropical cells, the\nequatorial upwelling system and the EUC) :\nI suggest here a few studies (among many others) :\nCapotondi, A., and B. Qiu, 2023: Decadal Variability of the Pacific Shallow Overturning Circulation\nand the Role of Local Wind Forcing. J. Climate, 36, 1001–1015,\nHong, L., L. Zhang, Z. Chen, and L. Wu (2014), Linkage between the Pacific Decadal Oscillation and\nthe low frequency variability of the Pacific Subtropical Cell, J. Geophys. Res. Oceans, 119, 3464–\nPacific Ocean. Nature 415(6872):603–608.\nDemonstrating that the AMO is a dominant forcing of the Eastern Pacific circulation (compared to\nthe PDO / PDV) would be an important finding. It should however be demonstrated in a thorough\nway, eventually by performing modelling experiments as in Sun et al., 2017 or analysing longer\nsimulations (e.g CMIP/PMIP), as the AMO frequency is of the same order as the SODA dataset. The\nFigure 4 (top) showing the positive AMO regressed SSTs shows confidence (cross hatching marks)\nthat AMO and SST are related in the western Pacific Ocean (which is in line with Sun et al, 2017).\nHowever in the eastern part, where the OMZ are located, the Fig 4 shows that the PDO plays a major\nrole (with confidence) in regulating SST which is line with many studies.\nI think that the authors need to focus on one specific point (1 or 2) as the articulation between 1 and\n2 is not clear to me at the moment. In both case, more discussion / comparison with literature is\nneeded.\n---\nMore specific comments :\nL24-36 : it is stated that global warming is driving the oxygen decrease while L38 it is stated that\nclimate oscillations are controlling oxygen trends. Please clarify: is climate change driving oxygen\ndecrease or climate oscillations ? Is there a consensus in the literature or is it still an open question\n?\nL40 : what do you mean by “apparent” decrease ? (is it not really decreasing ?)\nL42 : is it consistent with Fig 1 ? (it seems to me that the the PDO shift from positive to negative is\nassociated with an oxygen decrease)\nL43-44 and Figure 1: I agree, there is a trends in AMO and a trend in oxygen. However the AMO trend is\nopposite to the PDO. To me it is difficult to establish a causality link, so I think that “suggest”\nmay not be appropriate at least at this stage.\nL48 : The PDO / AMO characteristics likely changed in the past 55 Ka too !\nL59 : did OMZ intensity / location / oxygen mean state varied a lot during the last 55 Ka BP ?\nL89-90 : how does it compare with previous publications focusing on past OMZs ?\nL114 : what do you mean by “increasingly” : more studies are focusing on that point, or climate\nchange induces a stronger linkage ?\nL124 : Mann et al. (Ref 26) focuses on the last millenium. To me it is very speculative to extend their\nresults to the last 55 ky. Are there any publications focusing on paleo records (or paleomodelling) of\nclimate oscillations ? Are the PDO, AMO .. similar now and during the LGM for instance ?\nFurthermore, it has been shown that PDO does not necessarily have a multidecadal frequency even\nin the last millenium, e.g. Mac Donald and Case (2005) showed that its phase was negative from\n900 to 1300 AD.\nMacDonald, G. M., and R. A. Case (2005), Variations in the Pacific Decadal Oscillation over the past\nmillennium, Geophys. Res. Lett., 32, L08703, doi:10.1029/2005GL022478.\nL151 : what is the time range of CMIP5 historical simulations ? Are there any studies using data\nfrom PMIP ?\nL182 – 184: indeed, low variability have a larger impact on OMZ than ENSO. I think it is consistent\nwith litterature (e.g Deutsch et al., 2014; Ito et al., 2019).\nL192 – 194 : what do you mean by “can have broader effects” (be more specific).\nL232 – 234 : the discussion regional / large scale is very interesting and need to be developed. More\nspecifically, are there any reference of studies (even in present day) discussing the relative\nimportance of upwelling change / large scale circulation in constraining OMZs ? How do your\nresults compare with these studies ?\nL246 : I think that more references are needed here.\nL266-267 : in my understanding, the “isopycnal heave” (Ito et al., 2019) is related to variations in\nthermocline depth (related to the PDO phase). During PDO- the thermocline shallows and colder /\noxygen poor water is mixed toward the surface, decreasing oxygen levels. Other studies argue that\noxygen levels are determined by zonal oxygen transport (Duteil et al., 2014, 2018; Shigemitsu et al.,\n2017; Poupon et al., 2023).\nL312 : unclear\nL302 / 319-320 : many studies show a major impact of the PDO on tropical circulation !! They\nshould be discussed !\nL352 : Brandt et al. focus on the Atlantic Ocean, where the role of the AMO may be clearer.\nFigure 5 : text right of the figures, I think there are some confusion here : increased equatorial\nupwelling is directly related with fast EUC (strong trade winds / strong tropical circulation which\nincludes upwelling and EUC). If O2 supply increases, then nutrient supply should increase as well\nas productivity.\nFigure 5/6 : is the EUC total mean transport (integrated over depth) related to the maximum EUC\nvelocity ?\nSub panels should be consistently numbered (a,b,c …) avoiding top / bottom / left.", "reviewer_3": "NCOMMS-23-62151\n1st Review of “North Atlantic temperature control on deoxygenation in the Northern Tropical Pacific\n” by Pichevin et al.\nThe authors argued that the main challenge for modelling the decline in oxygen concentration\nlinked to future climate change is our lack of knowledge of the natural variability in marine oxygen\ninventory in the preindustrial past and how it relates to interannual and multidecadal modes of\nclimate variability. In the present study, the authors presented that the deoxygenation in the eastern\ntropical North Pacific is mainly controlled by sub-surface ventilation to the tropics triggering\nchanges in oxygen supply at multi-decadal timescales by analyzing 10 annually resolved 200 year-\nlong records of denitrification. The authors argued that the North Atlantic Temperature patterns are\nthe main control on the subsurface circulation in the tropical Pacific and therefore is the most likely\ndriver of natural oxygen variability in the Pacific Oxygen Minimum Zone (OMZ). While the topic is\ninteresting, it should be rejected to publish at Nature Communications because the analyses is not\nmature. The followings are major comments.\n1.Ambiguity: The abstract is a bit non-conclusive. In particular, it is necessary to clarify the role of\nnatural climate variability and global warming on the decline in oxygen concentration. The current\nabstract has no clear message on this issue. Please rewrite it.\n2. Figure 1: Indeed, the change in oxygen in ETNP OMZ since 1960 seems to be correlated with the\nAMO and PDO index, respectively. However, the detailed analysis is not seen in the main text.\nAccording to the previous literature, the AMO and PDO index is highly correlated with a lagged time.\nThe authors should examine the statistical analysis based on the time series shown in Fig. 1 at\nleast. And then, they are able to develop a hypothesis.\n3. As the authors mentioned, indeed, the Gulf of California is an ideal site to study multidecadal\noxygen changes. To support this notion, however, it is necessary to provide a solid analysis using\nthe historical observational or reanalysis dataset (oceanic and atmospheric reanalysis dataset). In\naddition to a multi-decadal climate oscillation, a local process would be important to influence the\nbiological and chemical state in the gulf of California.\n4. The subsection: Climatic oscillations in the modern Pacific Ocean:\nI am a bit surprised with this subsection which is full of a simple description based on a previous\nliterature without any statistical analysis at least.\n5. The subsection: Results\nBy the time series analyses in the frequency domain using spectral analyses with several\nwindowing settings and wavelet transform, While the authors introduced some important results,\nthe main results are limited to interpret the statistical relationship without any physical\nexplanation. This is not enough to publish this result to a high impact journal like Nature\nCommunications."}, "author_response": "REVIEWER COMMENTS\nReviewer #1 (Remarks to the Author):\nREVIEW OF MANUSCRIPT NUMBER “NCOMMS-23-62151”\nIn the manuscript titled “North Atlantic Temperature Control on Deoxygenation in the Northern\nTropical Pacific” Pichevin and coauthors present a high-resolution reconstruction of denitrification of\nthe Eastern Tropical Pacific (ETNP). The authors present an annually-resolved 55 kyr-long δ15N\nrecord based on ten core sections. After performing spectral analyses on their datasets, the authors\nwere able to find clear relationships between their proxy values and multi-decadal climate\noscillations.\nThe main strength of the work is the quality and resolution of the data series and the value of the\nspectral analyses performed. The authors found that multi-decadal signals in the form of PDO- and\nAMO-like periodicities dominate the records with only minor influence from NPGO and ENSO.\nWhile this could be true, the lesser influence of shorter-scale variabilities could also be related to the\nage model of the record. Additionally, the GC has its own local productivity dynamics, which could\ndominate the proxy records at the annual scale. Despite the latter, I was surprised to see not only\nPDO but also AMO play a role in the ETNP oxygenation, which is truly valuable information to\ncontinue moving the field forward.\nIn its present state, the manuscript exhibits certain weaknesses. The text is hard to read and feels a\nbit too raw and technical for the audience, so it could use some reorganization. For the scope of the\njournal, I would have expected the authors to focus more on the broader implications rather than\nthe technical aspects of the models and analyzed cores. To a lesser degree, I see other minor issues\nwith wording and structure, which will be easily solved after rephrasing and other refining efforts.\nWe have considerably re-worked the text and shortened it slightly. This includes : Cutting the\nintroductory subsections on climate oscillations, streamlining the discussion on circulation changes\n(from line 255) to make it clearer and more to the point and adding analysis to the “Implications”\nsection (from line 444).\nI consider the manuscript to be very promising, but it will need a bit more work before it can be\npublished in “Nature Communications”. Hereby, I suggest several ideas of how the manuscript could\nbe improved. I also upload a marked PDF highlighting some minor details.\nWe thank Reviewer 1 for this very supportive and constructive review and have made the requested\nchanges to the manuscript in order to clarify the points raised by the Reviewer and make the text\nand figures more readable and polished. These changes are delineated point by point below.\nI. GENERAL COMMENTS:\nA. What are the noteworthy results?\n- A truly high-resolution denitrification record of the past 50kyr was generated for the Eastern\nTropical North Pacific using the δ15N proxy.\n- Assuming that δ15N is able to show deoxygenation trends, clear responses to climate oscillations\nwere found for the ETNP OMZ after performing the spectral time series analysis.\n- The most promising and noteworthy results are finding statistical evidence for climatic periodicities\nsuch as PDO in the preindustrial past.\nB. Will the work be of significance to the field and related fields? How does it compare to the\nestablished literature? If the work is not original, please provide relevant references.\n- Although the manuscript aims to address crucial mechanistic questions, there is some uncertainty\nabout its novelty in relation to the journal's scope. Nevertheless, I believe that this study holds\nsignificant relevance and would contribute substantially to the scientific community if published here\nor elsewhere.\nThe Reviewer does not elaborate on why they are uncertain that the results presented in our article\nare novel enough for Nature Communications. Here, we present not one but ten 200-year long,\nannually resolved records of deoxygenation and productivity changes in the North Pacific OMZ\nduring key pre-industrial climatic intervals over the past 55,000 years. Our study for the first time\nhighlights the role of the AMO in oxygen changes in the Pacific OMZ at short timescales over the past\n55,000 years. This is in contrast to existing studies that mainly highlight the role of the PDO based on\nthe last 60 to 100 year of historical records, a time span that only covers 2 full PDO cycles at most.\nNot only do we show for the first time that the AMO is the main driver of oxygen changes but also\naddress the underlying mechanism. We argue this is novel and suitable for the broad readership of\nNature Communications. We now refine and strengthen our discussion of this mechanism from line\n311.\n- While I personally agree that many locations of the Gulf of California (GC) are excellent indicators of\npast ETNP variability, I would not generalize. I would still treat the GC as semi-restricted, which is\naccurately mentioned by the authors. It is also important to acknowledge that not every researcher\nfamiliar with modern GC dynamics agrees that bottom sediments can even accurately register\noxygenation changes. Many oceanographers argue that there are mesoscale dynamics that play a\nsignificant role in transporting nutrients and controlling the productivity (and deoxygenation) of GC\nbasins. This could be the reason why the NPGO and ENSO are not showing up as expected in the\nspectral analysis.\nWe note the Reviewer agrees that the GC is an excellent location to look at oxygen variability in the\nETNP, as other studies have suggested (Altabet, Pilskaln et al. 1999, Pride, Thunell et al. 1999).\nMaking sure that local processes impacting local primary productivity, upwelling and local water\ncolumn oxygen did not mask the broader scale changes we are aiming to examine is indeed crucial.\nThat is the reason we produced, and rigorously examined the variability seen in, biogenic silica\nrecords (productivity) for the same yearly sampled records used for d15N/oxygen reconstructions.\nWe also examined published sediment-trap productivity data from a nearby location in the GC\n(Thunell 1998) and compared it to SSTs/upwelling intensity data from the GC and ENSO & NPGO\nvariabilities. The results of these analyses and considerations are summarised in the original (and\nrevised) manuscript (from line 215) and in the supplementary material (section 2)\nFirstly, SST data in the GoC near Guaymas basin track ENSO and NPGO variabilities extremely well\n(see Figure 2 of the supplementary material) and spectral analyses between 1950 and 2012 for both\nENSO3.4 and GC SSTs give an almost exact correspondence, except for a particularly strong 12yr\nperiodicity in the SST record that corresponds to the NPGO periodicity. In other words, upwelling and\nsurface processes near our site respond strongly to short term Pacific climate variability such as\nENSO and NPGO. Secondly, opal export reconstructed from Thunell’s trap data and our records also\nshow a strong ENSO and NPGO signal (from line 215 and sup mat Fig 2C). This means that local\nproductivity is very much influenced by these large-scale climate variabilities, as opposed to local\nprocesses. Contrary to what the reviewer assumes in the last sentence of their comment, NPGO and\nENSO appear prominently in the biogenic silica records from our site but these modes of variability\ndo not stand out prominently in the corresponding δ15N/deoxygenation records. This is precisely this\ndiscrepancy that leads us to argue that the short-term Pacific climatic oscillations (ENSO-NPGO)\nwhich primarily affect biological productivity in the GC and the ETNP at large do not significantly alter\nthe oxygen budget in the water column of the ETNP overall, and other mechanisms involving\nsubsurface circulation, at longer timescales (multi decadal), must be examined.\n- The relationship between ETNP oxygenation and multi-decadal variability such as PDO is not new.\nOther authors have noted this before in the GC and the Mexican Pacific using geochemical (Tems et\nal., 2016; Choumiline, et al., 2019) and foraminiferal proxies (Almaraz-Ruiz et al., 2024). That being\nsaid, I am excited to finally see statistical evidence for PDO and potentially even AMO-like variability\nin the ETNP oxygenation records.\nWe agree with the Reviewer. We fully discuss the link previously made between PDO and ETNP\noxygenation in our responses to Reviewer 2 and strengthen our discussion about this aspect in the\nrevised manuscript from line 365.\nC. Does the work support the conclusions and claims, or is additional evidence needed?\n- An important part of the paper relies on time-resolved sedimentary records, so the validity of the\nage models is of the essence. However, despite referring to another paper, the authors do not\nmention much about how the age models were constructed (besides some mention in the\nsupplement) and how can we ensure the annual resolution that is promised in this manuscript.\nI think it is important to explain this important part in more depth, as many interpretations depend on\nthis.\nThe construction of the age model is now fully explained in section 1 of the supplementary material\nand more details on the sampling of the varves is given in the Method section (from line 448).\nD. Are there any flaws in the data analysis, interpretation and conclusions? Do these prohibit\npublication or require revision?\n- Overall, I consider the data analysis, interpretation and conclusions to be sound for the scientific\nquestions that want to be answered by the authors. In my opinion, the numerical and statistical\napproaches used in this manuscript are appropriate and are well explained in the Methods section.\nWe thank the Reviewer for highlighting the suitability of our methods for the aim of our study.\n- I am not surprised that the authors do not find many sub-decadal periodicities during Heinrich\nevents, which could be due to the fact that well-oxygenated waters rarely preserve a clear\noxygenation record. Most geochemical proxies fail to preserve a signal during well-oxygenated\ntimes such as the LGM. Moreover, there are processes in the water column found for sediment trap\nmaterial of the GC that show some authigenic proxy enrichments and isotopic fractionation)\nassociated with reducing microenvironments within marine snow aggregates (Choumiline et al.,\n2019). These additional particulate fluxes could contribute to the interannual noise of the\nsedimentary records.\nWe agree with the reviewer. This is what we explain in the manuscript (from line 183)\nE. Is the methodology sound? Does the work meet the expected standards in your field?\n- Overall, I consider the methodological approach to be adequate for the scientific questions that\nwant to be answered by the authors. The obtained and used data appears to be reliable. My only\nconcern would be the ability of generating a true annually-resolved proxy record, which seems hard\nto be achieved by varve counting given the presence of turbidites and recurring dust storms,\ntropical cyclones and earthquakes in the area that can lead to under or overestimation of the\nlaminations. I also understand that there are no other alternatives at this temporal resolution.\nThe 10 sections were carefully selected avoiding the disturbances the reviewer has mentioned. The\n10 sections (each a few decimetre-long) were selected amongst a 73m long sedimentary record\nspecifically because these particular sections showed regular laminations with no evidence of\ndisturbance, unconformity or bioturbation. We used Xray imagery to count the varves and ensure\nthat dark and light couplets (each couplet has been shown in modern sediment to represent a year\nof sedimentation) were always present and pristine throughout the chosen sections. More photos\nand Xray images of these sections are provided in the supplementary material (see S1 and the Figure\nS1)\nF. Is there enough detail provided in the methods for the work to be reproduced?\n- I think the detail provided in the methods is sufficient.\nII. MINOR COMMENTS\n- As a paper that has as one of its main focuses the past changes of deoxygenation, it is important to\nintroduce what is meant by anoxia, hypoxia, suboxia, etc. Different authors refer to these values very\ndifferently. An attempt was made by Canfield and Thamdrup, 2009 to suggest avoiding the use of\nvague terms like suboxic, so please refer to this paper. Typically using the dissolved oxygen values\nproposed by Hofmann et al., 2011 and mentioned in the Moffitt et al., 2015 paper would be a good\nidea. However, since we have to rely on proxies, it is nearly impossible to reconstruct exact values\nor even enough to affirm that a basin was either completely anoxic or oxic (see line 169). I suggest\nreferring to periods of the pre-instrumental past using terms like “more oxygenated” or\n“less oxygenated”.\nWe removed the terms anoxia and suboxia from the manuscript and now refer to “poorly\noxygenated water” instead. We now define anoxia (line 73).\n- The manuscript often repeats “10 annually resolved” denitrification records. I don’t think there is a\nneed to repeat that there are 10 records. I think just saying something along the lines of “our\nannually resolved records” or “annually resolved sections” would suffice. Speaking of these records,\nit might be a better idea to rename them from #46, #47 to A, B, C, D; 1, 2, 3, or any other code that\nwould help the reader.\nWe removed the repetitive phrase “10 annually resolved” as much as clarity permitted in the revised\nmanuscript. However, we did not modify the names of the sections as these correspond to positions\nin the sediment core that are or will be used in other publications relating to this core. Changing the\nname of these sections in the manuscript might lead to confusion for future comparisons.\n- Please rewrite the sentence that starts at line 152 and explain what you mean by “warm events of\nstage 3”? What stage 3 are we talking about? For instance, Marine Isotope Stage 3 comes to mind\nwhen reading the text.\nThe sentence was rewritten to clarify that the events in question are the Dansgaard/Oeschger events\nof isotopic stage 3.\n- Please be consistent when you refer to figures and materials from the supplement. For instance,\nyou have: “Fig 1”, “fig. 5”, “Figure 5”, “Fig.6A” and “Sup. mat 3”, “Sup. Mat 3”, “Sup. Mat. S3\",\n“Supplementary material 2”. Please pick one style and unify it throughout the text.\nThese have been harmonised across the manuscript.\n- Please be consistent with the use of uppercase or lowercase terms. For instance, on line 29 you\nrefer to “Oxygen Minimum Zones” by capitalizing each letter. However, in other parts of the text such\nas line 35, 54, and 168 you use lowercase “oxygen minimum zones”. Please be consistent with this\nand other terms. Same applies with the use of Biogenic Silica which also appears as “biogenic Silica”\nor “Biogenic silica”.\nThis has been harmonised in the revised text.\n- Several minor things in the text are not polished such as campaign names (e.g. “2002 MONA IMAGE\ncampaign” instead of “MD126/IMAGES VIII MONA”) or the brands of the analytical instruments (e.g.\n“CE instrument NA2500” instead of “CE Instruments NA2500”. Please go over the text and try to hunt\nfor these little mistakes. Some of these issues are on the following lines: 380, 425,\nWe have fully proofread the manuscript and are hoping the text is now free of such typos.\n- Some parts of the results would better go to the introduction. For instance, the first section that\nstarts on line 126 with “We carried out time series…” would better go to the bottom of the\n“Introduction” section.\nWe respectfully disagree with the reviewer that this sentence needs to go in the introduction. While\nit introduces the results presented in the following sentence, it does not pertain to the manuscript as\na whole but to the particular Result section that follows.\n- Please avoid starting sentences with element symbols or acronyms. For instance, in line 381 “N2\nand CO2 gases” would have to be “The N2 and CO2 gases”. Similarly, this happens throughout the\ntext in line: 276, and potentially elsewhere.\nThis has been amended.\n- Some sentences are a bit confusing, incomplete, or could use improvements. Please kindly rewrite\nthem. These start at the following lines: 170, 335, 377, and 411.\nThe sentence starting at line 170 was removed as it was redundant. The grammar in the sentence\nstarting at line 335 was improved. The sentence starting at line 377 was completed and the sentence\nstarting at line 411 was corrected.\n- The supplemental materials need work and organization. The figures and table are not captioned\n(under the figures and above the table).\nThis has been changed according to the Reviewer’s advice.\n- I suggest word-proofing the paper using Grammarly or similar software. It will help polish and find\nminor grammar issues.\nIII. COMMENTS ON FIGURES:\n- The figures need work. In my opinion, there are too many of them with some not showing relevant\ninformation. The figures have different font styles and sizes, which also does not help to get a sense\nof a final publishable product. It would be better to unify all of the styling differences using the\ngraphic design software of your choice. For instance, Figure 1 is composed of three figures that have\ndifferent font styles and sizes. Also, it would be better to refer to the subfigures separately and label\nthem as Figure 1a, Figure 1b, instead of having the reader figure out which one is which.\nWe have followed the Reviewer’s advice and redrawn the figures and changed the labels.\n- In Figure 1, the picture of the varved section from Core MD02-2515/17 does not really add any\nvaluable information. I suggest removing it completely by moving from the main text down to the\nsupplement.\nWe agree and have removed the picture.\n- Figure 6 needs to be redrawn. In its current state, it is composed of four figures showing a variety of\nrecords glued together. It would be better to unify the font size for the X-axes (years) and Y-axes (data\nvalues). Even removing the repetitive X-axes on all the records and only leaving one in the lower part\nwould help the figure tremendously. Please also adjust the Y-axes to the same vertical lines (please\nrefer to the marked manuscript PDF).\nThis has been done.\n- It would be nice to see the δ15N values for all the sections in the supplement (similarly to the\nprofile shown in Figure 2).\nThe yearly N isotopes records for the 10 sections have been added to supplementary material (S5).\nIV. ADDITIONAL REFERENCES:\nHere are the references mentioned during this review:\nAlmaraz-Ruiz, L., Machain-Castillo, M.L., Sifeddine, A., Ruiz-Fernández, A.C., Sanchez-Cabeza, J.-A.,\nRodríguez-Ramírez, A., Mendez-Millan, M. and Caquineau, S., 2024. Changes of bottom water\noxygenation during the last half millennium in the Gulf of Tehuantepec (Eastern Tropical North\nPacific): A multiproxy approach. Palaeogeography, Palaeoclimatology, Palaeoecology, 636: 111994.\nCanfield, D.E. and Thamdrup, B., 2009. Towards a consistent classification scheme for geochemical\nenvironments, or, why we wish the term 'suboxic' would go away. Geobiology, 7(4): 385-92.\nHofmann, A.F., Peltzer, E.T., Walz, P.M. and Brewer, P.G., 2011. Hypoxia by degrees: Establishing\ndefinitions for a changing ocean. Deep Sea Research Part I: Oceanographic Research Papers,\n58(12): 1212-1226.\nMoffitt, S.E., Moffitt, R.A., Sauthoff, W., Davis, C.W., Hewett, K. and Hill, T.M., 2015.\nPaleoceanographic Insights on Recent Oxygen Minimum Zone Expansion: Lessons for Modern\nOceanography. PLOS ONE.\nTems, C.E., Berelson, W.M., Thunell, R., Tappa, E., Xu, X., Khider, D., Lund, S., González-Yajimovich, O.\nand Hamann, Y., 2016. Decadal to centennial fluctuations in the intensity of the eastern tropical\nNorth Pacific oxygen minimum zone during the last 1200 years. Paleoceanography, 31(8): 1138-1151.\nI sincerely hope that my insights will be helpful to the authors and wish them good luck. I really\nenjoyed reviewing this important paper and would like to see it improved and ultimately be\npublished.\nReviewer #2 (Remarks to the Author):\nNorth Atlantic Temperature Control on deoxygenation in the Northern Tropical Pacific\nPichevin et al.\nThe thematic of the study is very relevant to the marine community. The Oxygen Minimum Zone\n(OMZ)increase is a cause of concern and has a large impact on marine life and biogeochemical cycles.\nThe impact of climate oscillations, such as the Atlantic Multidecal Oscillation (AMO) or the Pacific\nDecadal Oscillation (PDO), is still not clear compared to climate change. Based on a 55 ky sediment\nrecord and the analysis of the SODA 1950-2010 ocean reanalysis, the authors conclude that the\nAtlantic Multidecadal Oscillation plays a major role in controlling the Pacific Ocean tropical\ncirculation and hence the OMZ extension.\nIn its current form, the study is divided into two parts, which are mostly independent. One focuses\non the discussion of a 55 ky record of denitrification in the gulf of California. The other on the\nmechanisms relating AMO and oxygen levels based on a reanalysis of the last 6 decades (SODA). I\nhave major concerns in both parts :\nWe are a bit perplexed by this comment because it appears the reviewer is suggesting that\naddressing mechanisms that underpin palaeo-oceanographic data is a separate study. While the type\nof data examined in each section is of a different nature (paleo reconstruction of deoxygenation for\nthe first and historical velocity and temperature data reanalyses for the second), the sections\ncomplement and strengthen each other in order to (1) offer compelling evidence that the AMO is an\nimportant mode of variability for Pacific oxygen levels and (2) present a mechanistic explanation as\nto how this is the case. We now make the link between both sections clearer in the text line 268-273.\n1- The frequency of AMO / PDO in the present day is about 50-80 years and 20-30 years as stated by\nthe authors. It is very speculative to assume that this frequency does not change in the past 55.000\nyears. For instance, based on tree rings, a previous study has shown a long multi-centenial PDO\nnegative phase during the last millenium (McDonald and Case, 2005). Even if a “PDO-like” frequency\nis found in sediment record in one location, it does not mean that the PDO magnitude / geographical\npattern (and then implications for oceanic circulation and OMZs) is similar. Demonstrating the\nexistence of a AMO-like / PDO-like oscillation during the last 55 ky with a very constrained frequency\nrange would be a major finding. It would however require the analysis of several sediments cores at\ndifferent locations over the Pacific basin and temperature proxies in complement to denitrification.\nWe commend the Reviewer for putting forth the research agenda for the paleo-oceanographic\ncommunity for the next several decades in this comment. Our paper can be regarded as one of the\nfirst and important steps towards this agenda. Firstly, there are not many places where one can get\nannually varved sediments from the Pacific Ocean sector to achieve this goal. However, we do\nacknowledge in the manuscript that there are slight changes in the periodicities associated with the\nAMO and to a lesser extent the PDO (see frequency bands in Figure 3) especially during MIS 3 (text\nstarting line 168) compared to the Holocene, but these changes remain modest and within 10-20%\nof the average typical periodicity. In this comment, Reviewer 2 argues McDonald and Case (2005)\nfound that the PDO periodicity changes from multi-decadal to multi-centennial in records older than\n200 years BP. This is simply not what the authors of this article find. Here we quote from McDonald\nand Case, 2005: “The wavelet spectrum suggests that while the 50-year pattern of variability is a\nsignificant mode of variability over the past millennium, it is not consistently present, with notable\nweakening from approximately AD 1200 to 1300 and from AD 1500 to the early 1800’s”. This is\nconsistent with our findings that changes in frequencies are modest.\nWhile we agree with the Reviewer that PDO and AMO (and indeed ENSO) periodicities probably\nchanged in the past, there is only evidence that these changes were modest as supported by paleo-\nrecords (Beaufort and Grelaud 2017) and PMIP simulations (Zhang, Berntell et al. 2021), for instance.\n2- The authors argue that the AMO has a dominant impact on the equatorial circulation of the\nPacific Ocean. I agree that the AMO may have a significant (even dominant) impact more\nspecifically in the western part of the Pacific Ocean, as shown by Sun et al., (2017).\nSun, C., Kucharski, F., Li, J. et al. Western tropical Pacific multidecadal variability forced by the Atlantic\nmultidecadal oscillation. Nat Commun 8, 15998 (2017)\nOur results are entirely consistent with Sun et al. (2017) and together present a coherent mechanism\nlinking east and west tropical Pacific subsurface circulation. We would like to stress that the EUC\noriginates in the Western Equatorial Pacific where Sun finds a strong AMO influence. In the revised\nmanuscript we clarify further the mechanism that links AMO variations, the trade winds and the\nshallow zonal circulation (EUC) in the Pacific (from line 330, and particularly between l. 343-363).\nHowever many studies have demonstrated a major role of the PDO in the forcing of the equatorial\nPacific circulation (which includes the shallow overturning circulation and the subtropical cells, the\nequatorial upwelling system and the EUC) :\nI suggest here a few studies (among many others) :\nCapotondi, A., and B. Qiu, 2023: Decadal Variability of the Pacific Shallow Overturning Circulation and\nthe Role of Local Wind Forcing. J. Climate, 36, 1001–1015,\nHong, L., L. Zhang, Z. Chen, and L. Wu (2014), Linkage between the Pacific Decadal Oscillation and\nthe low frequency variability of the Pacific Subtropical Cell, J. Geophys. Res. Oceans, 119, 3464–\nPacific Ocean. Nature 415(6872):603–608.\nWe thank the reviewer for highlighting the paper by Capotondi et al to us. As it is a very recent paper,\nwe had not had time to include it to our manuscript (we do now). This paper, as well as McPhaden\nand Zhang (2022), makes a clear link between changes in the shallow overturning circulation in the\ntropical Pacific and changes in Ekman pumping resulting from interaction with the trade winds,\nwhich is what we also deduce from our SODA reanalyses of the EUC velocity (line 302 and 350-357\nfor instance). We believe there is no contradiction between our study and Capotondi’s or\nMacPhaden’s as we also propose that (1) the Trade winds modulate the Pacific subsurface circulation\naround the tropics and (2) the PDO also has an impact on the strength of the equatorial circulation\n(figure 4, C), albeit to a lesser extent than the AMO. But contrary to these other studies, we also look\nat the impact of the AMO phases on the Equatorial Circulation in SODA reanalyses and find that the\nAMO has a far greater impact on the strength of the zonal supply of oxygenated water eastward via\nthe EUC than the PDO (please see Figure 1 of the response below).\nSimilarly, Hong et al (2014) focus on the southward meridional subsurface circulation and, in\nparticular, its effect on tropical upwelling and SSTs. In our manuscript, we do not contest the link\nbetween PDO and SSTs in the ETNP (see next response below), we argue that it has little impact on\nsubsurface oxygen in the ETNP because of the competing effects of invigorated mixing and biological\nproductivity on oxygen. We now discuss this more clearly from line 366 of the revised manuscript.\nDemonstrating that the AMO is a dominant forcing of the Eastern Pacific circulation (compared to the\nPDO / PDV) would be an important finding. It should however be demonstrated in a thorough way,\neventually by performing modelling experiments as in Sun et al., 2017 or analysing longer\nsimulations (e.g CMIP/PMIP), as the AMO frequency is of the same order as the SODA dataset. The\nFigure 4 (top) showing the positive AMO regressed SSTs shows confidence (cross hatching marks)\nthat AMO and SST are related in the western Pacific Ocean (which is in line with Sun et al, 2017).\nHowever in the eastern part, where the OMZ are located, the Fig 4 shows that the PDO plays a major\nrole (with confidence) in regulating SST which is line with many studies.\nOur results are not in contradiction with Sun et al. In our study we find that upwelling and surface\ncirculation (via biological productivity and surface mixing), whether they are related to ENSO and\nNPGO variabilities or other causes have little impact on subsurface oxygen trends. The PDO controls\nSSTs in the ETNP via upwelling, heave and shallow circulation (Ito, Long et al. 2019). The associated\neffects of the PDO on oxidant demand and stratification tend to cancel each other when it comes to\ntheir impact on subsurface oxygen (Figure 5 of the manuscript). Therefore, while the PDO has a clear\nimpact on SSTs in the ETNP, it does not in fact modulate subsurface oxygen concentration in a\nsignificant. We argue that the AMO, via its impact on the trade winds, Equatorial upwelling and EUC\nstrength is the main driver of oxygen variability in the subsurface ETNP. This is supported by our\nSODA reanalyses of subsurface circulation We now make this clearer in the manuscript from line 337\nto 389 (also see Figure 1 of the response below).\nI think that the authors need to focus on one specific point (1 or 2) as the articulation between 1 and\n2 is not clear to me at the moment. In both case, more discussion / comparison with literature is\nneeded.\nWe follow the advice of Reviewer 2 and incorporate a more focused discussion of the suggested\nliterature (line 349-389). However we strongly disagree that we should focus on only one section of\nthe discussion for the reasons already expressed in our first answer to Reviewer 2’s comments.\n---\nMore specific comments :\nL24-36 : it is stated that global warming is driving the oxygen decrease while L38 it is stated that\nclimate oscillations are controlling oxygen trends. Please clarify: is climate change driving oxygen\ndecrease or climate oscillations ? Is there a consensus in the literature or is it still an open question ?\nThe literature - e.g. (Oschlies, Duteil et al. 2017, Oschlies, Brandt et al. 2018) - clearly shows overall\ndecreasing O trends worldwide, especially in the upper 300 m of the ocean which studies link to\n2\nglobal warming inducing increase in ocean temperature and driving a temperature related decrease\nin gas solubility in sea water. However, there is variability superimposed on this overall\ndeoxygenation trend depending on locations and on local to regional climate variability. We now\nclarify this, starting line 41.\nL40 : what do you mean by “apparent” decrease ? (is it not really decreasing ?)\nHere “apparent” is used in its literal sense: obvious, visible, evident. The word was removed to avoid\nconfusions.\nL42 : is it consistent with Fig 1 ? (it seems to me that the the PDO shift from positive to negative is\nassociated with an oxygen decrease). This sentence has been rephrased to avoid confusion.\nL43-44 and Figure 1: I agree, there is a trends in AMO and a trend in oxygen. However the AMO trend is\nopposite to the PDO. To me it is difficult to establish a causality link, so I think that “suggest”\nmay not be appropriate at least at this stage.\nThe fact that the AMO and PDO appear to be negatively correlated in parts of the record shown in\nFigure 1 does neither precludes nor suggests a causality between AMO and PDO. We do not make\nthis argument in the text here. Rather we propose that both changes in the PDO and the AMO\nindices appear to vary in tandem with oxygen anomalies in the ETNP, at least in parts of the historical\nrecord. We now simply state (line 49-50) “Intriguingly, similar trends of historical O anomalies in the\nsimply setting the scene for what we set out to do in this article: ground-truth the relationship\nbetween multi-decadal oscillations and ENP oxygen anomalies in the past and present and explore\nthe mechanisms through which these climatic variabilities can impact oxygen level in the North\nEastern Pacific subsurface.\nL48 : The PDO / AMO characteristics likely changed in the past 55 Ka too !\nPlease see our response above on the changes in periodicities.\nL59 : did OMZ intensity / location / oxygen mean state varied a lot during the last 55 Ka BP ?\nThe Gulf of California where our record is located is at the heart of the ETNP OMZ. Paleo-\noceanographic records from other sites in the ETNP (Ganeshram, Pedersen et al. 2000, Kienast,\nCalvert et al. 2002, Pichevin, Ganeshram et al. 2012), etc), agree with the long-term denitrification\nrecord presented in our paper.\nL89-90 : how does it compare with previous publications focusing on past OMZs ?\nWe are not sure what “it” refers to here. In the text we highlight that we are comparing our ability to\ntrack annual changes in denitrification to what was done in previous work (Deutsch, Berelson et al.\n2014) focused on the ETNP OMZ. We present the first annually resolved paleorecords of\ndeoxygenation in this area.\nL114 : what do you mean by “increasingly” : more studies are focusing on that point, or climate\nchange induces a stronger linkage ?\nThis sentence has been re-written for clarity.\nL124 : Mann et al. (Ref 26) focuses on the last millenium. To me it is very speculative to extend their\nresults to the last 55 ky. Are there any publications focusing on paleo records (or paleomodelling) of\nclimate oscillations ? Are the PDO, AMO .. similar now and during the LGM for instance ?\nFurthermore, it has been shown that PDO does not necessarily have a multidecadal frequency even\nin the last millenium, e.g. Mac Donald and Case (2005) showed that its phase was negative from\n900 to 1300 AD.\nMacDonald, G. M., and R. A. Case (2005), Variations in the Pacific Decadal Oscillation over the past\nmillennium, Geophys. Res. Lett., 32, L08703, doi:10.1029/2005GL022478.\nWe have addressed this point in detail in our response (above) to Reviewer 2’s comment where we\nstate that current evidence suggests only modest changes in the periodicity of the PDO and other\nclimatic oscillations in paleorecords.\nL151 : what is the time range of CMIP5 historical simulations ? Are there any studies using data from\nPMIP ?\nWe have modified the sentence in order to highlight that CMIP5 are modern simulation spanning the\nlast 150 years (Zhang and Wang 2013). A recent study using PMIP (Zhang, Berntell et al. 2021) for\ninstance shows (we quote) that Holocene, Last Interglacial and Pliocene simulation “reproduces\nreasonable spatial patterns and frequency features of several intrinsic climate variability modes such\nas ENSO, PDO and AMO”, indicating that they do not see a marked change in the frequency and\nstructures of these variabilities.\nL182 – 184: indeed, low variability have a larger impact on OMZ than ENSO. I think it is consistent\nwith litterature (e.g Deutsch et al., 2014; Ito et al., 2019).\nYes, we do not dispute this and indeed cite these 2 papers in our manuscript.\nL192 – 194 : what do you mean by “can have broader effects” (be more specific).\nWe have modified this sentence for clarity.\nL232 – 234 : the discussion regional / large scale is very interesting and need to be developed. More\nspecifically, are there any reference of studies (even in present day) discussing the relative\nimportance of upwelling change / large scale circulation in constraining OMZs ? How do your\nresults compare with these studies ?\nWe thank Reviewer 2 for this positive comment. The scale at which different mechanisms can impact\nsubsurface oxygen is often very briefly discussed in papers such as (Duteil, Böning et al. 2014) or\n(Oschlies, Brandt et al. 2018). While this is something we discuss in our manuscript in relation to our\nfindings that local upwelling/productivity appear to have little impact on subsurface oxygen trends\n(line 242-256) we feel that a broader discussion of this is beyond the scope of the paper.\nL246 : I think that more references are needed here.\nThis entire paragraph has been re-written (starting at line 260) as we felt it was unclear as to our\naims and approach, and slightly misleading.\nL266-267 : in my understanding, the “isopycnal heave” (Ito et al., 2019) is related to variations in\nthermocline depth (related to the PDO phase). During PDO- the thermocline shallows and colder /\noxygen poor water is mixed toward the surface, decreasing oxygen levels. Other studies argue that\noxygen levels are determined by zonal oxygen transport (Duteil et al., 2014, 2018; Shigemitsu et al.,\n2017; Poupon et al., 2023).\nWe agree with this assessment of the “isopycnal heave” by the reviewer and have rephrased this\nparagraph (line 376-380, please note that the discussion about the effect of the PDO has been\nmoved towards the end of the discussion). We however disagree with Ito (2019) that a shallow\nthermocline during PDO- “brings” poorly oxygenated waters to the ETNP. It simply brings the low\noxygen water already present in the subsurface upwards while isopycnal mixing linked to stronger\nupwelling facilitates mixing of oxygenated water from the surface. We postulate that the net impact\non oxygen would be small. In the manuscript, we propose that oxygen in the ETNP is driven by zonal\noxygen transport through the EUC which our re-analyses show to be particularly responsive to the\nAMO. It is not surprising given the findings of Sun et al, cited earlier by Reviewer 2, that the AMO\nforces changes in Western tropical Pacific (where the EUC originates) (Sun, Kucharski et al. 2017). The\nAMO- and PDO-regressed circulation (Figure 4, panels C and D) clearly shows that the east-west\n(zonal) equatorial circulation under the influence of the trade winds is mostly correlated with the\nAMO (in agreement with Sun et al, 2017).\nL312 : unclear\nThis entire paragraph was re-written.\nL302 / 319-320 : many studies show a major impact of the PDO on tropical circulation !! They\nshould be discussed !\nThroughout the original and the revised manuscripts, we discuss extensively several studies (Ito,\nDeutsch, Duteil…) that link PDO, STC and oxygen and explain why these studies might overestimate\nthe impact of PDO related mechanisms on subsurface oxygen (namely because they have\ncounteracting impact on oxygen budget via oxidant demand (productivity) and oxygen supply.\nWe have added clarifications to the text (see previous responses) and reorganised the discussion to focus\nspecifically on this aspect towards the end of the section (l. 366-390). In line 319-320 of the original\nmanuscript we simply describe the result of our SODA reanalyses.\nL352 : Brandt et al. focus on the Atlantic Ocean, where the role of the AMO may be clearer.\nWe agree.\nFigure 5 : text right of the figures, I think there are some confusion here : increased equatorial\nupwelling is directly related with fast EUC (strong trade winds / strong tropical circulation which\nincludes upwelling and EUC). If O2 supply increases, then nutrient supply should increase as well as\nproductivity.\nOur results show that the intuitive relationship between intense trade winds-strong upwelling-strong\nEUC is not straightforward (starting line 301 of the original manuscript), in agreement with other\nSODA reanalysis studies (Drenkard and Karnauskas 2014) which state that “the balance between the\neastward zonal pressure gradient force and westward surface stress [friction at the boundary\nbetween westward surface current and eastward flowing EUC] determines the strength as well as\nzonal and vertical structure of the EUC”. We argue that during positive AMO phases the balance is\nshifted towards greater friction (slowing down of the EUC in the east) due to strong trade winds in\nthe Eastern Pacific as put forward by Drenkard et al (2014). In such circumstances, the equatorial\nupwelling is strong but the eastward EUC transport is impaired by friction with the westward-flowing\nsurface current. This phenomenon decouples vertical nutrient supply induced by wind-driven\nupwelling and eastward oxygen supply via the EUC, specifically in the eastern part of the basin. We\nnow provide further evidence for this mechanism in the new Figure 5, the discussion starting line 350\nand in the figure below (new Figure S7 of the supplementary material).\nFigure 1: Change in the velocity gradient between the western and eastern side of the Equatorial\nPacific compared to AMO index.\nAbove, we present the maximum zonal velocity gradient between the western part and the eastern\npart of the Equatorial Pacific (Δu). The results show a strong correspondence with the AMO index\n(but not the PDO) and validates the mechanism we invoke in the paper (from line 350), namely\nincreased relative eastward transport of oxygenated water at times when the trade winds and\nupwelling are weaker (AMO-). During AMO+, while the upwelling is greater (more productivity), the\nwestward friction at the surface is also greater (Δu decreases) and the balance between zonal oxygen\nsupply and oxidant demand shifts towards oxygen decrease in the EEP and ETNP.\nFigure 5/6 : is the EUC total mean transport (integrated over depth) related to the maximum EUC\nvelocity ?\nMean velocity data reanalyses (SODA) at all available depths in the central and east Pacific show a\nconsistent pattern with the EUC velocity data we present in the paper, giving us confidence that the\nEUC total mean transport and max velocity are linked.\nSub panels should be consistently numbered (a,b,c …) avoiding top / bottom / left.\nThis has been modified for clarity.\nReviewer #3 (Remarks to the Author):\nNCOMMS-23-62151\n1st Review of “North Atlantic temperature control on deoxygenation in the Northern Tropical Pacific\n” by Pichevin et al.\nThe authors argued that the main challenge for modelling the decline in oxygen concentration\nlinked to future climate change is our lack of knowledge of the natural variability in marine oxygen\ninventory in the preindustrial past and how it relates to interannual and multidecadal modes of\nclimate variability. In the present study, the authors presented that the deoxygenation in the eastern\ntropical North Pacific is mainly controlled by sub-surface ventilation to the tropics triggering\nchanges in oxygen supply at multi-decadal timescales by analyzing 10 annually resolved 200 year-\nlong records of denitrification. The authors argued that the North Atlantic Temperature patterns are\nthe main control on the subsurface circulation in the tropical Pacific and therefore is the most likely\ndriver of natural oxygen variability in the Pacific Oxygen Minimum Zone (OMZ). While the topic is\ninteresting, it should be rejected to publish at Nature Communications because the analyses is not\nmature. The followings are major comments.\nWe are surprised that Reviewer 3 finds our analyses “not mature”. This is in opposition to Reviewer\n1’s assessment. We note that Reviewer 3 does not explain clearly here why they found our analyses\nto be unsatisfactory. We can only reiterate that we performed and presented the results of dozens of\ntime-series analyses, wavelet analyses and cross correlation analyses on several thousands of data\npoints in ten different annually-resolved records of both denitrification and productivity. We believe\nthe scale and thoroughness of such data analysis are unparalleled in the field. We systematically\npresent and interpret statistically significant results (95% confidence level) of these analyses. To\nstrengthen some aspects of our serial analysis we have now run additional coherence spectra (Figure\nS5 of the supplementary material) between δ15N and biogenic silica records which confirm our\ninterpretations derived from the cross-correlation analyses already performed by us between these\nrecords (namely the lack of relationship between local productivity and oxygen levels). We also\nrefined our SODA velocity reanalyses, including velocity comparisons at different longitudes and\ndepths and present additional velocity plots on Figure 5 of the manuscript. These new data\nreanalyses corroborate and strengthen the findings delineated in the original version of the paper\n(see also response to Reviewer 2).\n1.Ambiguity: The abstract is a bit non-conclusive. In particular, it is necessary to clarify the role of\nnatural climate variability and global warming on the decline in oxygen concentration. The current\nabstract has no clear message on this issue. Please rewrite it.\nWe now make a clear statement in the final sentence of the abstract on the implications of our\nfindings for future oxygen changes in the ETNP. We also add a final paragraph to the manuscript\ndelineating how rising Northern Hemisphere High Latitude Temperatures with global warming\nimpact the AMO and oxygen supply to the eastern tropical Pacific (line 341-end).\n2. Figure 1: Indeed, the change in oxygen in ETNP OMZ since 1960 seems to be correlated with the\nAMO and PDO index, respectively. However, the detailed analysis is not seen in the main text.\nAccording to the previous literature, the AMO and PDO index is highly correlated with a lagged time.\nThe authors should examine the statistical analysis based on the time series shown in Fig. 1 at\nleast. And then, they are able to develop a hypothesis.\nAs requested by Reviewer 3, figure 2 of the response (below) shows the results of the correlation\nanalyses between oxygen anomalies in the ETNP and either climatic oscillations shown in Figure 1 of\nthe paper (PDO and AMO) for the period 1950-2012. To prevent issues related to the large\nautocorrelation inherent in the time series, the significance of the correlations between the oxygen\nanomalies and either AMO/PDO is evaluated using a Monte Carlo approach with 1000 random\nsamples (following, for e.g., McCabe et al. 2004; Bollasina and Messori, 2018). The 90 and 95%\nconfidence levels are as follows for the PDO : 90% = 0.193367, 95% = 0.239286 and AMO: 90%=\n0.1834635, 95%= 0.2258225. The correlation coefficient R is much higher than the 95% confidence\nlevel for the AMO, but lower for the PDO. This is in agreement with the findings drawn from the\nthorough serial analyses we performed on our new paleo records, namely that oxygen responds\nmainly to the AMO and to a lesser extent the PDO. While this relationship between already\npublished data spanning a short (60 years) time period led us to formulate our hypothesis that the\nPDO and the AMO might control subsurface oxygen, such correlation it is simply not good enough to\no\noffer compelling evidences that it is the case. Indeed, 60 years barely covers one full PDO cycle and\nless than one AMO cycle, hence rendering this correlation unconclusive and we do not include this\nplot in the paper for this precise reason. Instead, we reconstructed several annually-resolved records\nspanning around 200 years each and performed thorough statistical and serial analyses on these\nrecords. That Reviewer 3 finds our analyses of these longer reconstructions “not mature” but\nrequests that we based our hypothesis on a simple correlation between these short records seems\ncontradictory.\nPDO AMO\n20\n10\nn\ne 5\ng\ny\nx 0\nO\n-0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1\na -5\nt\nl R² = 0.7156\ne\nD -10\n-15\n-20\nClimatic Index\nFigure 2 : Correlation analyses between oxygen anomalies in the ETNP and climatic oscillations (PDO\nin orange and AMO in blue) over the period 1950-2010.\n3. As the authors mentioned, indeed, the Gulf of California is an ideal site to study multidecadal\noxygen changes. To support this notion, however, it is necessary to provide a solid analysis using\nthe historical observational or reanalysis dataset (oceanic and atmospheric reanalysis dataset). In\naddition to a multi-decadal climate oscillation, a local process would be important to influence the\nbiological and chemical state in the gulf of California.\nThis comment is also raised by Reviewer 1 and we thoroughly respond on page 3 of this document.\n4. The subsection: Climatic oscillations in the modern Pacific Ocean:\nI am a bit surprised with this subsection which is full of a simple description based on a previous\nliterature without any statistical analysis at least.\nWe have now shortened this subsection which simply delineates the various interannual to\nmultidecadal variabilities that are known to modify atmospheric and surface / subsurface circulations\nand productivity in the ETNP, and how these interact together. This short section is primarily\ndesigned to introduce the reader to the various mechanisms and climatic phenomena that will be\ndiscussed in the result section and discussion. We believe the Reviewer would like us to show\nstatistical analyses such as shown in figure 2 of the response (above) in this section. We explain\nabove and in the introduction (line 52-54) that such analyses would be unconclusive as the historical\nrecords are too short, hence leading us to reconstruct and analyse longer deoxygenation records.\n5. The subsection: Results\nBy the time series analyses in the frequency domain using spectral analyses with several\nwindowing settings and wavelet transform, While the authors introduced some important results,\nthe main results are limited to interpret the statistical relationship without any physical\nexplanation. This is not enough to publish this result to a high impact journal like Nature\nCommunications.\nWe strongly disagree that we offer no physical explanation. An entire subsection from line 256 to 390\nis devoted to exploring and explaining the physical mechanism linking PDO and (mainly) AMO to\noxygen supply to the Eastern Tropical and Equatorial Pacific. We examine how subsurface circulation\nis impacted by both PDO and AMO oscillations by performing current velocity reanalyses (SODA data)\nregressed on either PDO+ or AMO+ (Figure 4) and reveal the physical process linking AMO changes to\nEUC velocity (transport) between the western and eastern part of the basin. We have now refined\nthese analyses and present further evidence linking the AMO to the West-East gradient in EUC\nstrength and eastward oxygen transport (new Figure 5). We further clarify this mechanism from line\n350.\nReferences:\nAltabet, M. A., C. Pilskaln, R. Thunell, C. Pride, D. Sigman, F. Chavez and R. Francois (1999). \"The\nnitrogen isotope biogeochemistry of sinking particles from the margin of the Eastern North Pacific.\"\nDeep-Sea Research Part I-Oceanographic Research Papers 46(4): 655-679.\nBeaufort, L. and M. Grelaud (2017). \"A 2700-year record of ENSO and PDO variability from the\nCalifornian margin based on coccolithophore assemblages and calcification.\" Progress in Earth and\nPlanetary Science 4(1): 5.\nDeutsch, C., W. Berelson, R. Thunell, T. Weber, C. Tems, J. McManus, J. Crusius, T. Ito, T. Baumgartner,\nV. Ferreira, J. Mey and A. van Geen (2014). \"Centennial changes in North Pacific anoxia linked to\ntropical trade winds.\" Science 345(6197): 665.\nDrenkard, E. J. and K. B. Karnauskas (2014). \"Strengthening of the Pacific Equatorial Undercurrent in\nthe SODA Reanalysis: Mechanisms, Ocean Dynamics, and Implications.\" Journal of Climate 27(6):\n2405-2416.\nDuteil, O., C. W. Böning and A. Oschlies (2014). \"Variability in subtropical-tropical cells drives oxygen\nlevels in the tropical Pacific Ocean.\" Geophysical Research Letters 41(24): 8926-8934.\nGaneshram, R. S., T. F. Pedersen, S. E. Calvert, G. W. McNeill and M. R. Fontugne (2000). \"Glacial-\ninterglacial variability in denitrification in the world's oceans: Causes and consequences.\"\nPaleoceanography 15(4): 361-376.\nIto, T., M. C. Long, C. Deutsch, S. Minobe and D. Sun (2019). \"Mechanisms of Low-Frequency Oxygen\nVariability in the North Pacific.\" Global Biogeochemical Cycles 33(2): 110-124.\nKienast, S. S., S. E. Calvert and T. F. Pedersen (2002). \"Nitrogen isotope and productivity variations\nalong the northeast Pacific margin over the last 120 kyr: Surface and subsurface paleoceanography.\"\nPaleoceanography 17(4).\nOschlies, A., P. Brandt, L. Stramma and S. Schmidtko (2018). \"Drivers and mechanisms of ocean\ndeoxygenation.\" Nature Geoscience: 1-7.\nOschlies, A., O. Duteil, J. Getzlaff, W. Koeve, A. Landolfi and S. Schmidtko (2017). \"Patterns of\ndeoxygenation: Sensitivity to natural and anthropogenic drivers.\" Philosophical Transactions of the\nRoyal Society A: Mathematical, Physical and Engineering Sciences 375(2102).\nPichevin, L., R. S. Ganeshram, B. C. Reynolds, F. Prahl, T. F. Pedersen, R. Thunell and E. L. McClymont\n(2012). \"Silicic acid biogeochemistry in the Gulf of California: Insights from sedimentary Si isotopes.\"\nPaleoceanography 27.\nPride, C., R. Thunell, D. Sigman, L. Keigwin, M. Altabet and E. Tappa (1999). \"Nitrogen isotopic\nvariations in the Gulf of California since the last deglaciation: Response to global climate change.\"\nPaleoceanography 14(3): 397-409.\nSun, C., F. Kucharski, J. Li, F.-F. Jin, I.-S. Kang and R. Ding (2017). \"Western tropical Pacific multidecadal\nvariability forced by the Atlantic multidecadal oscillation.\" Nature Communications 8(1): 15998.\nThunell, R. C. (1998). \"Seasonal and annual variability in particle fluxes in the Gulf of California: A\nresponse to climate forcing.\" Deep Sea Research I 45: 2059-2083.\nZhang, L. and C. Wang (2013). \"Multidecadal North Atlantic sea surface temperature and Atlantic\nmeridional overturning circulation variability in CMIP5 historical simulations.\" Journal of Geophysical\nResearch: Oceans 118(10): 5772-5791.\nZhang, Q., E. Berntell, J. Axelsson, J. Chen, Z. Han, W. de Nooijer, Z. Lu, Q. Li, Q. Zhang, K. Wyser and\nS. Yang (2021). \"Simulating the mid-Holocene, last interglacial and mid-Pliocene climate with EC-\nEarth3-LR.\" Geosci. Model Dev. 14(2): 1147-1169.\nBollasina, M.A., Messori, G. On the link between the subseasonal evolution of the North Atlantic\nOscillation and East Asian climate. Clim Dyn 51, 3537–3557 (2018). https://doi.org/10.1007/s00382-\n018-4095-5\nMcCabe GJ, Palecki MA, Betancourt JL (2004) Pacific and Atlantic Ocean influences on multidecadal\ndrought frequency in the United States. Proc Natl Acad Sci 101:4136–4141."}, {"round": 2, "reviewer_comments": {"reviewer_1": "REVIEWER COMMENTS\nReviewer #1 (Remarks to the Author):\nIn this version of the reviewed manuscript titled “North Atlantic Temperatures Control\nDeoxygenation in the Northern Tropical Pacific”, it is evident that Pichevin and coauthors have done\na significant amount of work addressing most of the suggestions. They responded clearly and\nconcisely to each comment, expressing their viewpoint when a suggested change was not\nimplemented. Significant effort has been made to enhance the manuscript's wording, grammar,\nand overall tone, greatly improving readability. Furthermore, the figures are now clearer and visually\nengaging. The manuscript has undergone substantial improvement, and I would be pleased to see\nit published. Here, I specifically highlight suggestions that were addressed or not addressed in the\ntext.\nI. GENERAL COMMENTS:\n- The authors added a more detailed description of how the age model was constructed in the\nSupplement. Moreover, they explained in detail how they minimized issues with potential\ndisturbances and bioturbation of the core records.\n- I agree with the second reviewer that climate oscillations are not necessarily directly driving\noxygen changes in each record, which is especially shown in recent high-resolution records of the\nlast millennia in the ETNP. However, as clarified by the authors in the response to the reviewer, there\nare local variabilities in productivity that are superimposed on the overall deoxygenation trends\ncaused by major climatic fluctuations. This is now clarified around line 41 in the manuscript.\n- The authors neatly removed repetitive and confusing words, as well as spent time rewriting\nparagraphs in order to improve the clarity of the text, as suggested by the reviewers.\n- I do not personally find the research as speculative, as referred to by some of the reviewers. While\nI might not personally agree with some of the conclusions of the authors, I definitely think their\nfindings were backed up by their analytical and statistical results.\nII. MINOR COMMENTS:\n- Please change “global warming” to “Global warming” in line 369.\n- I really like how Figure 6 looks now. Thank you for making this change!\nI sincerely hope that my comments will be helpful to the authors and wish them good luck. I\nenjoyed reviewing the manuscript and look forward to its publication.", "reviewer_2": "Reviewer #2 (Remarks to the Author):\nNorth Atlantic Temperatures Control on Deoxygenation in the Northern Tropical Pacific\nPichevin et al.\nThe authors improved the wording and presentation of the manuscript. My main comment is on the\nrole of the AMO and PDO in forcing the equatorial circulation. The authors state in their reply that\nthe AMO has a much larger role than PDO in modulating the Pacific Equatorial circulation. However\nat this stage, I find this part very speculative. I already made this comment in my first review. I can\nnot recommend publication either in Nat. Comm or in any scientific journal at this stage. A\nmanuscript demonstrating that AMO has a dominant impact on equatorial dynamics of the Pacific\nOcean compared to PDO (as stated by the authors) would be a major contribution and may change\nthe understanding of the tropical Pacific variability. It therefore needs to be proven, or at least\nthoroughly discussed using strong quantitative arguments.\nThe authors provide a reply on that specific point that I comment below :\n\"(1) the Trade winds modulate the Pacific subsurface circulation around the tropics\"\n> I agree. See for instance : England, M., McGregor, S., Spence, P. et al. Recent intensification of\nwind-driven circulation in the Pacific and the ongoing warming hiatus. Nature Clim Change 4, 222–\n227 (2014). https://doi.org/10.1038/nclimate2106\nThey discuss the potential role of IPO in modulating the wind stress trend: see in particular the Fig\n1. Would this figure make sense by using the AMO ?\n\"(2) the PDO also has an impact on the strength of the equatorial circulation (figure 4, C), albeit to a\nlesser extent than the AMO\"\n> Yes, the PDO has a large role on the strength of the equatorial circulation. It has been shown by\nmany studies and it is a classical view, which is likely the reason why many studies discuss PDO\nand oxygen levels in the Pacific Ocean (Deutsch et al., 2014; Ito et al., 2019; Duteil et al., 2018;\nPoupon et al., 2023)\n\"But contrary to these other studies, we also look at the impact of the AMO phases on the\nEquatorial Circulation in SODA reanalyses and find that the AMO has a far greater impact on the\nstrength of the zonal supply of oxygenated water eastward via the EUC than the PDO (please see\nFigure 1 of the response below).\"\n> In this figure the authors show the gradient of the maximum zonal velocity between the western\npart and the eastern part of the Equatorial Pacific (Δu). While this figure is interesting (it seems\nindeed that there is a correlation), it is not clear to me what would be the net impact. How do you\ncompute Δu ? Is it the maximal velocity between 170W-100W minus maximal velocity between\n140E-170W ? In this case, it does not take in account the whole EUC but is mostly a comparison of\nthe EUC in the central Pacific.\nIndeed, the velocity of the EUC presents an inflexion in the central Pacific (e.g Karnauskas et al.,\n2020 – their Figure 1). However, what matters for the OMZs is likely the termination of the EUC.\nKarnauskas, K. B., Jakoboski, J., Johnston, T. M. S., Owens, W. B., Rudnick, D. L., & Todd, R. E.\n(2020). The Pacific Equatorial Undercurrent in three generations of global climate models and glider\nobservations. Journal of Geophysical Research: Oceans, 125, e2020JC016609.\nhttps://doi.org/10.1029/2020JC016609\nIs the total transport of oxygen by the EUC increasing (In Figure 5a, you show the max velocity\nbetween 1950 and 2010 : is the max annual velocity the most relevant parameter ? Why not the\nmean ? Can you estimate the oxygen advection ? the geometry of the EUC may play a large role in\noxygen supply in complement to velocity see Busecke et al., 2019.\nBusecke, J. J. M., Resplandy, L., & Dunne, J. P. P. (2019). The Equatorial Undercurrent and the oxygen\nminimum zone in the Pacific. Geophysical Research Letters, 46, 6716–6725.\nhttps://doi.org/10.1029/2019GL082692\nFigure 5B, 6: Drenkard and Karnauskas, 2014 show a linear increase of the max EUC by c.a 16 % /\ncentury using the same SODA dataset that is used here, while you show the opposite result in\nFigure 6 (decrease from 1880 to 2010) which is very surprising. Can you elaborate on that ?\n\"Similarly, Hong et al (2014) focus on the southward meridional subsurface circulation and, in\nparticular, its effect on tropical upwelling and SSTs. In our manuscript, we do not contest the link\nbetween PDO and SSTs in the ETNP (see next response below).\"\n> I definitely agree with Fig 4A and 4B. It shows that PDO governs SST in the eastern tropical Pacific\nOcean (dashed areas). I do not understand how it relates to Fig 4C and 4D. A strong EUC is related\nto strong STCs and upwelling of cold water. If the EUC velocity does not change in PDO+/PDO-,\nit implies that the STCs do not change and that the SST change are not related to ocean dynamics,\nbut air / sea exchange. Is it consistent with literature ?\n\"We argue that it has little impact on subsurface oxygen in the ETNP because of the competing\neffects of invigorated mixing and biological productivity on oxygen. We now discuss this more\nclearly from line 366 of the revised manuscript\"\n> I appreciate the discussion. It could be the case that invigorated mixing and biological\nproductivity compensate during a PDO+/PDO- event. However why the same mechanisms would\nnot be at play during an AMO+/AMO- event ?\nL385 : “Our data reanalyses show that while the impact of the PDO multidecadal oscillation on the\nlarge-scale zonal subsurface circulation in the equatorial and tropical region appear to beis\nambiguous and/or small (Figures 4 and 5).”\n> In my understanding of the literature, the Pacific Decadal Oscillation has a strong impact on the\nlarge scale equatorial circulation. In complement, the sediment core data show a strong variability\nat PDO time scale too (spectral analyse - Figure 3). Which mechanisms can explain this variability if\nthe change of circulation due to PDO are weak / compensate ?", "reviewer_3": "Reviewer #3 (Remarks to the Author):\n2nd Review of “North Atlantic temperature control on deoxygenation in the Northern Tropical\nPacific ” by Pichevin et al.\nThe authors have done their best to address all the issues raised by the reviewer. Nevertheless, I am\nstill a bit unclear about how the lead-lagged relationship of PDO-AMO is related to the main results\npresented in this study. The reviewer asked to examine the lead-lagged relationship of AMO-PDO\nbased on the datasets mainly used by the authors. The authors have reconstructed several\nannually resolved records, each spanning about 200 years, so they are able to examine this\nrelationship and use it to interpret their results. In this way, the authors can give a more concrete\ninterpretation to their idea that North Atlantic temperatures control deoxygenation in the northern\ntropical Pacific."}, "author_response": "We thank the Reviewers for their remarks to which we respond in detail below each\ncomment (our response is in blue).\nREVIEWER COMMENTS\nReviewer #1 (Remarks to the Author):\nIn this version of the reviewed manuscript titled “North Atlantic Temperatures Control\nDeoxygenation in the Northern Tropical Pacific”, it is evident that Pichevin and coauthors\nhave done a significant amount of work addressing most of the suggestions. They\nresponded clearly and concisely to each comment, expressing their viewpoint when a\nsuggested change was not implemented. Significant effort has been made to enhance the\nmanuscript's wording, grammar, and overall tone, greatly improving readability. Furthermore,\nthe figures are now clearer and visually engaging. The manuscript has undergone\nsubstantial improvement, and I would be pleased to see it published. Here, I specifically\nhighlight suggestions that were addressed or not addressed in the text.\nI. GENERAL COMMENTS:\n- The authors added a more detailed description of how the age model was constructed in\nthe Supplement. Moreover, they explained in detail how they minimized issues with potential\ndisturbances and bioturbation of the core records.\n- I agree with the second reviewer that climate oscillations are not necessarily directly driving\noxygen changes in each record, which is especially shown in recent high-resolution records\nof the last millennia in the ETNP. However, as clarified by the authors in the response to the\nreviewer, there are local variabilities in productivity that are superimposed on the overall\ndeoxygenation trends caused by major climatic fluctuations. This is now clarified around line\n41 in the manuscript.\n- The authors neatly removed repetitive and confusing words, as well as spent time rewriting\nparagraphs in order to improve the clarity of the text, as suggested by the reviewers.\n- I do not personally find the research as speculative, as referred to by some of the\nreviewers. While I might not personally agree with some of the conclusions of the authors, I\ndefinitely think their findings were backed up by their analytical and statistical results.\nII. MINOR COMMENTS:\n- Please change “global warming” to “Global warming” in line 369. We thank Reviewer 1 for\ntheir positive assessment of the reviewed manuscript. We have now capitalised Global\nWarming in the text.\n- I really like how Figure 6 looks now. Thank you for making this change!\nI sincerely hope that my comments will be helpful to the authors and wish them good luck. I\nenjoyed reviewing the manuscript and look forward to its publication.\nReviewer #2 (Remarks to the Author):\nNorth Atlantic Temperatures Control on Deoxygenation in the Northern Tropical Pacific\nPichevin et al.\nThe authors improved the wording and presentation of the manuscript. My main comment is\non the role of the AMO and PDO in forcing the equatorial circulation. The authors state in\ntheir reply that the AMO has a much larger role than PDO in modulating the Pacific\nEquatorial circulation. However at this stage, I find this part very speculative. I already made\nthis comment in my first review. I can not recommend publication either in Nat. Comm or in\nany scientific journal at this stage. A manuscript demonstrating that AMO has a dominant\nimpact on equatorial dynamics of the Pacific Ocean compared to PDO (as stated by the\nauthors) would be a major contribution and may change the understanding of the tropical\nPacific variability. It therefore needs to be proven, or at least thoroughly discussed using\nstrong quantitative arguments.\nWe would like to first give a general response to Reviewer 2’s comments and we will then go\ninto the details of each point raised (numbered by us for clarity) below.\nWe agree with the Reviewer that tracking the changes in the EUC transport (mean velocity\nintegrated over depth and latitude/width) is pertinent and we now do this as well as looking at\nthe maximum velocity and Δu. The mass transport data (SODA) for the EUC agree well with\nthe max velocity data plotted in the manuscript. Further time series analyses on the EUC\ntransport confirms that its variability is in phase with the AMO rather than the PDO between\n1880 and 2012 (see detailed response to point 5 below).\nReviewer 2 seems to support their views on the variability of the oxygen budget in the ETNP\nOMZ mainly on modelling work (Deutsch et al., 2014; Ito et al., 2019; Duteil et al., 2018).\nWhile models are very valuable in helping us understand dominant mechanisms driving\nobserved climatic or geochemical signals, they have so far been notoriously inaccurate in\nrepresenting variations in oxygen distribution in the Ocean (Schmidt, Getzlaff et al. 2021).\nFor instance, current models still place the main Indian Ocean oxygen minimum zone (OMZ)\nin the Bay of Bengal rather than in the Arabian Sea (Schmidt, Getzlaff et al. 2021). Similarly,\nmodelling studies including those cited by the Reviewer (Oschlies, Duteil et al. 2017, Duteil,\nOschlies et al. 2018) find that simulations tend to place the Pacific OMZ at the equator\n(where there is no OMZ) and contract it further North due to an inaccurate representation of\nthe oceanic currents (Bopp, Resplandy et al. 2013). Moreover, the models cited by Reviewer\n2 below (point 2) test specifically the impact of the PDO on circulation and oxygen variability\nin the ETNP as an “a priory”. None of these modelling studies explore the impact of the\nAMO.\nIn our study, we observe the dominant occurrence of AMO-type variability based on 10\nsedimentary data records and use reanalysis data to propose a plausible mechanism as to\nhow the AMO could control oxygen distribution in the ETNP. Both our sedimentary and\nreanalysis data show that the AMO variability is strongly present in deoxygenation records\nand can explain oxygen changes via its impact on the EUC transport overall and in particular\nthe velocity gradient between the western and eastern parts of the basin. Given such a\nstrong body of evidence from Palaeo and modern data we argue that AMO has a larger role\nthan previously thought.\nThe authors provide a reply on that specific point that I comment below :\n1- \"(1) the Trade winds modulate the Pacific subsurface circulation around the tropics\"\n> I agree. See for instance : England, M., McGregor, S., Spence, P. et al. Recent intensification of\nwind-driven circulation in the Pacific and the ongoing warming hiatus. Nature Clim Change 4, 222–\n227 (2014). https://doi.org/10.1038/nclimate2106\nThey discuss the potential role of IPO in modulating the wind stress trend: see in particular\nthe Fig 1. Would this figure make sense by using the AMO ?\nThe relationship between atmospheric circulation and the strength/transport of the EUC is\nnot as straightforward as the Reviewer implies here. That is why, rather than plot average\nwind stress, we plot and interpret ocean circulation data in our paper. Once again, we do not\ndeny that the IPO/PDO has an impact on the equatorial circulation (atmosphere and upper\nocean). Our SODA reanalysis in figure 4 indeed shows a positive relationship between\nPDO+ and EUC strength in the East. However, the relationship between EUC strength and\nthe AMO is much stronger in our reanalysis, which we point out in the manuscript l.254.\n2- \"(2) the PDO also has an impact on the strength of the equatorial circulation (figure 4, C),\nalbeit to a lesser extent than the AMO\"\n> Yes, the PDO has a large role on the strength of the equatorial circulation. It has been\nshown by many studies and it is a classical view, which is likely the reason why many\nstudies discuss PDO and oxygen levels in the Pacific Ocean (Deutsch et al., 2014; Ito et al.,\n2019; Duteil et al., 2018; Poupon et al., 2023).\nFirstly, we do not disagree that the PDO impacts the tropical Pacific circulation. We say so in\nthe paper as quoted here by the reviewer. The studies the Reviewer cites above are based\non modelling work and hindcasts of the last 70 years (during which both PDO and AMO\nchanges signs -see Figure 1 of the manuscript). As discussed in the paper (lines 224-227)\nand in our responses to the previous round of reviews, these models do not take into\nconsideration the potential impact of the AMO on the circulation, ascribing all observed\nchanges in circulation and oxygen during this short time period to changes in the PDO\nphase (while the AMO also changes at more or less the same time during this interval, which\nthese studies disregard). Our data reanalyses on much longer records (1880-2012) of the\nEUC max velocity, EUC west-east gradient (supplementary material Fig. 7) and now mass\ntransport (see next response) all show variabilities in phase with AMO periodicities and\nstrong AMO-regressed anomalies.\n3- \"But contrary to these other studies, we also look at the impact of the AMO phases on the\nEquatorial Circulation in SODA reanalyses and find that the AMO has a far greater impact on\nthe strength of the zonal supply of oxygenated water eastward via the EUC than the PDO\n(please see Figure 1 of the response below).\"\n> In this figure the authors show the gradient of the maximum zonal velocity between the\nwestern part and the eastern part of the Equatorial Pacific (Δu). While this figure is\ninteresting (it seems indeed that there is a correlation), it is not clear to me what would be\nthe net impact. How do you compute Δu ? Is it the maximal velocity between 170W-100W\nminus maximal velocity between 140E-170W ? In this case, it does not take in account the\nwhole EUC but is mostly a comparison of the EUC in the central Pacific.\nWe thank the Reviewer for noting that Δu is indeed correlated with the AMO index rather\nthan the PDO which we do discuss in the manuscript (lines 274-276). To further clarify the\ninterpretation of the meaning of Δu changes we would like to stress that the EUC brings\noxygen to the eastern side of the basin from the more oxygenated western side. It\naccelerates eastwards to about 230E (130W), then decelerates sharply, due to friction with\nthe surface current fuelled by the eastern trade winds. In that respect Δu represents the\nefficiency in the transport of oxygenated waters from the western to the easternmost part of\nthe Equatorial Pacific and therefore is a pertinent parameter for oxygen budget in the ETNP.\n4- Indeed, the velocity of the EUC presents an inflexion in the central Pacific (e.g\nKarnauskas et al., 2020 – their Figure 1). However, what matters for the OMZs is likely the\ntermination of the EUC.\nKarnauskas, K. B., Jakoboski, J., Johnston, T. M. S., Owens, W. B., Rudnick, D. L., & Todd,\nR. E. (2020). The Pacific Equatorial Undercurrent in three generations of global climate\nmodels and glider observations. Journal of Geophysical Research: Oceans, 125,\ne2020JC016609. https://doi.org/10.1029/2020JC016609\nWe agree with the reviewer that both the EUC termination and volume transport are\nimportant. The SODA reanalysis plots shown in Figure 4 show the EUC velocity responds\nstrongly to the AMO including in the easternmost part of the basin (EUC termination).\nMoreover, our reanalysis Δu data and max EUC velocity data (Figure 5) encompass the area\nup to 100W (260E) to the east and therefore also account for the termination of the EUC. We\nnow mention EUC increase in volume transport (l. 272-276) as well as the termination in\neastern Pacific while discussing Figure 4 (l.247)\n5- Is the total transport of oxygen by the EUC increasing (In Figure 5a, you show the max\nvelocity between 1950 and 2010 : is the max annual velocity the most relevant parameter ?\nWhy not the mean ? Can you estimate the oxygen advection ? the geometry of the EUC may\nplay a large role in oxygen supply in complement to velocity see Busecke et al., 2019.\nBusecke, J. J. M., Resplandy, L., & Dunne, J. P. P. (2019). The Equatorial Undercurrent and\nthe oxygen minimum zone in the Pacific. Geophysical Research Letters, 46, 6716–\n6725. https://doi.org/10.1029/2019GL082692\nWe agree with Reviewer 2 that the mean EUC transport is a pertinent parameter. We have\nnow estimated the mean volume transport variation over the period 1880-2012. We define\nEUC transport as the mean eastward velocity integrated over ±2.5◦ latitude around the\nequator and between 0–350 m, similarly to (Karnauskas, Johnson et al. 2012, Stellema, Sen\nGupta et al. 2022). The results are plotted below in Sv. The transport derived from the mean\nvelocity shows a similar trend to the EUC max velocity we present in the paper (as\npreviously stated in our response to the first round of reviews, we now show it). We only plot\nthe mass transport rather than the oxygen transport as oxygen is not a conservative\nparameter and is influenced by biogeochemical consumption (oxidant demand). Broadly, the\nEUC transport increases during periods of AMO- by around 40% during the AMO- of 1900-\n1930 and roughly doubles during the AMO- period of 1960-1995 compared to AMO+\nperiods. Periodograms of the mean transport and Δu time series show prominent\nperiodicities corresponding to those of the contemporaneous AMO index record (see figure\nbelow) rather than the that of the PDO. We now succinctly discuss this finding of this result\nin the manuscript (l. 272-276) and include the figure below in the supplementary material 4\n(Figure S8).\nFigure 1: (top) EUC transport (Sv) and max velocity (m/s) between 1880 and 2012. Note the\naxes are reversed. (bottom) Periodograms of the EUC transport and Δu compared to the\nperiodograms of the PDO and AMO indices during the same time period show that the\ncharacteristic od the EUC vary strongly with the AMO rather than the PDO.\n6- Figure 5B, 6: Drenkard and Karnauskas, 2014 show a linear increase of the max EUC by\nc.a 16 % / century using the same SODA dataset that is used here, while you show the\nopposite result in Figure 6 (decrease from 1880 to 2010) which is very surprising. Can you\nelaborate on that ?\nWe think the reviewer did not note that the scale is reversed in Figure 6 (please see the\narrow added to point this out to the reader). The EUC velocity increases in agreement with\nDrenkard et al. (2014)\n7- \"Similarly, Hong et al (2014) focus on the southward meridional subsurface circulation\nand, in particular, its effect on tropical upwelling and SSTs. In our manuscript, we do not\ncontest the link between PDO and SSTs in the ETNP (see next response below).\"\n> I definitely agree with Fig 4A and 4B. It shows that PDO governs SST in the eastern\ntropical Pacific Ocean (dashed areas). I do not understand how it relates to Fig 4C and 4D. A\nstrong EUC is related to strong STCs and upwelling of cold water. If the EUC velocity does\nnot change in PDO+/PDO-, it implies that the STCs do not change and that the SST change\nare not related to ocean dynamics, but air / sea exchange. Is it consistent with literature ?\nThe Reviewer comment seems to be selective and ignore the AMO regressed patterns but\nonly point out to PDO patterns in Figure 4. Figure 4 A also shows that the AMO governs SST\nanomalies in the western north and tropical Pacific as pointed out by (Sun, Kucharski et al.\n2017). While we do not deny the role of PDO as pointed out by the Reviewer, our\ninterpretation is more nuanced in the manuscript, and we mention both climate cycles impact\non the SST patterns for instance in lines 236-243. Moreover, we would like to point out that\nthe dynamics that govern the Equatorial upwelling (involving the EUC) and the Boundary\ncurrent upwelling further north in the ETNP are not the same.\nThe PDO impact on tropical Pacific temperatures is partly governed by extra-tropical\natmospheric forcing (via intensified/weak Aleutian low) involving wind-evaporation-SST\nfeedbacks (Wills, Battisti et al. 2019, Dow, McKenna et al. 2023). Some model experiments\nshow that wind-stress curls anomalies related to the PDO phases have (we quote) “little\nimpact on the equatorial thermocline as positive and negative anomalies of comparable\nmagnitude drive opposite Sverdrup transport” (Sun and Okumura 2019). Therefore our\ninterpretations are consistent with literature, including modelling works.\n8- \"We argue that it has little impact on subsurface oxygen in the ETNP because of the\ncompeting effects of invigorated mixing and biological productivity on oxygen. We now\ndiscuss this more clearly from line 366 of the revised manuscript\"\n> I appreciate the discussion. It could be the case that invigorated mixing and biological\nproductivity compensate during a PDO+/PDO- event. However why the same mechanisms\nwould not be at play during an AMO+/AMO- event ?\nWe discuss this difference in the manuscript (lines 282-291 and 311-314) where we propose\nthat the effects of the AMO on the equatorial circulation and upwelling have an additive\nimpact on the oxygen while the PDO-related changes in the eastern tropical boundary\ncurrent upwelling (increased diapycnal mixing+productivity or stratification+less productivity)\nhave partly offsetting effects of oxygen.\n9- L385 : “Our data reanalyses show that while the impact of the PDO multidecadal\noscillation on the large-scale zonal subsurface circulation in the equatorial and tropical\nregion appear to beis ambiguous and/or small (Figures 4 and 5).”\n> In my understanding of the literature, the Pacific Decadal Oscillation has a strong impact\non the large scale equatorial circulation. In complement, the sediment core data show a\nstrong variability at PDO time scale too (spectral analyse - Figure 3). Which mechanisms\ncan explain this variability if the change of circulation due to PDO are weak / compensate ?\nOnce again, we do not argue that PDO has no impact on circulation and deoxygenation, and\nit is correct that our paleo-records of deoxygenation also show PDO-like variability. The PDO\ncan impact oxygen by strengthening Northern Pacific warming which would decrease the\nsubduction of oxygen from the North or by controlling stratification in the Eastern\nNorth/Tropical Pacific. While we argue that some of these PDO-related mechanisms can\nnegate each other, we stress in the concluding paragraph of the manuscript that the PDO\ncan indeed amplify the effect of the AMO on deoxygenation (from line 366).\nReviewer #3 (Remarks to the Author):\n2nd Review of “North Atlantic temperature control on deoxygenation in the Northern\nTropical Pacific ” by Pichevin et al.\nThe authors have done their best to address all the issues raised by the reviewer.\nNevertheless, I am still a bit unclear about how the lead-lagged relationship of PDO-\nAMO is related to the main results presented in this study. The reviewer asked to\nexamine the lead-lagged relationship of AMO-PDO based on the datasets mainly used\nby the authors. The authors have reconstructed several annually resolved records, each\nspanning about 200 years, so they are able to examine this\nrelationship and use it to interpret their results. In this way, the authors can give a more concrete\ninterpretation to their idea that North Atlantic temperatures control deoxygenation in the northern\ntropical Pacific.\nWe now articulate the interactions between AMO- and PDO-related mechanisms on\ndeoxygenation in the Tropical Pacific in several places in the manuscript (lines 254-259,\nlines 364-380) and describe how they can have additive impacts when AMO and PDO are in\nopposite phases.\nExamining the detailed relationship between the PDO and AMO is beyond the scope of this\npaper and has been done elsewhere, for instance in (Johnson, Chikamoto et al. 2020) where\nthe authors found that Atlantic temperatures are responsible for over 12% of the PDO\nvariance and lead the PDO over the last 70 years of historical records. As requested, we\nhave examined the lead-lag relationship between the PDO- and AMO-like periodicities in the\n10 sedimentary records by extracting the PDO and AMO signals from the deoxygenation\nrecords and calculating cross correlation plots. We found no consistent lead-lag relationship\nbetween AMO and PDO across the various records. However, it should be noted that\noverlapping oscillations at different frequencies (here PDO and AMO) expressed in\ncontinuous records are not expected to show a consistent lead-lag relationship throughout\nthese records. This is why we have not used this approach. This approach (lead-lag\nrelationships between climatic variabilities) is more suitable to understanding what climate\ncycle drives a specific events (for instance the latest phase change in PDO (Johnson,\nChikamoto et al. 2020) or the latest shift in ETNP oxygen. In this latter instance, the AMO\nleads.\nReferences\nBopp, L., L. Resplandy, J. C. Orr, S. C. Doney, J. P. Dunne, M. Gehlen, P. Halloran, C.\nHeinze, T. Ilyina, R. Séférian, J. Tjiputra and M. Vichi (2013). \"Multiple stressors of ocean\necosystems in the 21st century: projections with CMIP5 models.\" Biogeosciences 10(10):\n6225-6245.\nDow, W. J., C. M. McKenna, M. M. Joshi, A. T. Blaker, R. Rigby and A. C. Maycock (2023).\n\"Intensified Aleutian Low induces weak Pacific Decadal Variability.\" EGUsphere 2023: 1-25.\nDuteil, O., A. Oschlies and C. W. Böning (2018). \"Pacific Decadal Oscillation and recent\noxygen decline in the eastern tropical Pacific Ocean.\" Biogeosciences 15(23): 7111-7126.\nJohnson, Z., Y. Chikamoto, S.-Y. Wang, M. McPhaden and T. Mochizuki (2020). \"Pacific\nDecadal Oscillation remotely forced by the equatorial Pacific and the Atlantic Oceans.\"\nClimate Dynamics 55.\nKarnauskas, K. B., G. C. Johnson and R. Murtugudde (2012). \"An Equatorial Ocean\nBottleneck in Global Climate Models.\" Journal of Climate 25(1): 343-349.\nOschlies, A., O. Duteil, J. Getzlaff, W. Koeve, A. Landolfi and S. Schmidtko (2017). \"Patterns\nof deoxygenation: Sensitivity to natural and anthropogenic drivers.\" Philosophical\nTransactions of the Royal Society A: Mathematical, Physical and Engineering Sciences\n375(2102).\nSchmidt, H., J. Getzlaff, U. Löptien and A. Oschlies (2021). \"Causes of uncertainties in the\nrepresentation of the Arabian Sea oxygen minimum zone in CMIP5 models.\" Ocean Sci.\n17(5): 1303-1320.\nStellema, A., A. Sen Gupta, A. Taschetto and M. Feng (2022). \"Pacific Equatorial\nUndercurrent: Mean state, sources, and future changes across models.\" Frontiers in Climate\n4.\nSun, C., F. Kucharski, J. Li, F.-F. Jin, I.-S. Kang and R. Ding (2017). \"Western tropical Pacific\nmultidecadal variability forced by the Atlantic multidecadal oscillation.\" Nature\nCommunications 8(1): 15998.\nSun, T. and Y. M. Okumura (2019). \"Role of Stochastic Atmospheric Forcing from the South\nand North Pacific in Tropical Pacific Decadal Variability.\" Journal of Climate 32(13): 4013-\n4038.\nWills, R. C. J., D. S. Battisti, C. Proistosescu, L. Thompson, D. L. Hartmann and K. C.\nArmour (2019). \"Ocean Circulation Signatures of North Pacific Decadal Variability.\"\nGeophysical Research Letters 46(3): 1690-1701."}, {"round": 3, "reviewer_comments": {"reviewer_2": "Reviewer #2 (Remarks to the Author):\nMany thanks for your clear reply and balanced view regarding the role of PDO and AMO, with which I\nagree. It has been shown previously in many studies that PDO has an impact on the Pacific Ocean\ncirculation. However these studies did not consider AMO. You show that the AMO impact may be\neven larger, which is indeed possible.\nHowever, in the text you clearly state that the PDO has a marginal effect on circulation based on\nFigure 4.\nMore specifically :\n> L249-251 : “Subsurface circulation in the equatorial zone of the Pacific regressed on a positive\nPDO phase does not display a clear trend as shown by arrows pointing both westward and\neastward and with velocity anomalies close to noise”\n> L253 : “PDO seems to have an overall marginal impact on the EUC velocity (Figure 4D)”\nRegarding PDO and Figure 4 : The EUC is only 1-2 degree broad, so I see some consistency, at least\nin the middle of the Pacific (160W-140W): slow down of the eastward EUC (arrow pointing\nwestward) and the westward SEC branches (arrow pointing eastward). You mention “velocity\nanomalies close to noise” : are velocity anomalies close to noise statistically or is it a subjective\nanalysis ?\nIn complement, could you please detail the methodology and in particular which low-pass\n(temporal frequency) has been used (legend of Figure 4D) ? Why do you have a lot of white space on\nFig 4C,D (is it related to significance) ?\nI find the Figures 4C, 4D difficult to read. Could you please redo the Figure 4C, 4D (supplementary\nmaterial or main text as you wish) focusing on the tropical region (10°N – 10°S, 0.5° degree\nresolution : in Figure 4C, 4D resolution is currently 2° only) using only zonal velocities and a colorbar\nsimilar to Fig 4A ? The mean zonal velocity field should be included as contour. I think it will be\nmuch clearer.\nI will support publication provided that the authors implement my comments.", "reviewer_3": "3rd Review of “North Atlantic temperature control on deoxygenation in the Northern Tropical Pacific\n” by Pichevin et al.\nMajor comments\nThe author mentioned that they did not find consistent lead-lag relationship between AMO and\nPDO across the various records. I understand this result, however, without such a solid physical\nexplanation, the notion suggested in the current manuscript is a bit speculative and heavily\ndepended on their time series analyses. I strongly recommend to add a schematic diagram to\nclearly display how North Atlantic temperature controls deoxygenation in the Northern Tropical\nPacific with the detailed explanation."}, "author_response": "We are pleased to submit the revised version of our manuscript entitled “North Atlantic\nTemperature Control on Deoxygenation in the Northern Tropical Pacific” (NCOMMS-23-\n62151). Alongside this response, we submit the revised manuscript and supplementary\nmaterial files with (TRACK REVISED) and without (REVISED) highlighted changes for\nclarity. We respond to individual comments in blue.\nWe are thankful to Reviewer 2 for their positive assessment and for supporting publication of\nour manuscript upon revision. We have now amended the manuscript in accordance with the\nreviewer’s suggestion (lines 250-254) in order to clarify the relative impacts of PDO and\nAMO on the equatorial circulation. We have also amended Figure 4 following the reviewer’s\nadvice. We agree with Reviewer 2 that the new suggested figure presents the SODA results\nmore clearly.\nWe agree with Reviewer 3’s suggestion to add an explanatory figure (New Figure 6) at the\nend of the manuscript that summarises how the AMO can impact the Pacific oxygen budget\nvia Equatorial Pacific shallow overturning and North Pacific stratification. These mechanisms\nare supported by both modern SST and current velocity data (SODA) and by existing\nobservations (Keeling, Kortzinger et al. 2010, Schmidtko, Stramma et al. 2017, Sun,\nKucharski et al. 2017). Therefore, we disagree with Reviewer 3 that our finding that the AMO\ndrives Pacific oxygen changes is speculative and we re-iterate in our detailed response\nbelow why we believe our results and interpretations are supported by observational\nevidence.\nWe respond to individual comments in blue below.\nREVIEWER COMMENTS\nReviewer #2 (Remarks to the Author):\nMany thanks for your clear reply and balanced view regarding the role of PDO and AMO,\nwith which I agree. It has been shown previously in many studies that PDO has an impact on\nthe Pacific Ocean circulation. However, these studies did not consider AMO. You show that\nthe AMO impact may be even larger, which is indeed possible.\nHowever, in the text you clearly state that the PDO has a marginal effect on circulation\nbased on Figure 4.\nMore specifically :\n> L249-251 : “Subsurface circulation in the equatorial zone of the Pacific regressed on a\npositive PDO phase does not display a clear trend as shown by arrows pointing both\nwestward and eastward and with velocity anomalies close to noise”\n> L253 : “PDO seems to have an overall marginal impact on the EUC velocity (Figure 4D)”\nWe thank Reviewer 2 for this positive comment and are glad that they now agree with our\ninterpretations. We have rephrased the two statements quoted above by the Reviewer in\norder to reflect the relative role of the PDO and AMO (lines 250-254)\nRegarding PDO and Figure 4 : The EUC is only 1-2 degree broad, so I see some\nconsistency, at least in the middle of the Pacific (160W-140W): slow down of the eastward\nEUC (arrow pointing westward) and the westward SEC branches (arrow pointing eastward).\nYou mention “velocity anomalies close to noise” : are velocity anomalies close to noise\nstatistically or is it a subjective analysis ?\nWe thank the Reviewer for drawing our attention to the use of the word “noise” in the text.\nWe have now removed the mention of “noise” and modified the text to reflect that the\nvelocity anomalies associated with the PDO were small, in the order of 0.05 m/s or below,\nwhich is one order of magnitude smaller than the velocity anomalies associated with the\nAMO. This is confirmed in the new figure 4 C and D reproduced below (note the difference in\nvelocity scales between PDO- and AMO-regressed plots).\nIn complement, could you please detail the methodology and in particular which low-pass\n(temporal frequency) has been used (legend of Figure 4D) ? Why do you have a lot of white\nspace on Fig 4C,D (is it related to significance) ?\nIn the caption of Figure 1 we state: “The analysis focuses on decadal or longer scale\nvariability, isolated by applying a low-pass Lanczos filter with a cut-off frequency of 10 years\nto the detrended data.”\nThe reason for the presence of white areas in Fig 4C and D (both old and new versions) is\nthat values/colours are displayed only when the magnitude of the current exceeds 0.05 m/s\nfor the AMO and 0.01 m/s for the PDO.\nI find the Figures 4C, 4D difficult to read. Could you please redo the Figure 4C, 4D\n(supplementary material or main text as you wish) focusing on the tropical region (10°N –\n10°S, 0.5° degree resolution: in Figure 4C, 4D resolution is currently 2° only) using only\nzonal velocities and a colorbar similar to Fig 4A ? The mean zonal velocity field should be\nincluded as contour. I think it will be much clearer.\nWe thank the reviewer for this suggestion and we have now modified Figure 4 (C and D)\naccordingly (see new panels below, note the order of magnitude difference in velocity range\nbetween PDO and AMO anomalies). The old Figures 4C and 4D showing arrows as\nopposed to colours are now in the supplementary material (Figure S9).\nFigure: Blue-red colours denote velocity anomalies of observed low-pass filtered annual-\nmean ocean currents (m s-1) at 96 m below the surface regressed on the (C) positive AMO\nand (D) positive PDO indices shown in Figure 1 during 1950-2008. The contours denote the\nmean climatological current velocity (m s-1) at the same depth. All data were linearly\ndetrended prior to the analysis. Oceanic current data are from the Simple Ocean Data\nAssimilation (SODA) dataset version v2p2p4 at 0.5° × 0.5° resolution. I will support\npublication provided that the authors implement my comments.\nWe are grateful to Reviewer 2 for this positive feedback.\nReviewer #3 (Remarks to the Author):\n3rd Review of “North Atlantic temperature control on deoxygenation in the Northern Tropical\nPacific ” by Pichevin et al.\nMajor comments\nThe author mentioned that they did not find consistent lead-lag relationship between AMO\nand PDO across the various records. I understand this result, however, without such a solid\nphysical explanation, the notion suggested in the current manuscript is a bit speculative and\nheavily depended on their time series analyses.\nWe are surprised by this comment by Reviewer 3. While they state that they understand the\ninconsistency of the PDO-AMO lead-lag relationship, they still contend that without such\n“solid physical explanation” our interpretations are speculative. This seems contradictory. As\nstated in our response to the previous round of reviews, the aim of our study was not to\nexamine the relationship between PDO and AMO in the Pacific basin. This is beyond the\nscope of the paper and has been done elsewhere, and indicates that the AMO can indeed\nforce Equatorial Pacific and PDO variability (Johnson, Chikamoto et al. 2020).\nThe aim of our study was to show that the AMO is an important driver of multi-decadal\noxygen variability in the Pacific Ocean. We disagree with the reviewer that this finding is\nspeculative. First, our result that AMO variability is present in eastern tropical Pacific\noxygenation records stems from an unprecedented amount of palaeoceanographic data at\nannual resolution (10 century-long sedimentary records). Second, modern SST and current\nvelocity data (SODA) show a strong influence of AMO on the variability of the Pacific shallow\noverturning (see figures 4, 5, 6 and 7) as well as on North Pacific stratification. Both\nprocesses directly impact the oxygen supply to the Eastern tropical Pacific. This can not be\ndismissed as speculative and indicates that AMO variability impacts the Pacific oxygen\nbudget: we see this in the sedimentary denitrification data while modern oceanographic data\nprovide a plausible mechanism via which the AMO can drive oxygen changes. In summary,\nwe would like to stress that contrary to the majority of the recent literature on Pacific oxygen\ninterannual variability our finding that the AMO has an important impact on oxygen in the\nPacific Ocean stems from (a large amount of) data, both paleo-oceanographic records and\nmodern oceanographic reanalysis, rather than models.\nI strongly recommend to add a schematic diagram to clearly display how North Atlantic\ntemperature controls deoxygenation in the Northern Tropical Pacific with the detailed\nexplanation.\nWe thank the reviewer for this suggestion which we believe enhances the paper, and have\nadded an explanatory figure (new Figure 6 of the manuscript) summarising the impact of the\nAMO on sea surface temperature anomalies (SODA reanalysis), surface stratification in the\nNorth West Pacific (Sun, Kucharski et al. 2017), equatorial Pacific oxygen content\n(Schmidtko, Stramma et al. 2017) and shallow equatorial circulation (our SODA reanalysis)\nin the Pacific Ocean. All these processes impact the oxygen budget in the Eastern tropical\nPacific and are supported by SODA reanalysis and documented by existing published data\n(Schmidtko, Stramma et al. 2017, Sun, Kucharski et al. 2017). We reproduce this figure\nbelow.\nFigure: Schematic summary of the impact of (A) positive and (B) negative AMO phases on\nNorth Pacific Sea Surface Temperatures (SSTs, SODA, from Figure 4) and stratification,\nEquatorial biological productivity and oxidant demand and the Equatorial Under Current (EUC)\neastward velocity (SODA). Strong stratification in the North Pacific surface during positive\nAMO (A) results in warmer SSTs and decreased oxygen penetration into the western EUC\n[Schmidtko et al., 2017], decreased stratification in the Equatorial Pacific promotes biological\nproductivity and oxidant demand in the East while reduced EUC eastward transport limits\noxygen supply to the Eastern tropical Pacific resulting in the expansion of the Eastern Pacific\nOxygen Minimum Zone (OMZ). The anomalies are reversed during negative AMO phases (B).\nReferences:\nJohnson, Z., Y. Chikamoto, S.-Y. Wang, M. McPhaden and T. Mochizuki (2020). \"Pacific\nDecadal Oscillation remotely forced by the equatorial Pacific and the Atlantic Oceans.\"\nClimate Dynamics 55.\nKeeling, R. F., A. Kortzinger and N. Gruber (2010). \"Ocean Deoxygenation in a Warming\nWorld.\" Annual Review of Marine Science 2: 199-229.\nSchmidtko, S., L. Stramma and M. Visbeck (2017). \"Decline in global oceanic oxygen\ncontent during the past five decades.\" Nature 542(7641): 335-339.\nSun, C., F. Kucharski, J. Li, F.-F. Jin, I.-S. Kang and R. Ding (2017). \"Western tropical Pacific\nmultidecadal variability forced by the Atlantic multidecadal oscillation.\" Nature\nCommunications 8(1): 15998."}]

Decision letter