Origin and evolution of the North Atlantic Oscillation

Domain: earth_science
Published: 2025-03-03
Editorial outcome: major_revision

Abstract
The North Atlantic Oscillation (NAO), the dominant mode of atmospheric variability in the North Atlantic region, plays a crucial role in weather and climate. Here, to investigate when the NAO emerged and how it evolved over geological timescales, we analyzed time-slice paleoclimate simulations during the breakup of the supercontinent Pangea, starting 160 million years ago (Ma). Our findings indicate that a present-day-like NAO mode gradually formed between 80 Ma and 60 Ma, driven by the expansion of the North Atlantic Ocean and the enhanced land-ocean contrast. This expansion led to a regime transition in Northern Hemisphere winter circulation, characterized by a westward shift of the North Atlantic jet, a strengthening of the North Atlantic high pressure and storm track, and the emergence of NAO-like variability. The confluence of orographic effects of the Rocky Mountains also contributed to the strengthening of the NAO. This study depicts the evolutionary history of the NAO over geological time and reveals its coherent relationship with the evolution of continents and orography.

Introduction
The North Atlantic Oscillation (NAO), characterized by a dipole pressure anomaly over the North Atlantic 1, is the dominant atmospheric variability mode in the Northern Hemisphere (NH) mid-high latitudes during wintertime and has profound impacts on weather and climate. The phases of the NAO modulate the strength and position of the jet, resulting in significant weather anomalies over Europe and the Mediterranean 2–4. The NAO also can impact remote regions such as East Asia 5,6. Additionally, the NAO plays important roles in the climate system, affecting phenomena like the Atlantic Multidecadal Oscillation and Arctic sea ice 7–13.

The NAO dominates atmospheric variability in the North Atlantic sector across various timescales 1,14. The NAO index, based on instrumental station records, dates back to the 19th century 15 and can explain a significant portion of the variability in NH surface temperatures 2. Over the past millennium, the phase shift of the NAO has been linked to the European climate transition from the Medieval Climate Anomaly to the Little Ice Age 16. Proxy reconstructions indicate the existence of the NAO since the early Holocene 17,18. While the NAO is primarily controlled by atmospheric internal dynamics 19, its spatial-temporal variation can be influenced by external forcings such as volcanic eruptions 20, solar cycles 21, and changes in surface boundary conditions 22,23.

Previous studies have shown that surface boundary conditions, particularly the land-ocean contrasts and large-scale mountains, play a crucial role in shaping the NAO 23–25. The land-ocean thermal contrasts and orographic features like the Rocky Mountains (RM) can force atmospheric stationary waves 26,27 and increase baroclinicity and eddy activity in the North Atlantic sector 25,28,29. Idealized simulations suggest that both factors are pivotal in localizing storm tracks and the NAO 23,30. Glaciers are also possible orographic factors; for example, continental ice sheets may have affected the NAO during the Last Glacial Maximum 31.

Over geological timescales, continental distribution underwent drastic changes due to plate tectonics. When did the NAO first emerge in geological timescales? How did the evolution of continental distribution alter the NAO in the past, and what is the key mechanism? These unexplored questions are the focus of this study. Here, with a set of time-slice paleoclimate simulations (Methods), we examine the origin and evolution of the NAO over geological timescales, specifically during the breakup of the supercontinent Pangea and the expansion of the North Atlantic Ocean since 160 million years ago (Ma). By answering the abovementioned questions, this study establishes a coherent relationship between continental configurations and the characteristics of the NAO over geological time.

Results
We first obtain the leading empirical orthogonal function (EOF) of wintertime surface geopotential height anomalies over the North Atlantic sector in both the reanalysis data and paleoclimate simulations (Methods). The EOFs are normalized to ensure the standard deviation of the principal components is one. The leading EOF of the pre-industrial (PI) simulation exhibits a typical NAO pattern, closely matching the results from the reanalysis data (Fig. S1). This alignment, consistent with previous studies 32,33, validates the CESM’s capability in simulating the NAO.

The leading EOFs in the paleoclimate simulations display notable variabilities. Fig. 1a,b show two examples of the 150 Ma and 40 Ma, and Fig. S1 shows all the cases. As time period approaches PI, the spatial patterns of the EOFs increasingly resemble the meridional dipole of the modern NAO (Fig. S1). Before 80 Ma (except 130 Ma and 160 Ma), the midlatitude positive centers of the EOFs are too weak to be called NAO-like. An important feature of the modern NAO is its barotropic structure 1. At 130 Ma and 160 Ma, the leading EOFs at the middle troposphere (Fig. S2) do not show meridional dipole patterns, thus, they failed to pass the criterion of barotropicity. On the other hand, after 80 Ma, the leading EOFs exhibit similar meridional dipoles in the middle troposphere (Fig. S2). Given the lack of a strict threshold for paleoclimate NAO in the literature and the gradual change of the EOFs, we propose that a NAO-like mode emerges between 80 Ma and 60 Ma.

The evolution of the leading EOFs from a non-NAO-like pattern at 160 Ma to the modern NAO may be better demonstrated in a zonally averaged plot (Fig. 1c). Since all the EOFs have strong negative centers in high latitudes, the main difference among the EOFs is whether there is a localized and strong positive center in the midlatitudes (Fig. S1). In Fig. 1c, for each paleoclimate EOF, we first identify the maximum of the midlatitude positive anomalies, then zonally average the EOF over a 20° band to the east and west (e.g., the cyan lines in Fig. 1b). Consistent with Fig. S1, Fig. 1c shows the gradual emergence of the NAO. In the following, we refer to an EOF with larger positive midlatitude anomalies as a stronger NAO to represent its resemblance to the modern NAO.

To obtain a causal explanation for the emergence and evolution of the NAO, we link them to the variations of external parameters of the climate system. The main differences in the external parameters across the simulated time periods are the CO2 concentrations, continental distribution, and orography (i.e., large-scale mountains). The CO2 concentrations largely determine the global mean temperature, while continental distribution and orography break the zonal symmetry of general circulation by introducing background stationary waves 23,25,26,34. To examine the effects of climate, we conducted a set of simulations with fixed CO2 at 10 times the PI level (FixCO2, Methods), which exhibit much weaker climate variation than the default simulations (Fig. S3a). The evolution of the leading EOFs over the North Atlantic in FixCO2 shows a gradual emergence of an NAO-like pattern mode (Fig. S3b), similar to that in Fig. 1c. The cooling of the climate since 40 Ma seems to have greatly enhanced the NAO-like mode, consistent with results from current global warming studies 35. It indicates that climate may modify the characteristics of the NAO 35,36, but is unlikely to form the NAO itself. The impact of climate will be addressed in detail in a separate study. Here, we focus on the effects of continental distribution and orography on the evolution of the NAO under realistic paleoclimate history. In the following two sections, we demonstrate that the expansion of the North Atlantic Ocean was the main driver of the NAO’s origination and that the presence of the RM likely enhanced the NAO substantially.

As an internal variability mode, the NAO is closely related to storm tracks, jet streams, and stationary waves 23,24, whose regional features are heavily influenced by continental configurations. Here, we show that the expansion of the North Atlantic Ocean leads to a regime transition in the NH winter circulation: the dominant region of transient eddy energy dissipation (represented by the storm track) shifts from the North Pacific to the North Atlantic, accompanied by the emergence of an NAO-like variability mode.

We first examine the NH winter stationary waves and their co-evolution with continental drift (Fig. S4). Throughout all periods, the NH exhibits a dominant wavenumber-2 pattern, corresponding to the two major continents. Beginning at 110 Ma, the North Atlantic Ocean expands. The west coast of Eurasia remains near 0° E, while the east coast of North America shifts westward. Prior to the expansion, the North Pacific high pressure is significantly stronger than the North Atlantic high pressure (Fig. S4a–e). As the North Atlantic Ocean expands, the land-ocean contrasts over the North Atlantic sector increase, and the North Atlantic high pressure strengthens and shifts westward (Figs. 2a, S4). At the same time, the North Pacific high pressure weakens relatively. The area ratio of the North Pacific high to the North Atlantic high (Fig. 2b, with examples of the calculation method in Fig. S6) clearly illustrates the shift of high-pressure centers from the Western Hemisphere to the Eastern Hemisphere with the expansion of the North Atlantic Ocean.

The jets and storm tracks undergo a similar transition (Fig. S5). Prior to the expansion of the North Atlantic Ocean (Fig. S5a–e), in line with the weak North Atlantic high pressure, the North Atlantic jet extends eastward to Eurasia, close to the entrance of the Eurasian jet. On the other hand, the strong North Pacific high leads to a wide jet gap over the North Pacific, where transient eddy energy dissipates and a strong North Pacific storm track forms. As time progresses, the westward expansion of the North Atlantic high pressure pushes the North Atlantic jet westward. Consequently, the jet gap over the North Atlantic widens, while it narrows over the North Pacific. This results in the weakening of the North Pacific storm track and strengthening of the North Atlantic storm track (Fig. 2b). A strong North Atlantic storm track corresponds to significant transient eddy energy dissipation and increased jet meandering 23,37,38, thus the emergence of the NAO.

The shift of the North Atlantic jet over time is shown in Fig. 3. It should be noted that the above discussion pertains to midlatitude jets. During 100 Ma to 60 Ma, a subtropical jet develops over the North Pacific and merges with the North Atlantic jet, eliminating a distinct North Atlantic jet core. However, since the subtropical jet is less associated with storm tracks, the argument regarding the westward shift of the North Atlantic jet remains unaffected.

A set of aquaplanet simulations with two rectangular lands (Methods) is conducted to test the proposed mechanism in an idealized world. In these simulations (Fig. S7), the distance between the two lands increases, analogous to the expansion of the North Atlantic Ocean. As examples, the leading EOF shows a NAO-like pattern for an ocean width of 60° (analogous to the PI), but not for 20° width (Fig. 4a,b). Figure 4c shows a summary of the idealized simulations. The NAO-like EOF emerges as the ocean between the two lands becomes wide enough (approximately greater than 40°), consistent with the results of paleoclimate simulations. The aquaplanet simulations shows a regime transition of circulation qualitatively similar with that in the paleoclimate simulation. As the two lands move away from each other, a warm and high-pressure anomaly emerges and gradually strengthens over the expanding ocean basin (Fig. S8). During the same time, a strong storm track develops (Fig. S9), and a NAO-like variability mode emerges (Fig. S7). The consistency between the idealized simulations and the paleoclimate simulations confirms that a sufficiently wide North Atlantic Ocean forms the NAO due to stationary waves induced by the thermal contrasts between the ocean and land.

Large orographic barriers, such as the Rocky Mountains, forms stationary waves by blocking the westerlies and inducing diabatic heating anomalies 26,39,40. Modifying the orography of the RM in numerical simulations can substantially alter stationary wave, jet, and storm track 25,41. The modern RM formed through complex geological processes of mountain building and erosion 42–44. Based on the paleogeography used in our simulations 45 (Fig. S10), the average height of the RM has gradually increased since 100 Ma (method, Fig. S10). To evaluate the effects of the RM on the NAO, we conducted sensitivity experiments with modified orography in both the idealized and realistic setups.

In the idealized simulations, we added a two-dimensional Gaussian-shaped mountain mimicking the RM, with average heights of 412 m, 620 m, 825 m, and 1237 m in the cases of TL0, TL20, and TL60, respectively. The leading EOFs of these simulations are shown in Fig. S11 and summarized in Fig. 4d. With a single continent (TL0), even the tallest mountain does not lead to an NAO-like mode. In TL20, a mountain with a height of 1237 m is able to form an NAO-like mode. In TL60, the NAO-like mode is strengthened when the mountain height exceeds 825 m. The mountain induces an orographic stationary wave that superimposes on the land-ocean-induced stationary wave (Fig. S12). The higher the mountain, the stronger the orographic wave. Another important factor is the phase match between the orographic and land-ocean-contrast stationary waves. In TL0, the two waves are nearly out of phase, while in TL20 and TL60, their phases largely match (Fig. S13). Despite differences in experimental design, our results agree with those of Gerber and Vallis (2009) 23, highlighting the importance of the confluence of orographic and land-ocean-contrast waves in strengthening the NAO.

We also performed a set of high-resolution simulations in which the RM are flattened (NRM, method). Due to the limitation of computational resources, these simulations cover the period from the PI to 100 Ma with an interval of 20 Ma. The simulation results are shown in Fig. S14 and summarized in Fig. 1d. Comparing Fig. 1c and Fig. 1d, the removal of the RM leads to a notable weakening of the NAO-like mode from 40 Ma to PI. The effect of the RM is less significant from 60 Ma to 100 Ma, presumably because the NAO-like mode was weak and the RM were lower and narrower during these times. The orographic waves of RM are shown in Fig. S15. The results of the PI case are very similar to previous studies (e.g., Fig. 9 in Brayshaw et al. 2009 25, Fig. 11 in Jiang and Yang 2011 41). Throughout the examined time periods, the phases of the orographic waves approximately match those of the land-ocean-contrast stationary waves over the North Atlantic sector, indicating that the presence of the RM helps localize and strengthen the NAO.

Compared with the idealized simulations, the results in the realistic setup are much noisier and more complex. Many factors may contribute to the complexity, such as the varying background flow and orographic features of the real RM. Still, the conclusions from the realistic and idealized sensitivity experiments agree that the RM may have strengthened the NAO since 40 Ma by inducing orographic waves that enhance the land-ocean-contrast wave.

Methods
Reanalysis data and definition of the NAO
The reanalysis data from NCEP-2 53 are utilized. These data cover the period from 1979 to 2020 and are gridded at 2.5° × 2.5° resolution with 17 vertical pressure levels. In this study, our focus is on the wintertime (December to February). The NAO is defined 1,50,54 by calculating the leading mode of the EOF for the winter monthly Z1000 (geopotential height at the 1000 hPa level) over the North Atlantic region (20° N to 80° N, 70° W to 40° E for PI). For the paleoclimate period, we maintain the size of the regional box but shift its location so that it always centers on the North Atlantic Ocean. Our results are not sensitive to reasonable perturbations in the choices of the regional box. The modern NAO exhibits a robust equivalent barotropic structure 1. With this in mind, we posed a barotropic requirement for the NAO, stating that both the leading EOFs of Z1000 and Z500 (geopotential height at the 500 hPa level) shall exhibit similar meridional dipole patterns. The EOFs at 160 Ma and 130 Ma do not pass this criterion and are not considered NAO-like modes (marked by the white diagonal in Fig. 1c).

Paleoclimate simulations
Our main results are derived from a series of time-slice paleoclimate simulations using the Community Earth System Model 1.2.2 55 (CESM1.2.2). These time-slice simulations 56 cover the period from 160 Ma, predating the formation of the North Atlantic Ocean, to pre-industrial (PI), with a 10-million-year interval. The PI simulation’s climatology reasonably aligns with observations 56. Key differences between simulations lie in continental configurations and global mean temperature. Global mean surface temperatures for each era are tuned to match proxy reconstructions 57 by setting CO2 concentrations. Solar radiation increases linearly from 1344 W m-2 at 160 Ma to 1361 W m-2 at present. Paleogeographic maps are sourced from the paleo-digital elevation model 45, which provides the continental distribution, bathymetric and topography maps on the simulated time periods. The evolution of the large topographic features, such as the Tibetan Plateau and the Rocky Mountains, and oceanic gateways, such as the Drake Passage and Tasmanian Passage, are included in the paleogeographic maps used in the paleoclimate simulations. The paleogeographic map is available at https://zenodo.org/records/5460860.

Each paleo period includes both low- and high-resolution simulations. The low-resolution simulations consist of atmosphere (CAM4, with a horizontal 3.75° × 3.75°), ocean (POP2, with a horizontal resolution of g37), land (CLM4), sea-ice (CICE4) and river components (RTM), which are linked through a coupler. Since the paleogeographic maps of Scotese and Wright (2018) do not include information of ice sheets, there are no prescribed ice sheets in our simulation except the PI simulation that uses the CESM1.2.2’s default ice sheets. The impact of the lack of ice-sheets shall be small for this study, since the Greenland ice sheet emerges from 2.5 Ma 58. The Antarctic ice-sheet emerges from 40 Ma 59, however, it is too far away from the North Atlantic region. The low-resolution simulations are run to equilibrium (for several thousand model years) to obtain the climatological states. Then, the high-resolution simulations using an atmospheric model with a horizontal resolution of f09 (0.9375° × 1.25°) are driven by the prescribed climatological monthly mean sea surface temperatures (SSTs), sea-ice, and annual mean land vegetation, which are derived from the corresponding low-resolution simulations. Each high-resolution simulation also run to equilibrium, and we analyze the outputs of the last 60 years. The results of the low- and high-resolution simulations are similar. Here, we present the high-resolution simulations as the default simulations.

A set of simulations with fixed CO2 (short for FixCO2) is performed to examine the effect of background climate on the evolution of the NAO. These simulations follow the same setup as the default runs, except the CO2 concentration in all the time periods is fixed at 10 times the PI value. For each paleo time period, we also first run an atmosphere-ocean-coupled low-resolution simulation to obtain the climatological SSTs. Then, a high-resolution atmosphere model is driven by the climatology from the low-resolution simulations. The results of the FixCO2 high-resolution simulations are summarized in Fig. S3. Note that the global mean surface temperature in FixCO2 fluctuates with a standard deviation of 1.3 K, due to surface albedo changes associated with the evolution of continental configuration 60.

A set of simulations with flattened Rocky Mountains (short for NRM) is performed to examine the effect of the RM on the evolution of the NAO. These simulations are the same with the default high-resolution simulations (driven by low-resolution simulations with various CO2), except we set the areas over the RM with a surface elevation above 300 meters to 300 meters. The others surface properties, such as vegetation, are unchanged. The NRM simulations are performed for the time period from PI to 100 Ma with an interval of 20 Ma. The results of the NRM simulations are summarized in Fig. 1d.

Aquaplanet simulations
We conducted a series of aquaplanet simulations with CESM1.2.2 to explore the relationship between the width of the ocean basin and the NAO. Initially, we performed a slab ocean simulation (with a depth of 50 meters) with a prescribed ocean heat flux 61. The seasonal cycle is driven by an obliquity of 23.5°. Subsequently, a set of atmosphere-only simulations was run with SST prescribed as the monthly-mean SST obtained from the slab ocean simulation. Given the 50 m ocean depth, the seasonal cycle of SST in the slab ocean simulation lags behind observations by about 2 months 61,62. This SST lag is adjusted in the fixed SST simulations. The fixed SST simulations also feature two rectangular continents covering 20° N-70° N with a width of 60° and 120°, respectively. These continents may analogously represent the North American and Eurasian continents. The land elevation is set at 50 m, and the surface vegetation is grassland. In each simulation, the distance between the two continents varies from 0° to 160° with intervals of 20°. These simulations are named Two-Lands-simulations (TL*, where * refers to the distance between the two lands). Each simulation runs for 210 years, with the monthly outputs of the last 150 years analyzed.

To examine the effect of the RM on the NAO, we performed additional experiments with RM-like mountains in the TL0, TL20, and TL60 cases. The mountains are two-dimensional gaussian shaped with a half width at half maximum of 5° in the zonal direction and 15° in the meridional direction. The latitude of the mountain peak is 45° N, and the longitude is 15° west to the coastline of the smaller continent. In TL0, we run four simulations with the mountain peak of 523 m, 786 m, 1050 m, and 1500 m. The corresponding average heights (following the calculation below) are 412 m, 620 m, 825 m, and 1237 m, respectively. The same is done for the TL20 and TL60 cases. Thus, there are 12 simulations in total with idealized mountains. The results of the NRM simulations are summarized in Fig. 4d.

Calculation of the average height of the Rocky Mountains
Over geological time, the evolution of the orographic features of the RM is quite complicated (the gray dotted contours in Fig. S5). For simplicity, we calculate the average height of the RM as follows: first, we identify the peak of the RM between 30° N and 50° N. Next, we determine the highest grids at each latitude within a latitudinal band 5° north and south of the peak. Finally, we average the height across a meridional band 5° west and east of the peak at each latitude. The evolution of the average height of the RM is shown in Fig. S10. Different metrics of the RM height may yield different values, but the overall trends are similar.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "I commend the authors on their interesting and well written paper.", "reviewer_2": "This paper addresses an interesting long-standing question of the dynamical origins of the North Atlantic Oscillation in the context of paleoclimatic simulations, in which the geometry of the climate system undergoes drastic changes. The authors argue that their results demonstrate the dominant role of land-sea contrast, as compared to the orographic forcing) in the dynamics of a spatially localized NAO dipole over the North Atlantic. I believe this conclusion is flawed, or at least, not fully supported by the authors' arguments and analyses conducted thus far and suggest that major changes are required in the present submission to alleviate this concern.\n\nI've been more or less familiar with the topic of this paper, and, interestingly, have had a different interpretation of the relevant literature, some of which the authors are citing. In particular, I've been under impression that the scientific community consensus is that orographic forcing is the primary source of NAO localization, and the \"thermal topography\" (land-sea contrast) is secondary. This is diametrical opposite to what the authors suggest. My main concern here is that the demonstration of the conclusion the authors draw, of dominance of one of two possible explanations for a phenomenon, requires simulations that resolve both sets of dynamics (in this particular case, land-sea contrast and orography), then simulations in which one set of forcing is artificially suppressed, e.g., experiments with flat and actual topography are both required! It doesn't look like such two sets of experiments were conducted, so whatever arguments the authors make remain logically incomplete.", "reviewer_3": "Review for „Origin and Evolution of the North Atlantic Oscillation”\nThe article explores the origin and evolution of the North Atlantic Oscillation (NAO) over geological time scales, particularly focusing on the breakup of Pangea from 160 million years ago. It is argued that a present-day-like NAO pattern emerges around 80 million years ago as the North Atlantic widens enough to support a distinct high-pressure system.\n\nThe article has been interesting to read. However, I have some major concerns, starting with the usage of the low-resolution and high-resolution output. I feel that the outputs are sometimes contradicting each other and throughout the whole article it was not clear to me why both are used. For the low-resolution model the Figure S1 shows that the NAO like pattern is present constantly throughout 130 Ma, as the threshold for an NAO pattern was not given.\n\nI feel that the focus should be thoroughly on the high-resolution model and then the hypothesis of the authors should be put to the test. This would make it also a lot easier to follow the authors augmentation, because right now I am constantly looking if the statement applies to the high-resolution and the low-resolution model as well.\n\nMinor comments:\nll 31: remove the word of\nll 33: I suggest writing that the NAO is the most dominant mode during winter time.\nll 34: What do you mean by different phases? Are there more than two?\nLl 35: Distinct weather patterns is to unspecific.\nLl 37: Why is North America framed as a remote area?\nLl 37: May be it is just me but could you please specify teleconnection?\nLl 39-41: This abstract is lacking important references such as:\nLi et al. (2013): \"NAO implicated as a predictor of Northern Hemisphere mean temperature multidecadal variability\" in Geophysical Research Letters, Vol. 40, 5497-5502.\nPeings and Magnusdottir (2014): \"Forcing of the wintertime atmospheric circulation by the multidecadal fluctuations of the North Atlantic ocean\" in Environmental Research Letters, Vol. 9, 034018.\nSun et al. (2015): \"A delayed oscillator model for the quasi-periodic multidecadal variability of the NAO\" in Climate Dynamics, Vol. 45, 2083-2099.\nWills et al. (2019): \"Ocean–atmosphere dynamical coupling fundamental to the Atlantic multidecadal oscillation\" in Journal of Climate, Vol. 32, 251-272.\nDelworth and Zeng (2016): \"The Impact of the North Atlantic Oscillation on Climate through Its Influence on the Atlantic Meridional Overturning Circulation\" in Journal of Climate, Vol. 29, 941-962.\nDelworth et al. (2017): \"The central role of ocean dynamics in connecting the North Atlantic oscillation to the extratropical component of the Atlantic multidecadal oscillation\" in Journal of Climate, Vol. 30, 3789-3805.\nLl 42-43: Please give a reference.\nLL 46-47: The reference you are referring to is talking about the summer NAO (SNAO).\nLl 87-88: What do you mean by sufficiently strong?\nLl 96: Could this be because you have chosen 200 years of averaging for the low resolution instead of 100 which would represent a climate equilibrium?\nLl 100-102: I think that from 60Ma on, they clearly deviate.\nLl 114: What about the high-resolution simulation? Here it emerges at 160Ma, or am I mistaken?\nLl 113-115: For the low-resolution model, I would argue that it only disappears during 110Ma, and also here looking at S1 (m) it is probably related to the zonal average.\nLl 141; Why are these periods representative?\nLl 172: But what about the NAO pattern in Fig S1 13Ma?\nLl 172: If the atmospheric output is available for the high-resolution model, what is the motivation to use the low-resolution model?\nMethods:\nL248: I was just thinking, how would the Z500 look like?\nL250: I don’t get what is meant by regional box. The grid has a resolution of 2.5° x 2.5°. Do you mean the nearest grid box close to 60° Latitude and 110° Longitude?\nL250: In addition, what you mean by 110° longitude?\nL256-274: Please be a little bit clearer about how the simulations of Li et al. (2022) are conducted. If I am not mistaken it is like this: The study conducts simulations over a broad timeline (540Ma to 10Ma) in two main steps to align with paleoclimatic reconstructions:\n- 54 time-slice simulations are conducted at 10Myr intervals, without considering ice sheets, using paleogeographic maps and uniform soil and land surface conditions.\n- First simulation phase involves adjusting CO2 levels over 4000 to 6000 model years for equilibrium, with initial settings for ocean temperature, salinity, and no ice sheets, except in the pre-industrial simulation.\n- Second phase uses data from the first phase (averaged over the last 100 model years) to drive a 100-year simulation for atmospheric equilibrium, focusing on the atmosphere-land model and keeping paleogeography, CO2, and solar radiation constant.\nCan you also motivate why you choose 200 years of the low-resolution simulation? Li et al. (2022) used a threshold of 100 models years for determining a climate equilibrium."}, "author_response": "Responses to the reviewers:\nWe are grateful to the reviewers for their valuable comments and suggestions,\nwhich have been very helpful in our revision of this paper. The major revisions of\nthe manuscript include:\n1) Following the suggestions of the reviewers, in the revision, we focus on the\nresults of the high-resolution simulations. The high-resolution simulation results\nshow a regime transition of the wavenumber 2 pattern, with the strengthening of\nthe North Atlantic high pressure and storm track and weakening of the North\nPacific high pressure and storm track, which is qualitatively consistent with the\nlow-resolution simulation results.\n2) As reviewers pointed out, the emergence of the NAO-like mode is gradual. Thus,\nit is better not to specific a strict emergence time period (also, there is no strict\ndefinition or threshold for NAO). Instead, we describe the smooth nature of the\ntransition and provide a loose emergence time period of around 80 Ma to 60 Ma.\n3) We performed new sets of simulations to examine the effects of the Rocky\nMountains on the NAO. It was found that the presence of the Rocky Mountains can\nnotably strengthen the NAO since 40 Ma. The results are presented in the new\nSection 4.\nWe also improved the description of models and results. We hope that our efforts\nhave resulted in a satisfactory improvement of the manuscript. We would like to\nexpress our gratitude to the editor and reviewers once again for their time on this\npaper. The following are our point-by-point responses (in blue) to the reviewers’\ncomments. Unless otherwise specified, the figure numbers refer to those in the\nrevised manuscript.\nReviewer #1:\nThis paper investigates the evolution of the North Atlantic Oscillation over the past\nplanet simulations. The paper concludes that the NAO-like variability mode\nemerged around 80 Ma, when the North Atlantic Ocean had opened sufficiently to\nform a high-pressure system that separated the North Atlantic jet from the\nEurasian jet. In my view, it is both an interesting and important paper that\ngenerally is well written and easy to follow (see comments below). The\nconclusions generally appear robust and well supported, and I therefore support\npublication of this paper in Nature Communications.\nHowever, there are a number of issues that must be dealt with before the paper is\nready for publication. First, I think the lack of a rigorous definition of NAO-like\nvariability results in lack of clarity and some inconsistencies. I know that it is\ndifficult, but I encourage the authors to be more rigorous in their discussion of\ntheir results and figures. Second, as also stated by the authors, it is clear that other\nfactors potentially influenced the evolution of the NAO (presence of ice sheet,\nglobal mean temp, mountain ranges etc). Some of these other factors seem to have\nbeen included in the model (past temperature variations and paleo-elevation\nmodels), but the results are not really discussed or shown. Also, the results vary\nconsiderably over time (e.g. 130 Ma vs 110 Ma) and the low- and high-resolution\nmodels yield quite different results for many time slices. The source of this\nvariability should be addressed more thoroughly. Below, I describe these general\npoints in more detail.\nThank you very much for reviewing the manuscript.\nGeneral comments:\n1. Unclear how NAO-like conditions are defined. The section entitled “Origin of\nthe NAO” discusses the requirements that leading EOF patterns must meet to be\nconsidered NAO-like. This includes the statement “Secondly, the negative and\npositive anomalies must be sufficiently strong” (lines 87-88). However, it is never\ndefined what “sufficiently strong” actually means, and that results in some\ninconsistencies and what appears as semi-subjective interpretations\nThank you very much for your comments on the definition of the NAO-like pattern.\nThe NAO is well-studied, and there is a consensus on its definition under the\ncurrent climate and continental configuration. However, there are few existing\nstudies that define the paleoclimate NAO-like mode for reference. The results\nshow that the leading EOF patterns gradually approach the modern NAO, rather\nthan indicating a sudden change. The transition period is around 80 Ma to 60 Ma.\nTherefore, it is preferable not to specify a strict emergence time period. Instead,\nwe describe the gradual nature of the transition and provide an approximate\nemergence period of around 80 Ma to 60 Ma.\nLines 113-115 states that “In both the low- and high-resolution simulations (Fig.\n1c-d), the NAO-like pattern first appears at 130 Ma but subsequently weakens or\ndisappears”. In the high-resolution simulation, the NAO-like pattern is also present\nat 160 Ma – at least according to the colours in Fig. 1d and Fig. S2. Also, and\nperhaps more importantly, Fig. 1d indicates very strong NAO-like conditions at 60\nMa but looking at Fig. S2 there does not seem to be any positive anomaly at all\nduring this time period. The NAO-like conditions are strong at 60 Ma in the low-\nresolution simulations, as also indicated in Fig. S1, but that is not the case in Fig.\nS2. Something appears inconsistent here.\nThank you for raising these points. The advantage of low-resolution simulations is\nthat they include a dynamical ocean, allowing for atmosphere-ocean interaction in\nthe simulations. On the other hand, high-resolution simulations, although using\nprescribed SST, can better resolve synoptic-scale wave-flow interactions. Given\nthese differences, it may not be surprising that there are some inconsistencies\nbetween the low- and high-resolution modeling results for certain paleo periods.\nFollowing the suggestion of reviewer 3, in the revision, we focus on the results of\nthe high-resolution simulations. We have corrected a calculation error in the EOFs\nat 60 Ma.\nWe also looked into the leading EOFs at 500 hPa, following suggestion of reviewer\n3, to check if they have a robust barotropic structure. The leading EOF at 130 Ma\nand 160 Ma does not vertical extension to middle troposphere. Besides, Figure R1\nshows a zoomed-out view of the EOF (regression of the Z1000 on the PC1 time\nseries) at 160 Ma. The positive anomaly over the North Atlantic sector is not a local\nmaximum but instead extends from the upstream region. Thus, the EOFs at 130\nMa and 160 Ma are not NAO-like.\nFig. R1. The regression of wintertime Z1000 on the time series of the leading EOF\nof Z1000 over the red regional box. This is the high-resolution simulation results\nat 160 Ma.\nAlso, lines 117-118 state that “The explained variance of the NAO-like mode also\nconfirms a stable variability mode since 80 Ma (numbers in the subtitles of Fig. S1\nand S2)”. Looking at the numbers in the subtitles on Fig. S1, it seems that EOF1\nassociated with time slice 80 Ma explains 26.13% of the variance – the lowest\nnumber among all the time slices. For instance, EOF1 for time slice 10 Ma explains\n33.5% of the variance, where the EOF1 for time slice 150 Ma explains 34.62% of\nthe variance. It is thus impossible for me to reconcile these numbers with the\nstatement in lines 117-118 – or is there something I have misunderstood here?\nThank you for your comments. Here, we focus on the evolution of the spatial\npatterns of the leading EOFs. The explained variance of the EOFs is a separate issue\nthat is not examined in this study. We agree that the sentence you mentioned was\ninappropriate, and we have removed it in the revision.\nDespite the comments above, I find the interpretation of the general patterns\nconvincing, but I think a more rigorous description of the results is needed. It\nwould also be valuable if the authors can define more precisely what they mean by\nNAO-like conditions, and when the positive anomaly is “too weak to be considered\nan NAO-like pattern” (lines 105-106 and elsewhere). Moreover, there is no\ndiscussion of what causes the variability among the time slices, especially those\nthat are not aligned with the overall trend. For instance, why are the positive\nanomalies so pronounced at 120 Ma and 130 Ma in the low-resolution simulations\n– more pronounced than at 50 Ma and 90 Ma? For the high-resolution simulations,\nthe positive anomaly is as pronounced or more at 160 Ma and 130 Ma than during\nany other time slices until 40 Ma (judging from Fig. S2). What causes these\nvariations – minor effects related to the complex geometry of the continental\nconfigurations? I think these questions are important, because they have\nimportant bearings on the main conclusion of the paper.\nThank you very much for your valuable suggestion. This paragraph summarized\nseveral questions above. We have addressed these questions individual. Please see\nthe corresponding responses.\n2. Some key information on simulations is missing. I think that important\ninformation relating to the model setup used for the different time slices is missing.\nIn the introduction, the paper presents several different factors that can influence\nNAO conditions, such mountain ranges, sea-air interactions, and glacier\ntopography. Nevertheless, this aspect is not discussed at all in the presentation of\nthe model – it just mentions that paleo-geographic maps are sourced from the\npaleo-digital elevation model (Scotese and Wright, 2018). It is therefore unclear\nhow much the mountain-range configuration changes over time in response to\nplate tectonic activity. The same goes for the sea-air interaction, which may change\nas a function ocean gateways and changes in the circulation pattern.\nPerhaps more importantly, it is also unclear what ice-sheet configurations were\nused in the different simulations. The introduction mentions that “Due to the\ninfluence of glacier topography, significant differences exist in the spatial structure\nand temporal variability of the NAO during the Last Glacial Maximum compared to\nthe present climate”. The ice configuration clearly plays an important role in this\ncontext, but it is unclear what configuration was used in the model simulations. Do\nthey include the Greenland ice sheet in the model runs for the present-day, but no\nice sheets for earlier time slices?\nThe role of the above-mentioned factors for the evolution of the NAO may be minor\ncompared to the opening of the Atlantic, but it is currently impossible to assess if\nthat is the case. As a minimum, the paper should discuss what effect mountain-\nrange, ocean-gate, and ice-sheet configurations influenced the model results in the\ndifferent time slices. It would be good if they could show that these effects are\nsecond order compared to the opening of the Atlantic.\nThank you for the suggestion. Among the factors mentioned above, the effect of\nmountain range is perhaps the most important one. It has been examined in\nprevious studies under the modern continental configuration. Following your\nsuggestion (and that of reviewer 2), we conducted new experiments to examine\nthe effects of orography (specifically the Rocky Mountains) on the NAO. We found\nthat the Rocky Mountains can notably strengthen the NAO. These new results are\npresented in Section 4.\nThe ocean gateways may affect the NAO by modulating sea surface temperatures.\nThe three major ocean gateways—the Tasmanian Gateway, the Drake Passage, and\nthe Panama Gateway—are either in the Southern Hemisphere or near equator, so\ntheir effects on the Northern Hemisphere mid- and high-latitudes may be small. In\nour simulations, the Drake Passage is set to be open since 140 Ma (Scotese and\nWright 2018). We performed two sensitivity tests at 70 Ma and 100 Ma, in which\nthe Drake Passage was set to be closed. Fig. R2 shows that the setup of Drake\nPassage can lead to visible differences in the leading EOFs; however, the overall\npatterns are similar, and our conclusions are not affected. Still, the situation may\nbe different for other time periods or ocean gateways. We have added a discussion\non this point in the last paragraph.\nThe paleogeographic maps of Scotese and Wright (2018) do not include\ninformation on ice sheets. There are no prescribed ice sheets in our simulations,\nexcept for the PI simulation, which uses CESM1.2.2's default ice sheets. It is\nimportant to note that for most of the paleo periods examined here, the climate\nwas notably warmer than the present, so large-scale ice sheets did not exist. For\nexample, the Greenland ice sheet is estimated to have emerged around 2.5 Ma\n(Alley et al., 2010). The Antarctic ice sheet formed around 40 Ma (Zachos et al.,\n2001), but it is too far from the North Atlantic region to have a significant impact.\nWe have added a related description of the model setup and included a discussion\non these factors.\nFig. R2. The leading EOFs of Z1000 over the North Atlantic region. The left panels\nshow the default simulation results with the Drake Passage open, as presented in\nFig. S1. The right panels show the sensitivity simulations with the Drake Passage\nclosed. The upper panels represent simulations at 70 Ma, and the lower panels\nrepresent simulations at 100 Ma.\n3. The role of temperature for NAO-like conditions. I gather from lines 130-132\nand the Methods section that the temperature varies by ~10 K over the various\ntime slices (and that these variations are achieved by tuning the CO\n2\nconcentration). It would be very informative if the effect of these temperature\nvariations on the evolution of NAO-like conditions would be briefly discussed –\nand perhaps shown in the supplementary material. In this way, the reader can\nassess the importance of temperature variations. Now, the reader must rely the\nopaque statement in lines 133-135, which says that contemporary climate change\ncan influence the NAO, but had little impact on the evolution of the NAO over the\nlast 160 Ma (please re-phrase this sentence).\nThank you for your comment. Following your suggestion, we have added\ndiscussions (the last paragraph in section 2) and a supplementary figure (Fig. S3)\non the effects of temperature on the leading variability mode. We conducted a set\nof simulations with fixed 10-times-PI CO₂ concentrations. The global mean\ntemperature variation is much smaller in this set of simulations. The evolution of\nthe leading EOFs (Fig. S3) shows similar features to the results in the main text,\nsuch as the emergence of the NAO-like mode around 80 Ma to 60 Ma. These\nsimulations also indicate that a colder climate can strengthen the NAO, consistent\nwith the results based on the ongoing global warming (e.g., Hamouda et al., 2021).\nIn this paper, we mainly focus on the effects of continental distribution and\norography on the NAO; the impact of climate will be addressed in details in a\nseparate study.\nQuestions to figures:\nFigure 2. The description of Figs. 2a, c, and e is puzzling to me – and somewhat\ninconsistent. For instance, it says that “Without a detached North Atlantic jet, there\nis no NAO-like variability mode” (lines 150-151). First of all, it can be discussed if\nit is detached at 40 Ma. Secondly, the lines just above (146 -148) suggests that “the\nend of the spiral forms the North Atlantic jet, pointing toward high latitudes and\nwell separated from the head of the spiral”. The notion that “the end of the spiral”\nforming the North Atlantic jet is somewhat inconsistent with the notion that\n“Without a detached North Atlantic jet, there is no NAO-like variability mode.\nThank you very much. In the revision, we mainly present the high-resolution\nsimulation results. Thus, the figures and associated texts have been revised.\nFigure 3. I have a couple of questions that I hope the authors can clarify. First, the\nblue dots indicate the longitude of the NA jet core. Why is that 0 degrees for 160\nMa (Fig. 3a) – in the text on lines 165-169, the authors argue that there is only one\nhigh-pressure system over the North Pacific because the NA Ocean has not yet\nopened, “resulting in a gap there”. The blue line is different in Fig. 3b for approx.\n160-100 Ma, which makes sense because the NA has not yet got a core. But it looks\nstrange – maybe it would be better to omit the blue dots until the core has actually\nformed. Red dots in 3a and 3b – it appears a bit subjective “what it takes to get a\nred dot”, i.e. when the EOF patterns are deemed NAO-like. Looking at Fig. S2, I\nwonder why there is a red dot for 60 Ma and not 100 Ma.\nThank you for your suggestion. In the revised Fig. 3, we improved the presentation\nof the evolution of the jets.\nFigure 4c and Fig. S9. I find it a bit puzzling that the dark red anomaly persists all\nthe way to TL180. Why does it look so different from TL0, which had has no dark\nred patches (they have the same configuration, expect it is displaced 180\nlongitudinal degrees but that shouldn’t matter in this idealized setting).\nThanks. In the idealized simulations, the region of EOF analysis mimics the North\nAtlantic region before and after the separation of North America and the Eurasian\ncontinent. As you mentioned, the TL0 and TL180 cases have the same\nconfiguration (a single continent). The region of EOF analysis in the TL0 case is\nover the continent, while in the TL180 case, it is over the ocean. The location of the\nEOF region matters since the land-ocean contrasts induces stationary waves (Fig.\nS8, S9) breaking the zonal symmetry of the background circulation. In the revision,\nwe have removed the TL180 case from the figures since its results are similar to\nother cases and less relevant.\nMinor comments:\nL55: suggest “land-ocean contrasts…”, i.e. “contrasts”.\nThanks for your suggestion. We have revised it following your suggestion.\nL57: suggest “land-ocean thermal contrasts”.\nThanks for pointing it out. We have revised it following your suggestion.\nL68: suggest “These unexplored questions are the focus of this study”.\nThanks for your suggestion. We have revised it following your suggestion.\nL73 perhaps: “a coherent” instead of “the coherent”.\nThanks for pointing it out. We have revised it following your suggestion.\nL80: Please define “PI” prior to first use. I assume it means “Pre-industrial”?\nYes, you are right. We have revised it following your suggestion.\nL129: “CO ”.\nL141: Why are these periods representative?\nThe PI is the current climate. The 40 Ma case represents a period with a NAO-like\nmode, while the 150 Ma case represents a period without a NAO-like mode.\nL142: “the North Atlantic jet is situated upstream of the NAO”. It is unclear what\nyou refer to here. Is that the little yellow-green patch west of the positive anomaly.\nYes, you are right. In the left column of Fig. 2a of the previous manuscript, the color\nshading denotes the 250 hPa zonal wind (unit: m/s).\nL159: suggest “an associated upper-tropospheric…”.\nThanks for your suggestion. The sentence is no longer in the revised manuscript.\nL176 and elsewhere: suggest that “Euro-Asia jet” is replaced by “Eurasian jet”.\nThanks for your suggestion. The issue is fixed.\nL244: “The reanalysis data from NCEP-2 are also used”. What do you mean by “also”\nhere? I think you need to state all the data that are used here in the Methods\nsection.\nIt was a typo. It is removed in the revision. Thanks.\nReviewer #2:\nThis paper addresses an interesting long-standing question of the dynamical\norigins of the North Atlantic Oscillation in the context of paleoclimatic simulations,\nin which the geometry of the climate system undergoes drastic changes. The\nauthors argue that their results demonstrate the dominant role of land-sea\ncontrast, as compared to the orographic forcing) in the dynamics of a spatially\nlocalized NAO dipole over the North Atlantic. I believe this conclusion is flawed, or\nat least, not fully supported by the authors' arguments and analyses conducted\nthus far and suggest that major changes are required in the present submission to\nalleviate this concern.\nI've been more or less familiar with the topic of this paper, and, interestingly, have\nhad a different interpretation of the relevant literature, some of which the authors\nare citing. In particular, I've been under impression that the scientific community\nconsensus is that orographic forcing is the primary source of NAO localization, and\nthe \"thermal topography\" (land-sea contrast) is secondary. This is diametrical\nopposite to what the authors suggest. My main concern here is that the\ndemonstration of the conclusion the authors draw, of dominance of one of two\npossible explanations for a phenomenon, requires simulations that resolve both\nsets of dynamics (in this particular case, land-sea contrast and orography), then\nsimulations in which one set of forcing is artificially suppressed, e.g., experiments\nwith flat and actual topography are both required! It doesn't look like such two\nsets of experiments were conducted, so whatever arguments the authors make\nremain logically incomplete.\nRemarks on code availability:\nI'll have a look at the code if the authors will have addressed my major comments\nwith regards to flaws in the paper's current arguments.\nThank you very much for your constructive comments. Previous studies had\nemphasized the effects of orography (i.e., the Rocky Mountains). For example,\nflatting the Rocky Mountain leads to a weakening of the NAO (e.g., Brayshaw et al.\n2009, Valdes and Hoskins 1991). As you said, the numerical experiments in these\nstudies only modified the large-scale mountains and kept the continental\ndistribution fixed.\nYour suggestion to examine the relative importance of land-ocean contrast and\norography is very valuable, and the evolution of the paleoclimatic NAO provides a\nperfect opportunity to address this important question. Following your suggestion,\nwe conducted additional sets of simulations with a flattened Rocky Mountain in\nboth the idealized and paleoclimate simulations. The new results show that the\nNAO-like mode has been strengthened by the Rocky Mountain since 40 Ma. We\nbelieve this major revision has addressed your concern. These new results are\ndescribed in the new Section 4, and the revised conclusion includes this as a key\npoint of the study. We appreciate your suggestions, which have greatly improved\nthe manuscript.\nReviewer #3:\nThe article explores the origin and evolution of the North Atlantic Oscillation (NAO)\nover geological time scales, particularly focusing on the breakup of Pangea from\n160 million years ago. It is argued that a present-day-like NAO pattern emerges\naround 80 million years ago as the North Atlantic widens enough to support a\ndistinct high-pressure system.\nMajor comments\nThe article has been interesting to read. However, I have some major concerns,\nstarting with the usage of the low-resolution and high-resolution output. I feel that\nthe outputs are sometimes contradicting each other and throughout the whole\narticle it was not clear to me why both are used. For the low-resolution model the\nFigure S1 shows that the NAO like pattern is present constantly throughout 130\nMa, as the threshold for an NAO pattern was not given.\nI feel that the focus should be thoroughly on the high-resolution model and then\nthe hypothesis of the authors should be put to the test. This would make it also a\nlot easier to follow the authors augmentation, because right now I am constantly\nlooking if the statement applies to the high-resolution and the low-resolution\nmodel as well.\nThank you very much for the comments. Following your suggestion, in the revision,\nwe mainly focus on the high-resolution simulation results. We also performed new\nsimulations to investigate the effects of orography (the Rocky Mountains) on the\nevolution of the NAO.\nMinor comments\nll 31: remove the word of\nThanks for your suggestion. We have revised it following your suggestion.\nll 33: I suggest writing that the NAO is the most dominant mode during winter time.\nThanks for your suggestion. We have revised it following your suggestion.\nll 34: What do you mean by different phases? Are there more than two?\nWe meant positive and negative phases of the NAO. We have revised the sentence.\nLl 35: Distinct weather patterns is to unspecific.\nThanks. We have revised the sentence.\nLl 37: Why is North America framed as a remote area?\nThanks. Conventionally, the Europe is directly under the impact of the NAO. Other\nregions are framed as remote regions. We have revised the sentence to avoid\nmisleading.\nLl 37: May be it is just me but could you please specify teleconnection?\nThere are a few studies that propose mechanisms for the NAO's influence on East\nAsia. For example, Watanabe (2004) showed that the winter NAO signal can extend\ntoward East Asia through a quasi-stationary Rossby wave train along the Asian jet.\nLi et al. (2022) present that the NAO influences winter East Asian surface\ntemperature multidecadal variability through its accumulated delayed effect on\nthe Atlantic Multidecadal Oscillation and the Africa–Asia multidecadal\nteleconnection pattern. We agree with you that the term \"teleconnection\" is too\nvague to summarize these mechanisms. We have revised the sentence to avoid any\nmisunderstanding.\nLl 39-41: This abstract is lacking important references such as:\nLi et al. (2013): \"NAO implicated as a predictor of Northern Hemisphere mean\ntemperature multidecadal variability\" in Geophysical Research Letters, Vol. 40,\n5497-5502.\nPeings and Magnusdottir (2014): \"Forcing of the wintertime atmospheric circulation by the multidecadal fluctuations of the North Atlantic ocean\" in Environmental Research Letters, Vol. 9, 034018.\nSun et al. (2015): \"A delayed oscillator model for the quasi-periodic multidecadal\nvariability of the NAO\" in Climate Dynamics, Vol. 45, 2083-2099.\nWills et al. (2019): \"Ocean–atmosphere dynamical coupling fundamental to the\nAtlantic multidecadal oscillation\" in Journal of Climate, Vol. 32, 251-272.\nDelworth and Zeng (2016): \"The Impact of the North Atlantic Oscillation on Climate through Its Influence on the Atlantic Meridional Overturning Circulation\" in Journal of Climate, Vol. 29, 941-962.\nDelworth et al. (2017): \"The central role of ocean dynamics in connecting the\nNorth Atlantic oscillation to the extratropical component of the Atlantic\nmultidecadal oscillation\" in Journal of Climate, Vol. 30, 3789-3805.\nThanks a lot for providing these references. We have revised it following your\nsuggestion.\nLl 42-43: Please give a reference.\nReferences are added.\nLL 46-47: The reference you are referring to is talking about the summer NAO\n(SNAO).\nThanks. We have revised the sentence and updated the reference.\nLl 87-88: What do you mean by sufficiently strong?\nThanks. This issue is also pointed out by reviewer 1. We have clarified the\ndescription of the emergence of the NAO-like mode.\nLl 96: Could this be because you have chosen 200 years of averaging for the low\nresolution instead of 100 which would represent a climate equilibrium?\nThank you for your comment. Using a longer dataset is better for EOF analyses. We\nhave confirmed that the EOF results using 100 years of data are similar.\nLl 100-102: I think that from 60Ma on, they clearly deviate.\nThanks. Based on your suggestion, we have clarified the description of the\nemergence of the NAO-like mode.\nLl 114: What about the high-resolution simulation? Here it emerges at 160Ma, or\nam I mistaken?\nThanks. The EOF pattern at 130 Ma and 160 Ma is not NAO-like. Please see\nresponses to the main point of reviewer 1 and the 500 hPa leading EOFs (Fig. R3).\nLl 113-115: For the low-resolution model, I would argue that it only disappears\nduring 110Ma, and also here looking at S1 (m) it is probably related to the zonal\naverage.\nWe have clarified the description of the emergence of the NAO-like mode.\nLl 141: Why are these periods representative?\nThe 40 Ma represent the time periods during which the North Atlantic Ocean is\nwide enough and the leading EOFs show NAO-like patterns. The 160 Ma represent\nthe time periods that the leading EOFs are not NAO-like. They are chosen as\nexamples, and other time periods also works. We have revised the sentence for\nclarification.\nLl 172: But what about the NAO pattern in Fig S1 130Ma?\nThanks. The NAO-like pattern of Z1000 does not extend up to the middle\ntroposphere. Please see the responses to your comments L248 and Fig. R3.\nLl 172: If the atmospheric output is available for the high-resolution model, what\nis the motivation to use the low-resolution model?\nThe low-resolution simulations are atmosphere-ocean coupled, which is\nnecessary to obtain the SST distribution over different time periods. The\nclimatological SST distributions are then used to drive the high-resolution\nsimulations. We included the low-resolution simulations because some studies,\nsuch as Marshall et al. (2001), pointed out the importance of atmosphere-ocean\ninteractions. However, we agree that the focus of this study should be on the high-\nresolution simulations. In the revision, we focus on the results of the high-\nresolution simulations.\nMarshall J, Johnson H, Goodman J (2001) A study of the interaction of the North\nAtlantic Oscillation with Ocean Circulation. J Clim 14(7):1399–1421.\nMethods:\nL248: I was just thinking, how would the Z500 look like?\nThank you for your valuable suggestion. Fig. R3 shows the leading EOFs of Z500 in\nthese simulations. An important feature of the modern NAO is its barotropic\nstructure (Hurrell et al., 2003). After 80 Ma, the leading EOFs exhibit similar\nmeridional dipoles in the middle troposphere. Between 160 Ma to 130 Ma, the\nvertical extension is much weaker. At 140 Ma, the leading EOF of Z500 shows a\nzonal dipole pattern, totally decoupled from the leading EOF of Z1000.\nFig. R3. The leading EOFs of the wintertime Z500 over the North Atlantic region.\nThis is Fig. S2 in the revision.\nL250: I don’t get what is meant by regional box. The grid has a resolution of 2.5° x\n2.5°. Do you mean the nearest grid box close to 60° Latitude and 110° Longitude?\nIt means a rectangle (in term of latitude and longitude, not length in kilometers)\nbox with 110° length in the east-west direction and 60° length in the south-north\ndirection. For the current climate, the boundary of the box is 20°N to 80°N, 70°W\nto 40°E. We have revised this sentence.\nL250: In addition, what you mean by 110° longitude?\nThe length of the regional box in east-west direction is 110°. We have revised this\nsentence.\nL256-274: Please be a little bit clearer about how the simulations of Li et al. (2022)\nare conducted. If I am not mistaken it is like this: The study conducts simulations\nover a broad timeline (540Ma to 10Ma) in two main steps to align with\npaleoclimatic reconstructions:\n- 54 time-slice simulations are conducted at 10Myr intervals, without considering\nice sheets, using paleogeographic maps and uniform soil and land surface\nconditions.\n- First simulation phase involves adjusting CO2 levels over 4000 to 6000 model\nyears for equilibrium, with initial settings for ocean temperature, salinity, and no\nice sheets, except in the pre-industrial simulation.\n- Second phase uses data from the first phase (averaged over the last 100 model\nyears) to drive a 100-year simulation for atmospheric equilibrium, focusing on the\natmosphere-land model and keeping paleogeography, CO2, and solar radiation\nconstant.\nYes, you are right about the setups of the simulations. We have improved the\ndescription of the simulation setups in the method.\nCan you also motivate why you choose 200 years of the low-resolution simulation?\nLi et al. (2022) used a threshold of 100 models years for determining a climate\nThank you for your comments. Li et al. (2022) assessed that 100 model years are\nsufficient to obtain a stable mean climate state, such as the temperature and\nprecipitation distributions. However, for variability modes, such as the NAO\nexamined here, it is better to use a longer period if the outputs are available.\nBest wishes,\nJi Nie and all the authors\n2024.10.25"}, {"round": 2, "reviewer_comments": {"reviewer_1": "I have read the revised version of the paper, and I think the authors have done a good job addressing my comments from the first round of review (along with the comments by the other reviewers). It is generally a pleasure to read the paper, and I think the results are quite convincing. Also, I really like the addition of the summary figure (Fig. 5). However, there are still a couple of minor aspects that need to be addressed. Below, I list the minor aspects that I think needs to be addressed before the paper is ready for publication.\n\nThank you very much for reviewing the manuscript again.\n\nGeneral comments:\n1. Description of the main paleoclimate simulations. I think it is essential that the\nMethods section includes a brief description in plain words of the key parameters in\nthe main paleoclimate simulations, such as topographic features, forcing of long-term\nclimate, ocean gateways, and major glaciations (this part is included).\n\nThank you for the suggestion. We have added description of the key parameters and\npaleogeography of the main paleoclimate simulations in the first paragraph of the\n\"Paleoclimate Simulations\" section of the Methods. It provides readers a brief picture\nof the evolution of the paleogeographic maps. We also guided the readers to\nhttps://zenodo.org/records/5460860 for the high-resolution maps of the\npaleogeography in Scotese and Wright (2018).\n\nThe summary discusses the potential importance of ocean gateways and the uplift of\nthe Tibetan Plateau. But is the Tibetan Plateau not part of the paleogeographic maps\nused?\n\nThank you for raising this point. The Tibetan Plateau is included in the paleogeographic\nmaps (Scotese and Wright, 2018) used in this study. We have added clarification in the\ndescription of the paleoclimate simulations. In the discussion, we address the\npotential effects of the Tibetan Plateau on the North Atlantic storm track and the NAO.\nIt also says that “finer geographic features, such as ocean gateways, were not included”\n(L262-263), but what does that mean? The changing distribution of continents will\nhave changed the distribution of oceans – is that not included in the low-res\nsimulations (and hence also in the high-res models)? The response to the reviewer\ncomments show that the authors have thought more deeply about ocean gateways,\nand even conducted numerical experiments with changes in the gateways, but this is\nnot mentioned in the paper. It is ok if the authors don’t want to include the ocean\ngateway and Tibetan Plateau aspects in this paper, but I think it should be clearer what\nthe main paleoclimate simulations actually include.\n\nThank you for your comments. In our main paleoclimate simulations, we use the\npaleogeography from Scotese and Wright (2018), which includes maps of continental\ndistribution, bathymetry, and topography for each time period. However, there are\nuncertainties regarding the evolution of some paleogeographic features, which remain\ntopics of debate. For instance, in Scotese and Wright (2018) the Drake Passage has\nbeen open since 140 Ma, whereas other studies propose that it did not open until the\nCretaceous-Paleogene (~66 Ma, Livermore et al., 2005; Scher and Martin, 2006).\nIdeally, it would be beneficial to test the robustness of our results against all\nuncertainties in paleogeographic features. However, practical limitations in time and\ncomputational resources make this infeasible. In our previous response, we tested the\nsensitivity of the Drake Passage's configuration on our results, demonstrating that\nthese paleogeographic features do not significantly affect the main conclusions.\nRegarding the preliminary results on the ocean gateway and the Tibetan Plateau, we\nprefer not to include them in this manuscript to maintain its focus. These results may\nbe reported in a different format in the future.\n\nFollowing your suggestion, we have added descriptions and clarifications on the\npaleogeographic setup in the first paragraph of the \"Paleoclimate Simulations\" section\nof the Methods.\n\nLivermore, R., Nankivell, A., Eagles, G., & Morris, P. (2005). Paleogene opening of Drake\npassage. Earth and Planetary Science Letters, 236(1-2), 459-470.\nScher, H. D., & Martin, E. E. (2006). Timing and climatic consequences of the opening\nof Drake Passage. science, 312(5772), 428-430.\n\n2. Description of changes in the jets and storm tracks. I find it a bit hard to follow the\ndescription in lines 153-170 on the relevant figures (Figs. 2, 3, S4, and S5). For instance,\nthe area with storm tracks in the North Atlantic seems to have been higher during the\n40-10 Ma period than during the PI. That relates to another question – exactly what\npart is included in the calculation of the two curves in Fig. 2b? Also, looking at the high-\npressure changes in Fig. 2b, it appears to be more-or-less constant from 70 to 10 Ma –\nit only decreases from 10 Ma to the PI. I’m not sure how that fits with Fig. S5. Please\nclarify the description of these figures.\n\nThank you for raising these points. Let me first explain how Fig. 2b is calculated. Taking\nthe area ratio of the high pressure at PI as an example, we first identify the contour\nline (the bold blue line in Fig. R1a) corresponding to the 80th percentile of the N.H.\n(20°N to the North Pole) pressure anomaly. We then calculate the area enclosed by\nthe contour line over the North Pacific region (referred to as the area of the North\nPacific high pressure) and the area enclosed by the contour line over the North Atlantic\nregion (referred to as the area of the North Atlantic high pressure). The area ratio of\nthe North Pacific high pressure to the North Atlantic high pressure is 0.45 for PI, plotted\nas the lowest orange dot in Fig. 2b. The area ratio of storm track is calculated in a\nsimilar way, except that the area is based on the contour line of the 80th percentile of\nthe N.H. (20°N to the North Pole) eddy kinetic energy (right column in Fig. R1).\n\"For instance, the area with storm tracks in the North Atlantic seems to have been\nhigher during the 40-10 Ma than during the PI.\"\nYou are correct that the area of North Atlantic storm track is higher during the 40-10\nMa than during PI. At the same time, the area of North Pacific storm track is smaller\nduring the 40-10 Ma than during PI. Thus, the area ratio (North Pacific to North Atlantic)\nis smaller during the 40-10 Ma than during PI, shown as the purple dots in Fig. 2b.\n\"Also, looking at the high-pressure changes in Fig. 2b, it appears to be more-or-less\nconstant from 70 to 10 Ma – it only decreases from 10 Ma to the PI.\"\nYour observation is correct. The decrease of the area ration of high pressure is largely\ndue to the strengthening of North Atlantic high pressure from 10 Ma to PI (the right\ncolumn in Fig. R1).\n\nThe supplementary figures (Fig. S4 and Fig. S5) are consistent with the results in Fig.\n2b. We also want to point out that the choice of the x-axis in Fig. 2b (the area ratio)\nvisually suppresses the variability on the left side of 1. For example, at 150 Ma, the\narea of the North Pacific storm track is about twice the area of the North Atlantic storm\ntrack, so the area ratio is 2. At 10 Ma, the area of the North Atlantic storm track is\nabout twice the area of the North Pacific storm track, resulting in an area ratio of 0.5,\nwhich appears much smaller in Fig. 2b. We have not developed a simple metric that\ncan overcome this limitation.\n\nWe have clarified the above points in the revision, with an example similar to Fig. R1\nshown in the supporting figures (Fig. S6).\n\nFig. R1. Examples showing the area of North Pacific and North Atlantic high pressure\n(left column) and storm track (right column). The bold blue lines are the 80% percentile\ncontour on the map.\n\n3. FixCO2 simulations. In the methods section (and elsewhere), it is stated that “the CO2\nconcentration in all the periods is fixed at 10 times of the PI value”. However, Fig. S3 suggests\nthe CO2 concentration isn’t fixed in all periods – only after 70 Ma. Please clarify this.\n\nThank you for your comments. We have confirmed that in the FixCO2 simulations, the\nCO2 concentration is fixed at 10 times the PI value as designed. Figure R2 shows the\nspecified CO2 concentration and the simulated global mean surface temperature\n(GMST) in the default and FixCO2 simulations. As you pointed out, even with fixed\n10×CO2, the GMST still shows a fluctuation with standard deviation of 1.3 K (red line\nin Fig. R2b or Fig. S3). This variation is mainly due to the evolution of continental\ndistribution, which alters the surface albedo. Li et al. (2023) provided a detailed\ndecomposition showing that, after CO2 concentration, the evolution of continental\ndistribution is the second most important factor contributing to climate variation over\nthe past 250 Ma. We have clarified this point in description of FixCO2 simulations in\nthe methods.\n\nLi, X., et al. (2023). Climate variations in the past 250 million years and contributing\nfactors. Paleoceanography and Paleoclimatology, 38, e2022PA004503. https://doi.\norg/10.1029/2022PA004503\n\nFig. R2. (left) The specified CO concentration for the default simulations (black) and\n(GMST) in these simulations.\n\n4. Barotropicity. I think it is important that the authors briefly explain why the 160 Ma and\ncriterion of barotropicity (e.g. in the Methods section).\n\nThank you very much for the suggestion. Previous studies show that the NAO exhibits\na distinct equivalent barotropic structure (Hurrell et al., 2003), indicating that the\nseesaw pattern can extend into the upper troposphere. Following your suggestion, we\nhave added clarification in the method section of \"Reanalysis data and definition of\nthe NAO\".\n\nMinor comments:\nL72: suggest you write “this study”\nThanks for your suggestion. We have revised it following your suggestion.\nL91: suggest you delete “become”.\nThanks for your suggestion. We have revised it following your suggestion.\nL99: suggest you insert “the” before “literature”.\nThanks for pointing it out. We have revised it following your suggestion.\nL100: suggest you replace “may assert” with “propose”.\nThanks for your suggestion. The issue is fixed.\nL112: suggest you delete “shall”.\nThanks for your suggestion. The issue is fixed.\nL131: suggest “likely enhanced the NAO substantially”.\nThanks for your suggestion. We have revised it following your suggestion.\nL180: suggest “moves” should be “move”.\nThanks, and we have revised it.\nL189: suggest you start the sentence with “Large orographic barriers, such as…”\nThanks for pointing it out. We have revised it following your suggestion.\nL224: suggest you insert “the” before ”examined”.\nThanks for your suggestion. The issue is fixed.\nL256: suggest you delete “the” before “North Atlantic”. Also, please specify what\ntimescales you have in mind here.\nThanks. We have revised it following your suggestion. We have revised it as \" over\nother timescales, such as daily or interannual\".\nL31: suggest you delete “of”.\nThank you. There seems to be a typo in the line number. We believe you might be\nreferring to the text on L318, \", except the CO2 concentration in all the time periods\nis fixed at 10 times of the PI value. \" We have fixed it following your suggestion.\nL323: suggest “flattened” instead of “flatten”.\nThanks for your suggestion. We have revised it following your suggestion.\n\nFigure 1. The first impression of Fig. 1d is that something wrong due to the pattern\nwith missing colour bars for 10, 30, 50 Ma etc. I think it is important to point out clearly,\ne.g. in the caption and on the figure if possible, that these scenarios were not included\nin the analysis.\n\nThank you for your suggestion. We have added clarification to the caption to avoid\npotential misunderstandings.", "reviewer_2": "I believe the authors have addressed in a satisfactory fashion my own comments on the previous version of the paper, as well as the excellent comments by the other reviewers, and the paper has been much improved. For the sake of the argument, I mention here that single-mountain experiments may, nevertheless, give incomplete results, as, for example, present NH stationary wave is a byproduct of wavenumber-2 topography, alongside the associated storm tracks and the atmospheric low-frequency variability in their respective downstream regions. These effects may not be fully described by local dynamics (see Swanson 2007 - storm track dynamics chapter for a review). I suggest the authors take these arguments into account in the future. For now, I think this paper is suitable for publication in Nature Communications.\n\nThank you very much for your comments. Your points are well taken. Here, we\nattribute the dynamics of the storm track and the NAO primarily to local factors, which\ncertainly has its limitations. In future studies, we will bear your comments in mind. In\nthe discussion of the previous paragraph, we have revised the text to highlight the\nimportance of non-local dynamics (the last paragraph).\n\nRemarks on code availability\nMy apologies for not being able to get to the code...\nThanks. We have updated the data and codes deposit. Now it is available at\nhttps://doi.org/10.5281/zenodo.14507022."}, "author_response": "Responses to the reviewers:\nThank you very much for recognizing our efforts in revising this manuscript. We also\nappreciate your comments and suggestions during this round of review. We have\nrevised and improved the manuscript based on these suggestions. Below are our\npoint-by-point responses (in blue) to the reviewers' comments."}, {"round": 3, "reviewer_comments": {"reviewer_2": "I believe this paper is now suitable for publication in Nature Communications"}, "author_response": ""}]

Decision letter