Dear reviewer, 

We sincerely thank you for reviewing our manuscript and are grateful for your constructive comments recognizing the value of our work. We carefully accounted for all your comments (in italic) and questions and provided detailed answers here. 

The main remarks concerned the possible mechanisms associated with the areas of decreasing temperature seasonality and the presentation of these results with the limitations. To address this, we made new diagnostics which we may include as additional figures in the manuscript.

In addition, while correcting our manuscript, we felt it was unfortunate not to include data from the compilation of Pound and Salzmann (2017), given the small number of data available. We propose to increase the number of data and thus update the results accordingly as presented in a dedicated section after answering your comments. We are aware that this kind of practice is not usual, and we apologize for the extra work it may require, but we believe it will give more representative results of the changes of the Eocene-Oligocene Transition. The message of the paper and the conclusions remain the same.

Overall, we feel the manuscript is greatly improved by these substantial revisions.

Best regards, 

Agathe Toumoulin on behalf of all co-authors.

-------------------------------------------------------

Major comments:

#1 Overall, I think 3.1.3 is cutting corners, it might well be that some of the stated mechanisms are true, but it is difficult (if not impossible) to confirm the mechanisms based on the evidence presented. For this type of paper, it is not crucial to identify the exact mechanism, although it is valuable of course. I would suggest the following 1) to be able to make a bit more robust statement, the authors could check the correlation between surface air temperature change and latent heat flux change/P-E change/Primary prod. Change. 2) I would change the language towards ‘we suggest that this phenomenon could be explained by...’ rather than ‘this phenomenon is well explained by’.

Thank you for this comment. We recognize that the mechanisms could be better explored. In order to take this comment into account, we have performed additional diagnostics to better evaluate the potential mechanisms at stakes in our simulations. We focused particularly on the regions demonstrating a decreased seasonality in the early Oligocene (mostly at mid-latitudes), given that they constitute the most counter-intuitive result of this study. To do so, we extracted the anomalies in Precipitations, Evaporation, resulting P-E, NPP and surface temperature on land areas between 2X and 3X simulations (since the areas of decreasing seasonality appear as a result of decreasing CO2, see v1 of the manuscript, see Figure below).

We feel no evident correlation or single mechanism emerges from these diagnostics and the magnitude of these changes is highly variable from one region to the other which supports that decreasing temperature seasonality may result from various mechanisms depending on the considered area. The agreement between decreasing summer temperatures and increasing latent heat fluxes/net precipitation appears particularly good over the United States, and less so over Asia and Australia. In contrast, additional mechanisms are needed to explain the temperature changes over Europe and southern South America (as mentioned in the first version of the manuscript, see also our answer to your next comment). Finer analysis involving perhaps daily to hourly resolution might be necessary to provide a better understanding of the mechanisms at stake  

We have restricted Fig. 5 to subfigures (a-d) and made a new figure (see below) showing co-variations between temperature, latent heat, hydrological cycle (precipitation / net precipitation / evaporation), and net primary productivity. We propose to add this Figure to the supplementary material since it provides information on specific climate mechanisms that are not necessary for the understanding of the manuscript.

We modified the text of section 3.1.3 accordingly and added a short sentence in the discussion  (section 4.1.2) to discuss the changing extent of atmospheric cells in greenhouse climates. As suggested we also reformulated the sentence originally located l. 263.

New sentence section 4.1.2.:

“In parallel, the intensification and weakening of the Hadley cell extent in relation to changing pCO2 levels have been described numerous times (e.g., Lu et al., 2007; Frierson et al., 2007), but the implication of these mechanisms in the South American seasonality lowering zone appears non-obvious. Deeper analyses would be needed to understand the atmospheric dynamics in the simulations, which is out of the scope of the study”

We also modified the sentence l. 263 following your second suggestion:

“This phenomenon is well explained by two distinct chain reactions” by “This phenomenon could be explained by several chains of reaction, which are driven by both atmospheric and/or oceanic responses depending on the area”.

Additional diagnostics - Annual variability of multiple climate parameters within the different seasonality lowering terrestrial zones between 3X and 2X (a-c,g,h): surface atmospheric temperature (black), latent heat flux (soil to atmosphere; brown), hydrological cycle (incl. precipitation, evaporation and net precipitation, different shades of blue), and net primary production (green). (d-f) Temperature changes and ∆MATR between the simulations. Rectangles contour terrestrial zones (ocean zones are not included) analysed in subfigures (a-c,g,h). 

#2 Especially the argument of increasing cloud cover is not very convincing to me. In western Europe, there is a 10-20% increase, but that is not really seen in the southern hemisphere (small patches of 10% increase in austral summer). However, Fig 4b and magenta contours in Fig 5 seem to suggest that the negative MATR changes take place at the edge of the Hadley cell (and the associated ocean gyres/fronts) and the changes would be consistent with an equatorward/poleward shift of the Hadley cell – which would also impact the oceanic subpolar gyres. The Hadley cell extent has been well studied and can be related to changes in a latitudinal temperature gradient, which is clearly changing in these simulations. I would encourage the authors to rethink their results in this context.

We have performed additional diagnostics but the response does not seem to explain the subtropical trends, and in particular: the summer cooling signal observed in South America (and the associated decrease in temperature seasonality).

We observe changes, particularly in austral summer (JFM), with an increase in the intensity of the Hadley cell and a slight southward shift in the rising limb of the cell (new Fig. S4, h,i), in agreement with studies of the change in cell intensity and width under higher pCO2 (Lu et al., 2007; Frierson et al., 2007, and Chemke and Polvani, 2020, all three in Geophysical Research Letters). In parallel, there is a northward migration of the polar front (boundary between atmospheric polar cells and Ferrel cells), especially during the austral summer, and of the westerly wind maximum (by about 2° latitude, annually but less markedly during the austral winter, JAS; Figure S4). The Antarctic Circumpolar Current follows this northward shift (Figure S5), limiting the arrival of warm subtropical waters to the South Atlantic, between 40-45°S, but independently of the time of year.

The implication of changes in atmospheric and oceanic dynamics on temperature variability remains unclear as they have a small amplitude compared to the mid/high latitude anomalies and the latter, although small, would require a more detailed study which is - as you mention - out of scope here.

We may evoke these mechanisms in the results, in the form of two additional figures (Fig. S4 and S5 below), but nuancing their potential impact.

New Figure S4 - Changes in atmospheric temperature and vertical circulation patterns between 3X and 2X in the Southern Hemisphere. (a,b) latitudinal surface temperature gradient; (c) zonal winds; (d-g) Air temperature (shaded), atmospheric cell extent (zonal mean streamfunction, lines) and vertical winds (arrows) in austral summer and winter for the simulations 3X and 2X. (h,i) Temperature, atmospheric cell extent and wind changes between the simulations 3X and 2X. The white arrow shows the northward migration of the polar/ferrel cell boundary. Dashed lines indicate anticlockwise circulation, solid lines, clockwise circulation. Arrows correspond to vertical winds. Atmospheric circulation was calculated over the pacific sector, between 180-30 °W.

New Figure S5 - Annually 0–300 m depth averaged current velocity through the Southern Ocean (annual average, m.s-1). 

#3 In relation to comments #1-#2 I would encourage the authors to check the relative change in MATR. Since the MATR is usually small over the ocean, I would think that some of the signals would be emphasized, and maybe easier to appreciate, if one would look at the change relative to the baseline (i.e change in percentage).

Thanks for this interesting comment. We completed Figure 4 with the relative changes in MATR for 2X-3X and 2X-ICE-SL - 3X, and modified Figure S6 to enable a direct quantification of relative MATR changes associated to each forcing. Results are consistent with our previous figures although high-latitude seasonality increases tend to look more moderate.

We also provide relative ∆MATR values in the text, sections 3.1.2, 3.1.3 and 3.1.4

• “It [The large MATR increase at high northern latitudes] represents an increase in MATR of 5-20% between 3X and 2X and up to 40% between 4X and 2X (Figure 4 e and S6 b).”
• “The widest zones with decreasing MATR are located within the 30-50°N latitudinal band, across North America, Western Europe, Central Asia, and 30-50°S for South America and Australia (depending on the pCO2 lowering considered, 280 or 560 ppm, regionally up to 20 or 30% reduction of the MATR, Figure 4).” 
• “As visible from relative ∆MATR, seasonality strengthening takes place both in areas characterized by strong or weak seasonality during the Eocene (Figure 4 f and S6 f).”