Dear Dr. Cramwinckel:

Thank you for submitting your manuscript entitled « Surface-circulation change in the Southern Ocean across the Middle Eocene Climatic Optimum:  inferences from dinoflagellates cysts and biomarker paleothermometry» to Climate of the Past.

Your manuscript has been read by three reviewers, S. Fauquette, C. Hollis and G. Raquel Guerstein. While the three reviewers recognize the importance of your work, they also ask for substantial modifications of your manuscript. Different topics come out from these reviews, the interpretation of your records in terms of climate dynamics remain qualitative and it seems that your records may be interpreted differently. Please do account for these comments in your revised version. Also, two reviewers are skeptical about your continental record of the MECO claiming that 4 samples are not enough and that in the absence of a pre- and post-MECO intervals, it remains hard to say something about the MECO on land. Last, two reviewers also ask for more description of palynomorphs on which your interpretations are based.

I read your answers to the reviewers and I’ve appreciated the way you answered to each comment. Questions about uncertainties on the paleogeography and their impact on our understanding of the ocean-atmosphere dynamics at that time are non-trivial as you pointed out. I encourage you to clarify points in your revised paper, by providing the readers the level of confidence of your qualitative interpretations. Although Earth System Models have their own uncertainties, I also encourage you to use them to anchor your interpretations into the physics and to avoid over interpretation.

I encourage you to submit a suitably revised version of your manuscript taking into account as much as possible reviewer assessments (as you already did in your answers). I will probably send back the revised manuscript to one of the reviewers.

I look forward to receiving your revised manuscript. If you have any questions, please contact the editorial office at editorial@copernicus.org
.

Sincerely,

Yannick Donnadieu

Dear Authors,
First, I apologize for the long time it took to get this paper thoroughly reviewed. It was worth it seeing the discussion triggered by the reviewer 3 about the physical interpretation of your records. You will find the final review by the reviewer 3 attached to this email. It is still negative on the ground that your explanations for a more southward extent weren’t convincing and somehow misleading.

I spent a day reading the articles about the southward extension of the East Australian Current, especially those co-authored by Matt Huber. For the most part, they rather show that the existence of this current towards the Southeast edge of Australia in the Eocene is not possible unless 1) there is a strip of land joining Australia and Antarctica and 2) the existence of deep-water formation on the Eastern part of the Southern Pacific (Sijp et al., 2011). However, in this paper, the authors used wind fields from the CSSM1 to force the UVIC model. In addition, UVIC remains an EMIC (Earth Intermediate Complexity Model) with a poorly resolved hydrological cycle in the atmosphere, meaning that deep sinking locations should be taken with caution. Most recent modeling papers using CCSM3 (but also CCSM1, see Huber, 2004)) don't show this strange feature, i.e. southward flowing current while winds are blowing northward (See Fig. 7 of Hollis as pointed out by the rev3). I suspect some inconsistencies between the ocean and the atmosphere in UVIC potentially affecting their results. In any case, another argument can be used against the possibility of an EAC extended till the southern tip of Eastern Australia. Indeed, once TG is opened (even a few hundred meters), barotropic oceanic currents (which are the currents resulting from the onset of deep sinking in the Ross Sea) will be cancelled in this area. From what I got from the paper in review, the TG is already open and a land bridge can’t be applied.

My recommendations are the following for your paper to be accepted in Climate of the Past.
You need to disregard the hypothesis of an EAC located in the south of Australia if you’re unable to support it. This seems to be the case in the present state of the manuscript. Referring to modern behavior should be erased as you can’t base your explanations on our modern understanding as Australia was 15° southward to its modern position and wasn’t influenced by the same meteorological processes. In addition, in order to help the community to move forward, it may be nice to have a climate modeler joining your paper to do a synthesis of what we know about the EAC in deep times using all available simulations. Anna von Der Heydt and Michiel Baatsen may be the right person to help you doing this work. Last but not least, other explanations like the one suggested by the reviewer 3 makes sense and in the absence of other hypothesis, it may be wise to put it forward while keeping open your records for other physically based hypothesis. That’s to say that a fair and complete review of modeling papers should still let you propose the EAC but highlighting that conditions for its existence remain really weak and disagree with an open Tasmanian gateway.

I hope these comments will be useful and will help you to come up with an acceptable final version. The reviewer 3 won’t review the final version. I will do it. So please, don't hesitate to contact me if you need more information.

Best Regards,
 Yannick Donnadieu

Dear Dr. Cramwinckel:

Thank you for submitting your work to Climate of the Past. I am pleased to accept your manuscript for publication in the journal. You made a great job in accounting for all comments and summarizing all hypothesis we still have to test, to accept or to reject.

Sincerely,
-- Yannick Donnadieu