| 15 Oct 2025
Role of paleogeography on large-scale circulation during the early Eocene
Abstract. The configuration of continents and oceans has a major influence on Earth’s climate by shaping large-scale atmospheric circulation patterns. In this study, we investigate the effect of early Eocene paleogeography, specifically from the Ypresian stage, on extratropical eddies. We analyse the influence of the epicontinental West Siberian Sea as well as the impact of the absence of the Antarctic Circumpolar Current on mid-latitude cyclones and blocking events. Previous work from the Deep-Time Model Intercomparison Project (DeepMIP) has shown that during the early Eocene, heat transport through cyclonic systems was more intense than under modern conditions at the northern mid-latitudes, and less intense at the southern mid-latitudes. We analyse cyclone tracks and blocking systems of the early Eocene in an atmosphere-only CESM1.2 simulation, continuing the DeepMIP 1xCO2 experiment. Sea surface temperatures from the DeepMIP experiment are used as boundary conditions. The simulation output is six-hourly, which enables direct cyclone tracking in the pressure field. In parallel, a decrease in heat transport of stationary eddies at the northern mid-latitudes in DeepMIP data, motivates the analysis of blocking climatology based on the 500 hPa geopotential height field. Our results show that, through air–sea interactions, paleogeographic features of the early Eocene enhance cyclonic activity at northern mid-latitudes while reducing it at southern mid-latitudes compared to modern conditions.
This manuscript is well-organized contribution to our understanding of how early Eocene paleogeography shaped mid-latitude circulation, cyclone activity and blocking. The modeling setup is appropriate, and the comparison between pre-industrial (PI) and early Eocene conditions is clearly motivated. The main conclusions are convincing and broadly consistent with earlier DeepMIP work. That said, some sections are quite dense, a few claims could use clearer supporting evidence,and there are places where the writing becomes a bit repetitive. With some tightening and clarification.. the paper will be even stronger.
Major Comments
Line 10: The phrase “In parallel, a decrease in heat transport…” reads abruptly. Maybe say something like: “Motivating our blocking analysis, previous DeepMIP results showed…”
The core message.. more cyclones in the Northern Hemisphere and fewer in the Southern Hemisphere.. Is clear and well placed. The paleogeographic background (Lines 15–30) is good, but dense. A single summary line linking the geological setup to atmospheric circulation would help.
Line 21: The “C-shaped distribution of landmass” might confuse some readers. Adding a short explanation or pointing to a figure would help.
Lines 24–31: This section jumps quickly between monsoons, carbon cycle changes, and ice ages. Tightening it around “how paleogeography shapes climate circulation” would improve the flow.
Lines 35–45: The EECO description is well supported, but the number of proxies cited slows the narrative. You could trim a bit here.
Lines 45–55: When referring to Kelemen et al. (2023), it would help to briefly restate what changed in the heat transport components (e.g., stronger transient eddy transport in NH).
Lines 60–63: The goals are clear but could be merged to avoid repeating ideas.
Lines 75–90: Good justification for using 6-hourly data. If any spin-up was discarded, mention that explicitly.
Lines 80–85: Clarify whether atmospheric initial conditions came directly from the DeepMIP runs or were re-initialized.
Lines 95–101: The tracking thresholds ( ( >1 day) 20 hPa, 1000 kmdistance ) are standard, but a short sentence explaining why these values are chosen would help readers not familiar with cyclone tracking.
The blocking index description is long in lines 103–112. Consider moving most of it to supplementary material and keeping a short summary here.
Line 110: Explain why you remapped to 2.5° resolution; readers may wonder whether this reduces blocking sensitivity.
Lines 115–125: The explanation of NH vs SH cyclone changes is clear. Refer explicitly to Fig. 3a–d in the text to guide readers.
Line 120: 36% increase and 32% decrease are large values. Clarify whether these are annual averages or seasonal means? or include uncertainty if possible.
Since you used cyclones as a stand-in for transient eddies (Line 125).. it may help to point that out more directly
Lines 124–130: The eastward shift in NH blocking is interesting. A short explanation of why the West Siberian Sea environment favors blocking (thermal contrast ? moisture? ) would strengthen the argument.
Line 129: When you say “similar intensity,” specify whether this means statistically similar or just visually comparable.
Lines 130–140: The moisture transport explanation is good. Adding a mention of storm-track–orography interaction would make it even clearer.
Increased precipitation over western North America deserves a bit more detail @Line 134. It is likely linked to orographic lifting combined with more incoming tracks.
Lines 141–150: Nice summary of heat transport changes. You might add a simple clarifying sentence like: “These differences appear despite similar total MHT, due to Bjerknes compensation.”
Lines 155–160: The reason for increased NH baroclinicity could be elaborated—does it come mostly from stronger thermal contrasts or increased moisture supply?
Line 160: When discussing the absence of the ACC, briefly explain the physical tie between ACC, baroclinicity, and storm-track strength.
Lines 165–170: The statement that the Southern Hemisphere jet is weaker should be backed by a figure or numbers.
The link between OHT changes and deep-water formation is well made; one extra sentence tying this to energy balance constraints would be helpful (Line 170)
Lines 183–190: This summary is strong. A simple schematic showing the West Siberian Sea’s influence on baroclinicity would make the mechanism clearer.
Line 190: Instead of “We hypothesise…,” you might soften to “These results suggest…”
Line 195: The mention of future high-CO₂ simulations is good. Briefly state what specific questions those runs will address.
Figures 3 and 5: These are strong figures. Adding contour lines of precipitation would sharpen this spatial feature.
Figure 8: Latent + sensible heat fluxes are important, but splitting them or showing anomalies might make interpretation easier.
Figure 9: Adding wind vectors or streamlines would help readers visualize circulation, not just magnitude.
Minor:
Line 128: “More dispersed distribution” Lets specify whether you mean geographically or in frequency.
Throughout....: The explanation of OHT/AHT compensation appears multiple times; consider reducing repetition!!
References: Solid overall, though a couple more recent studies on Eocene storminess and SST gradients could be helpful.