the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 18 Mar 2026
WCD Ideas: Hydrologically Driven Throughflow in the Coupled Ocean–Atmosphere System
Abstract. Potential flow theory predicts bulk fluid motion driven by spatially separated sources and sinks of mass. In the atmosphere, such exchanges are dominated by the hydrological cycle: subtropical sources of water vapour combine with equatorial and high-latitude sinks to induce meridional source–sink flows in each hemisphere. Conventional gas-phase frameworks that represent mean meridional circulations as purely cellular neglect these throughflows. Here, these flows are identified as atmospheric branches of coupled ocean–atmosphere circulations termed Latent cells. Evidence from inert tracers indicates that they conduct significant large-scale mass transport, and they may influence atmospheric momentum budgets.
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Status: final response (author comments only)
- RC1: 'Comment on egusphere-2026-754', Anonymous Referee #1, 29 Apr 2026
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RC2: 'Comment on egusphere-2026-754', Anonymous Referee #2, 29 Apr 2026
As I will explain below, I feel extremely torn about the value of this paper. On one hand, the author highlights the intriguing fact that the atmospheric circulation drives a net meridional transport of water. Streamlines of meridional mass transport in the atmosphere do not close, unless one accounts for the return of water vapor by the ocean! This is a novel perspective that fits well with the WCD Ideas goal of raising interesting questions to the weather and climate community.
On the other hand, I am concerned by the near utter lack of quantitative analysis. I feel it is important that the author provide at least a rough estimate of the magnitude of these effects. Lacking something quantitative, the reader is left wondering if this is an important effect missing in our understanding and/and or ability to model the atmosphere, or a miniscule effect overwhelmed by uncertainties in other processes.
As a possible example of what I mean, the Earth is an oblate sphere. Should we be concerned by models that treat it as a sphere, or is this effect safely well below the rounding error/other uncertainties? This was considered in Gates 2004 (https://doi.org/10.1175/1520-0469(2004)061<2478:DOTEOA>2.0.CO;2,), who quantifies the effects in section 7 and table 1. [This is not meant as a request to cite this paper, rather, an example of how to quantify a neglected effect!] Quantitative estimation matters a lot: there are many things one could worry about: deviation of air from the ideal gas law, non-traditional Coriolis terms, Cosmic rays, etc., but some things matter and others don’t.
I think it should be straightforward to estimate the magnitudes of the latent cells and compare them to the mass transport by the atmosphere. Based on precipitation less evaporation, P-E, which could be taken from a model or reanalysis (albeit with caution), the net transport of water could be estimated and compared the transport by the mean meridional overturning cells and lagrangian transport by eddies (e.g., the TEM circulation, or isentropic circulation).
Another way to bolster the quantitative elements of the proposal would be to show more about the argon and oxygen fields. I was intrigued by the gradients in these species created by latent cells, and maps of their concentration relative to the latent overturning would be exciting to see.
Lacking something quantitative, I am hesitant to recommend publication of this article. To be constructive, I’ve made several suggestions/comments below for the author to consider.
- As discussed above, I feel the paper needs at least a back of the envelope estimate of the magnitude of these cells, and/or more quantitative discussion of the argon and oxygen gradients.
- The net transport of water vapor by the atmosphere is also intimately connected to energy transport, which can in turn be linked to mass transport by considering the circulation averaged on dry and moist isentropes (a more Lagrangian perspective). I think it would help to link the latent cells to energy transport, an even more fundamental aspect of the atmospheric circulation, or at least make a connection to this body of work.
For example, there is a series of nice work conducted by Pauluis and coauthors that consider the transport of mass by the circulation in potential temperature and equivalent potential temperature coordinates, in particular Pauluis et al. 2008 (10.1126/science.115964). They show that the amount of mass moved by the atmosphere from the equator to the pole (and back again) effectively doubles when you account for the fact that poleward moving air carries moisture relative to the returning, drier equatorward flow. - I did not find Figure 1 to be helpful; I think all readers will know what convergent/divergent flow looks like. This is just my opinion, but I felt the paper started too slowly, to the point of insulting the reader, and would appreciate jumping more quickly to the latent cells.
I appreciate the goal of Figure 2, but I also found that the schematic didn’t help much beyond the description in the text, which was informative. The problem is that the schematic diagram doesn’t include the essential sink and source elements, so I don’t see exactly how it helps.
To be constructive, I did like Figure 3 and would encourage the author to move it up in the text. This more quickly gets the reader thinking about the latent cells. Following up this schematic with an rough estimate of the net transport would be very nice, in my opinion.
Finally, here are a few points in the text where I believe I misunderstood the author. I point them out not to be critical, but to encourage the author to be more precise.
21-22 Baroclinic eddies are by no means the “smallest scale” of the atmosphere, nor of the transport of water vapor. Convective systems transport water, from the scale of an individual cloud to a tropical cyclone.
The notion of a “least degree of organization” is also vague. Baroclinic systems are very effective at transporting sensible and latent energy poleward. To argue that this transport/precess is “less organized” than the Ferrell or Polar Cells seems a matter of semantics. It is know that baroclinic systems transport far more mass than the Ferrel or Polar Cells. The polar cell is very weak, to the point that I believe one needs multi-year averages to see it. I was in general surprised by the amount of time discussing the polar cell throughout the paper given how small and insignificant it is relative to the eddy transport.
23 Baroclinic eddies transport angular momentum. This understanding dates back to Rossby, I think, and is textbook material, e.g., Vallis (2017) Chapter 15. I believe I misunderstand what the author meant by this statement.
28 Baroclinic eddies transport momentum upgradient, to generate the westerly surface winds.
29 The Hadley cell is a better example of a circulation which transports humidity upgradient.
30 I don’t understand what the author means by no net momentum? The Hadley cell advects angular momentum poleward, manifested the subtropical jets. I think I misunderstand what kind of momentum the author means?
35 I appreciate that the latent cells are hemispheric in scale, but in the midlatitudes I believe that the transport of water vapor is dominated by baroclinic eddies. I think it would be a mistake to think of these latent cells in a zonal mean sense.
152-155 Again, I think this description implies that the meridional transport in the atmosphere is dominated by the zonal mean cells. This is somewhat true for the Hadley cell, but demonstrably incorrect for the Ferrel and Polar Cells. I strongly encourage the author to emphasize that this meridional transport is not effected by the zonal mean circulation, but rather dominated by eddy transport.
Section 4 discusses the hydrological cycle. It’s begins a very basic level, but never seems to connect with the large body of work in the atmospheric literature on how to define mass transport. Equations 3 and 4 take baby steps towards the idea of a residual mean circulation, but never get past the idea of a single parcel.
180 I acknowledge that the author is getting to the point that eddies dominate the transport of water in the atmosphere, but it’s a rather oblique way of getting to this important fact.
197-203 I found this part of the manuscript intriguing – and getting near the point of being quantitative – but feel that I don’t fully understand it. (I point this out with humility, that this is a section where the author could help a reader like me.) I don’t fully understand why O2 is transported much more than CO2. Is this because there is vastly more O2, and thus the photosynthesis and cellular respiration thus have a stronger relative impact on CO2? Or is it because there are fossil fuel sources of CO2 (whose burning would affect O2, but again, the relative abundance is very different.) Quantifying the effect of biological vs. latent cells on O2 and CO2 would have helped me.
209 Why is the author talking about linear momentum when it is angular momentum that is conserved? I again refer to text book explanations of the momentum transport by the eddies and mean flow, mentioned above. I think I’m a bit confused, because the first part of the paragraph talks about weak zonal winds in the subtropic (implying less momentum) and then switches to a discussion of the abundant angular momentum. I think the switch from linear to angular momentum makes this overly confusing.
Citation: https://doi.org/10.5194/egusphere-2026-754-RC2 -
EC1: 'Editor comment on egusphere-2026-754', Stephan Pfahl, 03 May 2026
Dear Andrew Kowalski,
Your manuscript has been evaluated by two reviewers, and while both acknowledge the fact that it contains some interesting and novel thoughts, they also raise important points that should be improved. In particular, I think your work should be better connected with the more recent and relevant literature on the global circulation, and, as also suggested by both reviewers, you should try to obtain some quantitative estimate of the relevance of your suggested latent cells. Please also take the other, in my opinion very constructive, reviewer comments into account when preparing a new version of your WCD ideas submission that could then be re-evaluated for potential final publication.
Best regards,
Stephan Pfahl
Citation: https://doi.org/10.5194/egusphere-2026-754-EC1
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- 1
The paper notes that there is net (water) mass transport by atmospheric circulation and argues that such a throughflow should be accounted for. While the fundamental observation is correct and the author raises some interesting issues regarding the distribution of trace gases in the atmosphere, the proposal to address it through a 'latent' cell is rather unclear.
Main comments:
1. Discussion of the atmospheric circulation:
The description of the atmospheric circulation as consisting of three cells superimposed on an eddy "diffusive" flux is rather dated. Multiple authors have provided insight into the global circulation and, in particular, on the role of the hydrological cycles (see Pauluis et al. Science 2008, and Lalilberte etal Science 2015, among others). There is also extensive literature on 'freshwater transport' by the ocean circulation (see Schanze et al., Journal of Marine Research, 2010).
2. What exactly is the author's proposal for a throughflow?
While everyone would agree that there is indeed net transport of (water) mass by the atmospheric circulation, this transport is small when compared to overall mass transport. The author proposed addressing this by adding a throughflow. Less clear is how the author would estimate it, or whether it would have a significant impact on circulation itself. (Abeit the author seems to concede that the impact of the throughflow would be minor, "the direct contribution of atmospheric throughflow to the meridional transport of water vapour itself is minor" on line 180.)
The issue here is that there is a simple fix to the problem raised by the author: Given that there is no mass transport of 'dry air', one solution is to compute the circulation in terms of dry-air mass transport. This circulation would be 99% similar to the circulation obtained using the 'moist air transport', and would not involve any throughflow . In this context, the paper would benefit from some modicum of computations. Reanalysis datasets are widely available and should be used to provide preliminary results.
3. The discussion of trace gas distribution is welcome and could be more detailed.
The more interesting part of the paper is the discussion of trace gas distributions at the end of section 4. The discussion is fairly informal and lacks a clear testable hypothesis that would motivate the use of a thorough flow. At one point, the author claims that "Throughflow therefore dominates the net meridional transport budget of argon in the midlatitudes, even though it is not readily apparent in conventional circulation diagnostics". While this is an interesting claim, it is not backed by any data or supporting evidence.