the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 25 Apr 2025
Energetics of monsoons and deserts: role of surface albedo vs water vapor feedback
Abstract. Despite receiving similar solar energy, the top-of-the-atmosphere (TOA) radiation budget is negative over deserts and positive over monsoon domains. This contrast has been attributed to differences in the surface albedo between the two regions. Here, we show that this difference in TOA radiation budget is primarily driven by the absorption of longwave radiation by water vapor, while the surface albedo only plays a secondary role. As a greenhouse gas, water vapor absorbs the surface longwave radiation and enhances the local radiative heating of the atmosphere. Due to the aridity of the deserts and prevalent clear sky conditions, long wave energy is efficiently radiated to space. We demonstrate that this is the dominant cause of the net radiative cooling of the atmosphere. Our hypothesis is confirmed by a climate model experiment in which the Earth’s rotation is reversed. This flips the zonal asymmetries producing a monsoon over the Sahara (in spite of high albedo) and a desert over South East Asia (where low albedo persists). We find that radiative feedback of water vapor on the large-scale circulation drives these changes initially, with further amplification by cloud feedbacks. Our results suggest that this radiation-circulation feedback due to water vapor enhances monsoon-desert contrast.
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RC1: 'Comment on egusphere-2025-1734', Anonymous Referee #1, 21 May 2025
Review of the manuscript entitled "Energetics of monsoons and deserts: role of surface albedo vs water vapor feedback" by Jalihal and Mikolajewicz
In this manuscript, the authors present a compelling argument that the top-of-the-atmosphere (TOA) radiation budget contrast between monsoon and desert regions is primarily driven by water vapour feedback, with surface albedo playing only a secondary role. This challenges the classical Charney (1975, https://doi.org/10.1002/qj.49710142802) hypothesis, which emphasises albedo-driven desertification feedbacks. The study employs a combination of theoretical reasoning and a novel climate model experiment (RETRO, in which Earth’s rotation is reversed) to support its claims. While the hypothesis is intriguing and potentially significant for understanding monsoon-desert radiative dynamics, I have some serious concerns regarding the experimental design and interpretation of results.
Major Concern: Limitations of the RETRO Experiment
The central issue with this study lies in its reliance on the RETRO experiment to "confirm" the hypothesis. While reversing Earth’s rotation is a creative way to alter large-scale climate asymmetries, it is not an appropriate experimental framework for isolating the specific roles of water vapour versus surface albedo in TOA radiation budgets. My concerns are as follows:
- Fundamental Alteration of Planetary Dynamics
Reversing Earth’s rotation drastically changes the Coriolis force, jet stream pathways, ocean circulation, and storm tracks. These modifications introduce confounding dynamical effects that are unrelated to water vapour’s radiative role.
The resulting climate (e.g., a Sahara monsoon and Southeast Asian desert in the RETRO simulation) is influenced not just by humidity and radiation but also by completely reconfigured atmospheric and oceanic circulations. Thus, attributing the TOA budget differences solely to water vapour is problematic.
- Lack of a Clean Sensitivity Test
A more robust approach would involve directly perturbing water vapour concentrations (e.g., through a "dry world" vs. "moist world" experiment) while keeping Earth’s rotation unchanged.
Alternatively, radiative kernel analysis could quantitatively separate the contributions of water vapour, clouds, and surface albedo to the TOA budget. Please refer to Soden et al. (2008, https://doi.org/10.1175/2007JCLI2110.1) for further details.
- Overlooked Factors: Dust Aerosols and Clouds
The study does not account for dust aerosols, which are prevalent over deserts and significantly influence both shortwave (albedo) and longwave (trapping) radiation (Osborne et al., 2011, QJRMS, https://doi.org/10.1002/qj.771).
The role of cloud feedbacks, while briefly mentioned, is not rigorously disentangled from water vapour effects. Since clouds co-vary with humidity, their radiative impact could also explain part of the TOA contrast.
Citation: https://doi.org/10.5194/egusphere-2025-1734-RC1 -
RC2: 'Comment on egusphere-2025-1734', Anonymous Referee #2, 05 Jul 2025
Despite positioned along the same latitude in the norther hemisphere, dry-warm dessert exists over Sahara while wet monsoonal region is seen over south Asia in the present climate. In this manuscript, authors show that this difference is because of the presence of water vapor in the air. Water vapor is a strong absorber of longwave radiation emitted by the surface. Abundance of water vapor over South Asia enables high amount of longwave absorption such that the net radiation at the top of the atmosphere becomes positive, a necessary condition for monsoon to exist. Such a condition is not prevalent over Sahara on account of lack of water vapor. Authors confirm this by a GCM experiment where they performed two simulations with opposite rotation direction of the earth around its own axis.
The overall theme of the study is excellent. The results are useful to understand both monsoons and desert mechanisms. However, I have two primary concerns regarding its current version. Firstly, several figures are in the supplementary which make the text hard to follow. I suggest moving some important figures to the main text and expand discussions around them. Secondly, the data sets are from reanalysis. It is well known that reanalysis products, an output of numerical models, suffer from reasonably capturing clouds, a key component of the radiation budget of the climate. My detailed comments are given below. Although a figure is shown in Supplementary comparing ERA5 Fnet with that from CERES, I’m not convinced by it. The reason being, this study further divides its components, and it is highly likely that components of Fnet could be very different in ERA5 as compared to satellite-based estimates. It is advised to compare other components of radiation budget in the manuscript to make it more robust.
Specific Comments.
- Fig 1: Fnet is close to zero over Sahara. But the text all along says that Fnet is negative over Sahara (Lines 1, 13). Please correct the sentences accordingly.
- Overall, the manuscript can be expanded to make it a ‘long-paper’. I suggest
- moving the sub-sections under ‘Appendix’ under a new a section ‘Data and Methods’ after Introduction.
- moving some key figures from the supplementary to the main text.
- Organize the text such that the flow is a bit more natural. I suggest keeping the entire observation part at the start. Then the modeling part can start. Or keep the modeling component from the beginning to the end of the centerpiece and use observations only as evidence to the proposed theory.
- Line 72: Fic 2c and 2d show the relationship of OLR with CWV and Ts, respectively. Please correct.
- Line 74: “CWV over the Sahara does not exceed 25 kg m-2, and does not influence the clear sky OLR”. Figure 2b suggests that OLRclearsky increases sharply with an increase in CWV over the Sahara.
- Line 76: “As CWV increases during the onset and progression of the monsoon, clear sky OLR transitions from a state where it depends on surface temperature to a state where it depends on CWV.” It is not clear to me from the plot. I suggest making another figure to clearly visualize and justify this statement.
- It will be good to see the difference in SST between the two experiments. Is there any significant change over the Atlantic Oceans?
- There’s a key difference between the two descending regions in RETRO and CTL. Over Sahara in CTL, the descent does not have any seasonal cycle. However, over East Asia in RETRO, a strong seasonal cycle is prevalent. This indicates that the ‘monsoon’ happens during northern winter over East Asia in RETRO. What’s the reason for this difference?
Citation: https://doi.org/10.5194/egusphere-2025-1734-RC2
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