the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 27 Jun 2025
Non-zonal gravity wave forcing of the Northern Hemisphere winter circulation and effects on middle atmosphere dynamics
Abstract. Gravity waves (GWs) are a major yet poorly constrained driver of middle‑atmosphere dynamics. Using the high‑top UA‑ICON global circulation model, we conducted a set of six-member ensemble simulations in which orographic GW drag was selectively intensified over three Northern Hemisphere hotspots – the Himalayas (HI), Northwest America (NA), and East Asia (EA) – to assess their long‑term dynamical impacts on the stratosphere. The imposed forcing generated distinctive vertical–horizontal drag structures in each region, yet produced a coherent hemispheric response. Resolved waves compensated the local drag through compensation mechanisms. In all three cases, added westward momentum suppressed upward and equatorward propagation of planetary waves, particularly of wavenumber 1, strengthening westerlies in the upper stratosphere–mesosphere. The frequency of sudden stratospheric warmings remained unchanged in the HI and NA experiments, but increased notably in EA, while the ratio of split to displacement events was unaffected. These results highlight the sensitivity of stratospheric variability to non-zonal GW forcing and underscore the importance of improving our understanding of GW–climate interactions. The simulation dataset presented here offers a valuable resource for future studies on gravity wave–induced variability in the climate system.
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RC1: 'Comment on egusphere-2025-3005', Anonymous Referee #1, 18 Aug 2025
- AC1: 'Reply on RC1', Sina Mehrdad, 16 Sep 2025
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RC2: 'Comment on egusphere-2025-3005', Corwin Wright, 20 Aug 2025
In this manuscript by Merhdad et al, the authors asses the impact of scaled-up parameterised orographic forcing on the middle-atmospheric dynamics of the UA-ICON global model running at a ~160km grid size under climatological (i.e. repeating-annual) conditions. They find coherent responses at a hemispheric level with resolved waves compensating for the locally-induced drag and strengthened upper-stratospheric westerlies due to suppressed planetary wave propagation.
The paper is an interesting study, and I concur with Reviewer 1 that it is interesting and well-written, and worth accepting for publication. I have slightly more questions than Reviewer 1 though, and do think a minor set of corrections before acceptance would help strengthen this interesting study and help it better find an audience.
My main issue here relates to the structure of the text. I found the first half clear and easy to read, but the back half was much less structured - in particular sections 3.2 and 4 were ~155 lines and ~100 lines long respectively without a break and hence quite hard to read without losing track of where I was. I would strongly suggest restructuring the material here to be easier to read as I think a lot of the potential audience will get lost in this section, and I include a few suggestions below.
The written English and figure design are generally of an excellent quality - I do include a list of typos etc that I spotted below, but this is much lower than most papers I review!
Scientific/formatting comments
-------------------------------L003: how were the hotspots identified?
L125 (1): The sharpness of the modification spatially seems like it would introduce sharp discontinuities both horizontally and vertically - a real wave would likely extend over several surrounding pixels and would similarly not appear out of nowhere in the vertical. Does this lead to any unusual model response or stability issues in the region?
L125 (2): Just to check - when you introduce the scaling, you don't change the direction at all? I assume not, but it wouldn't hurt to be explicit.
L145: What is the impact of this being analysed as a 30 year mean? Presumably it leads to values being very smooth everywhere whereas a typical year would inevitably have anisotropies - could this affect anything about the subsequent model evolution? And would it matter to your results if it did - I suspect any effect would vanish within the spinup year?
L143, 149: six ensemble *members* per experiment? Or six ensembles of multiple members each? I am genuinely a bit puzzled here, and they're quite different things! I think it's members from the context of the rest of the manuscript?
Figure 2: it looks like the values in the boxes on these plots are extremely highly saturated even with the log scales. Could the scale be extended further so we can se where the values are actually peaking?
L238: I'm quite surprised that the non-orographic tendencies are so much larger than the orographic ones - is this normal for parameterisations of this type, or is it a feature of the increased magnitude of the drag you have produced (and if so, would the normal orographic contribution be even lower)? It feels odd given that in observations orographic sources seem to absolutely dominate the GW activity we see at these altitudes, even when averaged over zonal means. This is a genuine question as I'm not a parameterisation scientist - it's possible that this is normal for these schemes? Given my confusion here, it might be useful to maybe put in a second row showing what the tendencies are in the unmodified control run to help those who are similarly not entirely familiar with what "normal" is in this context.
Section 3.2 is extremely long and undifferentiated (155 lines, spread over >12 pages when figures are included), and this made it quite a hard read - I bounced off it several times, and I think a lot of readers would. I think there are two options here - either to break up the material into subsections, e.g. by region or variable, or (perhaps better) to try and synthesise broad conclusions form the figures and talk about them rather than going into detail about each individual panel. The content is interesting, but the way it's presented makes it quite hard to absorb as a reader, which is a shame as a lot of work has gone into it.Section 4 has the same problem - three pages and 100 lines of text going into quite a lot of detail but without a clear overarching structure. What might help in particular here might be to summarise the key finding from this material in a digestible way for the reader, perhaps as a schematic figure showing the key findings being discussed, or a summary paragraph flagging up the key findings. It's all very interesting, it's just quite hard to read due to the density and length without any breaks.
Typos etc
------------L173: Figure 3 "shows", not "represents", surely?
L223: you say you "follow the methodology of..." but then cite two papers - is it one of these or both?
L228: "slighly"
L283: "eastely"
L354: "schems"
L363: "similar than" -> "similar to" (preposition issue)
L479: "experimnets"
Citation: https://doi.org/10.5194/egusphere-2025-3005-RC2 - AC2: 'Reply on RC2', Sina Mehrdad, 16 Sep 2025
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The authors investigate the long-term impacts of intensified orographic gravity wave (GW) forcing in three hotspot regions—East Asia, Northwest America, and the Himalayas—on middle atmospheric dynamics using the UA-ICON high-top global model. The experiments reveal consistent enhancements of easterly subgrid-scale orographic (SSO) GW wind tendencies within the targeted hotspot regions. The non-orographic (NO) GW drag response is largely modulated by changes in the background zonal-mean zonal wind in the stratosphere and plays a critical role in shaping the net parameterized GW momentum tendency. In all three cases, the added westward momentum suppressed upward and equatorward propagation of planetary waves, strengthening westerlies in the upper stratosphere–mesosphere. Overall, I think this topic is interesting, and the manuscript is well written. I have only one minor comments.
Minor Comments:
Section 2.2 Data: The associated discussions of finite-amplitude wave activity are not easily understandable. It is recommended to provide a more detailed explanation of the method, along with the formula to enhance clarity for the reader.