| 23 Feb 2026
Co-occurrence of gravity waves, vertical wind shear and turbulence in the lowermost stratosphere over the North Atlantic
Abstract. This study focuses on the spatial and temporal co-occurrence of gravity waves (GWs) and of vertical shear and turbulence in the extratropical lowermost stratosphere (LMS). For this, one year of ERA5 reanalysis data is used to analyse the occurrence and variability of resolved GW momentum fluxes, vertical shear, and turbulence indices in the region of the North Atlantic. In the tropopause region GWs can lead to strong vertical wind shear and ultimately to the generation of turbulence, which may result in troposphere-stratosphere exchange and mixing and is a risk for commercial aviation. The occurrences of GWs, shear, and turbulence are analysed in terms of vertical, geographical, and seasonal occurrence frequency distribution and put in relation to the large-scale circulation and to processes related to GW appearances. The contribution of resolved GWs to shear is found to be notable, especially in the North Atlantic winter lowermost stratosphere, where vertical momentum flux convergence provides a peak zonal GW forcing up to −2 m s-1 day-1 around 45° N at tropopause altitudes. The prominent vertical propagation in the wintertime mid-latitudes substantially leads to the formation of belt-like structures of GW activity, as evident by momentum fluxes, and further contributes to the pronounced occurrence of shear in the LMS. Ultimately, this study discusses the role of small-scale dynamics in shaping a quasi-permanent layer of elevated shear above the extratropical tropopause and its potential to generate turbulence in this region.
This manuscript addresses a highly relevant topic: the relationship between gravity wave (GW) activity and the formation of tropopause shear layers (TSLs). The research question is well-posed and has the potential to contribute significantly to our understanding of upper-tropospheric and lower-stratospheric (UTLS) dynamics. However, in its current form, the link between the observed correlations and the physical mechanisms remains somewhat speculative. I believe the paper would be much stronger if the authors could move beyond describing "co-occurrence" and provide a more detailed mechanical explanation. I recommend a reject with an encouragement to resubmit to allow the authors time to incorporate high-resolution data and deepen the discussion on dynamics. Following are my comments for the authors to increase the impact and clarity of the work.
Major Comments:
C1.
The manuscript frequently highlights the "importance" of GWs in TSL formation. To make this argument more compelling, it is necessary to explore the specific physical processes involved. For instance, are these TSLs a result of GW breaking and subsequent momentum deposition, or are they the result of the superposition of large-amplitude waves? Additionally, providing information on the characteristics of the waves (e.g., their sources and properties) would greatly enhance the scientific depth of the study. What kind of GWs are responsible for the formation of TSLs?
C2.
While ERA5 offers hourly resolution, the current analysis relies on once-daily data. Since both gravity waves and turbulence are transient and evolve on much shorter timescales, I encourage the authors to utilize the high-frequency (hourly) data. This would allow for a more rigorous assessment of the temporal evolution of these phenomena and provide more robust evidence for the study's conclusions.
C3.
At present, some of the conclusions regarding the "role" of GWs appear to precede the physical evidence. I suggest re-evaluating the causal links to ensure that the data clearly supports each step of the argument. Framing the results more cautiously or providing the missing physical links will help avoid the impression of a circular reasoning.
Specific Points:
l. 29: Since the topic shifts after the phrase “On both hemispheres,” I suggest starting a new paragraph here.
l. 101: Is there a specific reason for defining the dynamical tropopause at 3.5 PVU instead of the more conventional 2.0 PVU (e.g., Hoskins et al., 1985; Kaluza et al., 2021)?
ll. 137-138 It appears there is a grammatical error; the sentence should likely read: “The shear perturbations are calculated…”
e.g., l. 173: It is difficult to characterize a monthly variation observed only in a single year (2017) as an “annual cycle,” as such a variation likely includes both the mean annual cycle and interannual variability. I recommend using a more appropriate term to describe this specific variation.
l. 175: This sentence (“This finding…”) is somewhat abrupt and lacks clarity. Please specify which large-scale dynamics are being referred to and what roles they play. Furthermore, there appears to be a logical leap regarding the definition of the Richardson number. It would be necessary to present the shear (or Ri) associated solely with the large-scale wind to demonstrate the contribution of large-scale processes to TSL formation.
Fig. 3: The magenta shading is difficult to distinguish from the shading used for the 200 hPa zonal wind.
l. 184: “A intensity” is grammatically incorrect. Did the authors mean “intensification of meridional baroclinicity” or “intense meridional baroclinicity”?
l. 188: The phrase “A planetary circulation feature” is too vague. Could the authors briefly elaborate on the discussion in Kaluza et al. (2021) to provide better context?
ll. 222-224: The interpretation of Fig. 5 is not entirely clear at this point of the manuscript. Why do the authors examine GWMF specifically in the vicinity of high-frequency strong shear events rather than showing the overall GWMF? Also, please clarify what is meant by “tropopause-relative,” given that the tropopause height itself varies in the figure.
l. 223: For clarity, please consider changing “shorter GWs” to “GWs with shorter horizontal wavelengths.
ll. 227-230: There is an inconsistency in the list format: point (i) is a full sentence, while (ii) and (iii) are noun phrases. Additionally, the latter half of point (i) lacks a verb (perhaps the authors meant “could also be related to”?). Regarding the interpretation, enhanced absolute GWMF “indicates” or “shows” intense upward propagation rather than just “hinting” at it. Furthermore, I am not convinced that convection directly contributes to the MF above the tropopause in the manner described, even if MF is shown as an absolute value.
ll. 239-240: A potential reason for the strong MF in shear regions is that GWs may be generated by, or have longer vertical wavelengths near, the jet stream where strong shear exists. This indirect relationship, which is a correlation rather than a causal "role" of GWs, should be explicitly noted.
l. 255: The term “smaller hotspots” is unclear. Furthermore, attributing them to “specific dynamical events” without further evidence feels speculative.
ll. 263-265: The last sentence does not make lots of sense. The vertical shear term S2 in the definition of the Richardson number Ri≡N2/S2 already includes contribution from shear associated with GWs. Therefore, the difference between the distribution of S2 and Ri is solely attributable to N2. While GWs may contribute to the distribution of N2, this difference alone does not provide sufficient evidence that GWs play an important role.
ll. 288-289: Could the authors clarify which specific results support the argument made in the sentence starting with “In fact, … GW activity”?
ll. 318-319: The statement "Nevertheless, …” is a fair acknowledgment, and it highlights the very question I had throughout this section. I recommend that the authors avoid overstating the causal link before this point is reached.
ll. 549-558: The final paragraph of the Summary seems slightly disconnected from the main discussion of the paper.