Review on "Impacts of Increasing CO2 on Diurnal Migrating Tide in the Equatorial Lower Thermosphere" by Kogure et al.

Reviewed by Yosuke Yamazaki, Leibniz Institute of Atmospheric Physics, University of Rostock

This paper examines the impact of increasing CO2 on the upward-propagating migrating diurnal tide (DW1) using a WACCM-X simulation with a prescribed long-term surface CO2 trend. The results indicate positive DW1 amplitude responses in the stratosphere and lower mesosphere (20-70 km) and negative responses at higher altitudes (90-110 km). The authors discuss in detail possible mechanisms underlying these altitude-dependent responses.

The study presents new and interesting findings, and I have no fundamental objection to its publication in Atmospheric Chemistry and Physics. However, I see two main weaknesses:

1. The results are not validated against observations, leaving their realism uncertain.

2. The conclusions are drawn from a single simulation without controlled experiments to isolate the suggested mechanisms.

Point (1) could be addressed by referencing existing tidal observations in the literature. Decades of ground-based and satellite measurements are available and should permit at least qualitative comparisons with the simulation results (e.g., consistency in the sign of responses). Point (2) may be more difficult to resolve, given the data storage limitations already noted by the authors. I suggest the authors address at least (1).

The following are other minor comments:

(l. 13) "... the negative trend appears to result from increased eddy diffusion in the mesosphere, ..."

This part is confusing. It first gives a reason for the negative trend, but the following sentence states there are two reasons. I suggest rephrasing for clarity.

(l. 47) "(Yamazaki et al., 2024; Yamazaki and Siddiqui, 2014)"

This should be "(Yamazaki et al., 2014; Yamazaki and Siddiqui, 2024)".

(l. 58) "absorption of solar heating"

Perhaps, "absorption of solar radiation"?

(l. 96) "and from 2015 onward, it is simulated by rewinding the solar forcing to the 1850 levels (Figure 1b)."

Could you elaborate on this? From 2015 onward, are the f10.7 data theoretical predictions, or are they actual observations from a different time period?

(l. 105) "These perturbations were then convolved with the (1,1) Hough mode function to derive the (1,1) mode amplitudes on pressure coordinates."

It should be explicitly stated that the amplitude of the (1,1) Hough mode is computed separately for each pressure surface.

(Equation 1)

The right-hand side should be multiplied by Z_{GP}.

(Equation 3)

Should the sign before sigma_i_{2003-2013} be "-" instead of "+"?

(l. 194) "tidal dumping"

Perhaps, "tidal damping"?

(Figure 4)

I cannot see the ticks on the x-axis. Also, the x-axes look unusual; neither the lower nor the upper x-axis appears to be linearly scaled.

(l. 223; also in the Figure 5 caption) "Eq. (2)"

Should this be "Eq. (5)"? Eq. (2) does not tell how to calculate the local vertical wavenumber.

(l. 247) "in zonal mean temperatures (7a) and their vertical gradients (7b)"

Should they be "(6a)" and "(6b)"?

(l. 256) "Temperatures in this layer are influenced not only by CO2 cooling but also by O3 heating via ultraviolet absorption (Garcia, 2021; Garcia et al., 2019; Jonsson et al., 2004; Lübken et al., 2013)."

Could you describe how O3 changes are produced in the model and how they are connected with the changes in CO2?

(l. 308) "In addition,... Therefore, increased CO2 concentrations... and reduce the tidal amplitudes."

I suggest rewriting this part. "Therefore" does not clearly connect the second sentence to the first.

Review on the manuscript of “Impacts of Increasing CO2 on Diurnal Migrating Tide in the Equatorial Lower Thermosphere” by Kogure et al.

General Comments:

It is an important topic that the global warming in the stratosphere has possible influences on the dynamics and electrodynamics in middle and upper neutral atmosphere and ionosphere. Tides, which are caused by the solar heating, are an important phenomenon of neutral atmosphere and influence the structures and circulations of middle and upper atmosphere. The migrating diurnal tide with Hough mode of (1, 1) is in the dominant position among the tidal modes in the tropic region. Based on the simulation results of WACCM-X under the scenario of RCP 8.5, this manuscript shows the trends of DW1 (1, 1) from 2000 to 2069. The main results are that the trends of DW1 (1, 1) are the 1% per decade at 20-70 km and -2% per decade at 90-100 km. The possible reasons are the depression in the atmospheric density and enhanced equatorial convective activity and the eddy diffusion.

However, there are some major issues should be fixed. Such as (1) the time period dependency of the linear trend, (1) the reliability of non-LTE CO2 cooling scheme when the concentration of CO2 approaching to its upper limit, (3) the enhanced gravity waves can amplify or depress the tidal amplitude. This is dependent on the relative phases between gravity waves and tides. Here only the depression effect is used. How to exclude the amplification effect. 

Please see the detailed comment below.

Comments:

1. L10-11: It is better to show the corresponding standard deviations or significant level.

2. L79-81: How about the vertical resolution at height above 1 hPa, since the model top is at ~500-700 km, which is much higher than 1 hPa.

3. L93: How about the reliability the non-LTE CO2 cooling scheme when the concentration of CO2 is approaching (such as, 600, 650, 700 ppm) to the upper limit (720 ppm). 

4. L96-97: “it is simulated by rewinding the solar forcing to the 1850 levels”. Please clarify the point of "rewinding".

5. L123: “It should be noted that the QBO is specified by climatology”. I cannot get this statement. Since the QBO's period ranges from 22 to 28 months, and the QBO's amplitude changes from one cycle to another, how about these variations change the trends of DW1? What’s the meaning of “specified by climatology”

6. L125-126: From Figure 2(a, c, d)，it looks like that the amplitude of DW1 (1, 1) mode decreases after ~2028 and more sharper after 2055. This raised the issue of time period dependent linear trend as supposed by Lastovicka & Jelínek (2019, JASTP, Problems in calculating long-term trends in the upper atmosphere, https://doi.org/10.1016/j.jastp.2019.04.01119.04.011). This point should be clarified. 

7. L150-152: This is related to the same issue of the time period dependent of the linear trend.

8. L200-205: From Figure 4b, it looks like that the differences of vertical wavenumbers are not different from zero under 1-sigma in most height range. This indicates that the changes of vertical wavenumbers are not significant.  Moreover, the below ~80 km the shorter vertical wavelengths in 2050-2061 contributed to stronger tidal dissipation, why the tidal amplitudes are larger in 2050-2061 (below ~80 km in Figure 3a)

9. L257, Equation (4)(7), O_{(\delta^2)} should be O(\delta^2)

10. Figures 5-7: It is better to indicate the regions where the significant level is at 1-sigma or 2-sigma.

11. L255-281: It is better to convert the concentrations of CO2 and O3 to their corresponding cooling rates, such that one can get their cooling effects more directly.

12. Figure 8: Please show the error bars or standard deviations in each panel.

13. L310-312: The enhanced gravity waves may amplify or depress the tidal amplitudes are also dependent on the relative phase between gravity waves and tides (Ortland & Alexander, 2006, JGR, Gravity wave influence on the global structure of the diurnal tide in the mesosphere and lower thermosphere, doi:10.1029/2005JA011467). Why the enhanced gravity waves can only reduce the tidal amplitudes?

14. Figure 9: Please show the confidence level in (a) and standard deviations in (b).

15. L329-330: This statement is questionable due to the time period dependent trend. Are the sharp decrease of the tidal amplitude above 70 km caused by the limitation of non-LTE CO2 cooling scheme in the mesosphere due to the CO2 concentration is large in later years.

16. L331-334: “We propose two …: a decrease in tidal vertical wavelength and an increase in diffusion due to GW breaking”. From Figure 4(a), the vertical wavelengths are longer in 2050-2061 than in 2003-2012 above ~95 km. This is contradictory to “a decrease in tidal vertical wavelength”. Why GW breaking can only dissipate tide? It can also amplify tide according to Ortland & Alexander (2006, JGR)