the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 27 Nov 2023
Changes in the tropical upper tropospheric zonal momentum balance due to global warming
Abstract. The zonal momentum budget of the deep upper tropics is studied in the context of present and future climates. In the zonal mean, as is known, a robust balance exists between the acceleration by the horizontal eddy momentum flux convergence and the deceleration by the mean meridional momentum advection. During summer, climatological stationary Rossby waves over the Asian monsoon longitudes converge westerly momentum into the tropics and are the primary contributors to the eddy term. During winter, anomalous westerly winds over the tropical East Pacific allow extratropical waves to propagate into the deep tropics, where they tend to break and decelerate the flow. When integrated over all longitudes, eddies from these two regions sum constructively in summer and destructively in winter, always yielding a net positive momentum forcing that balances the mean flow term. The state-of-the-art CMIP6 suite qualitatively captures these features in the historical run and shows that the momentum fluxes change due to global warming. In summer, stationary eddy circulations in the Asian monsoon zone weaken in the upper troposphere (UT) but strengthen in the lower stratosphere (LS). Greater upward mass flux from the troposphere forces a stronger divergence and a more intense circulation in the LS following a Sverdrup vorticity balance. This strengthening of summertime tropical and subtropical stationary waves in the LS is observed over all longitudes and is verified in an idealized aquaplanet general circulation model experiment. In winter, eddy westerlies over the East Pacific longitudes decrease in strength due to the expected weakening of the subtropical stationary waves with warming. This causes a significant decrease in the propagation of extratropical waves into this region, along with a drop in eddy potential vorticity fluxes associated with these waves. Thus, apart from the mean meridional flux, which weakens due to the projected weakening of the Hadley Cells, our analysis of warming simulations clearly suggests significant and robust changes in the eddy momentum fluxes in the deep tropics. Potential implications of these changes in the context of the zonal mean flow and regional circulations are discussed.
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RC1: 'Comment on egusphere-2023-2779', Anonymous Referee #1, 03 Jan 2024
General comments
The authors evaluate the upper tropospheric momentum budget for ERA-5 reanalysis, and CMIP6 historical and SSP585 simulations. The paper explores the magnitudes of the zonal mean state advection of absolute vorticity and the stationary waves throughout the year, and across longitudes. In wintertime, over the East Pacific, they find that under the SSP585 scenario, the stationary waves contribution strongly decreases, likely due to the future El Niño SST anomalies simulated in CMIP6 but not currently observed.
The paper is clearly written and structured, presents some useful analysis of reanalysis and model data, the methods generally seem appropriate, and the results support the conclusions.
Specific comments
Budget closure: The ERA5 momentum budget shows a large summertime residual, of similar magnitude to the eddy term. This is discussed in the text, but would it be possible to either compare closure against other studies using ERA5, or to look into the cause of this? For example, ERA5 does provide the ‘mean eastward wind tendency due to parametrisations’ on model levels, alongside code to interpolate this to pressure levels. Given the focus of the paper, this seems an important point to have some explanation for. Could it arise from sub-daily transient activity? How well does the budget close across the pressure level for JJA?
Novelty: The paper is well-contextualised in terms of the previous literature, and I believe presents some new and interesting results. The authors highlight that changes in the regional eddy fluxes have not received much attention. However, it would be helpful to promote more clearly in the abstract, introduction and conclusions the key new findings in this study, perhaps simply by rephrasing from the passive to active voice: ‘we find’.
CMIP6 simulations: r1i1p1f1 is not the main simulation for some centres, and its meaning is not standard across centres. It may be possible to expand the number of modules used, although the 23 selected seem to cover a reasonable range of modelling centres.
Methods
- Which pressure levels are used in the CMIP6 data, do these match the ERA5 levels used?
- wap is not listed in the CMIP6 table, were vertical fluxes not evaluated for CMIP6?
- The acronyms (ua, va, tos, etc.) for the CMIP6 variables given in the table are not explained
- In section 2.1.3 it would be useful to note that the simulation includes a stationary wave SST, and to refer to the supplementary information, rather than relying on knowledge of the paper referenced.
- Calculating the fluxes from daily means appears to give reasonable results. It could be noted that this excludes any short-lived, subdaily activities.
Fig 2/averaging regions: I initially found the averaging regions described in the text hard to interpret, vertical lines on Fig 2 could make these clearer.
Line 283: “A possible reason for these discrepancies is that the model fluxes tend to be slightly displaced than those for present-day reanalysis.” Not clear to me what is meant here, is this referring to spatial displacement, and in what sense?
Line 308: First sentence here refers to Fig 6, should note that this is discussing summer.
Fig 10: There is a shift to look at the Northern Hemisphere only here, is there a reason for this?
Line 386: “Quite clearly, comparing the balance for the two scenarios suggests a higher degree of compensation between the beta and stretching terms via a larger divergence in the forced ensemble than the control set.” This is not clear to me from Fig 10, the residual between the two terms may need to be shown to support this.
Fig. 11: Not fully clear what is shown here. Do a and b show a vertical slice of the horizontal streamfunction, or a lat-pressure streamfunction?
Fig. 12 caption: it needs to be made clear in the caption that quivers show differences in left column but absolute values in right column.
Fig. 13 caption: please could you specify that positive values correspond to northward fluxes in both panels, to make this simpler to interpret.
Line 446: It would be helpful to note the longitudes to look at for the westerly duct here, particularly given the change in longitude axis from -180-180 to 0-360.
Line 534-535: I found this sentence confusing. To me, “captured faithfully” implies models are consistent with the observations, but the sentence continues to say they are not. Is the intention to say that models consistently show this behavior?
Line 544-545: “Indeed, there is a fair spread amongst the models; some models in the ensemble indicate a switch with equatorial superrotation as an outcome of climate change” From Fig 14 I can’t see any that go from -ve in the control to +ve in the forced, as seems implied by this sentence.
Technical corrections
Equations 1 & 3 use x and y and lambda and phi respectively. Similarly deviations from the zonal mean are denoted by asterisks and primes differently in these equations. It would be good to make these consistent.
Section 2.2.3: should F by F_s in the sentence discussing WKB theory?
Line 225: “While the wave activity flux in (arrows in Figure 4b) captures stationary contributions” Delete ‘in’
Line 339: missing &
Fig. 5: Please add a legend to avoid readers scrolling to check Fig. 3.
Fig. 9: Could the same colorbar be used for both columns?
Citation: https://doi.org/10.5194/egusphere-2023-2779-RC1 - AC1: 'Reply on RC1', Abu Bakar Siddiqui Thakur, 20 Feb 2024
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RC2: 'Comment on egusphere-2023-2779', Anonymous Referee #2, 26 Jan 2024
* Summary
The authors diagnose the zonally resolved zonal momentum budget for the tropics in reanalysis data and CMIP models, both control and SSP8.5-forced. They split out the contributions from many different terms, including the rotational and divergent components.The writing is for the most part clear, and the Introduction sets up a potentially compelling story. But---and this might just be me---I struggled to keep focus/interest once the manuscript got into its results. The first few figures, there were already a number of things regarding their interpretation that were tripping me up, as noted below. So it was then hard to then make much of the remainder of the results.
That said, I don't see any glaring errors in the analyses, and the results presented are worthwhile contributions to the literature pending some cleaning up as detailed below.
* Major Comments
** Residual term and daily data
In your Fig. 1, in NH summer the leading balance is not, as you state, a two-term balance between the eddy momentum flux divergence and mean meridional advection...it's a three-term balance of those and the residual. In your discussion of this issue, you don't bring up what to me seems like a likely contributor: your use of daily averaged data. The standard is to use 6-hourly (or 3-hourly) covariances of instantaneous fields.I'm not necessarily saying you need to re-do the whole thing with hourly data, as that's a heavy lift indeed. But it would make everything much more compelling, and ERA5 hourly data is available to make this possible.
** Stationary vs. transient eddies
The framing is in terms of the mean meridional circulation on the one hand versus all eddies on the other hand. But stationary eddies and transient eddies are very different from one another, and you argue that one or the other plays more important roles in different locations and contexts. So I'm wondering if it's worth the effort to explicitly disentangle them, presenting results for both individual eddy terms.
* Minor comments
- On superrotation, see also Zhang and Lutsko, doi.org/10.1175/JAS-D-22-0066.1
- On the SSP8.5 scenario, just be aware that it is now virtually certain to not occur: doi.org/10.1038/d41586-020-00177-3
- Table 1 :: there's no need for the variable columns, since every single one is checked. Just list the model names and note either in the caption or main text that they all include the five variables you've listed.
- Text after Eq. 3, the F term is missing the "s" subscript
- L147 :: if it's already been published and discussed, it's not ``surprising''
- Fig. 3 :: If you're saying the other components that aren't plotted are small, then why doesn't the total visually add up to the sum of the plotted ur*vd and ur*vr lines in a lot of places?
- Fig. 3b :: the ur*vd legend label doesn't show up as dash-dotted, just as a shorter line than the total
- L259 :: "As long as ur-vd or tropical features dominate the eddy and mean fluxes, they should oppose each other in strength and symmetry" I don't understand this
- "tropical momentum balance is delicate" this is said a few times, but what does it really mean? Regarding the response to a forcing, the prevailing balance does not constrain the response in any way---a given forcing could operate primarily by a third term for example, so I don't follow e.g. L488-491.Citation: https://doi.org/10.5194/egusphere-2023-2779-RC2 - AC2: 'Reply on RC2', Abu Bakar Siddiqui Thakur, 20 Feb 2024
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