the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 11 Feb 2025
Shift in cold-point tropopause trends derived from radiosonde, satellite, and reanalysis data
Abstract. The tropical tropopause layer (TTL) is the transition region between the well-mixed convective troposphere and the radiatively controlled stratosphere and plays a crucial role for air mass transport between these layers. In this paper, we present updated trends of TTL and lower stratospheric temperature from radiosonde and Global Navigation Satellite System – Radio Occultation (GNSS-RO) data and evaluate temperature trends in the reanalysis data sets ERA5, JRA-3Q, and MERRA-2. Given its importance in determining the concentration of water vapor entering the stratosphere, we focused in particular on temperature trends at the cold point tropopause, which we determined from radiosonde observations by removing time-varying bias effects from trends based on unadjusted data.
From 1980 to 2023, cold point tropopause cooling is shown in radiosondes, overestimated by JRA-3Q and underestimated by MERRA-2 and ERA5. Splitting into two periods reveals a shift in TTL temperature trends: cooling (1980–2001) to warming (2002–2023) across all datasets, highlighting post-2002 changes in tropical tropopause dynamics. Tropical upwelling estimates from the three reanalyses show opposite trends for 1980–2001 compared to 2002–2023 consistent with the cold point and lower stratosphere temperature trends. While the vertical residual circulation increased before 2000 consistent with cold point cooling, the circulation trends turned to zero (MERRA-2, JRA-3Q) or became positive (ERA5) after 2000 consistent with cold point warming. Between 2002–2023, GNSS-RO and reanalysis show significant warming at the cold point and lower stratosphere, aligning with observed patterns and seasonality. Warming trends anticorrelate with tropospheric cooling, strongest where upper troposphere cooling appears.
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RC1: 'Comment on egusphere-2025-82', Anonymous Referee #1, 31 Mar 2025
The authors employed radiosonde, GNSS-RO, and reanalyses to update the long-term variations of temperature in the tropical upper troposphere and lower stratosphere, with a particular focus on cold point temperature. As the authors mention in the introduction, there are already many previous studies that use various data sources, including reanalysis, radiosonde, and GNSS-RO. However, a detailed comparison with previous studies is necessary to highlight the novelty of the present study. Furthermore, while the authors attempted to conduct their research using multiple datasets with high vertical resolution, they did not reveal novel or more refined features, nor did they propose new mechanisms for existing trends. Despite the study's objective to quantify trends in both space and time, it does not address the scientific questions that exist in this area. The current state of the manuscript lacks sufficient novelty for publication in the Atmospheric Chemistry and Physics.
Major Comments
1 As suggested by Highwood and Hoskins (1998), the cold-point is only a reliable tropopause definition when the lower stratosphere is not close to being isothermal, i.e., within the deep tropics. Beyond the deep tropics, the validity of the physical significance of cold point identified in the research region [30S-30N] must be assured.
2 What are the differences between the trend term derived from multivariate linear regression and that derived from conventional linear regression? Furthermore, it is crucial to ascertain the magnitude of the contributions of the additional terms (QBO, ENSO, SAOD) in the multivariate linear regression.
3 GNSS-RO measurements exhibit pronounced irregularities in the tropics compared to middle latitudes. The authors use a relatively large grid (30° longitude × 10° latitude) to ensure sufficient data sampling. However, early single GNSS-RO missions were capable of providing only about 100 profiles per day globally. For instance, the CHAMP (2001-2007) and GRACE (2006-2007) satellites each provided approximately 130 daily profiles. By contrast, the COSMIC constellation, operational since late 2007, provides about 2,500 measurements daily. This substantial disparity highlights significant sampling inhomogeneity. Given the use of multiple missions to construct long-term observations, how do the authors account for sampling biases arising from mission prioritization?
4 What implications might be drawn from the observation of significant upper troposphere warming in conjunction with insignificant temperature trends in both the cold point and lower stratosphere, as illustrated in Figure 6? Furthermore, it is important to consider the implications of these phenomena in relation to radiative or radiative-dynamical balances.
5 A discussion is also required regarding the discrepancy between the trends derived from different datasets.
6 An important question concerns the apparent shift in long-term trends around the year 2000. While the authors have proposed several potential mechanisms for this shift, a more thorough discussion is needed to fully explore these possibilities.
7 Given the apparent decoupling of upwelling trends and warming trends, it is necessary to explore alternative mechanisms that could explain the observed changes. The authors propose a correlation between the warming of the cold point and the weakening of BDC (Ln 305). This relationship should be quantitatively assessed through a rigorous analysis of reanalysis data.
Regarding the zonal-mean dynamical variables from Martineau et al. (2018) (covering 1958-2016), clarification is needed as to whether the vertical residual velocities were: (1) calculated following Martineau's methodology, or (2) obtained directly from the archived data. The relevant description in Lines 184-186 should be revised to clarify this point, or alternatively, the Data Availability statement should be updated accordingly.
Minor comments
Section 2.1 requires rewording. The methodology suggests that the cold point is determined for each radiosonde profile prior to the gridding procedure.
Ln 152-154 Please rephrase this sentence
Ln 158 balloon reading?
Ln 175 It is necessary to establish the reasons for the observation of a comparatively wider range of cold points in reanalyses as compared with radiosonde observations.
Ln 194 In fact, the short-term variations are left in the error term
It appears that reference to Table 1 has not been made in Ln 326-332.
Citation: https://doi.org/10.5194/egusphere-2025-82-RC1 -
RC2: 'Comment on egusphere-2025-82', Anonymous Referee #2, 02 Apr 2025
In this study temperature trends are derived from radiosonde, satellite and reanalysis data with special focus on the cold point tropopause. Thereby, the period for which the trend is estimated is extended compared to previous study and covers the time period from 1980-2023, thus more than 40 years. Although this study does not present any significant new results it merits in my opinion publication. The extension of the data considered and the according update in trend estimates justifies a publication since if I understand the summary of previous studies correctly the time periods previously considered where much shorter. Thus, analyzing here about 40 years of data is quite valuable for deriving reliable trend estimates. However, the writing of the manuscript should be improved to better point out what the highlights and major results of this study are.
Specific comments:
Abstract: The abstract needs to be improved. For example, the second paragraph is providing too many details on the results without making clear what the main result is and what the implications of this study are.
P3, L79: You consider here a time period of 40 years! This could be more clearly be pointed out. Especially since it seems that earlier studies considered significantly shorter time periods which were considering max ~ 20 years.
P3, L84: The authors published last year a paper on this topic. It would be worth to also clearly point out what the differences between your previous study and this study are.
P3, L86: Section 2, especially the sections about the radiosonde data could be improved. I had trouble to understand what the differences between the adjusted and unadjusted data is. I had the feeling that you here also got lost a bit in details. I would suggest to put 2.1 and 2.2 in one subsection (thus 2.1 and called “Radiosonde data”) and then start with an introductory sentence and then have two subsubsection headers (with or without section numbering) on the adjusted and unadjusted radiosonde data.
P4, L116ff: Does this paragraph really belong to this subsection? I had rather the feeling this is part of the result section.
P4, L120: Has the abbreviation “QBO” been introduced?
P6, 158-170: Is this information really necessary. I have the feeling also here too many details are provided which are not important for understanding this study.
P6, L171-181: I think it would be much more helpful to list these differences in a table.
P5-7: The reanalysis data section is too long and I had the feeling too many results are presented here that are not necessary for understanding your study.
P8, L219-220: If in both cases the same pressure levels have been used the sentence could be formulated much shorter and clearer.
P8, L234-236: The latter part in the sentence “The good agreement of the four trend estimates highlights the consistency of the CPT temperature trends derived from adjusted data sets, which are only half of the cooling trend suggested by unadjusted data.” not clear. Please rephrase.
P9, L244: Add “e.g.” before the reference “Randel et al.”.
P16, L374: Add here one or two sentences summarizing what has been done in this study.
P17 , L387-389: This is an important result and should be mentioned at several places in the manuscript and not solely in the middle of the discussion.
All figures: Put the units in the title in parentheses instead of brackets.
Technical corrections:
P6, L173: space between number and unit missing.
P8, L224: space between number and unit missing.
P11, L283: in the Appendix A section -> in Appendix A
P19, L445: ozon e -> ozone
P25, Appendix: Start with the Appendix on a new page.
Citation: https://doi.org/10.5194/egusphere-2025-82-RC2
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