Fig. 8. (a) and (b) are trends of eddy geopotential height (color shading) and trends of stream function (black contour lines, the values have been multiplied 10-6 units: m s-2 10yr-1) at 100 hPa. (c) and (d) are trends of temperature at 85 hPa and green dash lines indicate the location of the CPR. The area average trends in the CPR are shown in the top right-hand corners. (e) and (f) are trends of SWV entry at 85 hPa and the area average trends in the tropics (30°S-30°N) are shown in the top right-hand corners. The left column is from WACCM4 and the right column is from CAM5. Dotted regions indicate significance at the 90% confidence level.

Conclusions

In previous work, we have demonstrated that SST in IPWP has significant impact on stratosphere and tropical lower SWV in interannual scale (Xie et al. 2014; Xie et al. 2018; Zhou et al. 2018). And in this study, we reveal linkage between the decades-long drying trend in the tropical SWV entry and the substantial warming of the IPWP based on observations and transient experiments. Using merged satellite measurements SWOOSH and the newest reanalysis data set ERA5 we show a significant decreasing trend in the lower SWV entry since the satellite era (1984-2020), with a rate at 0.106 ± 0.021 ppmv per decade according to SWOOSH. The two data sets have relatively good agreement in the lower stratosphere, although they have large discrepancies in global. The trend in tropical SWV entry is captured by the tropical tropopause temperature well, which is concurrently experiencing significant cooling at a rate of ~0.32 K per decade. We found that cooling tropical tropopause, as a primary role, determines the decrease in the water vapour entry. Correlation analysis shows a good relationship between the warming tropical SST and the drying trend in the lower stratosphere. Importantly, the strong negative correlation is in the IPWP region. The SST over IPWP is substantially warming, at a higher rate than that over the eastern Pacific. Meanwhile, the drying trend in the tropical lower stratosphere is significantly reduced by 43% in SWOOSH and 61% in ERA5 after regressing out IPWP warming. And for global SWV, this contribution is very similar. It implies that the tropical SWV entry is very important for the global SWV budget.
We further validate the relationship by carrying out two groups of transient experiments with WACCM4 and CAM5, accompanied by a comparison between the two models in representing the response of SWV entry. The simulations by both models agree with the observations that IPWP warming can significantly reduce the SWV entry through the tropical pathway in the long-term. The simulated drying trend caused by IPWP warming is about 0.025 ppmv per decade using WACCM4. This further confirms the vital role of IPWP warming in dehydrating the lower stratosphere for the period 1984-2020. We explain the physical process using the bottom-up regime. It shows that IPWP warming can enhance the equatorial waves due to intensified convection and thereby cool the tropical tropopause, especially in the CPR. And IPWP warming finally reduces the transport of water vapour through the tropical pathway.
Our results differ from previous work on global warming causing TTL warming (Keeble et al. 2021). Here we focus more on the IPWP and find that the IPWP warming can cause cooling of the tropical tropopause, resulting in less water vapour entering the stratosphere. This suggests that although global warming causes TLL warming, regional SST warming may have different effects than global warming.
We have demonstrated a possible relationship between IPWP and tropical SWV entry, which can partially interpret the decreasing trend of SWV in the past decades. Nevertheless, owing to the limited instrumental measurements since the satellite era and the control role of multi-decadal variation in SWV short-term trend (Konopka et al. 2022), the long-term trend in the tropical lower SWV still exists large uncertainty and needs to be investigated in the future. Moreover, it would be of interest to assess the long-term changes of the tropical SWV entry in different models and analyze the role of the IPWP in the future. In addition, because there are large uncertainties in the model simulations on the tropical SST impacts on the tropical SWV entry (Garfinkel et al. 2021), a caveat that the results might depend on the models.
CRediT authorship contribution statement
Yangjie Jiang: Formal analysis, visualization, writing – original draft; Xin Zhou: Conceptualization, funding acquisition, investigation, methodology, supervision; Quanliang Chen: Funding acquisition, writing – review&editing; Wuhu Feng: Writing – review&editing; Xiaofeng Li: Methodology, writing – review&editing; Yang Li: Funding acquisition, methodology, writing – review&editing.
Declaration of Competing Interest
All authors declare no conflicts of interest.
Acknowledgements
This work was jointly supported by the National Natural Science Foundation of China (41905037; 42275059; 42175042), the China Scholarship Council (201908510031 and 201908510032), and Natural Science Foundation of Sichuan (2022NSFSC1056). We acknowledge the helpful suggestions from anonymous reviewers.
Data vailability
ERA5 reanalysis data can be obtained online fromhttps://cds.climate.copernicus.eu/cdsapp#!/dataset/reanalysis-era5-pressure-levels-monthly-means?tab=form. JRA55 can be downloaded online fromNCAR RDA Dataset ds628.1 Data Access (ucar.edu) . HadISST can been accessed fromhttps://www.metoffice.gov.uk/hadobs/hadisst/data/download.html.
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