| 01 Oct 2025
Tropical upwelling as seen in observations of the tape recorder signal
Abstract. Tropical upwelling constitutes the ascending branch of the global mean stratospheric circulation and governs the thermal and chemical properties of the tropical stratosphere. A lack of direct observations and a spread in upwelling structure across the modern reanalysis creates difficulties in determining variability and long-term changes of tropical upwelling. We have derived time series of effective vertical transport in the tropical lower and middle stratosphere from MLS and SWOOSH water vapour for 2005–2020 and 1995–2020. Our calculated upwelling is found to be in the range of 0.21–0.33 mm/s for 73–28 hPa in very good agreement with reanalysis vertical velocities (ERA5, JRA-3Q, MERRA-2) and other observation-based estimates (ANCISTRUS).
We show that interannual variations of upwelling in the middle stratosphere are dominated by the QBO signal, which explains a large fraction of the upwelling anomalies. In the lower stratosphere, tropospheric modes of variability also play a role with the QBO and ENSO being equally important for explaining interannual variability. Individual peaks of strongly enhanced upwelling in the lower stratosphere in 2000/2001 and 2011/2012 cannot be explained by QBO or ENSO variability and coincide with known drops in water vapour and cold point temperatures. We use independent observational data to show that tropical upwelling is anticorrelated with long-lived tracers such as ozone as expected, lending confidence to the derived values. A reduction in variability is observed for 2016–2020 in both our calculated upwelling and observed ozone, which is consistent with the disruption to regular QBO variability over this period.
Competing interests: One author is a member of the editorial board of ACP.
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This paper presents a straightforward analysis of tropical stratospheric upwelling derived from MLS water vapor (H2O) observations of the ‘tape recorder’, including analysis of variability and comparisons to reanalysis and another tracer derived dataset. The upwelling derived from MLS H2O was introduced many years ago but has received relatively little attention, and the study here can be useful to update results and serve as a reference for critically evaluating the derived upwelling. Regression analyses of the time series show important variability linked to the QBO throughout the lower stratosphere and ENSO close to the tropopause, which are well-known behaviors from other data sets. Anti-correlations are observed between upwelling and ozone in the tropical lower stratosphere, which is expected behavior but good to confirm with these data. Comparisons of the MLS H2O-derived upwelling to the other data sets show reasonable agreements along with some differences, and given the limitations of each of the data sets (including low vertical resolution and limited effective time sampling for MLS H2O) it is unclear which are the better estimates. Overall, the paper is a useful contribution that might help spur further uses of these data, but a somewhat more critical evaluation of the new data would be helpful for potential users. I recommend a minor revision, including addressing the following comments.
Specific comments
I suggest using ‘Tropical stratospheric upwelling…’ in the title.
It would be good to know more about the details and uncertainties of the MLS H2O upwelling calculations. The authors choose to use correlations between vertical levels ~ 4.5 km apart, so that the results represent mean upwelling over these broad layers. This detail is not well emphasized, and vertical profile results are shown with a 1-km grid (e.g. Fig. 1). What do the upwelling calculations show if narrower vertical layer differences are used (even between adjacent levels)? How sharply peaked are the lag correlations in time, and is there any corresponding information on uncertainties in derived upwelling? The 6-month window calculations look reasonable in the reanalysis upwelling comparisons, and why aren’t these used throughout the paper?
I think including the SWOOSH results prior to 2004 is interesting, but the results look problematic to me given the data gaps and much noisier character of the time series seen in Fig. 2. Figure 9 highlights poor coherence between adjacent pressure levels and poor agreement with reanalyses for the early period. What are the causes of the data gaps? Why is additional time smoothing (3rd order polynomial fits) needed for these data? A ‘spike’ near 2001 in the derived upwelling is discussed but this looks more like noise in the calculations to me. In my opinion the authors should be more critical of these issues in the results derived from SWOOSH.
Figure 1 left axis should be Pressure, not Altitude (also Figs. 7 and 10). Do the error bars in Fig. 1 represent the standard deviation of the calculated means, or the standard deviation of the monthly time series?
The regression results regarding QBO and ENSO impacts on upwelling are very similar to previous results of Abalos et al 2015 (doi:10.1002/2015JD023182)
I’m curious about the detailed results in Fig. 4 – how can regression results with R2 less than 0.1 be statistically significant? How are the degrees of freedom evaluated for these low-frequency variations?
I like the comparisons of upwelling and ozone time series in Fig. 5, but shouldn’t you use the ozone level closer to the midpoint of the upwelling calculation for the most meaningful comparison?
The residual vertical velocity from reanalyses is given in terms of pressure vertical velocity – does this include the eddy heat flux term, or is it just the zonal mean pressure velocity? (there are small differences in the deep tropics, but this should be clarified). For more direct comparisons to the H2O results, I recommend calculating and plotting the reanalyses upwelling in terms of mm/s.
The ANCISTRUS upwelling results (Figs. 10-11) seem overly smooth in terms of vertical structure and time evolution. While these estimates are derived from numerous chemical tracers, there are very small vertical gradients or seasonal variations in most of the tracers in the tropical lower stratosphere, and I frankly wonder about the information content in this region. Why is there no seasonal cycle in upwelling in the 100-46 hPa ANCISTRUS results in Fig. 11c? Also, the 6-month window MLS H2O results in Fig. 11 don’t seem to match the corresponding results in Fig. 8 in terms of seasonal cycle variations. As with the other parts of the paper, I think it would be useful to be somewhat more critical of the ANCISTRUS comparison results.