the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
29 Jul 2021
29 Jul 2021
Indices of the Hadley circulation strength and associated circulation trends
- 1Faculty of Mathematics and Physics, University of Ljubljana, Ljubljana, 1000, Slovenia
- 2Geophysical Institute and Bjerknes Centre for Climate Research, University of Bergen, Bergen, 5020, Norway
- 3Meteorological Institute, Center for Earth System Research and Sustainability, Universität Hamburg, Hamburg, 20146, Germany
- 1Faculty of Mathematics and Physics, University of Ljubljana, Ljubljana, 1000, Slovenia
- 2Geophysical Institute and Bjerknes Centre for Climate Research, University of Bergen, Bergen, 5020, Norway
- 3Meteorological Institute, Center for Earth System Research and Sustainability, Universität Hamburg, Hamburg, 20146, Germany
Abstract. This study compares the trends of Hadley cell (HC) strength using different HC measures applied to the ECMWF ERA5 and ERA-Interim reanalyses in the period 1979–2018. The HC strength is commonly evaluated by indices derived from the mass-weighted zonal-mean stream function. Other measures include the velocity potential and the vertical velocity. Six known measures of the HC strength are complemented by a measure of the average HC strength, obtained by averaging the stream function in the latitude-pressure (φ-p) plane, and by the total energy of unbalanced zonal-mean circulation in the normal-mode function decomposition. It is shown that measures of the HC strength, which rely on point values in the φ-p plane, produce unreliable long-term trends of both the northern and southern HCs, especially in ERA-Interim; magnitudes and even the signs of trends depend on the choice of HC strength measure. The two new measures alleviate the vertical and meridional inhomogeneities of the trends in the HC strength. In both reanalyses, there is a positive trend in the total energy of zonal-mean unbalanced circulation. The average HC strength measure also shows a positive trend in ERA5 in both hemispheres, while the trend in ERA-Interim is insignificant.
Matic Pikovnik et al.
Status: open (extended)
-
RC1: 'Comment on wcd-2021-50', Anonymous Referee #1, 17 Aug 2021
reply
Review of 'Indices of the Hadley circulation strength and associated circulation trends' by Pikovnik et al.
General comments:
This manuscript compares 8 measures of the HC strength derived from ERA5 and ERA-interim ranalysis datasets. Their main findings are that measures based on a single vertical level are more subject to uncertainty and inhomogeneity while measure based on spatial average or integration are more robust. They concluded that the measure of the average HC strength is best suited for studying variability and trends.
The comparison is interesting and the conclusions are pertinent. However, 7 out of the 8 measures are derived directly or indirectly from the zonal mean streamfunction, which explain the high correlations between measures. The one measure not derived from the streamfunction is deemed inadequate for the purpose of this study and needs further refinement. Perhaps it would have been important to compare independent measures of the HC strength and quantify their relative relevance rather than the 7 measures proposed here as it is intuitive that capturing the HC by taking into account both its meridional and vertical extent would be more robust than from a single location.
Specific comments:
I find it strange to chose 2 versions of the ECMWF reanalysis instead of 2 new generation products such as ERA5 and CFSR for a more independent comparison. It's been reported that ERA5 is an improved version of ERAI with many significant fixed errors therefore the discrepancies found by the authors maybe attributed to those improvements.
Technical corrections:
L46: suggest replace '…are the trend… the pressure level.' by '…the trend… the pressure level are.'
Section 2.2: this should go in the result section, not in the methods section
L177: suggest remove 'also'
L182: what do the authors mean by 'merely showcase'?
-
AC1: 'Reply on RC1', Žiga Zaplotnik, 09 Sep 2021
reply
We thank the reviewer for their constructive comments.
As the Reviewer points out, our suggestion that the measures of the HC strength that take into account both the meridional and vertical extent of the global HC are overall better indices than the HC measures based on local values may be intuitive. The time series of the stream function-based indices are aligned (Fig. 3) and highly correlated (Fig. 5), however, the differences become important when one computes the HC trends and quantifies their uncertainties. This is the first lesson from our comparison of independent measures of the HC strength. We applied the measures from previous studies to bring our results into the context of the reported trends in the HC strength. We agree that the trends based on other independent measures could be explored, such as the water vapor flow in Sohn and Park (JGR, 2013). This measure will be included additionally in the revised manuscript.
It is unclear how to quantify their relative relevance if we do not have a reference (or generally agreed) HC measure to which various other measures can be compared. It is in this context that we introduced a new, energy-based integral measure of the HC strength. The unbalanced energy of the zonal mean circulation is straightforward to derive for gridded datasets and it includes all 3 spatial dimensions of the unbalanced circulation. We are not sure what the reviewer finds confusing about the unbalanced energy measure, but we provide a further explanation here. Clearly, it is different from the stream function, but also omega and geopotential measure, and more research can be done to refine it, especially to differentiate between the northern and southern branches of the Hadley cell. But our figure A2 suggests that the global unbalanced energy is an adequate description of the HC. We believe that it is a suitable measure also for an intercomparison of reanalyses and climate model datasets analyzed in terms of the normal-mode functions.
In their specific comment, the Reviewer points out that ERA5 is a more advanced and therefore more reliable reanalysis dataset than ERA-Interim. We could not agree more and we will point this out in the revised paper. Although relatively few evaluations of the CFSR have been conducted and thus its performance is not well-known, we believe that CFSR is much more advanced than the NCEP-NCAR reanalyses. Yet, many researchers would argue that even the NCEP-NCAR reanalyses suffice for the description of the large-scale circulation. Even though ERA5 is available, many scientists rely on ERA-Interim and precisely a comparison of tropical aspects in the ERA5 and ERA-Interim, which have been the subject of several recent papers, motivated our study initially. The tropics remain the regions with the largest analysis uncertainties (e.g. Žagar et al., 2020, J. Clim) but the four modern reanalyses (ERA5, ERA-Interim, JRA55, and MERRA) agree relatively well regarding the large-scale tropical circulation. The total energy of the zonal mean unbalanced flow shows positive trends in both ERA5 and ERA-Interim, although weaker in the former, a result consistent with the overall quality of the representation of large-scale circulation in the reanalyses. Note, however, that the only aim of choosing another reanalysis besides ERA5 was to show that the strong sensitivity of HC strength trends are not an isolated feature of a particular (e.g. ERA5) reanalysis, as stated in lines 262-265.
Response to technical corrections:
> L46: suggest replace '…are the trend… the pressure level.' by '…the trend… the pressure level are.'
Will be corrected.
> Section 2.2: this should go in the result section, not in the methods section
We agree with the proposed reordering.
> L177: suggest remove 'also'
Will be corrected.
> L182: what do the authors mean by 'merely showcase'?
Will be corrected to a more neutral form: Figs. 2, A3 showcase the stronger year-to-year variability of monthly means...
-
AC1: 'Reply on RC1', Žiga Zaplotnik, 09 Sep 2021
reply
Matic Pikovnik et al.
Data sets
Hadley cell strength Žiga Zaplotnik, Matic Pikovnik https://doi.org/10.5281/zenodo.5135222
Matic Pikovnik et al.
Viewed
| HTML | XML | Total | BibTeX | EndNote | |
|---|---|---|---|---|---|
| 194 | 69 | 7 | 270 | 2 | 0 |
- HTML: 194
- PDF: 69
- XML: 7
- Total: 270
- BibTeX: 2
- EndNote: 0
Viewed (geographical distribution)
| Country | # | Views | % |
|---|
| Total: | 0 |
| HTML: | 0 |
| PDF: | 0 |
| XML: | 0 |
- 1