Abstract
The representation of upper tropospheric/lower stratospheric (UTLS) jet and tropopause characteristics is compared in five modern high-resolution reanalyses for 1980 through 2014. Climatologies of upper tropospheric jet, subvortex jet (the lowermost part of the stratospheric vortex), and multiple tropopause frequency distributions in MERRA (Modern Era Retrospective Analysis for Research and Applications), ERA-I (the ECMWF interim reanalysis), JRA-55 (the Japanese 55-year Reanalysis), and CFSR (the Climate Forecast System Reanalysis) are compared with those in MERRA-2. Differences between alternate products from individual reanalysis systems are assessed; in particular, a comparison of CFSR data on model and pressure levels highlights the importance of vertical grid spacing. Most of the differences in distributions of UTLS jets and multiple tropopauses are consistent with the differences in assimilation model grids and resolution: For example, ERA-I (with coarsest native horizontal resolution) typically shows a significant low bias in upper tropospheric jets with respect to MERRA-2, and JRA-55 a more modest one, while CFSR (with finest native horizontal resolution) shows a high bias with respect to MERRA-2 in both upper tropospheric jets and multiple tropopauses. Vertical temperature structure and grid spacing are especially important for multiple tropopause characterization. Substantial differences between MERRA and MERRA-2 are seen in mid- to high-latitude southern hemisphere winter upper tropospheric jets and multiple tropopauses, and in the upper tropospheric jets associated with tropical circulations during the solstice seasons; some of the largest differences from the other reanalyses are seen in the same times and places. Very good qualitative agreement among the reanalyses is seen between the large scale climatological features in UTLS jet and multiple tropopause distributions. Quantitative differences may, however, have important consequences for transport and variability studies. Our results highlight the importance of considering reanalyses differences in UTLS studies, especially in relation to resolution and model grids; this is particularly critical when using high-resolution reanalyses as an observational reference for evaluating global chemistry climate models.
Highlights
Variations in the upper tropospheric–lower stratospheric (UTLS) jets and extratropical tropopause influence highimpact weather and climate on regional and global scales: they play key roles in circulation changes, especially in the observed widening of the tropics (e.g., Staten et al, 2016)Published by Copernicus Publications on behalf of the European Geosciences Union.G
Because the frequency distributions are expressed as a percent, the arithmetic differences (i.e., Freqr1 − Freqr2, where r1 and r2 are two reanalyses) between two frequency distributions that are shown in the figures are expressed as “percentage points”; this should not be confused with the approximate percentage values for relative differences (e.g., (Freqr1 − Freqr2) / 0.5(Freqr1 + Freqr2) × 100) mentioned in the text
While the ASM products are recommended by Global Modeling and Assimilation Office (GMAO) for most studies, this distinction has not been widely recognized, so usage of one rather than the other has been inconsistent in existing studies
Summary
Transport processes that alter the extent and consequences of extratropical stratosphere–troposphere exchange (STE) are closely linked to the tropopause and jets, which are themselves sensitive to climate change and ozone depletion (e.g., Seidel and Randel, 2006; Lorenz and DeWeaver, 2007; Polvani et al, 2011; WMO, 2011; Hudson, 2012; Grise et al, 2013; Waugh et al, 2015). Much of the variability in UTLS ozone is inextricably linked to that of the UTLS jets
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