9.4.4.2 Changes in Tropopause Height

The height of the lapse rate tropopause (the boundary between the stratosphere and the troposphere) is sensitive to bulk changes in the thermal structure of the stratosphere and the troposphere, and may also be affected by changes in surface temperature gradients (Schneider, 2004). Analyses of radiosonde data have documented increases in tropopause height over the past 3 to 4 decades (Highwood et al., 2000; Seidel et al., 2001). Similar increases have been inferred from three different reanalysis products, the European Centre for Medium Range Weather Forecasts (ECMWF) 15- and 40-year reanalyses (ERA-15 and ERA-40) and the NCAR- National Center for Environmental Prediction (NCEP) reanalysis (Kalnay et al., 1996; Gibson et al., 1997; Simmons and Gibson, 2000; Kistler et al., 2001), and from model simulations with combined anthropogenic and natural forcing (Santer et al., 2003a,b, 2004; see Figure 9.14). In both models and reanalyses, changes in tropopause height over the satellite and radiosonde eras are smallest in the tropics and largest over Antarctica (Santer et al., 2003a,b, 2004). Model simulations with individual forcings indicate that the major drivers of the model tropopause height increases are ozone-induced stratospheric cooling and the tropospheric warming caused by greenhouse gas increases (Santer et al., 2003a). However, earlier model studies have found that it is difficult to alter tropopause height through stratospheric ozone changes alone (Thuburn and Craig, 2000). Santer et al. (2003c) found that the model-simulated response to combined anthropogenic and natural forcing is robustly detectable in different reanalysis products, and that solar and volcanic forcing alone could not explain the tropopause height increases (Figure 9.14). Climate data from reanalyses, especially the ‘first generation’ reanalysis analysed by Santer et al. (2003a), are subject to some deficiencies, notably inhomogeneities related to changes over time in the availability and quality of input data, and are subject to a number of specific technical choices in the reanalysis scheme (see Santer et al., 2004, for a discussion). Also, the NCEP reanalysis detection results could be due to compensating errors because of excessive stratospheric cooling in the reanalysis (Santer et al., 2004), since the stratosphere cools more relative to the troposphere in the NCEP reanalysis while models warm the troposphere. In contrast, the finding of a significant anthropogenic influence on tropopause height in the ‘second generation’ ERA-40 reanalysis is driven by similar large-scale changes in both models and the reanalysis. Detection results there are robust to removing global mean tropopause height increases.

Figure 9.14. Comparison between reanalysis and climate-model simulated global monthly mean anomalies in tropopause height. Model results are from two different PCM (Table 8.1) ensemble experiments using either natural forcings, or natural and anthropogenic forcings (ALL). There are four realisations of each experiment. Both the low-pass filtered ensemble mean and the unfiltered range between the highest and lowest values of the realisations are shown. All model anomalies are defined relative to climatological monthly means computed over 1890 to 1999. Reanalysis-based tropopause height anomalies estimated from ERA-40 were filtered in the same way as model data. The ERA-40 record spans 1957 to 2002 and was forced to have the same mean as ALL over 1960 to 1999. After Santer et al. (2003a) and Santer et al. (2004).