M. Free and J.R. Lanzante, 2008:
Effect of Volcanic Eruptions on the Vertical Temperature Profile in
Radiosonde Data and Climate Models.
Submitted to Journal of Climate.
Abstract:
Both observed and modeled upper-air temperature profiles show the tropospheric cooling and tropical stratospheric warming effects from the three major volcanic eruptions since 1960. Detailed comparisons of vertical profiles of Radiosonde Atmospheric Temperature Products for Assessing Climate (RATPAC) and Hadley Centre Atmospheric Temperatures, Version 2 (HadAT2) radiosonde temperatures with output from 6 coupled GCMs show good overall agreement on the responses to the 1991 Pinatubo and 1982 El Chichon eruptions in the troposphere and stratosphere, with a tendency of the models to underestimate the upper tropospheric cooling and overestimate the stratospheric warming relative to observations. Furthermore, the level of maximum stratospheric volcanic warming does not always correspond between models and observations. In addition, models and observations show a large disagreement at 100 mb for Pinatubo in the tropics, where observations show essentially no change, while models show significant warming of ~0.7 to ~2.6 K. This difference occurs even in models that accurately simulate stratospheric warming at 50 hPa. Most models overestimate the tropospheric cooling effect of the 1963 Agung eruption in the tropics and underestimate it in the Southern Hemisphere extratropics, but uncertainties in the observations and the volcanic forcings make meaningful comparisons difficult for that eruption. Overall, the Parallel Climate Model (PCM) is an outlier in that it simulates more volcanic-induced stratospheric warming than both the other models and the observations in most cases. Results for all three eruptions are sensitive to the methods used to remove ENSO effects.
The cooling effect at the surface in the tropics is amplified with altitude in the troposphere in both observations and models, but this amplification is greater for the observations than for the models. In contrast, amplification for the ENSO signal in the models is more similar to that in the observations.
Estimates of the effect of the eruptions on temperature trends are dependent on the method used and the choice of parameters for these methods. From 1979 to 1999 in the tropics, RATPAC shows a trend of less than 0.1 K/decade at and above 300 hPa while the mean of the models used here has a trend of more than 0.3 K/decade, giving a difference of ~0.2 K/decade. From 0.02 to 0.08 K/decade of this difference may be due to the influence of volcanic eruptions, with the smaller estimate appearing more likely than the larger. In the lower troposphere, none of the difference in trends appears to be attributable to volcanic effects.