Changes of the tropical tropopause layerin reanalysis and climate model data
The main entrance region for trace gases which are emitted in the troposphere reaching the stratosphere, is the tropical tropopause ayer (TTL). Chemical and dynamical processes within the TTL control the amount of trace gases entering the stratosphere. In this study a Lagrangian analysis of the upper TTL is presented, using backward trajectory calculations with a kinematic method. The Lagrangian approach is applied in order to determine the main entrainment regions of air masses into the stratosphere - the Lagrangian cold points (LCP)- with the new reanalysis product ERA Interim. Further, it is evaluated how well climate models are able to reflect the basic LCP temperature and distribution. Additionally, future simulations are analyzed in order to understand the evolution of the TTL in a changing climate. This calculation is carried out employing ERA Interim data for the period 1979-2011 and different CCMs from CCMVal-2 and one model from the CMIP5 project, analyzing one decade in the past (1990s) and two decades in the future (2040s and 2090s). As shown by ERA Interim, the main air mass entrainment is located over the tropical West Pacific, where more than 50% of the air masses enter the stratosphere on annual average. Generally, the analyzed climate models are able to capture this main entrance region for stratospheric air masses. Further, it is shown that also other convective regions, like South America, West Africa and the Indian Ocean play a larger role as entrainment regions in the reanalysis and the climate models compared to previous studies. During NH winter the West Pacific becomes even more important as entrance region for air masses to the stratosphere (62%), which is also simulated by most of the climate models. A new diagnostic method for analyzing the impact of the tropical widening on the upper TTL is presented in this thesis. By analyzing the meridional extension of the LCPs a significant broadening of the upper TTL is detected by ERA Interim of 1.2± lat/dec on the annual average between 1979-2011, indicating an increase of air mass entrainment into the stratosphere. However, the broadening of the upper TTL revealed larger transitions prior to the year 2000 than afterwards, possibly due to changes in the stratospheric circulations at the same time. Most of the climate models reveal a slight positive change of the width of the upper TTL as well for the time period 1990-1999, though the results are not significant. For the future scenarios the climate models project an increase of the LCP temperature around 1 K from the 1990s to the 2090s. Additionally, by determining the spatial evolution of the LCP occurrence, all climate models agree with each other by projecting an increase of the air mass entrainment over the West Pacific by 2% to 8% annually averaged. Simultaneously, a slight broadening of the upper TTL within the whole tropics and in particular over the main entrance region, the West Pacific is projected by the climate models on the annual average. These results suggest that the role of the tropical West Pacific as main entrance region for air masses into the stratosphere will be even more pronounced in a changing climate.
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