Simulated Trends of Stratospheric Water Vapor from 1960 to 1999 and their Impact on Ozone Chemistry

Abstract

Water vapor is an important controlling factor for climate feedback processes. Therefore, the understanding of the variability and of long-term changes of water vapor concentration in the UT/LS is essential for the understanding of the climate system. We applied the coupled chemistry-climate model E39/C of the troposphere and lower stratosphere in order to reproduce the main climate interactions involving water vapor, ozone and other species. E39/C includes the most relevant stratospheric homogeneous and heterogeneous chemical reactions. Some external forcings are prescribed, like the increase of greenhouse gases and chlorofluorocarbons, the 11-year solar cycle, major volcanic eruptions, the variability of sea surface temperatures, while others are nudged, like the QBO in the tropical lower stratosphere. A transient simulation from 1960 to 1999 aims at reproducing externally forced changes during the past decades as realistic as possible. In this simulation, observed climate variability features are indeed reproduced, like lower stratospheric water vapor anomalies with an approximate 2-year periodicity associated with the QBO. Lower stratospheric water vapor shows a decrease during the first 10 years of the simulation period, followed by nearly constant values in the 1970s. From 1980 on, a clear increase in lower stratospheric water vapor is simulated, which is of the same order of magnitude as observed in balloon soundings at Boulder, CO. Increasing methane emissions only explain about 30% of the simulated water vapor trend. Simulated temperature changes at the tropical cold point tropopause play the key role in controlling water vapor entry levels. The tropical tropopause in turn is controlled by a number of parameters, not only sea surface temperatures but also volcanic eruptions, stratospheric ozone depletion, and tropospheric ozone increase. Stratospheric water vapor changes affect the HOx budget which is important to ozone chemistry. Sensitivity experiments indicate that approximately 15% of the global total ozone decline during the last 20 years can be attributed to the stratospheric water vapour increase and its impact on ozone chemistry. A long-term water vapor increase does not only affect gas-phase chemistry, but also heterogeneous chemistry in the Antarctic stratosphere by enlarging the PSC area.