Abstract
Abstract. Studying the isotopic composition of water vapour in the lower stratosphere can reveal the driving mechanisms of changes in the stratospheric water vapour budget and therefore help to explain the trends and variations of stratospheric water vapour during recent decades. We equipped a global chemistry climate model with a description of the water isotopologue HDO, comprising its physical and chemical fractionation effects throughout the hydrological cycle. We use this model to improve our understanding of the processes which determine the patterns in the stratospheric water isotope composition and in the water vapour budget itself. The link between the water vapour budget and its isotopic composition in the tropical stratosphere is presented through their correlation in a simulated 21-year time series. The two quantities depend on the same processes; however, they are influenced with different strengths. A sensitivity experiment shows that fractionation effects during the oxidation of methane have a damping effect on the stratospheric tape recorder signal in the water isotope ratio. Moreover, the chemically produced high water isotope ratios overshadow the tape recorder in the upper stratosphere. Investigating the origin of the boreal-summer signal of isotopically enriched water vapour reveals that in-mixing of old stratospheric air from the extratropics and the intrusion of tropospheric water vapour into the stratosphere complement each other in order to create the stratospheric isotope ratio tape recorder signal. For this, the effect of ice lofting in monsoon systems is shown to play a crucial role. Furthermore, we describe a possible pathway of isotopically enriched water vapour through the tropopause into the tropical stratosphere.
Highlights
Variations in stratospheric water vapour alter the radiative heat budget (Forster and Shine, 1999) and the ozone mixing ratios (Shindell, 2001)
The link between the water vapour budget and its isotopic composition in the tropical stratosphere is presented through their correlation in a simulated 21-year time series
A sensitivity experiment shows that fractionation effects during the oxidation of methane have a damping effect on the stratospheric tape recorder signal in the water isotope ratio
Summary
Variations in stratospheric water vapour alter the radiative heat budget (Forster and Shine, 1999) and the ozone mixing ratios (Shindell, 2001). In Part 1 of this article (Eichinger et al, 2015), an extension of the global climate chemistry model (CCM) EMAC (ECHAM MESSy Atmospheric Chemistry; MESSy stands for “Modular Earth Submodel System”) was presented and evaluated This extension, namely the H2OISO (H2O ISOtopologues) submodel, comprises an additional hydrological cycle, including the water isotopologues H128O and HDO and their physical fractionation effects, based on previous studies by, e.g., Hoffmann et al (1998) and Werner et al (2011). The results revealed a stratospheric tape recorder which ranges between the pronounced signal of MIPAS (Michelson Interferometer for Passive Atmospheric Sounding) observations (see Steinwagner et al, 2010) and the missing upward propagation of the seasonal signal in the ACE-FTS (Atmospheric Chemistry Experiment Fourier transform spectrometer) retrieval (see Randel et al, 2012) The results of these simulations are further analysed, with the aim of identifying the processes which determine the patterns of the isotopic signatures in stratospheric water vapour. This study constitutes the first application of the isotopic composition of water vapour in order to explore the reasons for changes in the stratospheric water vapour budget with global atmosphere chemistry–climate models
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