The millennium water vapour drop in the stratosphere in chemistry-climate model simulations

Kurzfassung

This study investigates the millennium water vapour drop, the abrupt and severe water vapour decline in thestratosphere beginning in year 2000, by means of various simulations using the Chemistry-Climate Model (CCM)EMAC. Since the beginning 1980s, balloon borne stratospheric water vapour measurements and correspondingsatellite measurements starting in the early 1990s indicated a long-term steady increase of water vapour concentrations.However, the multi-year data sets also show significant fluctuations on different time scales. In theyear 2000, an extraordinary sudden drop of stratospheric water vapour concentration has been observed followedby persistent low values for several years. Solomon et al. (2010) showed that this drop slowed down the rate ofincrease in global surface temperature over the following decade by about 25%. So far, the stratospheric watervapour variations observed by satellite from 1992 to 2012 are not reproduced by CCM simulations forced byobserved changes in sea surface temperatures, greenhouse gases and ozone-depleting substances (Gettelman et al.,2010, Randel and Jensen, 2013).However, the CCM EMAC is able to reproduce the signature and pattern of the water vapour disturbances inagreement with those derived from observations. In this paper we present results of a hierarchy of simulationswith the CCM EMAC, demonstrating that it is possible to retrace the observed water vapour fluctuations in thestratosphere (incl. the millennium drop), if suitable inner and outer boundary conditions are applied.