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Abstract. Stratospheric water vapour influences the chemical ozone loss inthe polar stratosphere via control of the polar stratospheric cloudformation. The amount of water vapour entering the stratosphere through thetropical tropopause differs substantially between simulations fromchemistry–climate models (CCMs). This is because the present-day models, e.g.CCMs, have difficulties in capturing the whole complexity of processes thatcontrol the water transport across the tropopause. As a result there arelarge differences in the stratospheric water vapour between the models. In this study we investigate the sensitivity of simulated Arctic ozone lossto the simulated amount of water vapour that enters the stratosphere throughthe tropical tropopause. We used a chemical transport model, FinROSE-CTM,forced by ERA-Interim meteorology. The water vapour concentration in thetropical tropopause was varied between 0.5 and 1.6 times the concentration inERA-Interim, which is similar to the range seen in chemistry–climate models.The water vapour changes in the tropical tropopause led to about1.5  ppmv less and 2  ppmv more water vapour in the Arcticpolar vortex compared to the ERA-Interim, respectively. The change induced inthe water vapour concentration in the tropical tropopause region was seen asa nearly one-to-one change in the Arctic polar vortex. We found that the impact of water vapour changes on ozone loss in the Arcticpolar vortex depends on the meteorological conditions. The strongest effectwas in intermediately cold stratospheric winters, such as the winter of 2013/2014,when added water vapour resulted in 2 %–7 % more ozone loss due to theadditional formation of polar stratospheric clouds (PSCs) and associatedchlorine activation on their surface, leading to ozone loss. The effect wasless pronounced in cold winters such as the 2010/2011 winter because coldconditions persisted long enough for a nearly complete chlorine activation,even in simulations with prescribed stratospheric water vapour amountcorresponding to the observed values. In this case addition of water vapourto the stratosphere led to increased areas of ICE PSCs but it did not increasethe chlorine activation and ozone destruction significantly. In the warmwinter of 2012/2013 the impact of water vapour concentration on ozone loss wassmall because the ozone loss was mainly NOx -induced. Theresults show that the simulated water vapour concentration in the tropicaltropopause has a significant impact on the Arctic ozone loss and thereforeneeds to be well simulated in order to improve future projections of therecovery of the ozone layer.

Linking uncertainty in simulated Arctic ozone loss to uncertainties in modelled tropical stratospheric water vapour