Sensitivity of stratospheric water vapour to variability in tropical tropopause temperatures and large-scale transport

Abstract. Concentrations of water vapour entering the tropical lower stratosphere are primarily determined by conditions that air parcels encounter as they transported through tropopause layer (TTL). Here we quantify relative roles variations in TTL temperatures and transport determining seasonal interannual stratospheric vapour. Following previous studies, use trajectory calculations with concentration set Lagrangian dry point (LDP) along trajectories. To assess separate temperatures, LDP modified replacing either winds or those from different years to investigate wind temperature sensitivity and, correspondingly, months variations. Both ERA-Interim reanalysis data for 1999–2009 period generated a chemistry–climate model (UM-UKCA) investigated. Variations rather than transport, dominate variability, typically explaining more 70 % including individual events such 2000 drop. Similarly variation is shown be dominant driver annual cycle concentrations both reanalysis, but it also plays an important role reducing maximum (reducing range about 30 %). The quantitative dehydration sub-seasonal sub-monthly Eulerian variability examined using time-filtered fields calculations. Sub-monthly reduces mean 40 calculation calculation. As other aspects dehydration, simple measures not sufficient implications sampling must taken into account. These results indicate that, whilst capturing major factor modelling realistic concentrations, simulation cannot ignored.