F. Ploeger and co-authors compare MLS water vapour observations with simulations from the CLaMS model to investigate pathways of water vapour transport in the lower stratosphere during the boreal summer in a new JGR article. In the northern hemisphere (NH) mid- and high latitudes, a negative correlation is found between water vapour and ozone daily tendencies, indicative of frequent horizontal transport from the low latitudes. Analysis of the zonal mean tracer continuity equation shows that poleward of 50°N eddy mixing dominates the horizontal water vapour transport, while in the subtropics horizontal transport is largely due to the residual circulation. Model simulations with transport barriers confirm that most of the annual cycle in NH mid-latitude water vapour above 360K results from horizontal transport from low latitudes. The full abstract can be found here.