A connection has been found between puffing of CD4 at the outer mid-plane of JET and a rise in scrape-off layer (SOL) peripheral electron density. This observation led to the proposal that CD4 puffing might be used to improve lower hybrid coupling in optimized shear discharges. Results described elsewhere have demonstrated that this is indeed the case. The focus of this paper is on the causes of this peripheral density rise using two-dimensional fluid modelling and local measurements of turbulent transport in the JET SOL.