North Atlantic Integrated Water Vapor Transport-From 850 to 2100 CE: Impacts on Western European Rainfall

Moisture transport over the northeastern Atlantic Ocean is an important process governing precipitation distribution and variability over western Europe. To assess its long-term variability, the vertically integrated horizontal water vapor transport (IVT) from a long-term climate simulation spanning the period 850-2100 CE was used. Results show a steady increase in moisture transport toward western Europe since the late-nineteenth century that is projected to expand during the twenty-first century under the RCP8.5 scenario. The projected IVT for 2070-99 significantly exceeds the range given by interannual-interdecadal variability of the last millennium. Changes in IVT are in line with significant increases in tropospheric moisture content, driven by the concurrent rise in surface temperatures associated with the anthropogenic climate trend. On regional scales, recent and projected precipitation changes over the British Isles follow the global positive IVT trend, whereas a robust precipitation decrease over Iberia is identified in the twenty-first century, particularly during autumn. This indicates a possible extension of stable and dry summer conditions and a decoupling between moisture availability and dynamical forcing. The investigation of circulation features reveals a mean poleward shift of moisture corridors and associated atmospheric rivers. In particular, in Iberia, a significant increase in the frequency of dry weather types is observed, accompanied by a decrease in the frequency of wet types. An opposite response is observed over the British Isles. These changes imply a stronger meridional north-south dipole in terms of pressure and precipitation distributions, enhancing the transport toward central Europe rather than to Iberia.