Orographic distillation and spatio-temporal variations of stable isotopes in precipitation in the North Atlantic

Little is known about the spatial and temporal variability of the stable isotopic composition of precipitation in the North Atlantic and its relationship to the North Atlantic Oscillation (NAO) and anthropogenic climate change. The islands of the Azores archipelago are uniquely positioned in the middle of the North Atlantic Ocean to address this knowledge gap. A survey of spatial and temporal variability of the stable isotope composition of precipitation in Azores is discussed using newly presented analyses along with Global Network of Isotopes in Precipitation data. The collected precipitation samples yield a new local meteoric water line (delta H-2 = 7.1 * delta O-18 + 8.46) for the Azores region and the North Atlantic Ocean. The annual isotopic mean of precipitation shows a small range for the unweighted and precipitation mass-weighted delta O-18-H2O values. Results show an inverse relation between the monthly delta O-18-H2O and the amount of precipitation, which increases in elevation and into the interior of the island. Higher amounts of precipitation (from convective storm systems) do not correspond to the most depleted values of stable isotopes in precipitation. Precipitation shows an orographic effect with depleted delta O-18-H2O values related to the Rayleigh effect. Monthly delta O-18-H2O values for individual precipitation sampling stations show little relationship to air temperature. Results show a local source of moisture during the summer with the characteristics of the first vapour condensate. The stable isotope composition of precipitation is strongly correlated to the NAO index, and delta O-18-H2O values show a statistically significantly trend towards enrichment since 1962 coincident with the increased air temperature and relative humidity due to climate change. Results are in line with observations of increasing sea surface temperature and relative humidity.