Importance of boundary layer mixing for the isotopic composition of surface vapor over the subtropical North Atlantic Ocean
During the summer 2012, we carried out continuous measurements of the isotopic composition (δ) of water vapor over the near-surface subtropical North Atlantic Ocean (STRASSE cruise). In this region of excess evaporation, we investigate the control of evaporation and mixing with a lower troposphere-derived, isotopically depleted air mass on the near-surface δ. We use a simple model to simulate the near-surface δ as the result of a two end-member mixing of the evaporative flux with free tropospheric air. The evaporative flux δ was estimated by the Craig and Gordon equation while the δ of the lower troposphere was taken from the LMDZ-iso global atmospheric circulation model. This simulation considers instantaneous mixing of lower tropospheric air with the evaporated flux and neglects lateral advection. Despite these simplifications, the simulations allow to identify the controls on the near-surface δ. The d-excess variability is largely a consequence of varying kinetic effects during evaporation, even during a convection event when the input of tropospheric vapor was strong. Kinetic effects and mixing processes affect simultaneously the near-surface δ and result in the vapor occupying distinct domains in the δ18O-δD space. The relative humidity-d-excess relationship shows that the closure assumption overestimates the d-excess variability at short time scales (less than a day). We interpret this as due to an effect of the residence time of the near-surface water vapor on the d-excess. Finally, we highlight the importance of reproducing mixing processes in models simulating isotopes over the subtropical North Atlantic Ocean and propose an extension of the closure assumption for use in initial conditions of distillation calculations.