Disentangling different moisture transport pathways over the eastern subtropical North Atlantic using multi-platform isotope observations and high-resolution numerical modelling

Abstract. Due to its dryness, the subtropical free troposphere plays a critical role in radiative balance of Earth's climate system. But complex interactions dynamical and physical processes controlling variability moisture budget this sensitive region atmosphere are still not fully understood. Stable water isotopes can provide important information about several latter processes, namely subsidence drying, turbulent mixing, dry moist convective moistening. In study, we use high-resolution simulations isotope-enabled version regional weather prediction model Consortium for Small-Scale Modelling (COSMOiso) investigate predominant transport pathways Canary Islands eastern North Atlantic. Comparison simulated isotope signals with multi-platform observations (aircraft, ground- space-based remote sensing) from field campaign summer 2013 shows that COSMOiso reproduce observed stable vapour on timescales hours days, thus allowing us study mechanisms control free-tropospheric humidity. Changes isotopic along backward trajectories reveal behind synoptic-scale variability. We identify four mid-tropospheric air, each distinct signatures: air parcels originating boundary layer Saharan heat low (SHL) – these characterised by homogeneous composition particularly high ?D (median ?D=-122‰), which results mixing low-level diverse origin advected into SHL; above SHL although experiencing largest changes humidity during their over West Africa, typically have lower values ?D=-148‰) than outside region, descending tropical upper levels south SHL, often affected injections mesoscale systems Sahel is much less enriched heavy ?D=-175‰) those region; subsiding upper-level extratropical Atlantic pathway leads driest most depleted conditions ?D=-255‰) middle near Islands. The alternation explains synoptic large degree. further show different related specific large-scale flow conditions. particular, differences location African mid-level anticyclone Rossby wave patterns occur between pathways. Overall, demonstrates adopted Lagrangian perspective enhances our understanding mass offers sound interpretation days contrasting atmospheric