Unraveling the moisture transport in the North Atlantic trade-wind region using passive tracers and stable water isotopes

The atmospheric hydrologic cycle and the formation of shallow cumulus clouds in the marine trade-wind region are important for the Earth’s radiative budget and climate sensitivity. Furthermore, the understanding of air mixing and transport processes in the atmosphere is crucial for interpreting measurements and records of stable water isotopes. However, the representation of these processes in climate models is subject to large uncertainties. Here we investigate moisture transport and its impact on the isotopic signature in the North Atlantic trade-wind region. We use the regional COSMO model equipped with stable water isotopes and passive water tracers to quantify the contributions of the different evaporation sources to moisture contents and their isotope signals in the free troposphere of the western tropical Atlantic. For the time period of the EUREC4A field campaign (January-February 2020), convection-resolving high-resolution (5 km) nudged simulations are performed, allowing a comparison with field data. Passive tracers (water tagging) are combined with prognostic water isotope simulations to determine the specific isotopic fingerprints of the diagnosed moisture pathways. In January and February 2020, the tropical Atlantic region is characterized by alternating large-scale circulation regimes with distinct isotopic signatures. Humid conditions in the middle troposphere (300-650 hPa) over the island of Barbados are related to easterly and south-easterly moisture transport, while dry conditions correspond to extratropical transport from the north and west. Our modeling approach, together with the unprecedented observational data from the EUREC4A campaign, offers exciting new opportunities to evaluate and ultimately improve the representation of the tropical water cycle in climate models.