Marine boundary layer stable water isotope variability in the Southern Ocean: An investigation using the regional COSMOiso model

Abstract

The Southern Ocean overturning strength constitutes an essential control mecha- nism for future anthropogenic carbon uptake. The atmospheric water cycle, in turn, exerts an influence on upwelling intensity via buoyancy forcing induced by fresh- water fluxes. Stable water isotopes (H216O, H218O, HDO) are a powerful tool to constrain processes of the atmospheric water cycle and can therefore help to gain a more comprehensive picture on the Southern Ocean region. In order to fully ex- ploit this potential, simulations with the isotope-enabled, regional numerical model COSMOiso are combined with isotope and meteorological measurements of high temporal resolution, conducted during the Antarctic Circumnavigation Expedition (ACE) in austral summer 2016/2017. For the first time, the COSMOiso model per- formance with respect to meteorology and stable water isotopes is assessed in the Southern Hemisphere, along Leg 1 and Leg 2 of ACE. The validation confirms a re- liable representation of meteorology and isotope processes over the open ocean, but reveals model shortcomings with respect to isotopic variability in proximity to the Antarctic continent. Non-fractionating snow-atmosphere interactions are identified as a main factor limiting the model performance. Consequently, the implementa- tion of equilibrium fractionation during surface snow sublimation and deposition improves the skill of COSMOiso significantly (correlation increases from 0.1 to 0.7 for d-variables). This points out the potential of further model development in order to reproduce the entire spectrum of processes a↵ecting stable water isotopic variabil- ity in polar regions with COSMOiso. Furthermore, a newly developed Lagrangian budget model for isotopes and humidity is presented. The model framework incorpo- rates the processes of horizontal advection, ocean evaporation and marine boundary layer mixing. In a first, exploratory case study, the air parcel-based box model is used to study the isotopic signal along trajectories associated with a marine cold air outbreak. By comparing the box model simulations to COSMOiso, entrainment of depleted, free-tropospheric air is identified as an important factor, besides ocean evaporation, influencing the observed isotopic variability during the event. Down- ward mixing of air masses from the lower troposphere e ciently replaces the origi- nal Antarctic moisture along the transport pathway. The box model framework is a promising starting point for future e↵orts to further constrain the marine boundary layer moisture budget.