Natural marine bromoform emissions in the fully coupled ocean-atmosphere-model NorESM2
Abstract. Oceanic bromoform (CHBr3) is an important precursor of atmospheric bromine. Although highly relevant for the future halogen burden and ozone layer in the stratosphere, the global CHBr3 production in the ocean and its emissions are still poorly constrained in observations and are mostly neglected in climate models. Here, we newly implement marine CHBr3 in the state-of-the-art Norwegian Earth System Model (NorESM2) with fully coupled ocean-sea-ice-atmosphere biogeochemistry interactions. Our results are validated with oceanic and atmospheric observations from the HalOcAt (Halocarbons in the Ocean and Atmosphere) data base. The simulated mean oceanic concentrations (6.61±3.43 pmol L-1) are in good agreement with observations in open ocean regions (5.02±4.50 pmol L-1), while the mean atmospheric mixing ratios (0.76±0.39 ppt) are lower than observed but within the range of uncertainty (1.45±1.11 ppt). The NorESM2 ocean emissions of CHBr3 (214 Gg yr-1) are in the range of or higher than previously published estimates from bottom-up approaches but lower than estimates from top-down approaches. Annual mean emissions are mostly positive (sea-to-air), driven by oceanic concentrations, sea surface temperature and wind speed, dependent on season and location. During low-productivity winter seasons, model results imply some oceanic regions in high latitudes as sinks of atmospheric CHBr3, because of its elevated atmospheric mixing ratios. We further demonstrate that key drivers for the oceanic and atmospheric CHBr3 variability are spatially heterogeneous. In the tropical West Pacific, which is a hot spot for oceanic bromine delivery to the stratosphere, wind speed is the main driver for CHBr3 emissions on annual basis. In the North Atlantic as well as in the Southern Ocean region the atmospheric and oceanic CHBr3 variabilities are interacting during most of the seasons except for the winter months where sea surface temperature is the main driver. Our study provides improved process understanding of the biogeochemical cycling of CHBr3 and more reliable natural emission estimates especially on seasonal and spatial scales compared to previously published model estimates.
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