Chemical interaction between ocean and atmosphere

Abstract

The exchange of chemical constituents between ocean and atmosphere provides potentially important feedback mechanisms in the climate system. The aim of this study is to develop and evaluate a chemically coupled global atmosphere-ocean model. For this, an atmosphere-ocean general circulation model with atmospheric chemistry has been expanded to include oceanic biogeochemistry and the process of air-sea gas exchange. The calculation of seawater concentrations in the oceanic biogeochemistry submodel has been expanded from DMS, CO₂ , and N₂O to include also CO and isoprene (C₅H₈). Further new implementations comprise the possibilities to consider the nutrient import by rivers into the ocean, and the dust deposition as a source of silicic acid (Si(OH)₄) from silica (SiO₂) in addition to iron (Fe). Moreover, the climatological dust deposition can now be replaced by a simulated one from the atmospheric model. To evaluate the novel model system, a set of simulations under year-2000 conditions focusing on the major chemical interactions between ocean and atmosphere have been performed. With additional sensitivity studies the impact of a modified nutrient import from interactive dust deposition and from additional nutrient import by rivers has been assessed. To prepare the model system for studies on climatological time-scales, cloud parameterisation related parameters have been optimised for a suited model resolution. The simulations show robust and consistent responses of the model system to forcings in terms of interactive oceanic biogeochemistry. Deficiencies in the representation of oceanic net primary production (NPP) have been identified, depending strongly on the coupling frequency providing solar irradiance at the ocean’s surface. An approximately linear relationship between air-sea gas fluxes and surface water concentrations for the interactively simulated reactive species (DMS, C₅H₈, and CO) has been found. Additional nutrient import to the ocean by rivers shows only weak, insignificant impacts on the atmospheric composition, mostly due to the coarse resolution and the not well represented shelf areas. The modified dust deposition shows only a minor effect, although the interactively simulated amount of dust deposited into the ocean is twice as large as the previously used climatology. This weak impact of enhanced Fe and SiO₂ deposition is due to the limitation of phytoplankton growth by nitrate in aqueous phase (NO₃₋) in the main dust deposition regions. In summary, the chemically coupled model system is well prepared for applications in chemistry-climate research.