Seamless integration of the coastal ocean in global marine carbon cycle modeling
Contact: Moritz Mathis, Kai Logemann, Peter Korn, Tatiana Ilyina, Corinna Schrum
Global ocean-biogeochemistry models are powerful tools to investigate the marine carbon cycle of the open ocean. The coastal ocean, however, is poorly represented in global models to date, because of missing key processes controlling coastal carbon dynamics and too coarse grid resolutions to adequately resolve coastal circulation features. In CLICCS A5, we have developed the first global ocean-biogeochemistry model with a dedicated representation of the coastal ocean and associated marine carbon dynamics: ICON-Coast (Mathis et al., 2022). In this model, we globally apply a grid refinement in the coastal ocean (Fig. 1) and account explicitly for various shelf-specific physical and biogeochemical processes. These comprise tidal currents including bottom drag effects, sediment resuspension, temperature-dependent remineralization in the water column and sediment, riverine matter fluxes from land including terrestrial organic carbon, and variable sinking speed of aggregated particulate matter. The combination of regional grid refinement and enhanced process representation enables for the first time a seamless incorporation of the global coastal ocean in model-based Earth system research. In particular, ICON-Coast represents a new tool to deepen our mechanistic understanding about the role of the land-ocean transition zone in the global carbon cycle, and to narrow related uncertainties in possible and plausible climate futures.
Fig. 1: Non-uniform grid configuration used in ICON-Coast simulations. In this example, the highest horizontal resolution in the coastal ocean and continental margins is 10 km. The degree of grid refinement is locally dependent on the distance to the coastline, the water depth and the slope of the bottom topography (Logemann et al., 2021).
Further related achievements are the development of a variable-resolution grid generator (Logemann et al., 2020), and the implementation of generalized vertical coordinates in ICON-O to allow thin layer thicknesses for ICON-Coast grid configurations or general high-resolution ICON-O simulations (Singh & Korn, in prep).
As a precursor study, we applied historical river inputs to the ocean-biogeochemistry model MPIOM and run sensitivity experiments of the 20th century to estimate the impact of anthropogenic perturbations on the ocean sink for atmospheric CO2. We found that historical increases in land-derived nutrient inputs may have alleviated effects of physical climate change on the ocean carbon cycle (Lacroix et al., 2021). The weakening of global marine net primary production (NPP) by 3% as a result of increasing upper ocean stratification has been overcompensated by a NPP enhancement of 5% induced by increasing terrestrial nutrient inputs. The largest signal of an NPP increase by 14% is simulated in the coastal ocean, contributing to an enhanced ocean CO2 uptake of 0.02 Pg C yr-1 globally. Nevertheless, the imprint of enhanced river loads is also found further offshore, with a NPP enhancement of 4% in the open ocean. This implies that the perturbation of carbon fluxes through coastal eutrophication may extend further offshore than was previously assumed.
References:
Lacroix, F., Ilyina, T., Mathis, M., Laruelle, G. G., Regnier, P. (2021). Historical increases in land-derived nutrient inputs may alleviate effects of a changing physical climate on the oceanic carbon cycle. Global Change Biology, 27, 5491– 5513, doi: 10.1111/gcb.15822
Logemann, K., Linardakis, L., Korn, P., Schrum, C. (2021). Global tide simulations with 1292 ICON-O: testing the model performance on highly irregular meshes. Ocean Dynamics, 1293 71, 43–57, doi: 10.1007/s10236-020-01428-7
Mathis, M., Logemann, K., Maerz, J., Lacroix, F., Hagemann, S., Chegini, F., Ramme, L., Ilyina, T., Korn, P., Schrum, C. (2022). Seamless integration of the coastal ocean in global marine carbon cycle modeling. Journal of Advances in Modeling Earth Systems (under review), doi: 10.1002/essoar.10508736.2