Preindustrial Ocean CO2 Simulations with CYCLOCIM for River2Ocean Project

1. Model Implementation The model simulates ocean DIC at a cyclo-staionary state with sources and sinks from air-sea exchange, biological activity, evaporation and precipitation, and riverine inputs. See the detalis in the Model_Description.docx. 2. Scenario Runs NoRivC-ClimPI: The model configuration is similar to that in Devries (2022) and Devries et al. (2023). We use the climatological SST, SSS, surface TAlk, wind speed, and ice coverage to calculate Jgas. SST and SSS are from WOA18 (Locarnini et al., 2018; Zweng et al., 2018). TAlk is from OceanSODA product (Gregor & Gruber, 2021). Atmospheric forcings are from NECP2. No riverine carbon is added into the model. As our biological pump model does not simulate any nutrients and assimilates satellite-derived NPP, the riverine nutrient cannot be excluded from the model. Riv-ClimPI: Drivers of Jgas are the same as NoRivC-ClimPI. We add riverine semi-labile DOC and POC (converted into semi-labile DOC) into our biological pump model. For Jvir, dilution from river discharge (Dai, 2021) is added to balance the ocean evaporation and precipitation. Jriv includes both riverine DIC and labile DOC (converted into DIC). 3. Supplemental Notes for R2O project (1) As our model equation is solved as a steady-state annual cycle, the drift is negligible. (2) The model does not have any spin-up and its simulation length is only 1 year representing a steady-state annual cycle. (3) The atmospheric forcing is from NCEP2 and the runoff data is from Dai (2021). (4) The model does not include any nutrient forcings. (5) The model does not simulate alkalinity. The model outputs on PIC flux (epcalc100+eparag100, fric) are all zero. The surface alkalinity is prescribed for the calculation of air-sea CO2 exchange. (6) The model does not include burial fluxes for DIC/ALK. The biological pump model for Jbio has a burial flux of POC. (7) The riverine labile DOC is injected as DIC. The riverine POC is injected as semi-labile DOC. No riverine nutrients are included in the model. (8) The difference in air-sea fluxes between the two scenario runs is 0.95 PgC/yr, which is very close to the total riverine carbon input (0.96 PgC/yr) Reference: Dai, A. (2021). Hydroclimatic trends during 1950–2018 over global land. Climate Dynamics, 56(11), 4027-4049. DeVries, T., Yamamoto, K., Wanninkhof, R., Gruber, N., Hauck, J., Müller, J. D., ... & Zeng, J. (2023). Magnitude, trends, and variability of the global ocean carbon sink from 1985 to 2018. Global Biogeochemical Cycles, 37(10), e2023GB007780. DeVries, T. (2022). Atmospheric CO2 and sea surface temperature variability cannot explain recent decadal variability of the ocean CO2 sink. Geophysical Research Letters, 49(7), e2021GL096018. Gregor, L., & Gruber, N. (2021). OceanSODA-ETHZ: a global gridded data set of the surface ocean carbonate system for seasonal to decadal studies of ocean acidification, Earth Syst. Sci. Data, 13, 777–808. Huang, Q., Primeau, F., & DeVries, T. (2021). CYCLOCIM: A 4-D variational assimilation system for the climatological mean seasonal cycle of the ocean circulation. Ocean Modelling, 159, 101762. Lacroix, F., Liu, M., Ma, M., Resplandy, L., Beusen, A., Hauck, J., Lennartz, S., Li, Y., Tian, H., & Regnier, P. (2024). Biogeochemical river inputs for global ocean models (RivR2O) [Data set]. Zenodo. https://doi.org/10.5281/zenodo.13799103 Li, R., DeVries, T., Siegel, D., and Primeau, F. (2025). Seasonality in Marine Organic Carbon Export and Sequestration Pathways. Global Biogeochemical Cycles, under rivew. Locarnini, R. A., A. V. Mishonov, O. K. Baranova, T. P. Boyer, M. M. Zweng, H. E. Garcia, J. R. Reagan, D. Seidov, K. Weathers, C. R. Paver, and I. Smolyar, 2018. World Ocean Atlas 2018, Volume 1: Temperature. A. Mishonov Technical Ed.; NOAA Atlas NESDIS 81, 52pp. Zweng, M. M., J. R. Reagan, D. Seidov, T. P. Boyer, R. A. Locarnini, H. E. Garcia, A. V. Mishonov, O. K. Baranova, K. Weathers, C. R. Paver, and I. Smolyar, 2018. World Ocean Atlas 2018, Volume 2: Salinity. A. Mishonov Technical Ed.; NOAA Atlas NESDIS 82, 50pp. Kanamitsu, M., Ebisuzaki, W., Woollen, J., Yang, S. K., Hnilo, J. J., Fiorino, M., & Potter, G. L. (2002). Ncep–doe amip-ii reanalysis (r-2). Bulletin of the American Meteorological Society, 83(11), 1631-1644.