17 Dec 2021
17 Dec 2021
Status: a revised version of this preprint is currently under review for the journal BG.

Downscaling CMIP6 Global Solutions to Regional Ocean Carbon Model: Connecting the Mississippi, Gulf of Mexico, and Global Ocean

Le Zhang1 and Z. George Xue1,2,3 Le Zhang and Z. George Xue
  • 1Department of Oceanography and Coastal Sciences, Louisiana State University, Baton Rouge, LA 70803
  • 2Center for Computation and Technology, Louisiana State University, Baton Rouge, LA 70803
  • 3Coastal Studies Institute, Louisiana State University, Baton Rouge, LA 70803

Abstract. Coupled physical-biogeochemical models can significantly reduce uncertainties in estimating the spatial and temporal patterns of the ocean carbon system. Challenges of applying a coupled physical-biogeochemical model in the regional ocean include the reasonable prescription of carbon model boundary conditions, lack of in situ observations, and the oversimplification of certain biogeochemical processes. In this study, we applied a coupled physical-biogeochemical model (Regional Ocean Modelling System, ROMS) to the Gulf of Mexico (GoM) and achieved an unprecedented 20-year high-resolution (5 km, 1/22°) hindcast covering the period of 2000–2019. The model’s biogeochemical cycle is driven by the Coupled Model Intercomparison Project 6-Community Earth System Model 2 products (CMIP6-CESM2) and incorporates the dynamics of dissolved organic carbon (DOC) pools as well as the formation and dissolution of carbonate minerals. Model outputs include generally interested carbon system variables, such as pCO2, pH, aragonite saturation state (ΩArag), calcite saturation state (ΩCalc), CO2 air-sea flux, carbon burial rate, etc. The model’s robustness is evaluated via extensive model-data comparison against buoy, remote sensing-based Machine Learning (ML) predictions, and ship-based measurements. Model results reveal that the GoM water has been experiencing an ~ 0.0016 yr−1 decrease in surface pH over the past two decades, accompanied by a ~ 1.66 µatm yr−1 increase in sea surface pCO2. The air-sea CO2 exchange estimation confirms that the river-dominated northern GoM is a substantial carbon sink. The open water of GoM, affected mainly by the thermal effect, is a carbon source during summer and a carbon sink for the rest of the year. Sensitivity experiments are conducted to evaluate the impacts from river inputs and the global ocean via model boundaries. Our results show that the coastal ocean carbon cycle is dominated by enormous carbon inputs from the Mississippi River and nutrient-stimulated biological activities, and the carbon system condition of the open ocean is primarily driven by inputs from the Caribbean Sea via Yucatan Channel.

Le Zhang and Z. George Xue

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on bg-2021-339', Anonymous Referee #1, 10 Jan 2022
    • AC1: 'Reply on RC1', Z. George Xue, 02 Mar 2022
  • RC2: 'Comment on bg-2021-339', Anonymous Referee #2, 08 Feb 2022
    • AC2: 'Reply on RC2', Z. George Xue, 02 Mar 2022

Le Zhang and Z. George Xue

Le Zhang and Z. George Xue


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Short summary
We adapt a high-resolution regional carbon model to the Gulf of Mexico (GoM). We calculated the decadal trends of important carbon system variables in the GoM. The GoM surface CO2 values experience a steady increase from 2001 to 2019. Correspondingly, the ocean surface pH is declining. The surface carbonate saturation rates in the GoM remain supersaturated with aragonite during the time span of the model but with a slightly decreasing trend.