A Numerical reassessment of the Gulf of Mexico carbon system in connection with the Mississippi River and global ocean

Coupled physical-biogeochemical models can fill the spatial and temporal gap in ocean carbon observations. 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 degrees) hindcast covering the period of 2000 to 2019. The biogeochemical model incorporated the dynamics of dissolved organic carbon (DOC) pools and the formation and dissolution of carbonate minerals. The biogeochemical boundaries were interpolated from NCAR's CESM2-WACCM-FV2 solution after evaluating the performance of 17 GCMs in the GoM waters. Model outputs included carbon system variables of wide interest, such as pCO(2), pH, aragonite saturation state (Omega(Arag)), calcite saturation state (Omega(Calc)), CO2 air-sea flux, and carbon burial rate. The model's robustness is evaluated via extensive model-data comparison against buoys, remote-sensingbased machine learning (ML) products, and ship-based measurements. A reassessment of air-sea CO2 flux with previous modeling and observational studies gives us confidence that our model provides a robust and updated CO2 flux estimation, and NGoM is a stronger carbon sink than previously reported. Model results reveal that the GoM water has been experiencing a similar to 0.0016 yr(-1) decrease in surface pH over the past 2 decades, accompanied by a similar to 1.66 mu atm yr(-1) increase in sea surface pCO(2). The air-sea CO2 exchange estimation confirms in accordance with several previous models and ocean surface pCO(2) observations that the river-dominated northern GoM (NGoM) is a substantial carbon sink, and the open GoM is a carbon source during summer and a carbon sink for the rest of the year. Sensitivity experiments are conducted to evaluate the impacts of river inputs and the global ocean via model boundaries. The NGoM carbon system is directly modified by the enormous carbon inputs (similar to 15.5 Tg C yr(-1) DIC and similar to 2.3 Tg C yr(-1) DOC) from the Mississippi-Atchafalaya River System (MARS). Additionally, nutrient-stimulated biological activities create a similar to 105 times higher particulate organic matter burial rate in NGoM sediment than in the case without river-delivered nutrients. The carbon system condition of the open ocean is driven by inputs from the Caribbean Sea via the Yucatan Channel and is affected more by thermal effects than biological factors.

Automated summary

Coupled physical-biogeochemical models can bridge the spatial and temporal gap in ocean carbon observations. In this study, a coupled model was applied to the Gulf of Mexico, providing a high-resolution hindcast and evaluating model performance against observations. The results highlight the importance of the northern Gulf of Mexico as a carbon sink and the seasonal variability of carbon sources and sinks in the open Gulf.