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.
Highlights
Carbon dioxide (CO2) concentration in the atmosphere has increased approximately 150% from 1750 to 2019 (Le Quéré et al, 2018), the storage and transport of carbon in Earth’s ecosystem under the context of climate change has been receiving incremental attention over the past decades (Anav et al, 2013; Lindsay et al, 2014; Jones et al, 2016)
As by far most available observations are confined in the surface ocean, except for the GOMECC transects, in this study, we focus on the surface ocean carbon condition in the northern GoM (NGoM) and Open Gulf of Mexico (GoM) waters
We demonstrate that the regional high-resolution carbon model can reproduce the spatial and seasonal patterns of ocean surface pCO2 and generate reliable TA/dissolved inorganic carbon (DIC) profiles on the NGoM shelf
Summary
Carbon dioxide (CO2) concentration in the atmosphere has increased approximately 150% from 1750 to 2019 (Le Quéré et al, 2018), the storage and transport of carbon in Earth’s ecosystem under the context of climate change has been receiving incremental attention over the past decades (Anav et al, 2013; Lindsay et al, 2014; Jones et al, 2016). The ocean has intake some 170 ± 20 PgC (Le Quéré et al, 2018) since the industrial revolution This alleviates the CO2 accumulation rate in 35 the atmosphere while inducing a consequent increase in ocean carbon level and a decrease in ocean pH and calcium mineral saturation state (Ω, Doney et al, 2009). Extensive utilization of ESMs in hindcasting and coupled biogeochemistry provide pivotal information for understanding the carbon cycle on a global scale (Anav et al, 2013; Laurent, Fennel, & Kuhn, 2021; Lindsay et al, 2014; 45 Jones et al, 2016; Todd-Brown et al, 2014) Their relatively coarse spatial resolution is likely not appropriate to be directly compared with field measurements. While high-resolution regional models have been developed to represent the complex patterns of ocean circulation and elemental fluxes on the continental shelves, the regional ocean carbon system is challenging to model and predict due to its high sensitivity to the boundary and initial conditions, uncertainties in 50 the carbon pathway, and complex interactions between the atmosphere, ocean, and land (Hofmann et al, 2011)
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