Air-sea CO2 flux in the Gulf of Mexico from observations and multiple machine-learning data products

Abstract

Quantifying air-sea carbon dioxide (CO2) flux from observations is subject to uncertainties due to missing data, uneven data distribution, and a relatively short observation period in the Gulf of Mexico (GOM). Despite the publication of multiple seawater partial pressure of CO2 (pCO2sw) products, their reliabilities in the GOM have been relatively understudied. We compare the Surface Ocean CO₂ Atlas (SOCAT) observation-based synthesis with eight regional and global machine-learning pCO2sw data products in the GOM. SOCAT reveals significant spatial and seasonal variations in pCO2sw in the GOM owing to complex local nonthermal (physical and biological) dynamics, particularly in the Louisiana Shelf (LAS) and Western Florida Shelf (WFS). The regional pCO2sw data product outperforms the global products in capturing small-scale pCO2sw variations. When averaging climatology across the entire northern GOM, the spatial heterogeneity of pCO2sw and CO2 flux resulting from local nonthermal processes tends to counterbalance across the entire GOM in all pCO2sw data products. Consequently, the regional data product and the ensemble mean of seven global products yield pCO2sw climatology that closely aligns with the SOCAT observations with a small difference (< ±3 µatm). During the overlapping period from 2003 to 2017 (15 years), the average flux from the eight products indicates that the entire GOM is CO2-neutral, with an ocean uptake flux of 0.08 ± 0.12 mol C/m2/yr or 1.50 ± 2.25 TgC/yr, which is about 0.6 % of the global coastal ocean CO2 sink. Observations show that the pCO2sw trend also exhibits notable spatial differences, with the river plume area acting as an increasing CO2 sink and the WFS acting as an increasing CO2 source. Due to limited observations and large spatiotemporal variations, the true values of the decadal trend still have large uncertainties in the highly dynamic river plume area. In most other subregions, pCO2sw increases following atmospheric CO2. Uncertainties persist across all pCO2sw data products in simulating the decadal trend, given that the regional product displays essentially no trend (<0.5 µatm/yr), while the ensemble average of global products exhibits a trend that follows atmospheric pCO2 (∼+2.0 µatm/yr). Our findings demonstrate that existing pCO2sw data products effectively simulate the climatology of pCO2sw in the GOM, providing valuable information for CO2 flux quantification in the GOM. Future research should emphasize the development of pCO2sw data products designed to accurately predict small-scale variations and temporal shifts, while also delving into the underlying dynamics responsible for these changes.