Long term Strengthening of the CO₂ Sink and Spatiotemporal pCO₂ dynamics in the northern Gulf of Mexico: Insights from a 22 year Satellite based Machine Learning Reconstruction

Abstract. The northern Gulf of Mexico (nGOM) is a river‑dominated marginal sea with strong physical‑biogeochemical variability. We reconstruct sea surface partial pressure of CO₂ (pCO₂) at 4‑km, 8-day resolution from 2003 to 2024 using a satellite‑based, season‑specific random forest model (independent validation R² = 0.82, RMSE = 27.6 μatm). The climatological pCO₂ distribution exhibits a sharp coastal‑to‑offshore gradient: river‑influenced coastal waters (SSS < 33) have persistently low pCO₂ with high spatial variability, while offshore waters (SSS > 33) have higher pCO₂ with weaker heterogeneity and lower seasonal amplitude. The nGOM acts as a net CO₂ sink for atmospheric, largely concentrated in the river‑influenced plume region due to riverine nutrient‑stimulated biological uptake. Seasonal pCO₂ variation is dominantly controlled by temperature but counteracted by spring‑summer biological drawdown (reducing pCO₂) and autumn‑winter vertical mixing with CO₂‑rich deeper water (raising pCO₂). Interannual pCO₂ variability is dominantly affected by year-to-year changes in river discharge and nutrient loading, with higher discharge leading to lower pCO₂ via enhanced biological uptake. On a decadal timescale, sea surface pCO₂ increased at a rate of 0.50 ± 0.20 μatm yr^-1, much slower than atmospheric pCO₂ (2.13 ± 0.04 μatm yr^-1), leading to a strengthening oceanic CO₂ sink with the sea-to-air flux becoming more negative at −0.41 ± 0.06 mmol C m^-2 d^-1 yr^-1. Furthermore, a decreasing frequency of easterly winds has reduced the westward transport of the Mississippi River plume, causing a higher pCO₂ increasing rate on the western Texas‑Louisiana shelf.