Western Equatorial Atlantic: Changes in the CO2 sink/source potential along time

Increasing atmosphere and ocean carbon dioxide (CO2) concentrations have significant impacts on climate change and ocean chemistry. The Western Equatorial Atlantic Ocean (WEA) presents a patchiness regarding the sea-air CO2 exchange as the Amazon Plume (AP) supports primary production, turning the region into a CO2 sink. We analysed 20 years (2000-2019) of surface ocean CO2 partial pressure (pCO2sw) regional data and the difference between atmospheric and oceanic pCO2 (ΔpCO2 = pCO2atm–pCO2sw) and the effects over the processes controlling the exchange dynamics (biological and temperature effects) using the SOCATv.2020 database. Despite the regional large variability in pCO2sw (123–709 μatm) and ΔpCO2 (-99 - +269 μatm), our analysis of the ΔpCO2 time series suggest a difference between the rate of increase of the pCO2sw and the atmospheric pCO2. This result indicates an ocean sink increase around the plume region (i.e. where surface salinity < 35), whereas other regions showed a reduction in the ocean. The ΔpCO2 presented a general negative trend for WEA, but each sector showed a different pattern. The AP and North Equatorial Current (NEC) sectors showed a negative decadal trend (∆(∆pCO2)/dt = -0.93 (AP) and -0.16 (NEC), where dt = 10 years), indicating a decreasing sink capacity for atmospheric CO2. Other sectors in the WEA presented a positive trend, indicating a higher CO2 sink capacity (average ∆(∆pCO2)/dt = +0.38). Along the study period, the biological effect over the pCO2sw (pCO2bio) presented a significant increase in the AP (+9.2 µatm/yr-1) compared to pCO2temp (+1.8 µatm/yr-1), while in the other sectors, there were small differences among pCO2bio (+2.2 – +1.3 µatm/yr-1) and pCO2temp (+1.0 - +1.7 µatm/yr-1). Our study highlights the importance of long-term data analysis in understanding regional air-sea CO2 exchange, the role of biological processes in the AP region, and the carbon sink potential in the WEA region.