Kinematics of CO2 fluxes in the tropical Atlantic Ocean during the 1983 northern summer

Abstract

CO2 evasion within the Atlantic equatorial belt (5°N–5°S) increases from the east to the west [Andrié et al., 1986]. Many factors contribute the variations of pCO2 in the equatorial surface waters. To assess their relative importance, a kinematic box model is developed. A 2°×2° box whose depth is defined by the 24.90‰ isopycnal level flows westward from 4°W to 38°W within the Equator‐2°S band with the south equatorial current. Time (zonal) evolution of nitrate, total CO2, total alkalinity and mass, and of the corresponding water pCO2, are simulated taking into account advection, meridional divergence, diffusion, biological activity, and gas exchange. Initial and boundary conditions are taken from the FOCAL 4 (July‐August 1983) data set. The flow field is based on climatology. The ability of the model to reproduce the observed FOCAL data, as well as the correct orders of magnitude of the adjustable parameters KV (vertical mixing coefficient) and Jmax (maximum nitrate uptake rate), as compared with independent estimates, suggests that all major terms controling the equatorial Atlantic CO2 values are considered. Advection fluxes dominate the CO2 balance. The sinks due to biological consumption approximately balance the inflow from the core of the Equatorial Undercurrent by diapycnal mixing. Calculated new production JC values vary from east to west between 17 and 1 mol C/m2/yr, given Kv = 5 × 10−4 m2/s. Isopycnal mixing is negligible. Degassing plays a minor role in the total CO2 budget. Estimating the biological uptake of nutrients should be a very useful constraint for estimating equatorial divergence.