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The equatorial Pacific and Atlantic Oceans release significant amount of CO2 each year. Not much attention has been paid to evaluating the similarities and differences between these two basins in terms of temporal variability. Here we employ a basin-scale, fully coupled physical-biogeochemical model to study the spatial and temporal variations in sea surface pCO2 and air-sea CO2 flux over the period of 1984-2013 in the equatorial Pacific and Atlantic Oceans. The model reproduces the overall spatial and temporal variations in the carbon fields for both basins, including higher values to the south of the equator than to the north, the annual maximum sea surface pCO2 in boreal spring, and the annual peak in sea-to-air CO2 flux in boreal fall in the upwelling regions. The equatorial Pacific reveals a large interannual variability in sea surface pCO2, which is associated with the El Niño-Southern Oscillation. As a contrast, there is a strong seasonality but little interannual variability in the carbon fields of the equatorial Atlantic. The former is driven by the variability of dissolved inorganic carbon but the latter by sea surface temperature. Our model estimates an average sea-to-air CO2 flux of 0.521 ± 0.204 Pg C yr−1 for the tropical Pacific (18°S-18°N, 150°E-90°W), which is in good agreement with the observation-based estimate (0.51 ± 0.24 Pg C yr−1). On average, sea-to-air CO2 flux is 0.214 ± 0.03 Pg C yr−1 in the tropical Atlantic (10°S-10°N), which compares favorably with observational estimates. [ABSTRACT FROM AUTHOR]

Abstract: Satellite datasets show higher chlorophyll concentration in the surface water of the equatorial Atlantic relative to that of the equatorial Pacific. Is the phytoplankton biomass also higher in the Atlantic? To answer this question, a basin-scale ocean circulation-biogeochemistry model that has a phytoplankton dynamic model is utilized to compare the spatial and temporal variations of phytoplankton carbon biomass. We use field data collected in the equatorial Atlantic to derive a new set of biological parameters so that the model can reproduce the general features of the phytoplankton biomass and chlorophyll in this region. For instance, the model reproduces the observed deep chlorophyll a maximum (DCM) that is much deeper to the south (~100m) of the equator than to the north (~70m). The simulated surface chlorophyll also compares well with the satellite derived chlorophyll at basin scale for the equatorial Atlantic. Our comparative analyses demonstrate that for the upwelling region, the phytoplankton biomass in the surface water is considerably higher in the equatorial Pacific than in the equatorial Atlantic. However, for the entire euphotic zone, the integrated phytoplankton biomass is much higher in the equatorial Atlantic than in the Pacific. The difference in the surface water simply reflects larger phytoplankton carbon to chlorophyll ratios in the equatorial Pacific, due to strong iron limitation. The difference in the subsurface water is due to a pronounced deep biomass maximum (DBM) existing in the equatorial Atlantic, which is associated with higher nitrate in the lower euphotic zone. This modeling study emphasizes the importance of using variable C:Chl ratios to estimate carbon biomass at regional to global scales. [Copyright &y& Elsevier]