Influence of Antarctic Intermediate Water on the deoxygenation of the Atlantic Ocean

Abstract

Hydrographic trends in the Antarctic Intermediate Water (AAIW) layer that may be associated with changes in the thickness and oxygen content of oxygen minimum zones (OMZs) in the eastern tropical South Atlantic (ETSA) and eastern tropical North Atlantic (ETNA) are investigated by using historical data (1960 to 2015). Our results reveal that the thickness of these OMZs has continually increased (2.58±0.67myr−1 for the ETSA and 3.37±0.73myr−1 for the ETNA), the mean oxygen concentration has decreased (– 0.12±0.03μmolkg−1yr−1 for the ETSA and − 0.17±0.05μmolkg−1yr−1 for the ETNA), and the mean temperature has increased. The optimum multiparameter analysis method is used to track modifications in the AAIW along its path through the South Atlantic Subtropical Gyre. We observe an AAIW layer vertical expansion rate of 1.67±0.71myr−1, a decrease in the mean oxygen concentration of − 0.18±0.04μmolkg−1yr−1 and an increase in the mean temperature of 0.010±0.005°Cyr−1. Moreover, a similar decrease in oxygen concentrations is observed in the AAIW layer of the studied OMZ regions compared to those in the non-AAIW portions of these OMZs, which indicates strong deoxygenation in this water mass over time. Our results suggest that warming in the AAIW source region and in its extensive temporal displacement through the SASG to the eastern tropical Atlantic Ocean appreciably shifted this water mass toward lower densities with depleted oxygen (increases in ventilation age and oxygen consumption). The warming trend that is reported here suggests that global warming is one of the factors that influence oxygen solubility changes during the deoxygenation and expansion of OMZs.