Abstract
AbstractWe present the first meridional transects of atmospheric O2 and CO2 over the Atlantic Ocean. We combine these measurements into the tracer atmospheric potential oxygen (APO), which is a measure of the oceanic contribution to atmospheric O2 variations. Our new in situ measurement system, deployed on board a commercial container ship during 2015, performs as well as or better than existing similar measurement systems. The data show small short‐term variability (hours to days), a step‐change corresponding to the position of the Intertropical Convergence Zone (ITCZ), and seasonal cycles that vary with latitude. In contrast to data from the Pacific Ocean and to previous modeling studies, our Atlantic Ocean APO data show no significant bulge in the tropics. This difference cannot be accounted for by interannual variability in the position of the ITCZ or the Atlantic Meridional Mode Index and appears to be a persistent feature of the Atlantic Ocean system. Modeled APO using the TM3 atmospheric transport model does exhibit a significant bulge over the Atlantic and overestimates the interhemispheric gradient in APO over the Atlantic Ocean. These results indicate that either there are inaccuracies in the oceanic flux data products in the equatorial Atlantic Ocean region, or that there are atmospheric transport inaccuracies in the model, or a combination of both. Our shipboard O2 and CO2 measurements are ongoing and will reveal the long‐term nature of equatorial APO outgassing over the Atlantic as more data become available.
Highlights
The global ocean is a major long-term sink for anthropogenic carbon dioxide (CO2) [Le Quéré et al, 2016], and yet the spatial and temporal variability of the oceanic CO2 sink has large uncertainties [Landschützer et al, 2014; Rödenbeck et al, 2015]
The modeling study by Rödenbeck et al [2008] found a significant correlation between El Niño– Southern Oscillation (ENSO) variability and atmospheric potential oxygen (APO) anomalies, albeit in the opposite direction to that found by Tohjima et al [2015], and more recently, Eddebbar et al [2017] showed that the response of APO to ENSO variability differs between the western and central/eastern Pacific and confirmed that the suppression of the APO bulge over the western Pacific is mainly dominated by the weakening of trade winds
We have presented the first meridional transects of atmospheric O2, CO2, and APO over the Atlantic Ocean from a new, automated shipboard measurement system deployed on a commercial container ship traveling continuously between Europe and South America
Summary
The global ocean is a major long-term sink for anthropogenic carbon dioxide (CO2) [Le Quéré et al, 2016], and yet the spatial and temporal variability of the oceanic CO2 sink has large uncertainties [Landschützer et al, 2014; Rödenbeck et al, 2015]. High precision atmospheric oxygen (O2) and CO2 data can be combined to isolate the ocean influences on atmospheric O2 variability, by removing the influence of the terrestrial biosphere on O2. This is done using the tracer atmospheric potential oxygen (APO) which is defined as APO ≈ O2 + 1.1 × CO2 [Stephens et al, 1998]. Atmospheric O2 and CO2 measurements have been used to determine spatial and temporal features in air-sea O2 fluxes [Rödenbeck et al, 2008], test oceanic net primary productivity and ventilation fluxes in Earth system models [Nevison et al, 2015], and detect CO2 leaks from carbon capture and storage sites [Van Leeuwen, 2015]
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