Air‐sea flux of oxygen estimated from bulk data: Implications For the marine and atmospheric oxygen cycles

Abstract

Abstract. We estimate the annual net air-sea fluxes of oxygen for 13 regions on the basis of a steady state inverse modeling technique that is independent of air-sea gas exchange parameterizations. The inverted data consist of the observed oceanic oxygen concentration after a correction has been applied to account for biological cycling. We find that the tropical oceans (13øS-13øN) emit •212 Tmol 02 yr -•, which is compensated by uptake of 148 Tmol yr -• in the Northern Hemisphere (>13øN) and by uptake of 65 Tmol yr -• in the Southern Hemisphere (<13øS). These results imply that the dominant feature of oxygen transport in the combined oceanatmosphere system is the existence of a closed circulation cell in each hemisphere. These two cells consist of 02 uptake by the ocean in the middle and high latitudes of both hemispheres and transport in the ocean toward the tropics, where 02 is lost to the atmosphere and transported in the atmosphere back toward the poles. We find an asymmetry in the two cells involving 02 uptake in the temperate regions of the Northern Hemisphere versus loss of 02 in the temperate regions of the Southern Hemisphere. There is an additional asymmetry between the Atlantic basin, which has a net southward transport at all latitudes north of 36øS, in agreement with independent transport estimates, versus the Indian and Pacific Oceans, which have a strong equatorward transport everywhere. We find that these inverse estimates are relatively insensitive to details in the inversion scheme but are sensitive to biases in the ocean general circulation model that provides the linkage between surface fluxes and ocean interior concentrations. Forward simulations of 02 in an atmospheric tracer transport model using our inversely estimated oxygen fluxes as a boundary condition agree reasonably well with observations of atmospheric potential oxygen (APe m 02 + ce2). Our results indicate that the north-south asymmetry in the strength of the two hemispheric cells coupled with a strong asymmetry in fossil fuel emissions can explain much of the observed interhemispheric gradient in APe. Therefore it might not be necessary to invoke the existence of a large southward interhemispheric transport of e2 in the ocean, such as proposed by Stephens et al. [1998]. However, we find that uncertainties in the modeled APe distribution stemming from seasonal atmospheric rectification effects and the limited APe data coverage prevent the currently available APe data from providing strong constraints on the magnitude of interhemispheric transport. Stephens et al. [1998] recently proposed a new tracer combination, atmospheric potential oxygen (APO • 02 + CO2), that removes the terrestrial influence from the atmospheric 02 signal, including most of the signal from the burning of fossil fuels. Variations in APO therefore reflect mainly the air-sea exchange of oxygen and, to a smaller degree, also the air-sea exchange of CO2, N2, and a residual signal from the burning of fossil fuels. Annual mean observations of APO from 10 station...

In situ measurements of atmospheric O2 and CO2 reveal an unexpected O2 signal over the tropical Atlantic Ocean

“…The Pacific Ocean equatorial APO bulge predicted by Stephens et al [1998] and Gruber et al [2001], and later confirmed from observations by Battle et al [2006] and Tohjima et al [2005b], is not apparent in our Atlantic APO data in 2015. Owing to the development of strong El Niño conditions during 2015, it is possible that the equatorial APO bulge was suppressed by ENSO-related changes in ITCZ position and atmospheric transport, as was found to be the case in the western Pacific Ocean during 2010 [Tohjima et al, 2015].…”

“…For example, Stephens et al [1998] found that model-generated latitudinal gradients in annual-mean APO over the Pacific Ocean significantly underestimated the observed interhemispheric gradient in APO, suggesting that the models were underestimating the net southward transport of oceanic potential oxygen (the oceanic equivalent of APO [see Resplandy et al, 2016]) in the oceans. A later study by Gruber et al [2001] also found that modeled APO underestimated the observed APO interhemispheric gradient, particularly at southern hemisphere high-latitude stations. However, these authors found that robustly constraining oceanic transport of O 2 and CO 2 using APO data was not possible.…”

“…The contribution to modeled APO from the seasonal covariance of O 2 , N 2 , and atmospheric transport and mixing, also known as the "seasonal rectifier effect" (shown in Figure 6d), was simulated from the O 2 and N 2 climatological fluxes as explained in Gruber et al [2001]. The fluxes used in our model-generated APO are the same as those used to produce the model-generated APO presented in Gruber et al [2001] and in Battle et al [2006]; hence, one can directly compare our Atlantic results to those from these two previous studies over the Pacific.…”

“…The flux inputs to TM3 are as follows: fossil fuel emissions are from the Carbon Dioxide Information Analysis Center; annual ocean O 2 and N 2 fluxes are from Gruber et al [2001] and Gloor et al [2001], respectively; ocean seasonal O 2 and N 2 variabilities are from the climatologies of Garcia and Keeling [2001]; and ocean seasonal CO 2 variability is from the climatology of Takahashi et al [2009]. The contribution to modeled APO from the seasonal covariance of O 2 , N 2 , and atmospheric transport and mixing, also known as the "seasonal rectifier effect" (shown in Figure 6d), was simulated from the O 2 and N 2 climatological fluxes as explained in Gruber et al [2001]. The fluxes used in our model-generated APO are the same as those used to produce the model-generated APO presented in Gruber et al [2001] and in Battle et al [2006]; hence, one can directly compare our Atlantic results to those from these two previous studies over the Pacific.…”

“…This was owing to large uncertainties in the modeled APO, which stemmed from a combination of the seasonal O 2 rectifier effect (caused by the seasonal covariation of atmospheric mixing processes and surface ocean fluxes) and a lack of APO observations. Both studies by Stephens et al [1998] and Gruber et al [2001] found a significant equatorial APO bulge (between 5 and 15 per meg in magnitude) in modeled APO over the Pacific Ocean, caused largely by O 2 and CO 2 solubility-driven outgassing from the equatorial Pacific Ocean. Subsequent shipboard measurements confirmed the existence of this bulge, which occurred just south of the equator [Battle et al, 2006].…”