Atmospheric potential oxygen: New observations and their implications for some atmospheric and oceanic models

Abstract

[1] Measurements of atmospheric O 2 /N 2 ratios and CO 2 concentrations can be combined into a tracer known as atmospheric potential oxygen (APO % O 2 /N 2 + CO 2 ) that is conservative with respect to terrestrial biological activity. Consequently, APO reflects primarily ocean biogeochemistry and atmospheric circulation. Building on the work of Stephens et al. (1998), we present a set of APO observations for the years 1996-2003 with unprecedented spatial coverage. Combining data from the Princeton and Scripps air sampling programs, the data set includes new observations collected from ships in the low-latitude Pacific. The data show a smaller interhemispheric APO gradient than was observed in past studies, and different structure within the hemispheres. These differences appear to be due primarily to real changes in the APO field over time. The data also show a significant maximum in APO near the equator. Following the approach of Gruber et al. (2001), we compare these observations with predictions of APO generated from ocean O 2 and CO 2 flux fields and forward models of atmospheric transport. Our model predictions differ from those of earlier modeling studies, reflecting primarily the choice of atmospheric transport model (TM3 in this study). The model predictions show generally good agreement with the observations, matching the size of the interhemispheric gradient, the approximate amplitude and extent of the equatorial maximum, and the amplitude and phasing of the seasonal APO cycle at most stations. Room for improvement remains. The agreement in the interhemispheric gradient appears to be coincidental; over the last decade, the true APO gradient has evolved to a value that is consistent with our time-independent model. In addition, the equatorial maximum is somewhat more pronounced in the data than the model. This may be due to overly vigorous model transport, or insufficient spatial resolution in the air-sea fluxes used in our modeling effort. Finally, the seasonal cycles predicted by the model of atmospheric transport show evidence of an excessive seasonal rectifier in the Aleutian Islands and smaller problems elsewhere.

Seasonal and short-term variations in atmospheric potential oxygen at Ny-Ålesund, Svalbard

“…Such a difference in autumn was also found in previous studies ( Battle et al, 2006;Tohjima et al, 2012). Ishidoya et al (2016) attributed cause of this discrepancy to an intrusion of the shallow oxygen maximum layer, which exists in the subsurface ocean in summer, into the surface in autumn when the ocean mixed layer is deepened (e.g.…”

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

“…The CSL annual mean APO data do not exhibit a significant equatorial bulge, as observed over the Pacific Ocean [e.g., Battle et al, 2006;Tohjima et al, 2015] (Figure 6c). Although there is a small bulge in the CSL APO data at~2.5°N, it is not significant, given the uncertainty of the equatorial annual mean APO values, and because the bulge consists of only a single data point.…”

“…These model-observation discrepancies are most likely either caused by inaccuracies in the air-sea flux data used in TM3, and/or by inaccuracies in the atmospheric transport model itself, the latter of which were found by Battle et al [2006] to dominate differences in modeled and measured APO.…”

“…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 longest in situ atmospheric O 2 and CO 2 data set from a research vessel is measured from the Laurence M. Gould, which travels semicontinuously between Chile and Antarctica across the Drake Passage, and has been collecting O 2 and CO 2 data since 2012 [Stephens, 2017]. In contrast to research vessels, there have also been several atmospheric O 2 and CO 2 data sets collected from commercial container ships, including flask samples collected over the Pacific Ocean from 1996 to 2001 [Battle et al, 2006], and ongoing flask samples (2001 to present) and in situ O 2 and CO 2 measurements (2007 to present) from over the western and northern Pacific [Tohjima et al, 2005b[Tohjima et al, , 2015. The key advantage of deploying an in situ system on a commercial container ship, in contrast to both fixed stations and most research ships, is that multiple measurements can be combined into discrete latitude or longitude bands, thus forming time series from several virtual stations collected from a single measurement system.…”