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Supplementary Material: Estimating net community production in the Southern Ocean
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based on Atmospheric Potential Oxygen and satellite ocean color data
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C.D. Nevison1, R.F. Keeling2, M. Kahru2, M.Manizza2, B.G. Mitchell2, N. Cassar3,4
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Supplementary Material S2.
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Decomposition of the APO net biological signal at SPO and PSA
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S2.1 South Pole
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Our analysis at Cape Grim focused on the sensitivity of the decomposed N2O and APO
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cycles to uncertainty in the thermal signal. At the South Pole, the available thermal signals are
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limited to the two modeled heat flux-based curves. Ar/N2 data are measured at SPO, but have a
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problem with gravitational or thermal settling that prevents their use in our analysis. We
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therefore focus on the sensitivity of our results to uncertainty in the CFC-12 scalar which is
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used to estimated N2Ostrat (Equation S5). On the strength of the analysis at CGO, we assume in
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Figures S1-S2 that ATM-THERMQmod and ATM-N2OTHERMQmod give the best available
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estimates of the APO and N2O thermal cycles at SPO. We then experiment with values of 
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ranging from 0.6 to 1.0.
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If N2Ostrat and CFC-12 were governed purely by stratospheric influences, then the ratio of
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their normalized seasonal amplitudes (i.e., ) should be about 0.7, based on the slope of
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normalized N2O vs. CFC-12 data observed at the tropopause [Volk et al., 1997; Nevison et al.,
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2011]. However, the “stratospheric” terms also encompass seasonal variability in tropospheric
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transport and mixing, including boundary layer trapping and transport of enriched air from the
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Northern Hemisphere. The former probably affects both N2O and CFC-12 equally and thus
APO and Ocean Color
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might be expected to increase  toward 1, while the latter probably affects CFC-12 more, due to
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its larger relative interhemispheric gradient [Prinn et al., 2000; Nevison et al., 2007], and thus
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should tend to decrease . In practice,  is empirically constrained by the use of N2Ovent in the
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APO seasonal decomposition, since there is a limited range of values that yields a reasonable
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solution for APONCP. Values of  = 0.6 and 1.0 reflect the extreme limits of the acceptable
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range, since the inferred APONCP signal becomes oddly-shaped at either extreme, with
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unexplicable minima in June and November (Figure S1b).
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Within this acceptable range, with a best guess value of  ~ 0.8, the inferred APONCP
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signal at SPO agrees well in shape and phase with independent ocean model and satellite-based
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estimates (Figure 8b). This remarkable result again invites a direct comparison of the
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independent estimates of NCP to the APO-based constraint. As at CGO, APONCP tends to agree
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best in amplitude with the smaller of the two ocean color-based estimates, ATM-NCPEP12, while
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the two larger estimates, ATM-NCPEP17 and ATM-NCPMIT, tend to exceed the upper range of
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uncertainty in APONCP.
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As at CGO, the phasing of APOvent and N2Ovent (using ATM-THERMQmod and ATM-
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N2OTHERMQmod for the thermal signals) agrees well with the independent ocean model
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estimates (Figure S1c,d). The amplitude of the ventilation cycles increases by a factor of two
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thirds as  is decreased from 1.0 to 0.6, revealing a strong sensitivity to this parameter. The
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best-guess decomposition of the APO cycle, using =0.8, shows staggered NCP and ventilation
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signals, in which the former drives the springtime rise leading to the summer maximum, while
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the latter dominates the observed early spring minimum in APO (Figure S2b).
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S2.2 Palmer Station
APO and Ocean Color
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Palmer Station (PSA) is of particular interest due to its unusually large amplitudes for
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both observed APO and modeled ATM-NCP tracers (Figures 2,4a,b). For the latter, the large
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amplitudes seem to result in part from the strongly seasonal concentration of the NCP flux,
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which is probably enhanced by the tendency of MATCH to trap the associated outgassing O2 in
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the shallow summertime marine boundary layer. Between November and February, nearly 90%
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of the annual total air-sea O2 flux associated with ocean color-derived NCP is emitted from the
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60-65ºS latitude band near PSA, compared to only about 50% from the 40-45ºS latitude band
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near CGO. The actual magnitude in moles O2 m-2 day-1 of the air-sea flux FO2 from the 60-65ºS
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band is only about half that from the 40-45ºS band during these peak months. These results
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suggest that APONCP provides a constraint on a combination of the seasonality and overall
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magnitude of NCP, and that a larger APO amplitude at a given station does not necessarily imply
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a larger regional NCP flux. They also suggest that results at PSA are particularly sensitive to
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uncertainties introduced by atmospheric transport models, such as summertime boundary layer
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trapping.
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Atmospheric N2O and CFC-12 data are available at PSA, but are measured by fortnightly
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flask sampling programs, rather than in situ gas chromatography, as at CGO and SPO, and yield
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seasonal amplitudes that are uncertain and vary depending on the time span of data subject to the
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harmonic fit. With these caveats in mind, we proceed with our decomposition analysis at PSA
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(Figure S3). We assume as a best guess that the observed NOAA/CCGG N2O seasonal
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amplitude at PSA should be scaled down by a factor of 0.65, based on the average amplitude
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ratio of in situ N2O data to NOAA/CCGG N2O data at common CGO and SPO sites [Nevison et
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al., 2011]. As at CGO, we assume a best guess  scalar of 0.7 and present results in terms of
APO and Ocean Color
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their sensitivity to three alternative estimates of N2Otherm and APOtherm based on 1) SIO Ar/N2
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data, 2) the QST/Cp formula and 3) the modified QST/Cp formula.
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The spread among the Ar/N2 and heat flux-based thermal signals is more pronounced at
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PSA than at CGO both in phase and amplitude (Figure S3a). The three alternatives lead to a
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greater than 2-month spread in the maximum of the inferred N2Ovent signal, with best phase
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agreement with independent ocean model estimates achieved using ATM-N2OTHERMQmod for
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the thermal signal (Figure S3c). The N2Otherm,Ar thermal signal gives an N2Ovent peak in late
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November, a result that seems physically unreasonable and that is largely independent of the
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choice of  or the assumed scale factor for N2Oobs.
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The large spread in N2Otherm at PSA leads to a large range of uncertainty in the inferred
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APONCP signal (Figure S3b), even without taking into account the large additional uncertainty in
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the amplitude of N2Oobs. We ruled out the APONCP signal derived using ATM-N2OTHERMQ as
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physically unreasonable, since it has an inexplicably late phasing (April maximum) compared to
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the satellite-based estimates. The smaller ocean color-based estimate, ATM-NCPEP12, falls within
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the remaining range of uncertainty in APONCP, although the amplitude tends toward the upper
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end of the allowed range (Figure 8c). The larger ocean color-based estimate, ATM-NCPEP17, and
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ATM-NCPMIT fall well outside the range of uncertainty. Of the three alternative views of the
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partitioned APO cycle (Figures S4b-d), only one (Figure S4d, using the modified QST/Cp
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thermal signal) seems consistent with the results at CGO and SPO in terms of the relative
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phasing and magnitude of APONCP and APOvent. Again, our confidence in the APO
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decomposition at PSA is low, as a result of the uncertainty in the observed N2O seasonal cycle at
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this site and the sensitivity of the results to divergent estimates of the thermal signal.
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APO and Ocean Color
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Figure S1. Selected components of the N2O (a,c) or APO (b,d) mean seasonal cycles at the South
Pole, showing sensitivity to choice of scalar  used to estimate N2Ostrat from CFC-12n,  = 0.6
(dash), 0.8 (solid), 1.0 (heavy dash-dot). All colors are the same as in Figure 7.
APO and Ocean Color
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Figure S2. Decomposition of the mean seasonal cycle in APO at the South Pole into its
biological, showing sensitivity to choice of scalar  used to estimate N2Ostrat from CFC-12n,  =
0.6 (dash), 0.8 (solid), 1.0 (heavy dash-dot). See Figure S2 for details of corresponding N2O
cycle.
APO and Ocean Color
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Figure S3. Selected components of the N2O (a,c) or APO (b,d) mean seasonal cycles at Palmer
Stations, Antarctica, showing sensitivity to choice of thermal signal, assuming a CFC-12n scalar
of =0.7. All colors and line styles are the same as in Figure 7. The mean observed N2O cycle
is shown as a black window of uncertainty because it is estimated based on flask data, which
have relatively low precision compared to in situ data and tend to overestimate the mean cycle
[Nevison et al., 2011].
APO and Ocean Color
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Figure S4. Decomposition of the mean seasonal cycle in APO at Palmer Stations, Antarctica
into its biological components, showing sensitivity to choice of thermal signal. See Figure S3
for details of corresponding N2O cycle.
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APO and Ocean Color
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