Abstract
AbstractThe Southern Ocean plays an important role in the global oceanic uptake of CO2. Estimates of the air‐sea CO2 flux are made using the partial pressure of CO2 at the sea surface (), but winter observations of the region historically have been sparse, with almost no coverage in the Pacific or Indian ocean sectors south of the Polar front in the period 2004–2017. Here, we use summertime observations of relevant properties in this region to identify subsurface waters that were last in contact with the atmosphere in the preceding winter, and then reconstruct “pseudo observations” of the wintertime . These greatly improve wintertime coverage south of the Polar Front in all sectors, improving the robustness of flux estimates there. We add the pseudo observations to other available observations of and use a multiple linear regression to produce a gap‐filled time‐evolving estimate of from which we calculate the air‐sea flux. The inclusion of the pseudo observations increases outgassing at the beginning of the period, but the effect reduces with time. We estimate a 2004–2017 long‐term mean flux of −0.02 ± 0.02 Pg C yr−1 for the Southern Ocean south of the Polar Front, similar to comparable studies based on shipboard data. However, we diverge somewhat from an estimate which utilized autonomous float data for recent years: we find a small sink in 2017 of −0.08 ± 0.03 Pg C yr−1 where the float‐based estimate suggested a small source.
Highlights
The ocean is an important sink for anthropogenic CO2, having absorbed 25% of that emitted between 1750 and 2018, and it, has a mitigating impact on climate change (Friedlingstein et al, 2019)
We have found that the addition of wintertime pseudo observations of pCO2surf increased the strength of winter outgassing in estimates of the 2004–2017 mean seasonal cycle of air-sea CO2 flux south of the Polar Front in the Southern Ocean
Our results with pseudo observations show stronger outgassing/weaker uptake and a stronger downward trend compared with fluxes calculated from earlier estimates of pCO2surf between 2004 and 2013, and do not replicate the strength of a reversal of the trend to a reducing sink/becoming a source between 2013 and 2017 suggested by other studies
Summary
The ocean is an important sink for anthropogenic CO2, having absorbed 25% of that emitted between 1750 and 2018, and it, has a mitigating impact on climate change (Friedlingstein et al, 2019). The Southern Ocean is the largest oceanic sink of anthropogenic CO2, representing around 40% of the contemporary sink (Devries, 2014), and accounting for 43% of the global oceanic uptake from 1861 to 2005 according to models (Frölicher et al, 2015). The variability in the air-sea flux of CO2 in the Southern Ocean is dominated by the seasonal cycle of ΔpCO2, the difference between the partial pressure of CO2 in the atmosphere and the surface ocean (Mongwe, Chang, & Monteiro, 2016) This in turn is driven by the seasonal cycle of the sea surface pCO2, which depends on sea surface temperature (the thermal component) and biogeochemistry, including DIC, alkalinity, and salinity (the non-thermal component). Observational estimates find that pCO2surf in the high-latitude Southern Ocean has a peak in August (Austral winter) and a minimum in January (Austral summer), with the changes dominated by the non-thermal component
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