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Abstract:
The Southern Ocean (44–75 S) plays a critical
role in the global carbon cycle, yet remains one of the most
poorly sampled ocean regions. Different approaches have
been used to estimate sea–air CO2 fluxes in this region: synthesis
of surface ocean observations, ocean biogeochemical
models, and atmospheric and ocean inversions. As part of
the RECCAP (REgional Carbon Cycle Assessment and Processes)
project, we combine these different approaches to
quantify and assess the magnitude and variability in Southern
Ocean sea–air CO2 fluxes between 1990–2009. Using
all models and inversions (26), the integrated median annual
sea–air CO2 flux of −0.42±0.07 PgC yr−1 for the 44–
75 S region, is consistent with the −0.27±0.13 PgC yr−1
calculated using surface observations. The circumpolar region
south of 58 S has a small net annual flux (model and
inversion median: −0.04±0.07 PgC yr−1 and observations:
+0.04±0.02 PgC yr−1), with most of the net annual flux located
in the 44 to 58 S circumpolar band (model and inversion
median: −0.36±0.09 PgC yr−1 and observations:
−0.35±0.09 PgC yr−1). Seasonally, in the 44–58 S region,
the median of 5 ocean biogeochemical models captures the observed sea–air CO2 flux seasonal cycle, while the median of 11 atmospheric inversions shows little seasonal change in
the net flux. South of 58 S, neither atmospheric inversions
nor ocean biogeochemical models reproduce the phase and
amplitude of the observed seasonal sea–air CO2 flux, particularly
in the AustralWinter. Importantly, no individual atmospheric
inversion or ocean biogeochemical model is capable
of reproducing both the observed annual mean uptake and the
observed seasonal cycle. This raises concerns about projecting
future changes in Southern Ocean CO2 fluxes. The median
interannual variability from atmospheric inversions and
ocean biogeochemical models is substantial in the Southern
Ocean; up to 25% of the annual mean flux, with 25% of
this interannual variability attributed to the region south of
58 S. Resolving long-term trends is difficult due to the large
interannual variability and short time frame (1990–2009) of
this study; this is particularly evident from the large spread
in trends from inversions and ocean biogeochemical models.
Nevertheless, in the period 1990–2009 ocean biogeochemical
models do show increasing oceanic uptake consistent
with the expected increase of −0.05 PgC yr−1 decade−1. In
contrast, atmospheric inversions suggest little change in the strength of the CO2 sink broadly consistent with the results of Le Qu´er´e et al. (2007).
