4.1. Natural CO2
 We investigate the mechanisms responsible for the trend in the flux of natural CO2 by estimating the contributions to the total trend (F′) from the trends in wind speed (WS), sea ice fraction (Ice), air pressure (p), surface dissolved inorganic carbon (DIC), alkalinity (Alk), temperature (T), and salinity (S). The trend in the CO2 flux, F, can be deconvolved using a linear Taylor expansion:
where the partial derivatives are determined from model equations and mean values in the Southern Ocean [see Lovenduski et al., 2007], and the trends represent the slope of a linear regression to the data. As the Taylor expansion is only strictly correct for small perturbations, the sum of the terms of the right hand side is often not exactly equal to the left hand side. Cross correlations among the variables and the approximations used can cause differences as well [see Lovenduski et al., 2007].
 The analysis of the contributions to the long-term trend in natural CO2 flux from 1979 to 2004 (Table 2) demonstrates that the largest term is the trend toward elevated natural DIC. However, a large portion of the DIC term is canceled by the trend toward elevated alkalinity. The trend in surface temperature also contributes to reducing the total trend, while the impact of trends in wind speed, sea ice, air pressure, and salinity do not have a large impact on the CO2 flux trend. Similar results are found from the estimated contributions to the 10-year CO2 flux trends during this period (Figure S5). Prior to 1979, however, the driving factors for the 10-year and long-term trends in natural CO2 flux are not as clear (Figure S1 and Table S1), but this is where we have much less confidence in the atmospheric forcing.
 The positive trends in surface DIC and Alk are primarily caused by trends in Southern Ocean circulation. We find a positive trend in the rate of Southern Ocean meridional overturning, upwelling around 60°S, and northward Ekman transport between 50°S and 60°S (Figure 3), as well as a significant trend in Antarctic Circumpolar Current strength (0.008 cm s−1 a−1; Figure S6) from 1979 to 2004. These trends lead to enhanced upwelling of Circumpolar Deep Water (CDW) in the southernmost portions of the Southern Ocean. The DIC and Alk trends are also enhanced by a trend toward deeper mixed layers (Figure S7a). The strong response of surface DIC and Alk to these changes in ocean circulation is because the upwelled CDW is characterized by high DIC and Alk owing to its source waters, i.e., North Atlantic Deep Water and return flows from the deep Pacific and Indian. In our model, anomalously high DIC and Alk persist near the surface (see Figure 4a), as biological production remains largely unaltered in response to the enhanced upwelling (Figure S7b), perhaps because of light limitation (Figure S7a). Thus Southern Ocean biology in our model simulation is not compensating to the degree that the climate change simulation of Sarmiento et al.  would have suggested.
Figure 4. Linear trends in the zonally averaged (a) natural, (b) anthropogenic, and (c) contemporary DIC from 1979 to 2004 (mmol m−3 a−1).
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 These trends in circulation and air-sea fluxes of natural CO2 are consistent with those expected from previous studies of the response of Southern Ocean circulation and carbon cycle to interannual changes in Southern Ocean winds [Lenton and Matear, 2007; Lovenduski et al., 2007; Verdy et al., 2007]. We investigate this link between changes in Southern Ocean winds and air-sea fluxes of natural CO2 by estimating how much of the natural CO2 flux trend can be explained by projecting the response of the fluxes to interannual changes in the winds onto the trend in wind speed, i.e., we estimate the congruence of the natural CO2 flux trend with the trend in wind speed. When spatially integrated over the Southern Ocean (<35°S), we find that 130% (0.006 Pg C a−2) of the trend in the flux of natural CO2 can be explained by the linear trend in the wind speed. The spatial congruence of the two is highest in the southernmost Southern Ocean (Figures 2a and 2b). This very large fraction implies that other processes, such as changes in buoyancy forcing may play a role in mitigating the trends caused by the winds.
 The mechanisms that control the fraction of the natural CO2 flux trend related to the wind speed are largely the same as those that control the total trend (Table 2), particularly over the recent period (1979–2004; Table S1). The methods for this study are identical for those of the total trend, with the exception that contributions from each component were estimated using only the portion that is congruent with the linear trend in the wind speed. The congruent portion was then multiplied by its associated partial derivative to determine the contribution from each component.
4.2. Anthropogenic CO2
 The negative trend in anthropogenic CO2 uptake is not surprising given the increasing trend in the atmospheric CO2 concentration, which continuously increases the air-sea difference in the partial pressures of CO2. However, it is of interest to know whether the changes in winds and ocean circulation have altered the uptake trend of anthropogenic CO2 relative to a situation with constant physical forcing. We estimate the expected oceanic uptake of anthropogenic CO2(Fexptanth(t)) under constant physical forcing using the following scaling:
where Foanth is the flux of anthropogenic CO2 in 1958, and χCO2anth(t)/χCO2,oanth is the ratio of the anthropogenic perturbation in atmospheric CO2 at a given time with the atmospheric perturbation in 1958. This scaling was developed for the inverse modeling of the oceanic uptake of anthropogenic CO2 [Gloor et al., 2003; Mikaloff Fletcher et al., 2006] and was successfully tested by using results from forward model simulations under constant physical forcing.
 The spatial pattern of the expected trend in anthropogenic CO2 flux (Figure 2d) shows a close correspondence with that of the total anthropogenic trend (Figure 2c). We find that the linear trend in the spatially integrated (<35°S) values of Fexptanth(t) can explain a very large fraction (98%) of the linear trend in anthropogenic CO2. The remaining trend is mostly one of ocean uptake, with the exception of the Amundsen/Bellingshausen sector and the western Atlantic at ∼45°S, where the trend is toward ocean outgassing (not shown). Only a small amount (15%) of this remaining trend in the region south of 35°S is congruent with the linear trend in the wind speed (not shown).
 Thus, in sharp contrast to natural CO2, the flux of anthropogenic CO2 appears to be only marginally affected by the changes in wind and ocean circulation. This is surprising given that the uptake of anthropogenic CO2 by the ocean is primarily determined by how fast anthropogenic CO2 is ultimately transported from the surface toward the interior of the ocean [Sarmiento et al., 1992]. Thus, one would have expected an enhanced uptake of anthropogenic CO2 in response to the enhanced meridional overturning. However, residence times of Southern Ocean surface waters with regard to the exchange of gases with the atmosphere [Ito et al., 2004b] tend to be shorter than the ∼9 months it takes to equilibrate the surface ocean with the overlying atmosphere [Sarmiento and Gruber, 2006], because of the presence of sea ice preventing air-sea exchange. As a result, surface waters in the Southern Ocean tend to fail to take up anthropogenic CO2 up to their capacity [Gruber, 1998; Ito et al., 2004a]. Hence, the reduction of the surface residence time due to the enhanced overturning circulation could have compensated for the enhanced wind speeds and enhanced surface to deep transports, so that the total uptake of anthropogenic CO2 remained largely unaltered, relative to a situation with constant physical forcing.
4.3. Contemporary CO2 and Summary
 The combination of the natural and anthropogenic flux trends creates a complex spatial pattern in the trends of the contemporary CO2 flux (Figure 5) that is difficult to interpret. The mechanisms driving this contemporary trend pattern are a superposition of the mechanisms driving the natural and anthropogenic CO2 flux trends, namely the trends in wind speed and atmospheric pCO2. Since only the natural CO2 flux component is congruent with wind speed, while the anthropogenic CO2 flux component is not, the contemporary CO2 flux trend has a low congruence with wind speed or the SAM, explaining the low congruence number (20%) reported by Le Quéré et al. .
Figure 5. Trends in the air-sea flux of contemporary CO2 from 1979 to 2004 (mol m−2 a−2). Only those trends with significance ≥95% are shown. Negative values indicate trends toward ocean uptake.
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 In summary, our results indicate that there is a positive trend in the natural CO2 outgassing over the course of our simulation, and that a large fraction of it is congruent with the linear trend in the wind speed, owing to the wind-change-induced trends in ocean circulation. Meanwhile, anthropogenic CO2 has exhibited an ingassing trend over the same period, mostly due to the increasing anthropogenic perturbation in atmospheric CO2, with changes in wind speed and ocean circulation playing only a minor role. Therefore, the wind speed trend has led to a reduction in the ability of the Southern Ocean to absorb CO2, while the trend in the anthropogenic perturbation of atmospheric CO2 has led to an increase in the strength of the Southern Ocean carbon sink.