Tests for the Interpolated Values

Comparison with Climatological SST. To evaluate the validity of our results, the SST values that have been computed using the measurements made concurrently with ApCO2 are compared with the climatological SST compiled by Shea et al. (68). Consistent with the procedures used for ApCO2 interpolation, the temperature field was preloaded using mean monthly values for temperature values measured concurrently with ApCO2 in each climatic zone. In Fig. 3a, the difference between the mean monthly SST observed in 4° x 5° pixels and the corresponding climatological SST are plotted against the climatological SST; and in Fig. 3b, the difference between the interpolated value and the corresponding climatological SST is plotted. Fig. 3a shows that the measured temperatures are consistent with the climatological SST yielding a mean difference of -0.07°C (n = 5,780) with a rms deviation of ±1.5°C. The small mean difference indicates that our temperature observations are consistent with the climatological SST.

Fig. 3. Comparisons between (a) the mean monthly climatological SST data (68) and the mean monthly temperatures measured concurrently with ApCO2; and (b) the mean monthly climatological SST data and those interpolated for pixels with no measurements. The solid and dashed horizontal lines indicate the mean and rms deviation values, respectively.

Fig. 3. Comparisons between (a) the mean monthly climatological SST data (68) and the mean monthly temperatures measured concurrently with ApCO2; and (b) the mean monthly climatological SST data and those interpolated for pixels with no measurements. The solid and dashed horizontal lines indicate the mean and rms deviation values, respectively.

Fig. 3b shows that our interpolated temperatures are, on average, warmer than the climatological SST by 0.43 ± 1.89°C. This may be taken as a measure of errors attributable to our computational method and limited time-space database. The temperature dependence of pCO2 in surface waters varies regionally and seasonally from about +3% C-1 in temperate gyres to about -6% C-1 in polar oceans (31), and is about 3.5% C-1 (in the absolute magnitude) on the average over the global oceans. This gives that the SST error of 0.43°C corresponds to a ApCO2 of 5 j atm. If a mean global sea-air CO2 exchange rate of 19 mol-m-2yr-1 (69), which is consistent with the gas transfer formulation used by Tans et al. (1), is assumed, this error would correspond to a flux error of about 1 Gt-C-yr-1 or 75% of the net global ocean CO2 flux estimated in this study. Because of the sparseness of time-space coverage, the uncertainty in the global ocean ApCO2 depends critically on the availability of observations especially in data poor areas. The error could be reduced significantly by additional seasonal observations especially over the southern mid- and high-latitude oceans.

Sensitivity to the Interpolated ApCO2 Values. The sensitivity of the interpolated ApCO2 values to the number of observations made along ship's tracks has been tested by eliminating a measurement from every 10 observations. The interpolated values obtained with 90% of the full database have been compared with those computed using 100% of the database. The effect of this reduction on the interpolated ApCO2 is small with a mean difference of -0.01 ± 2.2 j atm and is independent of latitude. Therefore, the estimated ApCO2 values over the global oceans are not sensitively affected by the number of measurements made along ship's tracks.

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