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OCADSAccess DataNarrative of DIC Adjustments

4.1 Narrative of DIC Adjustments

In consideration of all the analyses described in this report, we believe that cruises with consistent offsets in DIC of greater than 4 µmol/kg should be considered for adjustments. The cruises with offsets of this magnitude are discussed in the following subsections.

A06 and A07. The 11 crossovers showed high variability and often large differences along with large standard deviations, suggesting significant variability over the density range investigated. The MLR indicated that these cruises had biases of 11 to 15 µmol/kg with large standard deviations compared with other cruises in this region. These results suggest significant station-to-station differences and possible variability with depth. Therefore, these cruises are not recommended for use.

A08. A08 had crossovers with five other cruises. It was 8 µmol/kg low compared with A17, but the other crossings were not more than 3 µmol/kg lower than other cruises. The MLR showed the cruise to be in line with other cruises in the tropical region. No adjustment is recommended.

A09. A09 had crossovers with five other cruises. These crossovers showed that A09 had lower values than A16S (-5 µmol/kg) but higher values than A15 (+8 µmol/kg). It was almost 5 µmol/kg high with respect to the regional MLR but had a standard deviation of 8 µmol/kg. An adjustment of -4 µmol/kg would bring the values in line with most of the other transects, but this is at the borderline of suggested adjustments. Given the variability and inconsistency of the different comparisons, no adjustments are recommended.

A10. A10 had crossovers with six other cruises. A10 was 4 µmol/kg higher than A15 but was slightly low relative to the other cruises. It did not appear anomalous in the MLR; therefore, no adjustment is recommended.

A12B. A12B had crossovers with three other cruises. It was 7 µmol/kg high relative to A13 but showed no offset in multiple crossovers with A12 and A12BR. A12B also compared favorably with the MLR. Therefore, no adjustment is recommended.

A13. A13 had four crossovers with other cruises. It was low by 7 µmol/kg relative to A12B but compared favorably in crossovers with three other cruises and in the MLR. It is our assessment that the stations associated with the A13-A12B crossover must be slightly anomalous relative to the rest of the data from these cruises. No adjustment is recommended for A13.

A14. A14 had four crossovers with other cruises. It was 5 µmol/kg high with respect to A10 but compares favorably with A08 and A09 as well as in the MLR. Therefore, no adjustment is recommended.

A15. A15 had six crossovers with other cruises. It was 8 µmol/kg low compared with A09 and was consistently low compared with other cruises except crossover A08, which showed a large standard deviation. The regional MLR suggested that adding 4 µmol/kg would put the values in line with other cruises. This is a borderline case, but no adjustment is recommended.

A16S. A16S had five crossovers with other cruises. A16S was 5 µmol/kg higher than A09 but was consistent with crossovers from four other cruises. A16S was 48 µmol/kg low in the tropical Atlantic MLR, but only 33 samples were included in this analysis. The A16S MLR analysis for the South Atlantic was within 3 µmol/kg. The internal consistency and overlap with A16N showed good agreement; thus no adjustment is recommended.

A16N. A16N had seven crossovers with other cruises. It was 6 µmol/kg lower than A05 but with a large standard deviation. The six other crossovers looked very good, as did the overlap MLR and regional MLR. No adjustments are recommended.

A17. A17 had seven crossovers with other cruises. A17 was 9 µmol/kg higher than A08 but was in good agreement with three other crossovers. The MLR showed good agreement in the southern region and a -3 offset in the tropical region. Visual inspection of the crossover plots did not show systematic trends. The internal consistency comparison suggested that either pH, DIC, or TAlk had a bias (Fig. 6). We suspect that the calculation involving pH is the culprit. The cruise covered a large region and traversed a series of boundary currents that could contribute to the differences. No adjustments are proposed.

A20. A20 had three crossovers. It was 4 µmol/kg lower than A05R and 2 to 3 µmol/kg lower than the other overlaps. In the regional MLR it was 3 µmol/kg lower, but the internal consistency with fCO2 was excellent. Applying an adjustment of 2 to 3 µmol/kg would improve the consistency among cruises, but this adjustment is smaller than the overall consistency criteria. No adjustment is recommended.

A22. This cruise was 2 to 3 µmol/kg low based on two crossovers and the internal consistency check with fCO2. The northern MLR also suggested that the A22 DIC values were low by 2 µmol/kg, but the tropical MLR suggests that the values are high. Applying an adjustment of 2 to 3 µmol/kg would generally improve the consistency among cruises but this is beyond the overall consistency criteria. No adjustment is recommended.

A23. A23 had three crossovers. There were significant problems with the coulometer (Robertson and Watson, personal communication), and the offsets were not consistent for this cruise. The high-latitude crossover with A12BR at 67° S showed a very consistent offset of -30 µmol/kg throughout the water column. However, the crossover with A17 at 29° S showed that an offset increased with density, ranging from 0 to -10 µmol/kg. The DIC data for A23 is not recommended for use in the synthesis product.

Last modified: 2021-03-17T18:30:24Z