North Atlantic WOCE Chlorofluorocarbon Data Comparison

by Deborah A. LeBel




WOCE and non-WOCE CFC data, 1996-1998 Go directly to:








The work presented on this site is supported by NSF grant OCE 98-11034.
 






Description of Quality Control Methods


The initial quality control included a number of steps. First, and most simply, plots of CFC-12 and the CFC-11/12 ratio vs CFC-11 were used to identify anomalous points. Vertical profiles of potential temperature, salinity, oxygen, and CFC-11 and CFC-12 were used to confirm suspicious values. Data were then grouped into classes, almost always temperature ranges, which were used to predict CFC-12 concentrations using CFC-11 as an independent variable. The rms deviation from the model fit line was used to quantify variability; note that this measure includes natural variability at the regional and mesoscale levels as well as measurement variability. The class with the least variability was used to evaluate the measurement variability.

Second, equilibrium atmospheric concentrations were calculated from surface water measurements and CFC solubilities (Warner and Weiss, 1985) at each station and compared to the atmospheric time history curve (Walker et al, 2000), and to shipboard air measurements when available (Table 1). Shipboard measurements were within 2.5% of the expected values on all but two cruises. Mean surface saturation values were compared for CFC-11 and CFC-12 as an additional quality control measure. Cruises with CFC-11 and CFC-12 saturations that differed by more than 3% were considered suspect.

Finally, data from different cruises were compared at section cross-over points. Some difficulties were encountered, particularly near rough topography, in obtaining enough data to make a useful comparison. In some cases, such as the approach of sections A02 and A20 southeast of Newfoundland, we were unable to make estimates with any degree of confidence. These comparisons are not reported. Suspect data identified by any of these processes were reported to the principle investigator responsible for the measurement, the PI checked the data and provided corrected data if warrented. Plots were then redone with the corrected data. Finally revised data sets and data quality flags were submitted to the WOCE Hydrographic Program Office.





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Shipboard Air Measurements





Shipboard air measurements (parts per trillion) for A02 (M30/3), AR04F, A02 (M39/3), Hudson 1996 and 1997, A24, A20, A22, and AR01. The two curves show the Northern (upper) and Southern Hemisphere (lower) atmospheric values (Walker et al., 2000). Click on the thumbnail for a full-scale picture.
F11
CFC-11 latitudinal distribution
F12 F113
CFC-12 latitudinal distribution CFC-113 latitudinal distribution





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Surface Water Saturations





Table 1. Comparison of shipboard air measurements and predicted surface water saturations with the Walker et al. (2000) atmospheric curve.

An equilibrium 100% saturation has been assumed. Click on the section labels for a plot of station versus % saturation for CFC-11 and CFC-12 (solid line - CFC-11, dotted line - CFC-12).
F11 F12
measured air all surface water samples measured air all surface water samples
vs. Walker vs. Walker vs. Walker vs. Walker
A02 Roether 100.9% 102.4% 99.6% 101.8%
24N Bullister 99.7% 100.4% 100.0% 100.4%
A25 Smythe-Wright unavailable 97.2% unavailable 98.7%
KN147 Smethie unavailable 81.7% unavailable 83.9%
A20 Smethie 98.6% 102.6% 99.4% 103.5%
A22 Smethie 99.3% 99.4% 99.7% 99.3%
A24 Weiss 100.3% 102.8% 98.6% 100.9%
AR04F Andrie 100.8% 100.9% 98.3% 90.6%
M39/4 Rhein unavailable 106.2% unavailable N/A
V161 Rhein unavailable 112.1% unavailable N/A
M39/5 Rhein unavailable 103.2% unavailable N/A
V172 Rhein unavailable 106.9% unavailable N/A
M39/2 Rhein unavailable 102.0% unavailable 97.8%
Hudson 1996 Jones 90.5% 81.9% 86.9% 84.1%
Hudson 1997 Jones 98.0% 95.7% 101.7% 101.6%


Almost all the sections show some degree of supersaturation. One exception is the KN147 cruise in the Labrador Sea during March 1997. Vertical profiles of temperature, salinity, and CFCs show that the cruise occurred either during or immediately after a period of active convection. Some stations show an essentially homogeneous water column down to 800 m and undersaturation during convection is commonly observed. The 1996 occupation of AR07 in the Labrador Sea also shows similar saturation values at the surface. These measurements are not easily explained, however. The cruise took place in mid- to late May, after the deep convection is expected to have ceased. Also, the air measurements are 10-15% low compared to the atmospheric time history, suggesting that the low values are not attributable to undersaturation during a convective event.



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Evaluations at Section Cross-Overs


Once intra-section anomalies had been identified, it remained to single out any possible differences among the nine different laboratories contributing to the North Atlantic WOCE data set. We quantified these differences using the method described by Johnson et al. (2001). Briefly, a nexus of stations within 350 km of a cross-over was selected from each section; using potential temperature-salinity relationships, this group was further narrowed so that a single water mass was represented. Vertical profiles of salinity, CFC-11, and CFC-12 were interpolated to a set of 99 potential temperature surfaces using an Akima spline. At each level, a polynomial was fit to the data using distance from the cross-over point as the independent variable. A mean profile for each section was formed by evaluating the fitted polynomial at the cross-over point. Finally, a weighted average of the difference profile over a temperature range gave the final estimate of difference between the two sections. Throughout the subpolar gyre, the comparison was made in the Overflow Waters. In the subtropical region, we used the 18 Degree Water, as the section cross-overs were well away from the formation region and the sections presumably sampled the same vintage of mode water.

Comparisons were made for CFC-11, CFC-12, and salinity. In all cases but one, salinity offsets were less than about 4 per mille, comparable to the variability between standard sea water batches. The one case with a salinity difference greater than this was the comparison between KN147 in 1997 and V161 in 1996 from the deep Labrador Sea, where the overflow waters became markedly more saline between 1996 and 1997. The CFC-12 data on the M39/4, M39/5, and V161 were all flagged questionable by the PI, and these comparisons are not reported.

The uncertainty on the CFC difference estimates ranges from 1 to 4 %. This was determined in two ways. Three self-cross-overs of KN147 in the Labrador Sea yielded values of (2%, 0.5%, 1%) for CFC-11 and (0.5%, 0.6%, 4%) for CFC-12. The fairly coarse station spacing limited the ability of the method to smooth out small variations in the standard curves. The second check involved subsampling a portion of line A20, with alternate stations forming two "new" sections. The cross-over value was estimated using the two subsampled sections and the full number of stations. In regions with a smoothly varying field, the cross-over value of each subsection agreed to within 1%, while the agreement degraded to almost 4% in eddy-rich regions. Results of the comparisons at section cross-overs are summarized in Table 2.





Table 2. Percent difference of CFC-11 and CFC-12 at section cross-over points.

Signs indicate relationship of the first section to the second section list in column 1. Cells with blue backgrounds are indirect estimates, calculated by comparison of two sections at their cross-overs with a common third section, rather than by comparison of differences between two sections. Click on the highlighted links to see a map of the stations used, the potential temperature/salinity, potential temperature/CFC-11, and mean profile plots for each cross-over. See Table 3 for cruise name and ship.
Cross-Over Stations CFC-11 % Difference CFC-12 % Difference
A02/A24 302-310/128-139 +0.7 -2.8
A02/A25 292-305/43-54 +0.3 -7.6
A02/M39-2 -1.0 -0.3
A02/M39-4 -1.8 NA
A02/M39-5 -0.7 NA
A20/AR01 43-51/62-70 +0.3 -0.6
A22/AR01 39-48/81-88 +0.4 +1.8
A24/A25 6-21/37-46 +0.5 -7.8
A24/M39-2 74-80/219-226 -1.5 -0.3
A24/M39-4 121-125/409-413 -2.7 NA
A24/M39-5 35-46/566-574 +1.6 NA
A24/AR21 -0.7 -1.2
A25/M39-2 -2.2 +6.6
A25/M39-4 -3.0 NA
A25/M39-5 -1.9 NA
KN147/M39-4 5,90,92,97,100,103/341-349,370-380 -1.6 NA
KN147/AR07W 5,90,92,97,100,103,106/60,61,66-71 +1.5 -6.6
M39-2/M39-4 -0.8 NA
M39-2/M39-5 -0.3 NA
M39-4/M39-5 +1.1 NA
M39-4/AR07W +8.1 NA





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WOCE Standards


In addition to the quality control procedures described here, other groups applied similar procedures to the other WOCE CFC data sets. The 1% goal for precision and accuracy was achieved on some cruises, as was the case for the North Atlantic, but it was also often exceeded. Most data sets had an accuracy and precision better than 3%, which was adapted as the relaxed WOCE standard for CFCs 11 and 12. Data quality classifications for the North Atlantic cruises are summarized in Table 3.


Table 3. CFC Data quality classifications for WOCE North Atlantic sections.
WOCE
Section		 Cruise Name Ship CFC-11
within 1%
WOCE std.		 CFC-11
within 3%
WOCE std.		 CFC-12
within 1%
WOCE std.		 CFC-12
within 3%
WOCE std.
AR02 OC269 Oceanus Y Y
AR04F Etambot II Edward Link Y Y
AR07E V161 Valdivia Y N
AR27 KN147 Knorr Y Y
AR12 M39/2 Meteor Y Y
AR07W Hudson 97006 Hudson N N
A24 KN151/1 Knorr Y Y
A02 M39/3 Meteor Y Y
A22 KN151/2 Knorr Y Y
A20 KN151/3 Knorr Y Y
A25 DI230/1 (FOUREX) Discovery Y Y
AR12 M39/4 Meteor Y N
AR25 M39/5 Meteor Y N
AR02 EN311 Endeavor Y Y
AR01 OCEACES Brown Y Y
AR21 DI233/1(CHAOS) Discovery N N
AR13 V172 Valdivia Y N





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References


Johnson, G. C., Robbins, P. E., and Huff, G., 2000. Systematic adjustments of hydrographic sections for internal consistency, Journal of Atmospheric and Oceanic Technology, 1234-1244.

Walker, S. J., Weiss, R. F., and Salameh, P. K., 2000. Reconstructed histories of the annual mean atmospheric mole fractions for the halocarbons CFC-11, CFC-12, CFC-113, and carbon tetrachloride. Journal of Geophysical Research 105, 14285-14296.

Warner, M. J. and Weiss, R. F., 1985. Solubilities of chlorofluorocarbons 11 and 12 in water and sea water. Deep-Sea Research PartII 32, 1485-1497.

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