Air–sea gas exchange in the North Atlantic: ³He/SF₆ experiment during GasEx-98

Figures & data

Fig. 1 Map showing the cruise track of the second leg of GasEx-98, where the ³He/SF₆ experiment was conducted. The cruise left from Lisbon, Portugal on 25 May 1998, and arrived in Ponta Delgada, Portugal on 26 June 1998.

Fig. 2 Wind speeds measured and adjusted to u ₁₀ on the NOAA Ship Ronald H. Brown during GasEx-98. The shaded areas mark the six segments that were used to evaluate the ³He/SF₆ data.

Table 1. Summary of wind speeds and gas transfer velocities from GasEx-98

				Wind Speeds u 10 (m s^−1)							Schmidt numbers^a Sc		Gas transfer velocities^b k(600) (cm h^−1)
Segments	Stations^c	Date range	h ^d (m)	Mean	Range	σ ^e	ɛ ^f	#^g	Temp^h (°C)	Sal^h	^3He	SF6	Mean±error
1	152, 154, 157	152.57–154.36	18.3	4.6	1.9–8.6	1.5	1.11	226	15.11	35.668	175.3	1370.7	7.6±2.7
2	161, 163	155.74–156.53	15.3	7.9	2.9–14.1	2.5	1.10	114	15.41	35.663	173.2	1349.3	18.8±2.8
3	165, 167,169, 171, 172	157.36–159.68	28.6	8.8	0.9–18.3	4.2	1.23	322	15.54	35.657	172.1	1338.5	18.7±3.2
4	173, 176	160.35–161.40	21.7	10.7	1.1–14.9	2.3	1.04	138	15.10	35.657	175.9	1376.5	30.4±4.6
5	178, 179, 180	162.37–163.35	30.8	8.3	2.3–12.8	2.7	1.10	136	15.40	35.670	173.6	1353.8	18.6±8.5
6	184, 190	164.36–166.35	30.0	5.0	0.8–9.3	2.3	1.20	270	15.50	35.679	172.5	1342.8	10.7±1.6

^aAveraged Schmidt numbers of all stations in each segment.

^bEnhancement-corrected k(600) for each segment (i.e. k(600)/ɛ, where ; error in k(600) is determined from the least-squares fit to the ³He/SF₆ decrease for each segment, and the uncertainties in the mixed layer depth.

^cStations in each segment.

^dAveraged mixed layer depth determined from SF₆ profiles.

^eStandard deviation.

^f

^gNumber of 10-minute averaged wind speed measurements in each segment.

^hAveraged temperature and salinity from the surface of all stations in each segment.

Fig. 3 Gas transfer velocities for GasEx-98 (filled red circles), corrected to a Schmidt number of 600, plotted along with all published ³He/SF₆ derived k(600) values from the coastal and open oceans against wind speed u ₁₀. The 2-week averaged k(600) presented in McGillis et al. (2001a) is also shown (filled red square). The points for GasEx-98 were determined using a least-squares fit to the ³He/SF₆ ratios from each of the six segments to determine , and then using eqs. (1 and 2) to calculate k(600). The error estimation for the GasEx-98 data is described in the footnotes to Table 1.

Fig. 4 Observed and modelled ³He/SF₆ ratio during GasEx-98. Each data point is the mean of the ³He/SF₆ profile from the mixed layer of an individual station, with the error bars representing the standard deviation of ³He/SF₆ in the profile. The open symbols are stations that were not used in the analysis, either because they occurred at the beginning of the experiment before the patch has mixed sufficiently (i.e. the first two stations), or because they were deemed to be outside of the tracer patch based on the surface SF₆ concentrations.

Fig. 5 Model and data comparison of ³He/SF₆ ratios for segments 1 and 3 of GasEx-98. For both segments, the parameterisations of Nightingale et al. (2000b), Ho et al. (2006) and Wanninkhof et al. (2009) are most able to model the data.

Table 2. Relative root mean square errors of the model/data comparison for each segment of the ³He/SF₆ data for each of the parameterizations listed

	rRMSE
Parameterisations	Segment 1 (%)	Segment 2 (%)	Segment 3 (%)	Segment 4 (%)	Segment 5 (%)	Segment 6 (%)
Liss and Merlivat (1986)	7.8	14.3	11.9	17.3	4.1	15.4
Wanninkhof (1992)	2.5	1.3	9.8	6.7	4.6	6.9
Wanninkhof and McGillis (1999)	8.7	5.5	9.4	10.4	2.7	14.7
Nightingale et al. (2000b)	0.7	7.3	2.1	4.7	2.9	9.8
Ho et al. (2006)	1.8	6.7	2.4	2.3	2.9	10.5
Wanninkhof et al. (2009)	2.2	9.9	1.7	6.9	3.0	10.3

The italicised values are segments 2, 4 and 6 that only contain two stations each, and segment 5, which occurs over the course of less than a day, and have greater measurement uncertainty. The bold values in each segment denote the top 4 parameterisations in terms of having low rRMSE.

Fig. 6 Wind speed distributions of segments 1 and 3 of the ³He/SF₆ data, shows that segment 1 covered a more narrow range of wind speeds than segment 3, with a mean wind speed that was about half of segment 3.

Table 3. Wind speed/gas exchange parameterisations that are able to reproduce the ³He/SF₆ results from GasEx-98 and other studies in the coastal and global oceans, and global relationships based on ¹⁴C

Citation	Approach	Region	Parameterisations^a
Nightingale et al. (2000b)	^3He/SF6	North Sea and Georges Bank	^b
Ho et al. (2006)	^3He/SF6	Southern Ocean
Sweeney et al. (2007)	^14C	Global	(±30%)^c
Wanninkhof et al. (2009)	Synthesis	Global
Wanninkhof (2014)	^14C	Global	(±30 %)^c

^aFor all parameterisations, u ₁₀ is in m s⁻¹ while the resulting k is in cm h⁻¹.

^bIn the literature, investigators have chosen to express wind speed/gas exchange parameterisations as either k(600) or k(660), corresponding to Schmidt numbers of 600 (CO₂ in freshwater at 20 °C) or 660 (close to 666, which is CO₂ in seawater at 20 °C), respectively. This choice is arbitrary. The conversion from k(600) to k(660) requires a multiplication of 0.953 assuming the dependence of gas exchange on Schmidt number scales to Sc^−1/2, for example, for Ho et al. (2006), is equivalent to .

^cThe uncertainty in the global ¹⁴C relationships includes both the uncertainty in wind speeds and global bomb ¹⁴C inventory.

McGillis W. R. , Edson J. B. , Hare J. E. , Fairall C. W . Direct covariance air–sea CO2 fluxes. J. Geophys. Res. 2001a; 106: 16729–16745. DOI: http://dx.doi.org/10.1029/2000JC000506 .

 Google Scholar

Nightingale P. D. , Malin G. , Law C. S. , Watson A. J. , Liss P. S. , co-authors . In situ evaluation of air–sea gas exchange parameterizations using novel conservative and volatile tracers. Glob. Biogeochem. Cycle. 2000b; 14: 373–387. DOI: http://dx.doi.org/10.1029/1999GB900091 .

 Google Scholar

Ho D. T. , Law C. S. , Smith M. J. , Schlosser P. , Harvey M. , co-authors . Measurements of air–sea gas exchange at high wind speeds in the Southern Ocean: implications for global parameterizations. Geophys. Res. Lett. 2006; 33: 16611. DOI: http://dx.doi.org/10.1029/2006GL026817 .

 Google Scholar

Wanninkhof R. , Asher W. E. , Ho D. T. , Sweeney C. , McGillis W. R . Advances in Quantifying Air–Sea Gas Exchange and Environmental Forcing. Ann. Rev. Mar. Sci. 2009; 1: 213–244. DOI: http://dx.doi.org/10.1146/annurev.marine.010908.163742 .

 Google Scholar