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

David T. Ho; Rik Wanninkhof

doi:10.3402/tellusb.v68.30198

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

Tellus B: Chemical and Physical Meteorology

Volume 68 (2016): Issue 1

By: David T. Ho and Rik Wanninkhof

Open Access

|Jan 2016

Figures & Tables

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.

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 u₁₀ (m s⁻¹)Schmidt numbers^aScGas transfer velocities^bk(600) (cm h⁻¹)SegmentsStations^cDate rangeh^d (m)MeanRangeσ^eɛ^f#^gTemp^h (°C)Sal^h³HeSF₆Mean±error1152, 154, 157152.57–154.3618.34.61.9–8.61.51.1122615.1135.668175.31370.77.6±2.72161, 163155.74–156.5315.37.92.9–14.12.51.1011415.4135.663173.21349.318.8±2.83165, 167,169, 171, 172157.36–159.6828.68.80.9–18.34.21.2332215.5435.657172.11338.518.7±3.24173, 176160.35–161.4021.710.71.1–14.92.31.0413815.1035.657175.91376.530.4±4.65178, 179, 180162.37–163.3530.88.32.3–12.82.71.1013615.4035.670173.61353.818.6±8.56184, 190164.36–166.3530.05.00.8–9.32.31.2027015.5035.679172.51342.810.7±1.6

[i] ^aAveraged Schmidt numbers of all stations in each segment.

[ii] ^bEnhancement-corrected k(600) for each segment (i.e. k(600)/ɛ, where $ε = \bar{u_{10}^{2}} / {\bar{u_{10}}}^{2}$ ; 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.

[iii] ^cStations in each segment.

[iv] ^dAveraged mixed layer depth determined from SF₆ profiles.

[v] ^eStandard deviation.

[vi] ^f $ε = \bar{u_{10}^{2}} / {\bar{u_{10}}}^{2}$

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

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

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 $\frac{d}{d t} (^{3} He / {SF}_{6})$ , 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.

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.

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

rRMSEParameterisationsSegment 1 (%)Segment 2 (%)Segment 3 (%)Segment 4 (%)Segment 5 (%)Segment 6 (%)Liss and Merlivat (1986)7.814.311.917.34.115.4Wanninkhof (1992)2.5 1.3 9.8 6.7 4.6 6.9 Wanninkhof and McGillis (1999)8.7 5.5 9.410.4 2.7 14.7Nightingale 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)2.29.91.7 6.9 3.0 10.3

[i] 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.

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

CitationApproachRegionParameterisations^aNightingale et al. (2000b)³He/SF₆North Sea and Georges Bank

k (600) = 0.333 u_{10} + 0.222 u_{10}^{2}

^bHo et al. (2006)³He/SF₆Southern Ocean

k (600) = (0.266 \pm 0.019) u_{10}^{2}

Sweeney et al. (2007)¹⁴CGlobal

k (660) = 0.27 u_{10}^{2}

(±30%)^cWanninkhof et al. (2009)SynthesisGlobal

k (660) = 3 + 0.1 u_{10} + 0.64 u_{10}^{2} + 0.011 u_{10}^{3}

Wanninkhof (2014)¹⁴CGlobal

k (660) = 0.251 u_{10}^{2}

(±30 %)^c

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

[ii] ^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), $k (600) = 0.266 u_{10}^{2}$ is equivalent to $k (600) = 0.254 u_{10}^{2}$ .