Toward a universal relationship between wind speed and gas exchange: Gas transfer velocities measured with³He/SF₆during the Southern Ocean Gas Exchange Experiment

Abstract

He/SF 6 dual-gas tracer injections were conducted during the Southern Ocean Gas Exchange Experiment (SO GasEx) to determine gas transfer velocities. During the experiment, wind speeds of up to 16.4 m s −1 were encountered. A total of 360 3 He and 598 SF 6 samples were collected at 40 conductivity-temperature-depth (CTD) rosette casts and two pumped stations. The gas transfer velocity k was calculated from the decrease in the observed 3 He/SF 6 ratio using three different approaches. Discrete points of wind speed and corresponding k were obtained from the change in 3 He/SF 6 ratio over three time intervals. The results were also evaluated using an analytical model and a 1-D numerical model. The results from the three approaches agreed within the error of the estimates of about ±13%-15% for Patch 1 and ±4% for Patch 2. Moreover, 3 He/SF 6 dual-tracer results from SO GasEx are similar to those from other areas in both the coastal and open ocean and are in agreement with existing parameterizations between wind speed and gas exchange. This suggests that wind forcing is the major driver of gas exchange for slightly soluble gases in the ocean and that other known impacts are either intrinsically related to wind or have a small effect (<20% on average) on time scales of the order of days to weeks. The functionality of the wind speed dependence (quadratic or cubic) cannot be unequivocally determined from SO GasEx results.

Transfer Across the Air-Sea Interface

“…Sweeney et al 2007), dual/multiple tracer experiments (e.g. Ho et al 2011b) and 222 Rn distributions (e.g. Bender et al 2011) into reasonable agreement, all broadly in line with the commonly used parametrisation of Nightingale et al (2000b), at least to within the experimental uncertainty in mass balance techniques (Asher 2009;Johnson et al 2011).…”

“…Experiments have been conducted in a variety of locations, the North Sea (Watson et al 1991;Nightingale et al 2000b), the Georges Bank (Asher and Wanninkhof 1998), the Florida Shelf , the Atlantic Ocean (McGillis et al 2001;Salter et al 2011), the equatorial Pacific (Nightingale et al 2000a) and the Southern Ocean Ho et al 2006Ho et al , 2011b.…”

“…In the ocean environment these processes are directly or indirectly related to wind via turbulent forcing and wave breaking and as such wind speed is found to correlate well with transfer velocity in observational data . Wind is routinely measured and can be derived from remotely sensed data and ground-truthed by high precision instruments on ships (Wanninkhof et al 2009;Bender et al 2011;Ho et al 2011b); hence it is likely to remain the key variable used in empirical parameterisations of transfer velocities for the foreseeable future. The first parameterisation of environmental airwater transfer velocity from wind speed (U) were theoretical or based on simple laboratory experiments (e.g.…”

“…2.10) -approximately a factor of 2 at a typical average wind speed of 7 ms À1 (e.g. Yang et al 2011;Ho et al 2011b;Johnson et al 2011). However, this might be considered rather good agreement considering the differing scales of space and time over which the various methods operate e.g.…”

Spatial variability in surface-water &lt;i&gt;p&lt;/i&gt;CO&lt;sub&gt;2&lt;/sub&gt; and gas exchange in the world's largest semi-enclosed estuarine system: St. Lawrence Estuary (Canada)

“…Gas transfer is thought to be regulated by turbulence at the air-water interface (Wanninkhof, 1992). Wind stress plays a key role in the generation of turbulence at the ocean surface through the transfer of momentum to waves and currents (Ho et al, 2011), whereas, in estuarine environments and especially macrotidal estuaries, surface turbulence can be created by interactions of wind forcing, tidal currents, and boundary friction (Zappa et al, 2003(Zappa et al, , 2007Borges et al, 2004a, b). In turbid estuaries, surface turbulence can be attenuated by sus-pended material ).…”