Air‐sea gas transfer: Its dependence on wind stress, small‐scale roughness, and surface films

Abstract

[1] The influence of wind stress, small-scale waves, and surface films on air-sea gas exchange at low to moderate wind speeds (<10 m s À1 ) is examined. Coincident observations of wind stress, heat transfer velocity, surface wave slope, and surface film enrichments were made in coastal and offshore waters south of Cape Cod, New England, in July 1997 as part of the NSF-CoOP Coastal Air-Sea Chemical Fluxes study. Gas transfer velocities have been extrapolated from aqueous heat transfer velocities derived from infrared imagery and direct covariance and bulk heat flux estimates. Gas transfer velocity is found to follow a quadratic relationship with wind speed, which accounts for $75-77% of the variance but which overpredicts transfer velocity in the presence of surface films. The dependence on wind stress as represented by the friction velocity is also nonlinear, reflecting a wave field-dependent transition between limiting transport regimes. In contrast, the dependence on mean square slope computed for the wave number range of 40-800 rad m À1 is found to be linear and in agreement with results from previous laboratory wind wave studies. The slope spectrum of the small-scale waves and the gas transfer velocity are attenuated in the presence of surface films. Observations over largescale gradients of biological productivity and dissolved organic matter show that the reduction in slope and transfer velocity are more clearly correlated with surface film enrichments than with bulk organic matter concentrations. The mean square slope parameterization explains $89-95% of the observed variance in the data and does not overpredict transfer velocities where films are present. While the specific relationships between gas transfer velocity and wind speed or mean square slope vary slightly with the choice of Schmidt number exponent used to scale the heat transfer velocities to gas transfer velocities, the correlation of heat or gas transfer velocity with mean square slope is consistently better than with wind speed.

The FuGas 2.3 Framework for Atmosphere–Ocean Coupling: Comparing Algorithms for the Estimation of Solubilities and Gas Fluxes

“…This fits the general theory that under low-moderate wind speeds, the k w is set by surface renewal and micro-scale wave breaking (i.e., by k wind ), increasing with steeper younger waves [28,51,65], and only under higher wind speeds is k w set by bubbles from breaking waves (i.e., k bubble ) [41][42][43][44][45][46][47][48][49][50]. The preponderance of surface renewal and micro-scale wave breaking at the coastal ocean turn it more susceptible to additional factors driving k w .…”

“…The preponderance of surface renewal and micro-scale wave breaking at the coastal ocean turn it more susceptible to additional factors driving k w . As an example, under the low-moderate wind speeds often verified at the coastal ocean, the hydration reaction enhances k w [19] and surfactants attenuate surface renewal [28,58,59,65,66]. [20] to compilation by Sarmiento and Gruber [2].…”

“…In addition to their single influence, these factors also interact. For instance, while surfactants increase surface tension, although decreasing the water surface renewal and micro-scale wave breaking [28,65], there is no report of them affecting whitecap from breaking waves. Surfactants are expected to be more relevant on the coastal ocean due to their supply from land and their effect being more pronounced under low-moderate winds [60,66].…”

“…Such are the formulations proposed by Carini et al [30] or Raymond and Cole [31], accounting only for u 10 , or by Borges et al [32], adding current drag with the bottom. Meanwhile, new evidence emerged for the effects of the sea surface cool-skin and warm-layer [33][34][35][36][37][38], atmospheric stability [39,40], wave-breaking [41][42][43][44][45][46][47][48][49][50], water surface renewal [28,[51][52][53][54][55][56][57], surfactants [28,[58][59][60] and rain [61][62][63][64], just to mention a few examples from an extensive list of published works. In addition to their single influence, these factors also interact.…”

“…The solubility of gases at the surface can be estimated by algorithms with different physical and chemical backgrounds [20][21][22][23][24][25][26][27]. The transfer velocity depends on numerous processes occurring at the sea surface [17][18][19][20][21][22]28]. However, large-scale simulations of relevant processes for the air-water gas exchange at the coastal ocean and their inherent variability poses serious difficulties for Earth System Models (ESM) and marine biogeochemical models.…”