How well does wind speed predict air‐sea gas transfer in the sea ice zone? A synthesis of radon deficit profiles in the upper water column of the Arctic Ocean

Abstract

We present 34 profiles of radon‐deficit from the ice‐ocean boundary layer of the Beaufort Sea. Including these 34, there are presently 58 published radon‐deficit estimates of air‐sea gas transfer velocity (k) in the Arctic Ocean; 52 of these estimates were derived from water covered by 10% sea ice or more. The average value of k collected since 2011 is 4.0 ± 1.2 m d−1. This exceeds the quadratic wind speed prediction of weighted kws = 2.85 m d−1 with mean‐weighted wind speed of 6.4 m s−1. We show how ice cover changes the mixed‐layer radon budget, and yields an “effective gas transfer velocity.” We use these 58 estimates to statistically evaluate the suitability of a wind speed parameterization for k, when the ocean surface is ice covered. Whereas the six profiles taken from the open ocean indicate a statistically good fit to wind speed parameterizations, the same parameterizations could not reproduce k from the sea ice zone. We conclude that techniques for estimating k in the open ocean cannot be similarly applied to determine k in the presence of sea ice. The magnitude of k through gaps in the ice may reach high values as ice cover increases, possibly as a result of focused turbulence dissipation at openings in the free surface. These 58 profiles are presently the most complete set of estimates of k across seasons and variable ice cover; as dissolved tracer budgets they reflect air‐sea gas exchange with no impact from air‐ice gas exchange.

Wave Attenuation and Gas Exchange Velocity in Marginal Sea Ice Zone

“…Eddy covariance data from Butterworth and Miller () and Prytherch et al () squares and circles mark the median and error bars show the standard error. (c) Ratio of effective gas exchange over gas exchange in open water versus ice fraction for WAGT model, compared with radon data from Loose et al () and Rutgers Van Der Loeff et al (). Data are marked with circles, triangles, and stars and are binned based on SI concentration and grouped and colored based on ratio of sea ice to wind speed.…”

“…This complexity is due to variability in mixed layer depth and sea ice forcings that are acting on radon budget prior to sampling. Here we simply employ the weighted time average (Bender et al, ; Loose et al, ) of k as k open for each of n = 53 samples (Loose et al, ; Rutgers Van Der Loeff et al, ) and normalized the k from radon deficit by this weighted k. The details of this procedure can be found in supporting information Text S2. The results have been binned by sea ice concentration and grouped based on ratio of time averaged ice speed to wind speed (Figure c).…”

“…Two recent studies support this approximation (Butterworth & Miller, ; Prytherch et al, ). However, there are other observations that show enhancement (Else et al, ; Fanning & Torres, ; Loose et al, ) as well as reduction (Rutgers Van Der Loeff et al, ) of gas exchange beyond the linear relationship due to sea ice. One possible sea ice effect on gas exchange is surface wave attenuation and reduction of breaking wave induced turbulence in the presence of sea ice.…”

Variations in Rates of Biological Production in the Beaufort Gyre as the Arctic Changes: Rates From 2011 to 2016

“…Gas transfer velocities in open water have uncertainties of approximately 15 to 20% (Ho et al, 2011;Stanley et al, 2009;Wanninkhof, 2014). In partially ice-covered waters, gas exchange could be enhanced by up to 40% (Loose et al, 2014;Lovely et al, 2015) but parameterizations that take these processes into account are not well established and there is much debate as to whether the open water fraction (Butterworth & Miller, 2016;Prytherch et al, 2017) or the enhanced turbulence model is correct (Fanning & Torres, 1991;Loose et al, 2017). Thus, the uncertainty in the gas transfer calculation is by far the largest source of uncertainty in the GOP and NCP estimates.…”

Harnessing remote sensing to address critical science questions on ocean-atmosphere interactions

“…At low wind speeds (<5 m s -1 ), buoyancy-driven convection has been proposed as a dominant transfer mechanism (McGillis et al, 2004). The picture is even more complicated in polar oceans due to the presence of sea ice, which itself directly exchanges gases with the atmosphere (e.g., Loose et al, 2011) and alters the propagation of kinetic energy within the water through processes that are poorly understood (Loose et al, 2017;Prytherch et al, 2017). The relatively poor predictive skill of wind-speed based k-parameterisations has triggered the development of alternative approaches.…”