Currents and convection cause enhanced gas exchange in the ice–water boundary layer

Abstract

A B S T R A C TThe presence of sea ice acts as a physical barrier for airÁsea exchange. On the other hand it creates additional turbulence due to current shear and convection during ice formation. We present results from a laboratory study that demonstrate how shear and convection in the iceÁocean boundary layer can lead to significant gas exchange. In the absence of wind, water currents beneath the ice of 0.23 m s(1 produced a gas transfer velocity (k) of 2.8 m d(1 , equivalent to k produced by a wind speed of 7 m s (1 over the open ocean. Convection caused by airÁsea heat exchange also increased k of as much as 131 % compared to k produced by current shear alone. When wind and currents were combined, k increased, up to 7.6 m d (1 , greater than k produced by wind or currents alone, but gas exchange forcing by wind produced mixed results in these experiments. As an aggregate, these experiments indicate that using a wind speed parametrisation to estimate k in the sea ice zone may underestimate k by ca. 50 % for wind speeds B8 m s

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

“…The results of Loose et al . [] also support this observation. However, the mathematical implication of equation that as must be bounded by an upper limit.…”

Changes in gross oxygen production, net oxygen production, and air-water gas exchange during seasonal ice melt in the Bras d'Or Lake, a Canadian estuary

“…The ice was likely melting even at the beginning of the time-series since the surface water temperature was always above the freezing temperature of water ( Figure 3). Changes in surface ice cover and total ice volume are both important factors during the study; changes in ice volume/thickness will affect 15 stratification and convection in the mixed layer as well as light penetration through the ice, and the surface ice cover affects the rate of gas exchange (Smith and Morison, 1993;Butterworth and Miller, 2016;Loose et al, 2016) CTD profiles at Little Narrows channel near the water pump intake showed substantial changes in stratification during the time-series (Figure 3). From 25 March through 8 April the water column was strongly stratified and we estimated the mixed layer depth to average 0.8(0.3) m. During this period, it was often difficult to determine the exact mixed layer depth because 20 the mixed layer depth was similar to the length of the CTD and obtaining a stable CTD response so close to the surface was challenging.…”

“…From the submersible pump, water flowed through flexible high-pressure PVC tubing submerged underwater 15 to a 3-port pressure-relief valve (on shore) and was then pumped along shore (∼50 m) to our sampling apparatus. The water passed through three 10" canister filters (100, 20, and 5 µm nominal pore size) and then into a sampling manifold containing valves for distributing water for measurement of O 2 /Ar (continuously, by mass spectrometry) and for discrete sampling.…”

“…or an e-ratio (Dugdale and Goering, 1967;Laws et al, 2000) and provides information on the fraction of GOP that is available 15 for export out of the mixed layer (Figure 7f). The uncertainties in the NOP/GOP ratio on each day are quite large.…”

“…We selected the value of k3 He yielding the smallest root mean square deviation (RMSD) between the measured ratio and modeled ratio for each injection. The model ran 1000 times using a Monte Carlo simulation where the measured excess concentration ratios, including the initial condition, are 15 varied with a Gaussian distribution, with the standard deviation being the estimated measurement error in the ratio (7.3 %).…”