Abstract
The presence of sea ice acts as a physical barrier for air–sea 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 ice–ocean 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 air–sea 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 <8 m s−1.
Highlights
Ocean storage of biogenic gases such as CO2 and O2 is heavily influenced by high latitude ocean processes, including deep water formation, and the seasonal sea ice cycle
The estimates of k during wind ' current forcing were not included in Fig. 7, because we lack the information from literature or observations to convert measurements of the wind speed to estimates of o at the ocean surface, which is a significant gap in our understanding of airÁsea gas exchange in the ice ocean boundary layer (IOBL)
It is worth pointing out that enhancement of gas exchange is thought to occur by eddies that impinge on the viscous mass boundary layer at the airÁsea interface
Summary
Ocean storage of biogenic gases such as CO2 and O2 is heavily influenced by high latitude ocean processes, including deep water formation, and the seasonal sea ice cycle. The airÁsea gas transfer in the open ocean is typically parametrised using wind speed as well as the entrainment of air bubbles, certain hydrodynamics are known to produce turbulence in the absence of direct forcing by wind These processes include surf zone and riverine turbulence (Zappa et al, 2003), the convective oceanic mixed layer (McGillis et al, 2004) and rain on the water surface (Ho et al, 2004). We present experimental results that demonstrate how gas exchange is produced in the ice ocean boundary layer (IOBL), where a variable percentage of ice cover in free drift alternately stimulates and obstructs airÁsea gas exchange These results highlight the role of shear and convection in gas exchange, in a region where interactions between wind and water are reduced by the physical barrier presented by ice. This article is organised as follows: Section 2 describes the experimental methods, including configuration of the Ice Engineering Test Basin, channel and wind tunnel, and a description of the tracer methods used to estimate the gas transfer velocity (k).
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