|Diagram of the experimental setup in the sea ice test basin (Brice Loose)|
Gas transfer Across Polar Sea ice (GAPS)
Laboratory Studies at USACE Cold Regions Research and Engineering Lab.
Collaborators: Wade McGillis1, Don Perovich2, Peter Schlosser1, Chris Zappa1 1Lamont-Doherty Earth Observatory, Columbia Univeristy 2US Army Corps of Engineers Cold Regions Research and Engineering Lab (CRREL)
The seasonal ice zone (SIZ) of the Arctic and Southern Oceans are both considered important regions for air-sea CO2 transfer and thus important components in the global carbon cycle. For example, the Southern Ocean (SO) is thought to be the most important oceanic region in regulation of atmospheric CO2 by restricting the ventilation of the deep ocean carbon reservoir and through biological export production [Marinov et al., 2006]. Yet, there are many outstanding questions about the transport pathways of CO2from the atmosphere to the deep ocean and back again. In particular, it is difficult to constrain the carbon budget in the SIZ because of (1) our incomplete understanding of gas transfer and its modulation by sea ice formation, deformation and melt, and (2) difficulty in estimating the strength and timing of net community production during the spring bloom. For lack of an alternative, most studies rely upon the wind-speed parameterization of gas exchange developed for the open ocean to estimate the gas transfer velocity (k), but there is little theoretical basis nor empirical evidence to support its application in the presence of sea ice. In fact, wind speed has a diminishing contribution to oceanic turbulence as fetch is reduced, and processes such as current shear between the water and the ice appear to dominate in some cases. The goal of this study is to explore the fundamental processes that contribute to turbulent kinetic energy (TKE) production and its control on the gas transfer velocity, k, in the presence of sea ice. The complex interplay between momentum inputs at the ocean surface and their mitigation by sea ice type and coverage indicate a new hierarchy of turbulence production, driven by ice-water current shear and convection, as well as wind and waves. This project seeks to quantify the tendency between these inputs and gas flux through simultaneous measurements of TKE dissipation, ε, and k in a laboratory seawater basin, and thereby provide answers to the following questions:
- What are the turbulent mechanisms driving gas exchange through leads and polynyas?
- Under which conditions of reduced fetch does wind-driven turbulence cease to be the dominant mechanism for gas exchange?
- Does shear in the ice-water boundary layer contribute significantly to the magnitude of k?
- Does buoyancy production and residual circulation promote air-sea exchange by providing a renewal mechanism of water exposed to the opening in the ice?
- To what degree do pancake ice, brash and other free floating, unconsolidated features dampen the surface wave field and restrict gas exchange?
- How does stable density stratification during melt alter k and the rate of mixed-layer ventilation?