Abstract
Abstract. Carbon budgets for the polar oceans require better constraint on air–sea gas exchange in the sea ice zone (SIZ). Here, we utilize advances in the theory of turbulence, mixing and air–sea flux in the ice–ocean boundary layer (IOBL) to formulate a simple model for gas exchange when the surface ocean is partially covered by sea ice. The gas transfer velocity (k) is related to shear-driven and convection-driven turbulence in the aqueous mass boundary layer, and to the mean-squared wave slope at the air–sea interface. We use the model to estimate k along the drift track of ice-tethered profilers (ITPs) in the Arctic. Individual estimates of daily-averaged k from ITP drifts ranged between 1.1 and 22 m d−1, and the fraction of open water (f) ranged from 0 to 0.83. Converted to area-weighted effective transfer velocities (keff), the minimum value of keff was 10−55 m d−1 near f = 0 with values exceeding keff = 5 m d−1 at f = 0.4. The model indicates that effects from shear and convection in the sea ice zone contribute an additional 40% to the magnitude of keff, beyond what would be predicted from an estimate of keff based solely upon a wind speed parameterization. Although the ultimate scaling relationship for gas exchange in the sea ice zone will require validation in laboratory and field studies, the basic parameter model described here demonstrates that it is feasible to formulate estimates of k based upon properties of the IOBL using data sources that presently exist.
Highlights
The regulation of air–water gas exchange occurs within a small vertical region of ∼ 100 μm at the air–water interface – a region that is too small for direct measurement of the properties governing this exchange, namely partial pressure and diffusive length scale
We have argued that the sea ice covered polar oceans 14 are another region where wind speed parameterizations are not expected to be adequate (Loose et al, 2009; Loose and Schlosser, 2011), and in this contribution we present an alternative parameter model that is based on the mechanisms that produce turbulence in the ice–ocean boundary layer (IOBL): (1) shear caused by the velocity differential between drifting ice and the water column beneath (McPhee and Martinson, 1992), (2) buoyant convection caused by heat loss and phase changes at the ocean surface (Morison et al, 1992), and
Our goal is to explore what effect these turbulent production mechanisms may have on the magnitude of k by utilizing the significant advances in theory and observation of turbulence in the IOBL, developed over the past 30 yr (McPhee, 2008)
Summary
The regulation of air–water gas exchange occurs within a small vertical region of ∼ 100 μm at the air–water interface – a region that is too small for direct measurement of the properties governing this exchange, namely partial pressure and diffusive length scale. The air–water interface is continually being deformed by waves and surfactants and the profile of turbulence and gas abundance are difficult to determine for a given surface condition (Jähne et al, 1987). This limitation at the critical scale has led to a proliferation of rate approximations based on quantities that can be more readily measured. Wind speed parameterizations are widely used where wind is the first order mixing mechanism, and may be adequate under certain open ocean conditions
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