JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 108, NO. C2, 3051, doi:10.1029/2001JC000887, 2003
[2] Sea ice is generally understood to be a key element in the earth's climate system. Large-scale model simulations indicate that it is not only sensitive to climate change, but that it also contributes to such change [Ingram et al., 1989; Manabe et al., 1991; Rind et al., 1995]. Because the sea ice cover is relatively thin, small changes in thermal forcing can produce large and rapid changes in thickness and areal extent which can have a major impact on the transfer of heat, moisture, and momentum between the ocean and the atmosphere on both local and regional scales. Of particular interest are interactions between shortwave radiation and the ice cover which have the potential to amplify small changes in climate. In addition to the well-established positive feedback between ice extent and solar energy absorbed at the surface [e.g., Ewing and Donn, 1956, 1958; Budyko, 1969; Sellers, 1969], the state of the ice pack is strongly influenced by a variety of analogous feedback processes involving the absorption, transmission, storage, and redistribution of solar heat within the ice-ocean system [e.g., Maykut, 1982; Ebert and Curry, 1993]. Collectively, these processes are often referred to as the “ice-albedo feedback mechanism” and have become an important topic of current experimental and theoretical research [e.g., Moritz and Perovich, 1996; Holland et al., 1997]. These processes affect not only ice and climate, but also ocean mixed layer structure [Morison and Smith, 1981], biological activity in the ice and water [Cota, 1985], and oceanic heat flux at the underside of the ice [Maykut and McPhee, 1995].
[3] Ice albedo feedback processes depend directly on the way in which shortwave radiation is absorbed, transmitted, and backscattered by the ice cover. While the apparent optical properties of sea ice vary with ice type and temperature throughout the annual cycle, they depend more fundamentally on the number and distribution of inhomogeneities within the ice. Sea ice is a complex material composed of numerous inclusions of brine, gas, precipitated salt crystals, and various impurities embedded within a matrix of nearly pure ice. Neither the distribution of these inclusions nor their effect on the optical properties is precisely known. It is known, however, that temperature changes in the ice can produce large changes in the size, number, spatial distribution, and chemistry of these inclusions which, in turn, can affect the optical properties and radiative transfer in the ice [Light, 1995; Maykut and Light, 1995; Light et al., 1998]. Changes in the way solar energy is partitioned within the system is clearly of fundamental interest in a wide variety of geophysical problems involving the polar oceans.
[4] While treatment of radiative transfer in sea ice in large-scale GCMs and climate models remains relatively crude, detailed multistream and multilayer radiative transfer models [Grenfell and Maykut, 1977; Grenfell, 1983, 1991; Perovich, 1990; Jin et al., 1994; Light, 1995; Grenfell et al., 1998] have been formulated and tested with reasonable success in cases with specified optical properties. But to realistically treat radiative transfer in large-scale models, it is also necessary to be able to predict how the optical properties of the ice evolve with time in response to environmental changes. This evolution is determined entirely by how the microstructure responds to induced changes in temperature, salinity, and density of the ice. Structural models [e.g., Grenfell, 1983; Light, 1995] to predict changes in the distribution of brine, gas, and precipitated salts in colder ice have been formulated using equations of state for the ice [Cox and Weeks, 1983] and brine [Nelson and Thompson, 1954], but there has been no experimental verification of the results and a quantitative understanding of fundamental structural-optical relationships has yet to emerge.
[5] In an effort to provide more detailed information that can be used to develop an improved understanding of sea ice microstructure and its effects on ice optics, this paper presents laboratory results from high resolution imagery of interior first-year ice. Not only do the data obtained from vertical thin sections reveal significantly different distributions of brine pockets and gas bubbles than previously reported, but some temperature-dependent changes are also different than predicted, suggesting that there may be important processes involved in the structural and optical evolution of the ice which have not been considered in earlier models. It is hoped that these results will lead to more precise treatments of radiative transfer in sea ice and to better estimates of heat and mass exchange within the ice-ocean-atmosphere system.
Citation: Effects of temperature on the microstructure of first-year Arctic sea ice, J. Geophys. Res., 108(C2), 3051, doi:10.1029/2001JC000887, 2003.