Gary A. Maykut

The Surface Heat and Mass Balance

Formation of a sea ice cover produces profound changes in the state of the atmosphere and ocean, primarily by altering the surface heat exchange. The high albedo of the ice and its insulation of the atmosphere from the underlying water give rise to a climate over the polar oceans that is more characteristic of the continental ice sheets than of a marine environment. Unlike the ice sheets, however, sea ice is only a thin veneer whose thickness and areal extent are sensitive to small changes in heat input. Variations in sea ice extent have the potential to amplify small changes in climate through a variety of positive feedback mechanisms (Kellogg, 1975), leading to their central role in several ice age theories (Brooks, 1949; Ewing and Donn, 1956, 1958; Budyko, 1966).

Solar heating of the Arctic mixed layer

Data from the 1975 Arctic Ice Dynamics Joint Experiment (AIDJEX) are used to examine energy exchange between the Arctic mixed layer and the ice pack. Conductivity-temperature-depth profiles from four drifting stations reveal significant heat storage in the upper 50 m of the water column during summer, with mixed layer temperature elevation above freezing δT reaching as high as 0.4°C. Combining δT with turbulent friction velocity obtained from local ice motion provides an estimate of heat flux from the ocean to the ice Fw which was found to be strongly seasonal, with maximum values reaching 40–60 W m−2 in August. The annual average value of Fw was 5.1 W m−2, about half again as large as oceanic heat flux inferred from bottom ablation measurements in undeformed ice at the central station. Solar heat input to the upper ocean through open leads and thin ice, estimated using an ice thickness distribution model, totaled about 150 MJ m−2, in general agreement with integrated values of Fw. Results indicate that oceanic heat flux to the ice in the central Arctic is derived mainly from shortwave radiation entering the ocean through the ice pack, rather than from diffusion of warm water from below. Indeed, during the AIDJEX project the mixed layer appears to have contributed 15–20 MJ m−2 of heat to the upper pycnocline. During the summer, Fw was found to vary by as much as 10–30 W m−2 over separations of 100 to 200 km and thus represents an important term in the surface heat budget not controlled by purely local deformation and thermodynamics.

The effect of included participates on the spectral albedo of sea ice

Sediments and other participates are often entrained into sea ice formed over shallow shelves in the Arctic, causing significant changes in the albedo of the ice and in the amount of shortwave radiation absorbed and transmitted by the ice. A structural-optical model was used in conjunction with a four-stream radiative transfer model to examine the effects of such particulates on the optical properties of sea ice. Albedo data from well-characterized ice with moderate particulate loading were combined with model calculations to infer a spectral absorption coefficient and effective size for the particulates. Results indicate that sediment particles contained in the ice have an effective radius (R) of ∼9 μm, assuming absorption coefficients similar to those of Saharan dust. With these values, model predictions are in close agreement with spectral albedo observations over a broad range of particulate loading. For particle size distributions commonly observed in sea ice, the calculations indicate that particles with R>30 μm have little effect on the bulk optical properties of the ice. The albedo data also suggest that even apparently “clean” ice contains trace amounts (5–10 g m−3) of particulates that reduce albedos by as much as 5–10% in the visible part of the spectrum. The calculations show that particulates in sea ice primarily affect radiative transfer at visible wavelengths, whereas apparent optical properties in the near-infrared tend to be governed by ice structure rather than by the presence of particulates. Particle-bearing layers occurring below ∼20–30 cm are found to have little effect on albedo, although they can still have a substantial effect on transmission. Estimates of total particle loading cannot be obtained from reflectance data without some additional information on particle size, vertical distribution, and ice structure.

Passive microwave remote sensing of thin sea ice using principal component analysis

Time sequences of surface based measurements of passive microwave emission from growing saline ice reported by Wensnahan et al. (1993) are used to explore the possibility of developing a satellite based sea ice concentration algorithm which solves for the presence of thinner ice. It is shown that two classes of thinner ice can be distinguished from mixtures of open water (OW), first-year (FY) ice, and multiyear (MY) ice. The two classes do not necessarily correspond to specific World Meteorological Organization ice types; rather, newly formed ice represents a brief transition spectrum between OW and thin ice. Newly formed ice appears to be optically thick at 37 and 90 GHz and has a relatively dry surface. The thin ice spectrum occurs when the ice is greater than 4 cm thick and appears to result from the accumulation of brine at the surface of the ice. Thin ice has a relatively stable spectrum characterized by high brightness temperatures, a near-zero spectral gradient at vertical polarization, and a large difference between vertical and horizontal polarizations. Supervised principal component analysis (PCA) was done of laboratory data using 10 channels of passive data: vertical and horizontal polarization at 6.7, 10, 19, 37, and 90 GHz. Analyses were also done on subsets of the laboratory data at 6.7 to 37 GHz as well as 19 to 90 GHz, representing the scanning multichannel microwave radiometer (SMMR) and special sensor microwave imager (SSM/I) satellite frequencies, respectively. Using all of the channels or the SMMR subset makes it possible to solve for mixtures of OW and FY, MY, newly formed and thin ice but with large errors. However, any four of these scene types can be distinguished with reasonable accuracy. The SSM/I frequencies allow determination of at most four of these scene types but with moderate errors. PCA was used in a case study of SSM/I data from the Bering Sea for April 2, 1988. Winds from the north formed thin ice areas which the NASA Team algorithm interprets as large amounts of OW and MY ice. With PCA, these same areas are interpreted as 20–30% OW near the lee shores but otherwise as consisting almost entirely of thin ice. We conclude that thin ice can be detected using satellite data. However, questions remain as to how the thin ice spectrum varies with environmental conditions, how it evolves to that of FY, and how this evolution affects the predicted concentrations of thin ice.

Bio‐optical observations of first‐year Arctic sea ice

The interrelationships among snow cover, ice structure, optical properties, and biological activity are of critical importance in understanding the response and behavior of sea ice systems. Of particular concern in this regard are the optical properties of the algae and the effect of snow thickness on biomass accumulation. In this paper we present the first direct in situ measurements of biomass specific diffuse attenuation spectra for arctic ice algae. The data show in situ attenuation values that are about 3 times larger than those obtained from corresponding in vivo absorption measurements, apparently reflecting differences in the geometrical distribution of the algae or the influence of skeletal ice and dissolved organic material. Observations also confirm that maximal algal accumulation occurs when there is a thin layer of snow covering the ice. A new technique to separate the effects of snow and algae in observed transmission spectra is presented. The ratio of transmittance between 600 nm and 450 nm is a sensitive indicator of biomass, while the 700 nm to 600 nm ratio is strongly affected by snow depth.

Optical Properties Of Ice And Snow In The Polar Oceans. I: Observations

Optically sea ice is a complex material with an intricate and highly variable structure which includes brine pockets, air bubbles, brine channels and internal platelet boundaries. Large variations in the optical properties of the surface layer can occur on horizontal scales of only a few meters, complicating efforts to quantify larger scale interactions between shortwave radiation and the ice-ocean system. Radiative transfer in sea ice is dominated at visible wavelengths by scattering rather than absorption. Because scattering in the ice is essentially independent of wavelength, spectral variations in the optical properties are primarily the result of differences in absorption. Observations show that albedos are particularly sensitive to the presence of liquid water in the surface layers, the effect being most pronounced at wavelengths above 600 nm. Albedos and extinction coefficients in the ice vary inversely with brine volume, and thus temperature. Below the eutectic point, precipitation of solid salts causes a sharp increase in scattering and corresponding increases in albedo and absorption. Biological activity in natural sea ice often affects light transmission and absorption, particularly in coastal regions and in the Southern Ocean. Phase function measurements indicate that the scattering distribution in sea ice is only weakly dependent on wavelength and brine volume.

Observations and theoretical studies of microwave emission from thin saline ice

Observations of time-dependent changes in microwave emission from thin (0–9 cm) saline ice were made at frequencies of 6.7, 10, 19, 37, and 90 GHz for both vertical and horizontal polarizations. Experiments were carried out on artificial sea ice in the laboratory and on natural ice in the Greenland Sea. Coincident data were also collected on ice thickness, temperature, and salinity together with a variety of basic meteorological data. Several phenomena were found to be common to all the laboratory measurements, with some indication that similar features were also present in the field data. These include (1) a sharp rise in surface temperature when the ice was between 1 and 2 cm thick, apparently unrelated to any environmental changes, (2) a decrease in brightness temperature (Tb) at 37 GHz during or just after the surface temperature rise, and (3) an initial increase in Tb with increasing ice thickness, followed by substantial decreases in Tb at the higher (19, 37, and 90 GHz) frequencies. The maximum Tb values observed were higher than those previously reported for young ice. Tb was also found to be much more sensitive to variations in ice properties at horizontal polarization than at vertical polarization. Numerical modeling results indicate that the high-frequency decreases in Tb were not due to bulk changes in either ice temperature or salinity. The most likely explanation for the observed rise in surface temperature and decrease in Tb was the formation of a salinity-enhanced ice or brine surface layer caused by the upward transport of brine as the ice grows.

Spectrophotometers for the Measurement of Light in Polar Ice and Snow

Two portable spectrophotometers have been designed to record light scattering and absorption in polar ice and snow. In the first instrument optical fibers are used to transmit light from the interior of the ice to the spectrophotometer. Such an arrangement allows light measurements up to 2 m away from the instrumentation with minimal disturbance of the natural environment. A miniaturized, submersible spectrophotometer was also built for in situ measurements under floating sea ice. This version, except for the recording apparatus, is entirely self-contained and is housed in a cylindrical tube 9 cm in diameter and 60 cm in length. The unit can be lowered into the ocean through a small borehole in the ice; position and orientation are controlled from the surface. Both spectrophotometers are designed to measure light intensities in the visible spectrum (400-1000 nm). Wavelength resolution is adjustable down to 8 nm at a wavelength of 400 nm, with a field of view of less than 3 degrees . Sensitivities in the pr sent versions are sufficient for measurements through several meters of sea ice with a relative accuracy of 1%. Instrument operation has been tested in the Arctic down to temperatures of -25 degrees C.