The Baltic Sea is a semi-enclosed brackish water basin where sea ice occurs annually. The sea-ice study discussed here was conducted as a Finnish-Japanese cooperative research programme entitled "Ice Climatology of the Okhotsk and Baltic Seas’’ to investigate the structure and properties of the brackish ice in the Baltic Sea. Ice, snow and water samples were collected at Santala Bay, near the mouth of the Gulf of Finland, once a week from 20 January to 12 April 1999. The salinity and oxygen isotopic composition (δ18O) of the samples were measured. The ice samples were analyzed stratigraphically. The ice was composed of a granular upper layer, occupying approximately one-third of the entire ice thickness, and underlying columnar ice toward the bottom. The crystallography structure and δ18O values reveal that the granular ice consisted of two layers with different origins, i.e. snow ice and superimposed ice. The fraction of snow relative to the total thickness was estimated. The limited data show a significant contribution of the snow cover to the sea-ice development. The salinity of the granular ice was higher than that of the columnar ice, implying that the mechanism of entrapment of brine may be different between the two ice types.

Sea-ice cores from 11 sites in McMurdo Sound, Antarctica, were collected in 1982 and their crystallography examined. All but one were first-year sea ice. The cores, approximately 2 m long, consisted typically of a thin layer of granular or snow ice (approximately 0.1 m) followed by columnar-grained ice in the top metre and platelet ice in the bottom metre. Salinity and temperature measurements are reported. The columnar-grained ice usually had a strong preferred c-axis orientation in the horizontal plane and also showed a change in this preferred direction with depth in the ice. The mean c-axis orientation, however, usually aligned well with measured or implied currents in the Sound. The platelets were usually aligned with c axis horizontal or close to horizontal, and did not exhibit as marked a preferred orientation as the columnar-grained ice.

This work investigates the possibility of brine-channel formation and development during the freezing of granular ice from a loose frazil-ice suspension in an NaCl solution at set-water concentration. Three experiments were performed at various constant growth rates in a purpose-built vessel with computer-controlled thermal driving. High-resolution chlorinity measurements are used as a proxy for the bulk salinity of the samples. These show clear brine-segregation processes in the ice, with very high salinity gradients for the fast (10 mm h−1) to medium (2 mm h−1) freezing rates, provided that a suitable sampling scale is adopted. Weak segregation was found at the low freezing rate (0.5 mm h−1). The spatial distribution of the bulk salinity fits the visual appearance of brine channels in the ice adequately, in both horizontal and vertical sections. In a similar way to columnar-congelation sea ice, the number of brine channels significantly decreases with growth rate, but the density of channels is systematically lower in the granular ice than that found at equivalent freezing rates in the columnar ice. This is attributed to the lower geometrical constraints on brine transport in the granular medium. Contrasts between brine-channel geometry and density at different growth rates are discussed in light of the "mushy-layer" concept adapted to sea-ice growth from the solidification of alloys

Dendritic crystals of platelet ice appear beneath the columnar land-fast sea ice of McMurdo Sound, Antarctica. These leaf-like crystals are frozen into place by the advancing columnar growth. The platelets most probably begin to appear during July although in some parts of the Sound they may not appear at all. In addition, the amount and extent of platelet ice within the Sound varies from year to year. Previous authors have suggested that the formation of platelet ice is linked to the presence of the nearby ice shelf. It is a matter of debate whether these platelets form at depth and then float upwards or whether they grow in slightly supercooled water at the ice/water interface. The phenomenon is similar to that observed in the Weddell Sea region, but previous authors have suggested the two regions may experience different processes. This paper presents the results of field-work conducted in McMurdo Sound in 1999. Ice-structure analysis, isotopic analysis and salinity and temperature measurements near the ice/water interface are presented. Freezing points are calculated, and the possible existence of supercooling is discussed in relation to existing conjectures about the origin of platelets.

Sea ice is distinguished from many other porous composites, such as sandstones or bone, in that its microstructure and bulk material properties can vary dramatically over a small temperature range. For brine-volume fractions below a critical value of about 5%, which corresponds to a critical temperature of about −5°C for salinity of 5 ppt, columnar sea ice is effectively impermeable to fluid transport. For higher brine volumes, the brine phase becomes connected and the sea ice is permeable, allowing transport of brine, sea water, nutrients, biomass and heat through the ice. Over the past several years it has been found that brine transport is fundamental to such processes as sea-ice production through freezing of flooded ice surfaces, the enhancement of thermal and salt fluxes through sea ice, nutrient replenishment for sea-ice algal communities, and to sea-ice remote sensing. Motivated by these observations, recently we have shown how percolation theory can be used to understand the critical behavior of fluid transport in sea ice. We applied a percolation model developed for compressed powders of large polymer particles with much smaller metal particles, which explains the observed behavior of the fluid permeability in the critical temperature regime, as well as Antarctic data on surface flooding and algal growth rates. Moreover, the connectedness properties of the brine phase play a significant role in the microwave signature of sea ice through its effective complex permittivity and surface flooding. Here we review our recent results on brine percolation and its role in understanding the fluid and electromagnetic transport properties of sea ice. We also briefly report on measurements of percolation we made on first-year sea ice during the winter 1999 Mertz Glacier Polynya Experiment.

A two-dimensional convective transport model has been been developed to investigate the phase and compositional evolution of a flooded slush layer overlying sea ice. The processes governing the solutal and compositional evolution of the flooded layer are investigated in an attempt to explain field observations of salinity and oxygen isotope ratios. Simulations indicate fairly vigorous convective transport in the slush layer, which may lead to the formation of porous channels, depending on permeability. It is found that the salinity and δ18O composition of the freezing slush layer is determined primarily by the sea-ice permeability. Simulated salinity and δ18O profiles produce varied results, though consistent with observations. The influence of the slush on the salinity and δ18O composition of the underlying ice was found to be minimal.

During a field experiment in July 1994, while the R.V. Nathaniel B. Palmer was moored to a drifting ice floe in the Weddell Sea, Antarctica, data were collected on sea-ice and snow characteristics. We report on the evolution of ice which grew in a newly opened lead. As expected with cold atmospheric conditions, congelation ice initially formed in the lead. Subsequent snow accumulation and large ocean heat fluxes resulted in melt at the base of the ice, and enhanced flooding of the snow on the ice surface. This flooded snow subsequently froze, and, 5 days after the lead opened, all the congelation ice had melted and 26 cm of snow ice had formed. We use measured sea-ice and snow salinities, thickness and oxygen isotope values of the newly formed lead ice to calculate the salt flux to the ocean. Although there was a salt flux to the ocean as the ice initially grew, we calculate a small net fresh-wlter input to the upper ocean by the end of the 5 day period. Similar processes of basal melt and surface snow-ice formation also occurred on the surrounding, thicker sea ice. Oceanographic studies in this region of the Weddell Sea have shown that salt rejection by sea-ice formation may enhance the ocean vertical thermohaline circulation and release heat from the deeper ocean to melt the ice cover. This type of deep convection is thought to initiate the Weddell polynya, which was observed only during the 1970s. Our results, which show that an ice cover can form with no salt input to the ocean, provide a mechanism which may help explain the more recent absence of the Weddell polynya.

Between austral late winter 1993 and austral autumn 1998, during five cruises aboard the research vessel Nathaniel B. Palmer, almost 300 m of core was obtained from first-year ice floes in the Ross, Amundsen and Bellingshausen Seas. Analysis of the texture, stratigraphy and stable-isotopic composition of the ice was used to assess the magnitude of the role of flooding and snow-ice formation at the base of the snowpack in the thickening of the ice cover and the thinning of the snow cover. Snow ice occurred in all ice-thickness categories and made a significant contribution to the total ice mass (12−36%) in both autumn and winter. Although the amount of snow ice was often exceeded by the amount of frazil ice and congelation ice, the thickness of individual layers of each ice type indicated that snow ice often made a greater contribution to the thermodynamic thickening of the ice cover than the other ice types. The larger quantities of frazil ice and congelation ice were primarily the result of dynamic thickening. Flooding and snow-ice formation reduced the snow cover to 42−70% of the total snow accumulation depending on time and location. On the basis of this information, ship-based snow-depth estimates were adjusted to estimate the total snow accumulation on different ice-thickness categories.

Snow-cover characteristics on ice floes in the Ross Sea, Antarctica, were examined during cruises in autumn 1998 and summer 1999. The autumn snow cover was shallower, colder and had higher and more variable salinity, and smaller single and composite grain-sizes than the summer snow cover. The autumn snow cover was dominated by rounded particles in chains of grains and clusters, while the summer snow cover was composed primarily of melt clusters. There was extensive flooding of the summer snow cover at the snow/ice interface. The summer snow cover was nearly isothermal and close to the melting point. It exhibited obvious signs of melting and refreezing in the form of ice lenses, pipes and superimposed ice, although no melt ponds were evident. Many of the ice lenses were located directly above the saline standing water found on most of the summer ice floes.

Time series of satellite radar backscatter coefficients of the European Remote-sensing Satellite (ERS-1/-2) scatterometer are presented for perennial sea-ice regions in the Antarctic from June 1991 to June 1999. There is a pronounced seasonal cycle, with higher backscatter in summer than in winter. On average, backscatter increases from spring values of −16.3 dB to summer values of −10.7 dB within 96 days. This rapid rise and the summer maximum are occasionally interrupted by sudden strong signal drops. After late summer/early autumn, backscatter decreases again and slowly approaches winter values. The seasonal cycle is interpreted in terms of processes at the ice surface. The spring backscatter rise is associated with internal snowmelt and the formation of superimposed ice. This process commences once the surface energy balance becomes positive. Sudden backscatter drops are caused by temporary meltwater saturation of the snow during episodic events of warm-air advection. In the autumn, when superimposed-ice formation ceases due to surface cooling, gradual surface flooding with sea water becomes the dominant process, causing the observed decreasing backscatter. The reoccurrence of the seasonal backscatter cycle in most regions points to the widespread formation of superimposed ice on perennial Antarctic sea ice.

Climate models suggest surface warming in the Arctic will be rapid and pronounced, implying substantial changes in snowmelt onset are likely. This research therefore examines spatial and temporal variability in passive-microwave derived snow-melt-onset dates over Arctic sea ice. The objectives are to understand better the regional characteristics of snowmelt and to document whether the snowmelt-onset record shows signs of climate change. Snowmelt-onset dates are derived with Scanning Multichannel Microwave Radiometer and Special Sensor Microwave/Imager brightness-temperature data, and they are subsequently stratified into 13 regions to analyze spatial and temporal variability. Results illustrate significant spatial variability in snowmelt onset, with the median annual snowmelt-onset date in one region of the Arctic typically being statistically different from most other regions. The examination of temporal variability also shows large interannual differences in the median snowmelt-onset date in most regions. Additionally, trends towards earlier snowmelt onset are documented in the West Central Arctic, Lincoln Sea, Beaufort Sea and Canadian Arctic Archipelago regions.

The snowmelt-onset date represents an important transitional point in the Arctic surface energy balance, when albedo decreases and energy absorption increases rapidly in response to the appearance of liquid water. Interannual variations in snowmelt onset are likely related to large-scale variations in atmospheric circulation, such as described by the Arctic Oscillation (AO). This research therefore examines the relationship between monthly-averaged AO values and mean annual snowmelt-onset dates over Arctic sea ice in 13 regions, from 1979 to 1998. The objective is to statistically relate variations in mean annual regional snowmelt-onset dates to variations in the AO. Additionally, monthly-averaged 500 hPa heights and 2 m air temperatures are used to illustrate a physical link between snow-melt onset and a positive AO phase. Regression analyses demonstrate that variations in the AO explain a significant portion of the variations in snowmelt onset in the West Central Arctic, Laptev Sea, East Siberian Sea, Hudson Bay and Baffin Bay. Synoptic analyses suggest earlier (later) than average snowmelt onset occurs where warm (cold) air advection and increased (decreased) cyclonic activity are present.

Daily acquisitions from satellite microwave sensors can be used to observe the spatial and temporal characteristics of the Arctic sea-ice snowmelt onset because the initial presence of liquid water in a dry snowpack causes a dramatic change in the active-and passive-microwave response. A daily sequence of backscatter coefficient images from the NASA scatterometer (NSCAT) clearly shows the spatially continuous progression of decreasing backscatter associated with snowmelt onset across the Arctic Ocean during spring 1997. A time series of the active NSCAT backscatter and a scattering index from the passive Special Sensor Microwave/Imager (SSM/I) show similar trends during the time of the melt onset. An NSCATsnowmelt-onset detection algorithm is developed using the derivative of the backscatter with respect to time to select a melt-onset date for each pixel, generating a melt map for the Arctic sea ice. Comparison between this melt map and one previously generated from an SSM/I scattering index shows the NSCAT algorithm predicts the onset occurs 1−10 days earlier than the SSM/I-based algorithm for most portions of multi-year ice.

Floe size within the Antarctic pack ice is an important parameter that affects both the ocean-ice-atmosphere energy exchange and the mechanical properties of pack ice. In this paper we present a computer-based algorithm to extract floe-size distribution information from digital aerial photographs of Antarctic sea ice. The algorithm segments digital images of sea ice into distinct floes from which the size of each floe is calculated and floe-size distributions are derived. Through a recursive application of the morphological erosion operator, each floe is eroded towards its centre while each pixel’s erosion number is recorded. This ensures no information is lost while individual floes are identified. The algorithm combines image-processing techniques with some manual input to accurately identify the boundaries of individual floes within an image. To demonstrate the functionality of the algorithm, six images, representing regions along a 22 km south-to-north transect through a transitional zone of pack ice near 65° S, 140° E, have been processed. Regional variations in the floe-size distributions show an increase in the number of smaller floes relative to larger floes and a decrease in the dominant floe size (in terms of areal coverage) from south to north. These results are consistent with ship-based observations.

The United States National Ice Center (NIC) provides weekly ice analyses of the Arctic and Antarctic using information from ice reconnaissance, ship reports and high-resolution satellite imagery. In cloud-covered areas and regions lacking imagery, the higher-resolution sources are augmented by ice concentrations derived from Defense Meteorological Satellite Program (DMSP) Special Sensor Microwave/Imager (SSM/I) passive-microwave imagery. However, the SSM/I-derived ice concentrations are limited by low resolution and uncertainties in thin-ice regions. Ongoing research at NIC is attempting to improve the utility of these SSM/I products for operational sea-ice analyses. The refinements of operational algorithms may also aid future scientific studies. Here we discuss an evaluation of the standard operational ice-concentration algorithm, Cal/Val, with a possible alternative, a modified NASA Team algorithm. The modified algorithm compares favorably with Cal/Val and is a substantial improvement over the standard NASA Team algorithm in thin-ice regions that are of particular interest to operational activities.

Using data from the 85 GHz channels of the Special Sensor Microwave/ Imager (SSM/I) allows a resolution improvement by at least a factor of four compared to the other channels. Consequently higher-resolution sea-ice concentration data can be obtained which in turn can be used to improve the results of numerical weather-prediction (NWP) and global circulation models. The proposed new sea-ice concentration retrieval algorithm (SEA LION algorithm) uses the polarization at 85 GHz (p). Emission from atmospheric water and scattering at the wind-roughened sea surface (weather effect) decrease p and cause an overestimate of the sea-ice concentration. We quantify the weather effect with a radiative transfer model and atmospheric data obtained from NWP models and the other SSM/I channels, and correct p for this effect. Tie points of open water and sea ice are determined for each month separately from daily gridded 85 GHz SSM/I brightness temperatures. Sea-ice concentrations are calculated with the new algorithm for the entire Southern Ocean for each day of the period 1992−98 with a spatial resolution of 12.5 × 12.5 km2. Comparisons of these ice concentrations with Operational Linescan System visible images reveal convincing results concerning the monitoring of coastal polynyas and the break-up of the pack ice in spring. SEA LION sea-ice extents and areas, and comparisons between SEA LION sea-ice concentrations and ship observations, agree with those obtained by the NASA Team and the Bootstrap algorithms:

Multi-frequency, multi-polarization airborne synthetic aperture radar (SAR) observations of sea ice in the southern Sea of Okhotsk were carried out in February 1999 in conjunction with RADARSAT SAR observations. The final goal of this study is to clarify the backscattering characteristics and to understand the scattering mechanisms of sea ice in the Sea of Okhotsk by using microwave multiparametric SAR. The airborne SAR (Pi-SAR) has two frequencies (X- and L-band) and multi-polarization (HH, VV, HV, VH) with 1.5 m (X-band) and 3.0 m (L-band) resolution. It was developed by the Communications Research Laboratory (X-band) and the National Space Development Agency of Japan (L-band). We show the frequency dependence and polarization properties of radar backscattering from sea ice. We find that it is possible to distinguish ice types by comparing backscattering from sea ice in the X- and L-bands. Investigation of the polarization characteristics at X-band was very useful for detecting the thin-ice area (e.g. nilas and gray ice).

We have acquired ground-truth data at Lake Saroma, northeast Hokkaido, Japan, and the surrounding area since 1993 in order to collect data on regional sea ice in the Sea of Okhotsk. The data were acquired in 1999 by polarimetric and interferometric SAR (Pi-SAR), the dual-frequency, fully polarimetric airborne SAR system jointly developed by the National Space Development Agency of Japan (NASDA) and the Communications Research Laboratory (CRL), simultaneously with ground experiments. This paper describes the results of polarimetric data analysis of typical sea ice observed in the offshore region near Lake Saroma. The polarimetric parameters used were correlation coefficient and phase difference. Based on the analysis of these parameters, we found that the correlation coefficient between RR and LL polarizations can discriminate four categories including three types of ice and open water.

Several years of daily microwave satellite ice drift are combined with moored upward-looking sonar (ULS) ice drafts into an ice-volume flux record at points along a flux gate across the Weddell Sea, Antarctica. Monthly ice transport varies at the mooring locations from a maximum export of 0.4 m3 s−1 near joinville Island to −0.4 m3 s−1 imported along the Fimbul and Riiser-Larsen ice-shelf margins. Winter peaks are observed at each end of the flux gate, where high concentrations of deformed ice are advected in and out of the basin along the coastline. The central gyre, in contrast, exhibits negligible seasonality and much smaller volume transports. During the period of overlapping ULS operation, the mean monthly integrated ice export west of the gyre center is 59 × 103 m3 s−1, and the import in the East Wind Drift is −17 x103 m3s-1. ULS data are compared with ERS satellite observation of radar backscatter to obtain an empirical relationship between ice thickness and the rate of change of backscatter with incidence angle. Resulting proxy ice-thickness data are combined with Special Sensor Microwave/Imager-derived ice velocities to obtain seasonally varying estimates of net ice-volume flux for the period 1992−98. Significant interannual variability is observed in ice-volume flux expressed as fresh-water transport. A maximum annual mean of 0.054 Sv is observed in 1992; with a minimum of 0.015 Sv in 1996. A 6 year mean transport of 0.032 Sv is observed. Maximum seasonal ice export occurs in July 1992, with a minimum in November 1996. The 10 year mean area flux is 30 × 103 m2s–1 Interannual variations in net volume flux closely follow variations in area flux, with summer minima in 1990/91 and 1996/97. Maximum area transport occurs in 1991, and although this predates the ERS-1 scatterometer data, ice-thickness estimates by Harms and others confirm 1991 as a decadal peak in net integrated fresh-water transport.