US ITASE Research Disciplines
US ITASE comprises four major research disciplines, namely: meteorology, remote sensing, ice coring, and surface glaciology and geophysics. Pertinent aspects of these disciplines that relate to US ITASE are described below.
Meteorology
The US National Science Foundation supports automatic weather station (AWS) units in remote areas in Antarctica (Figure 2). The basic AWS units measure air temperature, wind speed, and wind direction at a nominal height of 3 meters above the surface. Air pressure is measured at the height of the electronics enclosure. Some units measure relative humidity at 3 meters above the surface and the air temperature difference between 3 meters and 0.5 meters above the surface (distances at the time of installation). These data are collected by the ARGOS Data Collection System on board the National Oceanic and Atmospheric Administration series of polar-orbiting satellites. These data have been used to develop synoptic reconstructions, applied to the understanding of atmosphere-snow transfer functions and the interpretation of ice cores, and have served a wide variety of other specific science goals.
Glaciological accumulation determinations have been able to depict the time-averaged spatial distribution of Antarctic accumulation, although the temporal averaging is not well constrained. Accumulation time series of annual variations are limited in number and spatial coverage. Atmospheric methods for estimating accumulation (actually precipitation minus evaporation/sublimation, and omitting drift snow transport effects) have recently been developed, particularly for Greenland (e.g., Calanca and Ohmura, 1994; Robasky and Bromwich, 1994; Chen et al., 1997), and show good agreement with time-averaged accumulation depictions. Similar efforts are underway for Antarctica and show substantial promise in depicting broadscale accumulation patterns (Yamazaki, 1992; Bromwich et al., 1995; Budd et al., 1995). ITASE offers a unique opportunity to compare atmospheric and glaciological accumulation estimates for the same time periods and for well-sampled, very large areas.
Atmospheric numerical analyses are a comprehensive assimilation of all available meteorological observations (from surface weather stations, radiosonde balloons, satellites, etc.) in a consistent fashion. Recent evaluations have shown that the analyses produced by the European Centre for Medium-Range Weather Forecasts (ECMWF) provide a good broadscale description of the atmospheric circulation in and around Antarctica (Bromwich et al., 1995; Turner et al., 1996; Cullather et al., 1997). From these analyses the accumulation rate over Antarctica can be determined indirectly. It has been found that on average about 40% of the water vapor falling as snow on Antarctica enters the continent through West Antarctica (Bromwich et al., 1995), in line with earlier estimates (Lettau, 1969). This sector is subject to the largest interannual variability in Antarctica, particularly in conjunction with the El Nino-Southern Oscillation (ENSO) phenomenon. As shown by Figure 3 (from Cullather et al., 1996), the derived accumu-lation rate over part of West Antarctica (180-120oW, 75-90oS) facing the South Pacific Ocean varied in phase with the Southern Oscillation Index (a tropical Pacific pressure index related to ENSO) from 1982-1990 and then became anticorrelated after 1990 in association with the prolonged series of El Nino events of the early 1990s (Trenberth and Hoar, 1996). Further, ENSO events observed in the South Pacific Ocean (White and Peterson, 1996) appear to affect both the sea ice cover in this sector (Gloersen, 1995) and pressure and temperature on the continent (Smith and Stearns, 1993).
Uncertainties exist concerning the quality of the pre-1982 ECMWF analyses. These diagnosed accumulation variations are associated with zonal migrations of the Amundsen Sea low for distances of up to 2000 km (Figure 4). The accumulation variations experienced by other parts of West Antarctica depend on their spatial relationship to the Amundsen Sea low, thus illustrating the complexity in space and time of the ENSO signal in West Antarctic precipitation. Much more diagnosis is needed to understand the dynamics and teleconnections associated with the poleward propagation of the ENSO signal from the tropical to the high latitude parts of the Pacific Ocean (Chen et al., 1996), as well as its impact on the climate of West Antarctica. Particularly important is the study of interannual variations of cyclone behavior in relation to the large-scale atmospheric circulation.
As already noted, almost half of the moisture precipitating on Antarctica moves poleward across the coast of West Antarctica between the Ross Ice Shelf and the Antarctic Peninsula. This poleward advection of moist, warm air has a profound impact on the surface temperature regime over West Antarctica, as well as the boundary layer dynamics near the Siple Coast (Bromwich and Liu, 1996). Figure 5 shows the results for May 1995 from an AWS array deployed upslope from the Siple Coast to study the dynamics of the katabatic winds. Only one month is considered in order to maximize the data coverage and because a similar pattern is present in the long-term average (Radok, 1973; Hogan, in review). The potential temperature accounts for the effect of elevation on air temperature readings. The potential temperature pattern in Figure 5 shows that warm air is present near the ice divide upslope from Siple Coast and that its effects steadily decrease downslope toward the Ross Ice Shelf. This warm air signature is the inland expression of the large poleward flux of warm, moist air across the coast and is thought to arise because of the combined radiative effects of clouds and moist air (Liu and Bromwich, 1994; Bromwich and Liu, 1996) on the surface energy balance. In the long-term average this feature follows the ridgeline into East Antarctica (Hogan, in review). Because of the strong ENSO variability in this area described above, there is likely to be strong interannual variability in the location and intensity of this warm air advection pattern.
Remote Sensing
Remote sensing is expected to be an integral part of ITASE, contributing to route and site selection for the ground-based sampling, and extending parameters measured during this sampling. The remote sensing contributions planned for the ITASE effort are: detailed image maps of the ice surface along ground-based sampling routes; mean annual and mean seasonal ice surface temperature values for the entire continent at 1.25 km resolution; and accumulation rate estimates over the continent at 25 km resolution. Additional remote sensing data and techniques will also be used to make site-specific contributions to the ITASE effort. In turn, the ITASE ground-based sampling experiments contribute to the development of improved analyses of remote sensing algorithms by providing ground truth and validation information on, for example: accumulation rate, grain size and temperature, and other parameters which can be used to improve the analysis algorithms.
Several reasonable digital elevation maps (DEMs) exist of the Antarctic continent, at least for regions north of 81.5oS. These are derived mostly from ERS-1 and other satellite radar altimeters, with additional survey data near most of the mountain ranges. In the best of the current DEMs, the spatial resolution (grid cell spacing) is 5 km. If a need is identified for better elevation mapping along the ground-based sampling routes, it may be possible to use photoclinometry (e.g., Bindschadler and Vornberger, 1994) to improve the existing DEMs along the traverse routes.
The Advanced Very-High Resolution Radiometer (AVHRR) is the logical sensor for a significant portion of the mapping required for ITASE. Its resolution (1.1 km pixel size at nadir) is appropriate for the vast interior of the ice sheet. In this area there are few crevasses or other features of importance at small (500 m) spatial scales, and where the dominant surface features are subtle, 2-5 km undulations are well-resolved with AVHRR¹s high sensitivity for brightness changes. One-kilometer AVHRR data for portions of West and East Antarctica are available from three sources: National Snow and Ice Data Center (NOAA, University of Colorado), Antarctic Meteorological Research Center (AMRC, University of Wisconsin), and Antarctic and Arctic Research Center (Scripps Institution of Oceanography). East Antarctica is covered by the 4 km Global Area Coverage (GAC) data archived by NOAA. Despite coverage limitations, Landsat and SPOT imagery can also contribute significantly in areas near mountains and crevasse fields where smaller pixel size would be important. The upcoming mapping of the entire continent at 25 m pixel size by the Radarsat Antarctic Mapping Project (RAMP) would provide the definitive map set for Antarctica if it becomes available prior to ITASE ground-sampling projects; this seems feasible, but acquisition of the RAMP image data has not yet begun.
Along with the ice surface feature mapping, AVHRR may also be useful for grain size mapping and determining the extent of blue ice areas (Warren et al., 1993). Reflectivity of snow in the near infrared (0.9-1.6 Êm) is a strong function of grain size (Wiscombe and Warren, 1980; Grenfell et al., 1994) as well as of illumination, and a band ratio of AVHRR channel 1 (~0.8 Êm) and channel 2 (~1.1 Êm) gives a strong signal related to grain size while reducing the illumination variation effects (Hall and Martinec, 1985). Grain size is an important component of the accumulation mapping from passive microwave emission, as discussed below.
As part of the NASA Mission to Planet Earth Pathfinder project, investigators at the National Snow and Ice Data Center (NSIDC) are currently using AVHRR data to calculate ice surface temperature data that will yield a 5 km grid cell size for Antarctica on a twice-daily basis for the period 1982-present, with the last four years at 1.25 km resolution over most areas. The algorithm for this work is derived from Key and Haeflinger (1992). Accuracy of the temperature measurement from the satellite data is expected to be Ò1oK. Data from the Pathfinder-funded research should be available by the end of 1997.
As part of ITASE, statistical analysis of this data set would be undertaken to determine the mean annual temperature, mean seasonally-averaged temperature, and some measure of the interannual variability over the entire Antarctic. This data base could then be compared with the existing 10m temperature data sets as well as the 10m temperatures gathered on ITASE ground-based sampling projects.
Knowledge of the optical properties of Antarctic snow can be applied to determine the vertical distribution of radiative heating in the snow during the Antarctic summer (Brandt and Warren, 1993) and should provide a link between spectral albedo and wavelength-integrated albedo and satellite radiances in visible and near-IR channels (Grenfell et al., 1994).
Passive microwave emissivity has been shown to be a function of grain size, which in turn depends on accumulation rate (e.g., Zwally, 1977). Recently, two papers have been published on the subject of extracting snow/water equivalent accumulation from these data (Davis, 1995; Zwally and Giovinetto, 1995). With grain size and mean annual temperature available in the near future from AVHRR-derived algorithms, it should be possible to derive accurate estimates of the accumulation rate of 25 km resolution from the SSM/I data record available from 1987-present.
Mean annual and mean seasonally-averaged temperature from passive microwave brightness can be compared with δD and δ180 isotopic variations in surface snow to better constrain the relationship of these isotopic climate indicators with average annual temperature (Shuman et al., 1996).
As part of the verification of the above algorithms, the remote sensing portion of the ITASE effort will include some field measurements of parameters pertinent to the above data sets, for example: elevation mapping, various snow properties, 10 m temperature, and shallow-core gamma-ray logging for accumulation rate determination.
Ice Cores
High resolution ice core records are now recognized as the most direct means for procuring records that document the soluble, insoluble, and gaseous components of the atmosphere at resolutions as fine as seasonal and, potentially, on time scales as long as a million years. Such records provide us with the resolution needed to interpret past environments and the perspective we need for predictive modeling. While ice core records are potentially available from a wide variety of geographic locations, those developed from polar glaciers generally have the best preserved records. The components transported by the atmosphere and captured in glacial ice document both the responses to environmental change and many of the forcing factors that control and/or modify this change.
Previous Antarctic deep drilling efforts (e.g., Vostok, Byrd, Dome C, Taylor Dome; Figure 6) have provided excellent steppingstones to our understanding of climate change. In addition, heightened international awareness of climate change issues plus the quality of the new Summit, Greenland, GISP2/GRIP ice core records have stimulated a new series of proposed and underway deep ice coring efforts at Dome C (France and Italy), Vostok (Russia, France and the US), Filchner-Ronne (England and Germany), Law Dome (Australia), Dome Fuji (Japan), and West Antarctica (US) (Figure 6). Coupled with all of these proposed deep drilling efforts are a series of shallow to intermediate ice cores (Figure 6). Linking these ice cores and developing an understanding of the spatial gradients in ice core properties are essential to the interpretation of these ice core records.
Properties that will be examined using US ITASE ice core sampling are divided into two groups. Primary properties (Table 1) include those to be determined at most of the ice core sites via surface (S) or time-series (TS) sampling, and secondary (Table 2) implies more exploratory sampling. Properties valuable for dating control are distinguished by an asterisk (*).
| Table 1 - Primary US ITASE ice core properties | |
|---|---|
| Annual accumulation | TS* |
| Gamma-ray detection | TS* |
| Laser-light scattering | TS* |
| Electrical conductivity (ECM) | TS* |
| Physical properties (size, shape, arrangement of grains, c-axis fabrics, depth-density analyses, melt layers, visible strata) | TS* |
| Stable isotopes (δD, δ180 and deuterium excess) | TS* |
| Major chemistry (Ca, Mg, Na, NH4, K, Cl, SO4, NO3) | TS* |
| Microparticles | TS* |
| Other chemistry (F, I, Br, MSA, H2O2, HCHO) | TS* |
| Cosmogenic isotopes (10Be, 36Cl, 26Al) | S* |
| Temperature | TS |
| Table 2 - Secondary US ITASE ice core properties |
|---|
| Radionuclides * |
| Tephra * |
| Trace metals (Se, Pb, Hg, V, Mn) |
| Trace elements (Cs, Rb, Ba, Sr) |
| Isotopes (Nd, Sr, Pb) |
| Gases (CO2, CH4, N2O, CFCs, CO, methyl-halides) |
| Biological particles (pollen, diatoms) |
| Biogenic compounds (DMSO, DMSO2) |
| Organic acids |
Despite the importance of ice core research, our current understanding of the spatial distribution of ice core properties over Antarctica is limited to a general knowledge of the surface distribution of δ180 and a sparse surface sampling of selected major ions presented below.
The δ180 of ice has classically provided the basic stratigraphy and paleo-climatology (temperature, moisture source) of ice cores. Morgan (1982) provides a survey of the sites from which mean annual surface values of δ180 have been recovered revealing the relative sparsity of samples from all but a few selected regions (Figure 7). Stable isotope measurements from a limited number of Antarctic ice cores covering the last ~2 kyr (Figure 8) reveal, however, the regional complexity available from these records.
The sources of the chemical species deposited in polar snow and ice have been summarized in numerous papers (Herron, 1982; Lyons and Mayewski, 1984; Mayewski et al., 1992; Whitlow et al., 1992; Legrand and Mayewski, 1996). Based upon our present knowledge of the chemistry of the atmosphere, polar precipitation is expected to be composed of various soluble and insoluble impurities which are either introduced directly into the atmosphere as primary aerosols, such as seasalt (mainly sodium and chloride; some magnesium, calcium, sulfate and potassium) and continental dust (magnesium, calcium, carbonate, sulfate and aluminosilicates), or are produced within the atmosphere along various oxidation pathways involving numerous trace gases primarily derived from the sulfur, nitrogen, halogen and carbon cycles (e.g., Crutzen and Bruhl, 1989). Secondary aerosols and gases (H+, ammonium, chloride, nitrate, sulfate, fluoride, CH3SO3--, HCOO- and other organic compounds (Legrand et al., 1993)) are derived from a variety of biogenic and anthropogenic emissions or from volcanic activity. Some chemical species have multiple sources. For example, sulfate present in snow can be linked to primary marine seasalt (as Na2SO4) or continental dust (as CaSO4). It can also arise due to the presence of H2SO4 produced by atmospheric oxidation of SO2 introduced directly into the atmosphere during volcanic eruptions, through anthropogenic activity, or via atmospheric oxidation of various other S compounds emitted from the biosphere.
The chemical composition of an individual air mass provides a fingerprint that documents the history of the source area over which the transporting air mass passed. Therefore, atmospheric circulation systems can be labeled by the identification of the source areas that contribute to their chemistry. In the simplest case, marine versus continental air masses can be differentiated based on the identification of seasalts (e.g., NaCl) versus continental dusts (e.g., CaSO4), respectively, in the chemistry of these air masses. More complex atmospheric circulation patterns can be differentiated by the addition of other tracers such as specific chemical species indicators that record, for example, continental biogenic inputs (e.g., ammonium) as noted by Mayewski et al. (1983) in the Ladakh Himalayas.
Glaciochemical analyses provide a powerful tool for understanding the history of environmental change (e.g., marine storminess, volcanic activity, anthropogenic pollutants). Further, by examining glaciochemical time-series as indicators of atmospheric circulation patterns, records of highly dynamic response and forcing of climate change can be developed (Mayewski et al., 1997).
Mineralogy and certain radiogenic isotopes (of Sr, Nd and Pb) constitute additional tracers of continental dust sources. Given their multiplicity and the fact that the several source area characteristics that determine them are completely independent, these tracers have the potential of being fairly source-specific. This has been shown in East Antarctica, where the Late Glacial Maximum (LGM) dust at Dome C has been shown to have come from Patagonia in South America, and has been related to transport by the Antarctic Circumpolar Vortex (Grousset et al., 1992), and in Greenland, where the LGM dust at GISP2 appears to have originated in eastern Asia (Chinese loess plateau, Gobi Desert; Biscaye et al., in review).
Essential to the foregoing is an understanding of the spatial distribution of the soluble and insoluble constituents in Antarctic snow and ice. However, the data available on which to base such an understanding are severely limited. Measurements of soluble ionic constituents in snow and ice rank second to stable isotopes in number of sites sampled over Antarctica, yet few data are available for overlapping time periods (Figure 9). Interpretations requiring spatial understanding of ice core properties are, therefore, severely hindered.
Further, because spatial gradients in ice core chemistry reflect both spatial gradients in local atmospheric chemistry and spatially varying atmosphere-snow transfer functions, atmospheric measurements should be conducted at selected surface sampling sites. Variations in the chemical composition of the atmosphere and snow arise from differences in temperature, wind, and snow deposition processes (Bales and Wolff, 1995; Waddington et al., 1996; Wolff and Bales, 1996). For example, the large differences between concentrations of photochemically produced H2O2 in the Byrd and Siple Station ice cores cannot be explained simply by considering the different latitudes and temperatures of the two sites (Neftel et al., 1995; Fuhrer et al., 1996). The possibility of making both surface snow and atmospheric measurements over the latitudinal, temperature and accumulation gradients of West Antarctica offers an excellent opportunity to deconvolve through both forward and inverse methods (Waddington, 1996) the various factors noted above, thus adding greater understanding to ice core chemistry.
Surface Glaciology and Geophysics
The mass balance of an ice sheet is important because of its control on global sea level and because ice thickness change distorts the ice core record. ³Observed² sea-level rise (tide-gauge records corrected for isostatic effects) exceeds ³explained² sea-level rise (all non-Antarctic terms) by roughly 2x, or 1 mm/yr. Models and some marine-based Antarctic measurements argue that the Antarctic closes the budget and is causing sea-level rise; Antarctic land-based measurements suggest that Antarctica actually is contributing to sea-level fall, compounding the problem.
The most difficult term in mass balance determination is net surface accumulation rate. Typically, accumulation rates vary both spatially and temporally. Long (>100 years), well-dated cores are needed to properly assess this variation. Thus far, results indicate that variations on the 10 km scale are tied to surface slope. Depth-density determinations are useful for separating satellite-derived surface elevation changes into ice-sheet-mass-balance and firn-density-change terms. Results also improve the interpretation of trapped-gas records during those critical times when rapid climate changes were producing rapid changes in the pore-close-off depth and thus the diffusive column height.
Ice-sheet boundary and internal layer geometries are necessary inputs for regional ice sheet models that provide context for century/millennial-scale ice coring and temperature studies. High-resolution internal layer studies can give direct verification of the spatial variation of the century-scale and finer accumulation rates that is necessary for valid interpretations of widely spaced shallow ice cores. Digital elevation models for the ice-sheet surface and bed as well as internal layers are available, nominally at three resolutions (whole ice sheet, top one third and top one tenth). The folding in the firn and ice revealed by radar can distinguish between unsuitable (Figure 10) and suitable (Figure 11) sites for ice coring.
Temperature histories determined by stable isotope temperature proxies should be calibrated at each site by borehole temperature measurements (Cuffey et al., 1995; Clow et al., 1996) because stable isotopes can also reflect other changing climate parameters in addition to temperature. Continuous temperature logs of boreholes several hundreds of meters deep can provide thermal calibration over the past several hundred years.