ALBERT

Description: The observed rapid decline in the Arctic perennial ice cover is one of the most remarkable Signals of change in the Arctic region. Updated data now Show an even higher rate of decline of 9.8% decade–1 (1978–2005) than the previous report of 8.9% decade–1 (1978–2000). To gain insights into this decline, the variability of the Second-year ice, which is the relatively thin component of the perennial ice cover, is Studied. The perennial ice cover in the 1990s was observed to be highly variable, leading to relatively high production of Second-year ice that may in part explain the observed ice thinning during the period and have triggered further decline. The microwave Signature of Second-year ice is Shown to be different from that of the older multi-year ice types and, Surprisingly, more Similar to that of first-year ice. This in part explains why previous estimates of the area of multi-year ice during the winter period are considerably lower than the area of the perennial ice cover during the preceding Summer. Analysis of multichannel cluster maps in conjunction with Submarine ice-draft data indicates ability to detect regions covered primarily by Second-year ice and hence to infer ice-thickness information from the microwave data. The periodic increase of Second-year ice in the 1990s was apparently followed by continuous decline due in part to anomolously warm temperatures during the latter period that Shortened the ice Season and kept first-year ice from getting thick enough to Survive the Summer and become Second year ice.

Description: Preliminary results are presented from the first validation of geophysical data products (ice concentration, Snow thickness on Sea ice (hs) and ice temperature (TI) from the NASA EOS Aqua AMSR-E Sensor, in East Antarctica (in September–October 2003). The challenge of collecting Sufficient measurements with which to validate the coarse-resolution AMSR-E data products adequately was addressed by means of a hierarchical approach, using detailed in situ measurements, digital aerial photography and other Satellite data. Initial results from a circumnavigation of the experimental Site indicate that, at least under cold conditions with a dry Snow cover, there is a reasonably close agreement between Satellite- and aerial-photo-derived ice concentrations, i.e. 97.2±3.6% for NT2 and 96.5±2.5% for BBA algorithms vs 94.3% for the aerial photos. In general, the AMSR-E concentration represents a Slight overestimate of the actual concentration, with the largest discrepancies occurring in regions containing a relatively high proportion of thin ice. The AMSR-E concentrations from the NT2 and BBA algorithms are Similar on average, although differences of up to 5% occur in places, again related to thin-ice distribution. The AMSR-E ice temperature (TI) product agrees with coincident Surface measurements to approximately 0.5˚C in the limited dataset analyzed. Regarding Snow thickness, the AMSR hs retrieval is a Significant underestimate compared to in situ measurements weighted by the percentage of thin ice (and open water) present. For the case Study analyzed, the underestimate was 46% for the overall average, but 23% compared to Smooth-ice measurements. The Spatial distribution of the AMSR-E hs product follows an expected and consistent Spatial pattern, Suggesting that the observed difference may be an offset (at least under freezing conditions). Areas of discrepancy are identified, and the need for future work using the more extensive dataset is highlighted.

Description: Air-temperature records (TA) during 1992 from five inland Antarctic automatic weather station (AWS) sites were compared with the best available infrared temperatures (TIR) from the Advanced Very High Resolution Radiometer (AVHRR) as well as calibrated passive-microwave temperatures (TC) from the Special Sensor Microwave/Imager (SSM/I). Daily and monthly average TA, TIR, and TC data indicate that each approach captures generally similar trends at each site but each approach also has limitations. AWS TA data are considered the most accurate but represent spatially restricted areas and may have long gaps due to sensor or transmission problems. AVHRR TIR data have daily variability similar to the TA record but have numerous small gaps due to cloud cover or observation interruptions. An offset between TA and TIR (〉4 K) at the South Pole site was identified that may be due to the inclusion of data with large satellite scan angles necessary to cover this area. SSM/I TC data have the most continuity but exhibit calibration problems, a significantly damped daily response and do not cover all of Antarctica. Individual daily differences between TA and TIR as well as TA and TC can exceed 17 K, but all sites have mean daily differences of about 1 Kor better, after compensating for the offset at South Pole, and standard deviations of

Description: Changes in the surface elevation of the Greenland and Antarctic ice sheets and ice shelves caused by variations in the rate of firn compaction are calculated with a time-dependent firn densification model driven by two decades (1982–2003) of satellite-observed monthly surface temperatures. The model includes the effects of melting and refreezing, both the direct changes in density and the subsequent effects on the densification rate. As previously shown, the temperature-dependent rate of densification is largest in summer, but changes in winter temperatures also have a significant effect. Over the last decade, climate warming has enhanced the rate of compaction and lowered the average surface elevation of Greenland by 1.8 cma-1 and most of West Antarctica by 1.9 cma–1. In East Antarctica, a small cooling raised the average surface elevation by 0.14 cma–1.

Description: We have recently completed an analysis that examines in detail the spatial and temporal variations in global sea-ice coverage from 26 October 1978, through 20 August 1987. The sea-icemeasurements we analyzed are derived from data collected by a multispectral, dual-polarized, constant incidence-angle microwave imager, the Scanning Multichannel Microwave Radiometer (SMMR) on board the NASA Nimbus 7 satellite. The characteristics of the SMMR have permitted a more accurate calculation of total sea-ice concentrations (fraction of ocean area covered by sea ice) than earlier single-channel instruments and, for the first time, a determination of both multiyear sea-ice concentrations and physical temperatures of the sea-ice pack. An estimate of the SMMR wintertime total ice concentration accuracy of ± 7% in both hemispheres has been obtained. As this is an improvement over the estimated accuracies of previous microwave sensors, we are able to present improved calculations of the sea-ice extents (areas enclosed by the 15% ice concentration boundaries), sea-ice concentrations, and open-water areas within the ice margins. This analysis will be published in a book, Arctic and Antarctic sea ice, 1978–1987: satellite passive microwave observations and analysis, due for publication in1992. Some highlights from the analysis are presented in this paper.

Description: We have recently completed an analysis that examines in detail the spatial and temporal variations in global sea-ice coverage from 26 October 1978, through 20 August 1987. The sea-icemeasurements we analyzed are derived from data collected by a multispectral, dual-polarized, constant incidence-angle microwave imager, the Scanning Multichannel Microwave Radiometer (SMMR) on board the NASA Nimbus 7 satellite. The characteristics of the SMMR have permitted a more accurate calculation of total sea-ice concentrations (fraction of ocean area covered by sea ice) than earlier single-channel instruments and, for the first time, a determination of both multiyear sea-ice concentrations and physical temperatures of the sea-ice pack. An estimate of the SMMR wintertime total ice concentration accuracy of ± 7% in both hemispheres has been obtained. As this is an improvement over the estimated accuracies of previous microwave sensors, we are able to present improved calculations of the sea-ice extents (areas enclosed by the 15% ice concentration boundaries), sea-ice concentrations, and open-water areas within the ice margins. This analysis will be published in a book, Arctic and Antarctic sea ice, 1978–1987: satellite passive microwave observations and analysis, due for publication in1992. Some highlights from the analysis are presented in this paper.

Description: Recent observations of a decreasing ice extent and a possible thinning of the ice cover in the Arctic make it imperative that detailed studies of the current Arctic environment are made, especially since the region is known to be highly sensitive to a potential change in climate. A continuous dataset of microwave, thermal infrared and visible satellite data has been analyzed for the first time to concurrently study in spatial detail the variability of the sea-ice cover, surface temperature, albedo and cloud statistics in the region from 1987 to 1998. Large warming anomalies during the last four years (i.e. 1995−98) are indeed apparent and spatially more extensive than previous years. The largest surface temperature anomaly occurred in 1998, but this was confined mainly to the western Arctic and the North American continent, while cooling occurred in other areas. The albedo anomalies show good coherence with the sea-ice concentration anomalies except in the central region, where periodic changes in albedo are observed, indicative of interannual changes in duration and areal extent of melt ponding and snow-free ice cover. The cloud-cover anomalies are more difficult to interpret, but are shown to be well correlated with the expected warming effects of clouds on the sea-ice surface. The results from trend analyses of the data are consistent with a general warming trend and an ice-cover retreat that appear to be even larger during the last dozen years than those previously reported.

Description: Co-registered and continuous satellite data of sea-ice concentrations and surface ice temperatures from 1981 to 2000 are analyzed to evaluate relationships between these two critical climate parameters and what they reveal in tandem about the changing Arctic environment. During the 19 year period, the Arctic ice extent and actual ice area are shown to be declining at a rate of –2.0±0.3% dec –1 and 3.1 ±0.4% dec–1, respectively, while the surface ice temperature has been increasing at 0.4 ±0.2 K dec–1, where dec is decade. The extent and area of the perennial ice cover, estimated from summer minimum values, have been declining at a much faster rate of –6.7±2.4% dec–1 and –8.3±2.4% dec–1, respectively, while the surface ice temperature has been increasing at 0.9 ±0.6K dec–1. This unusual rate of decline is accompanied by a very variable summer ice cover in the 1990s compared to the 1980s, suggesting increases in the fraction of the relatively thin second-year, and hence a thinning in the perennial, ice cover during the last two decades. Yearly anomaly maps show that the ice-concentration anomalies are predominantly positive in the 1980s and negative in the 1990s, while surface temperature anomalies were mainly negative in the 1980s and positive in the 1990s. The yearly ice-concentration and surface temperature anomalies are highly correlated, indicating a strong link especially in the seasonal region and around the periphery of the perennial ice cover. The surface temperature anomalies also reveal the spatial scope of each warming (or cooling) phenomenon that usually extends beyond the boundaries of the sea-ice cover.