A Method To Determine Lake Depth and Water Availability on the North Slope of Alaska with Spaceborne Imaging Radar and Numerical Ice Growth Modelling

Spaceborne synthetic aperture radar (SAR) images and numerical ice growth modelling were used to determine maximum water depth and water availability in two areas of the North Slope in northwestern Alaska. SAR images obtained between September 1991 and May 1992 were used to identify when and how many lakes froze completely to the bottom, and how many lakes did not freeze completely to the bottom. At Barrow, on the coast, 60% of the lakes froze completely to the bottom in mid-January alone, and by the end of winter 77% of the lakes were completely frozen. In contrast, 100 km to the south in the 'B' Lakes region, only 23% of the lakes froze completely, and there was no sudden freezing of many lakes as occurred at Barrow. A physically based, numerical model was used to simulate ice growth on the lakes. The simulated maximum ice thickness is 2.2 m. Consequently, any lake where some part of the ice cover does not freeze to the bottom has some water more than 2.2 m deep. For those lakes where the ice cover had frozen completely at some time in the winter, the simulated ice growth curve provides the ice thickness at the time each lake had frozen completely to the bottom and thus the lake's maximum water depth. At Barrow, 60% of the lakes are between 1.4 and 1.5 m deep, and 23% are more than 2.2 m deep. At the 'B' Lakes, 77% of the lakes are more than 2.2 m deep. Thus, there is a considerable contrast in lake depth and water availability between the Barrow and the 'B' Lakes regions. This method is simple to implement, and the relatively inexpensive SAR data have good spatial and temporal coverage. This method could be used to determine lake depth and water availability on the entire North Slope and in other polar and subpolar areas where shallow lakes are common. On s'est servi d'images prises au radar à antenne latérale synthétique (RALS) spatioporté et d'une modélisation numérique de la formation de la glace pour déterminer la profondeur d'eau maximale et la disponibilité de cette eau dans deux régions du versant Nord dans le nord-est de l'Alaska. Des images RALS obtenues entre septembre 1991 et mai 1992 ont servi à identifier quand et comment un grand nombre de lacs avaient gelé sur toute leur profondeur et comment cela ne s'était pas produit pour bien d'autres. À Barrow, sur la côte, 60 p. cent des lacs avaient déjà gelé sur toute leur profondeur à la mi-janvier et, à la fin de l'hiver, 77 p. cent des lacs avaient complètement gelé. Par contre, à 100 km plus au sud, dans la région des lacs "B", seulement 23 p. cent des lacs avaient complètement gelé, et on n'observait pas l'engel soudain de nombreux lacs comme c'était le cas à Barrow. On s'est servi d'un modèle numérique fondé sur des critères physiques pour simuler la formation de la glace sur les lacs. L'épaisseur maximale de la glace simulée est de 2,2 m. En conséquence, tout lac où une partie du manteau glaciel n'atteint pas le fond a une profondeur supérieure à 2,2 m. Pour les lacs dont le manteau glaciel atteignait le fond à un moment quelconque de l'hiver, la courbe de formation simulée de la glace donne l'épaisseur de la glace au moment où chaque lac a gelé sur toute sa profondeur, et donc, la profondeur maximale de ce lac. À Barrow, 60 p. cent des lacs ont entre 1,4 et 1,5 m de profondeur et 23 p. cent ont plus de 2,2 m de profondeur. Dans la région des lacs "B", 77 p. cent des lacs ont plus de 2,2 m de profondeur. Il y a donc un fort contraste dans la profondeur des lacs et la disponibilité de l'eau entre la région de Barrow et celle des lacs "B". Cette méthode est facile à appliquer et les données RALS - relativement bon marché - offrent une bonne couverture spatiale et temporelle. On pourrait utiliser cette méthode pour déterminer la profondeur des lacs et la disponibilité de l'eau sur tout le versant Nord et dans d'autres zones polaires et subpolaires où se trouve un grand nombre de lacs peu profonds

Ice shelf break-up and ecosystem loss in the Canadian high arctic

Over the last 3 years, extensive fractures have appeared in the ∼3000‐yr‐old Ward Hunt Ice Shelf (83°N,75°W).The largest fracture, a north‐south‐oriented serpentine feature (Figure l), now forms an obvious dividing line between the west and east sides of the ice shelf. Secondary fractures extending westward from the central fracture have fragmented a large area of the ice shelf into free‐floating ice blocks. The fractures have severely weakened the ice shelf, although for the moment it remains pinned in place by a number of islands and ice rises. An immediate consequence of the fracturing was the catastrophic drainage of a fresh water lake that was dammed behind the ice shelf. This “epishelf” lake represented a rare ecosystem type in the northern hemisphere, which was particularly vulnerable to climate change. In a recent paper in Geophysical Research Letters [Mueller et al., 2003], a recent 30‐year period of accelerated warming, part of a longer 20th‐century warming trend, is implicated as a factor in the fracturing of the ice shelf and the drainage of the epishelf lake

Break-up of the largest Arctic ice shelf and associated loss of an epishelf lake

Field observations and RADARSAT imagery of the Ward Hunt Ice Shelf (lat. 83°N, long. 74°W), Nunavut, Canada, show that it broke in two over the period 2000 to 2002, with additional fissuring and further ice island calving. The fracturing caused the drainage of an ice-dammed epishelf lake (Disraeli Fiord), a rare ecosystem type. Reductions in the freshwater volume of Disraeli Fiord occurred from 1967 to the present and accompanied a significant rise in mean annual air temperature over the same period in this far northern region. The recent collapse of ice shelves in West Antarctica has been interpreted as evidence of accelerated climate change in that region. Similarly, the inferred thinning and observed fragmentation of the ice shelf, plus the drainage of the epishelf lake, are additional evidence for climate change in the High Arctic

High Arctic lakes as sentinel ecosystems: Cascading regime shifts in climate, ice cover, and mixing

Climate and cryospheric observations have shown that the high Arctic has experienced several decades of rapid environmental change, with warming rates well above the global average. In this study, we address the hypothesis that this climatic warming affects deep, ice-covered lakes in the region by causing abrupt, threshold-dependent shifts rather than slow, continuous responses. Synthetic aperture radar (SAR) data show that lakes (one freshwater and four permanently stratified) on Ellesmere Island at the far northern coastline of Canada have experienced significant reductions in summer ice cover over the last decade. The stratified lakes were characterized by strong biogeochemical gradients, yet temperature and salinity profiles of their upper water columns (5-20 m) indicated recent mixing, consistent with loss of their perennial ice and exposure to wind. Although subject to six decades of warming at a rate of 0.5°C decade-1, these lakes were largely unaffected until a regime shift in air temperature in the 1980s and 1990s when warming crossed a critical threshold forcing the loss of ice cover. This transition from perennial to annual ice cover caused another regime shift whereby previously stable upper water columns were subjected to mixing. Far northern lakes are responding discontinuously to climate-driven change via a cascade of regim

An exceptional winter sea-ice retreat/advance in the Bellingshausen Sea, Antarctica

The exceptional sea-ice retreat and advance that occurred in the Bellingshausen Sea, Antarctica during August 1993 was the largest such winter event in this sector of the Antarctic during the satellite era. The reasons for this fluctuation of ice are investigated using passive microwave satellite imagery, ice motion vectors derived from the satellite data, in-situ meteorological reports and near-surface winds and temperatures from the European Centre for Medium-range Weather Forecasts (ECMWF) numerical weather prediction model. The ice edge retreat of more than 400 km took place near 80degreesW from approximately 1-15 August, although the southward migration of the ice edge was not continuous and short periods of advance were also recorded. Between 16 August and 2 September there was almost continuous sea-ice recovery. The rate of change of the ice edge location during both the retreat and advance phases significantly exceeded the southward and northward velocity components of ice within the pack, pointing to the importance of ice production and melting during this event. During the month, markedly different air masses affected the area, resulting in temperature changes from +2degreesC to -21degreesC at the nearby Rothera station. 'Bulk' movement of the pack, and compaction and divergence of the sea ice, made a secondary, but still significant, contribution to the observed advance and retreat. The ice extent fluctuations were so extreme because strong meridional atmospheric flow was experienced in a sector of the Southern Ocean where relatively low ice concentrations were occurring. The very rapid ice retreat/advance was associated with pronounced low-high surface pressure anomaly couplets on either side of the Antarctic Peninsula