Balance , and the Canadian Equilibrium - Line Altitudes , Mass July Freezing - Level Heights in High Arctic

Equilibrium-line a ltitudes on the White Glacier, Axe! H eiberg Island, a nd the north-west sector of the Devon Ice Cap a re shown to be closely related to mean July freezinglevel h eights at nearby upper-air weather stations. An inverse relationship between July freezing-level heights a nd mass balance on the Devon I ce Cap is also sh own. R easons for such correlations are suggested and som e limitations of the relationship are ou tlined . R ecent lowering of the freez ing level in July is discussed in relation to the theoretical "steady-state" equilibrium-line altitudes in the Canadia n high Arct ic. It is suggested that positive mass-ba lance years have predominated over a la rge part of northern Ellesmere I sla nd a nd northcentral Axel H eiberg Island since 1963, and some glac iological evidence supporting this hypothes is is given. R EsuME. Altitude des lignes d'equilibre, bilalls et hauteurs des niveaux de gel enjuillet dalls le ha ut Arcti que Calladien. On montre que les altitudes des lignes d'equilibre du White Glacier d a ns Axel Heiberg Island, e t le secteur Nord-Ouest d e la ca lotte de D evon sont en etroite relation avec les h a ute urs du niveau moyen du gel en juillet da ns les sta tions climatologiq ues du voisinage. On trouve egalem ent une relation inverse entre les hauteurs du niveau de gel en juille t e t le bilan de la calotte glacia ire de Devon. On suggere quelques explicalions a ces relations et on en esquisse quelques limites. Le recent abaissement d u niveau du gel en juillet es t discute par reference aux alti tudes theoriques de la ligne d'equilibre d 'un "stade stable" dans le ha ut Arctique Canadien. On suggere que les annees de bilan positif ont predomine d epuis 1963 sur une large part de l'Ellesmere I sland du Nord et du Centre et du Nord de I ' Axel Heiberg I sland; quelques observations glaciologiques corroborant ces hypotheses sont presentees. ZUSAMMENFASSUNG. Hohen. der Gleichgewichtslillie, Massenhaushalt lInd J uli-Frostgrellze in der kanadischen Hoch-Arktis. Es wird gezeigt, d ass die Hohen der G leichgewichtslinie auf d em W hite Glacier , Axel Heiberg Island, und a uf d em nordwestlichen Sektor des D evon I ce Cap eng mit den H ohen der mittleren JuliFrostgrenze uber nahegelegenen Wetterstationen fur die hohere Atmosphare zusammenhangen. Ebenso wird eine inverse Beziehung zwischen den H ohen d er Juli-Frostgrenze und dem Massenhaush a lt auf dem Devon I ce Cap nachgewiesen . Grunde fill' diese Korrelationen werden vorgeschlagen und einige Beschrankungen des Zusammenhanges dargelegt. Das neuerli ch e Absinken del' Frostgrenze im Juli wird im Hinblick auf die H ohen der lheoretischen "stationaren" G leichgewichtslinie in del' ka nad ischen Hoch-Arktis diskutiert. Es wi rd vermutet, dass seit 1963 a uf einem grossen Teil del' nordlichen E llesmere Island und del' nordlichen zentralen Axel Heiberg Island J a hre mit positivem Massenhausha lt uberwogen; einige glaziologische Fakten zu r Stutzung dieser H ypothese werden angefuhrt. INTRODUCTION Over much of the Canadian Arctic Archipelago, surface temperatures are b elow 0 ° C from September to May (i. e. the 0 ° C isotherm orfreezing level does not rise above the surface). T he freezing level reaches a maximum elevation in July, and h ence changes in the mean elevation of this surface can greatly influence the area of snow and ice affected by melting. With freezing levels near the ground surface, sensible heat exchange is low and receipts of long-wave radiation are reduced , resulting in less energy available for melting (Paterson, 1969). T he height of the freezing level also determines the relative proportions of precipitation reaching the surface as rain or snow. As the elevation of the freezing level lowers, the percentage of precipitation falling as snow at the surface increases; when the freezing level is below approximately 250 m , 50 % of the precipitation at the surface is in the form of snow. T his has been sh own for both mid-latitudes (Murray, 1952, 1959) and Arctic regions (persona l communication from R . G. Barry) . By controlling the altitudinal zonation of rain and snow during the peak of the ablation season, the height of the July freezing level also has an importan t influen ce on surface a lbedo which remains high where summer snowfall occurs and thus further diminishes the energy available for m elting. 267 https://www.cambridge.org/core/terms. https://doi.org/10.1017/S0022143000021754 Downloaded from https://www.cambridge.org/core. IP address: 54.191.40.80, on 17 Sep 2017 at 05:30:13, subject to the Cambridge Core terms of use, available at JO U RNAL O F GLACIOL O GY ELA, MASS BALANCE AND THE FREEZING LEVEL In any balance year the equilibrium line is the bounda ry between the zone of net a ccumulation and the zone of n et ablation on a glacier. T he height of the equi librium line is thus an integrated measure of the rela tive amounts of accumulation and ablation throughout the mass-balance year. In negative mass-balance years the eq uilibrium line a ltitude (ELA) will be high, whi le in positive mass-balance years the reverse is true. By examining variations of climatic parameters with changes in equi librium line altitude, the principal climatic influences on the ELA can be d e termined. F ew records of equilibrium line altitude variations a re available for Canadian A rctic. H owever, on the White G lacier, Axe! H eiberg Island (Figure 1) ELA m easurements have b een kept for about a decade (Muller, 1966, 1967, 1968, 1970). The White G lacier extends from 75 m to I 400 m a .s.l. , with an average gradient of 1 in 10 (Adams, 1966). Surface and upperair climatological data are available for I sachsen (lat. 780 47' N., long. 1030 32' W. ), 300 km to the south-west, and Eureka (I at. 80° 00' N ., long. 85° 65' W.), 110 km to the nor th -east . Table I shows correlation coeffi cients between the Whi te Glacier ELA a nd various clima tic p a rameters. The equi librium-line altitude is poorly correlated with winter precipita tion, p articu larly at Eureka, which is situated in the lee of the Axel Heiberg moun tains. H owever,