ABSTRACT: Influence of summer inversion frequency on melt season parameters and enhanced arctic glacial melt.

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Abstract

We investigated the vertical variation of temperature in the High Arctic to identify differences between weather records from stations near sea level and higher elevations where ice caps are located. Mean daily lapse rates were determined from twice daily rawinsonde ascents at Resolute, Nunavut from May 1 to September 30 from 1961 to 2003. Results indicated that thermal inversions are critical for determining the length and intensity of the melt season at the level of low-elevation ice caps (e.g., 300 m asl). Increased observed July inversion frequency (the ratio of days with positive lapse rate to the totalnumber of days in the set) resulted in a 72% increase in the total melting degree-days as opposed to estimates using a fixed linear lapse rate. For the 1961- 2003 period (May through September), a mean increase in total melting degree-days of 41% was observed. Also, intraseasonal lapse rates demonstrate a control on the initiation and conclusion of the melt season. Reduced inversion frequency in June and September reduced the incidence of positive temperatures and effectively limited the melt season to July and August. In years with increased June and September inversion frequency, a commensurate lengthening of the melt season was observed. The increasing trend of vertical lapse rates observed since the late 1980’s implies a general lengthening of the Arctic melt season since that time. Increased inversionfrequency leading to shallower vertical lapse rates since the late 1980’s are explained by observed synoptic scale climate patterns. The troposphericcircumpolar vortex is a dominant control on Arctic atmospheric circulation. When centered over the Canadian Arctic, the circumpolar vortex blocks northward moving warm air masses and maintains cooler surface temperatures. However, a 40% decrease in the incidence of a Canadian Arcticcentered vortex since 1987 has been observed and was identified as an important factor in increasingly negative glacier mass balance since that time(Gardner and Sharp, Journal of Climate, in press). The net mass balance of four Canadian Arctic ice masses decreased from –0.07 m·a-1 to –0.20 m·a-1from 1963-1986 to 1987-2001, respectively. The net mass balance is highly correlated with the summer mass balance on Devon Ice Cap (r2 = 0.97) andstrongly suggests that the increased mass loss is due to enhanced summer ablation. Summer thermal inversions are likely the result of the incursion of warm air masses to higher latitudes as a consequence of the position of the circumpolar vortex in the Eurasian Arctic, and are a potential cause of enhanced summer glacial ablation.

Bibliography of Canadian Geomorphology