Ninth International Conference on Permafrost ... - IARC Research

Twelve-Year Thaw Progression Data from Zackenberg, Northeast GreenlandIntroductionHanne H. ChristiansenThe University Centre in Svalbard, UNISCharlotte SigsgårdInstitute of Geography and Geology, University of CopenhagenIn Greenland, it is only in the northeast part, at 74°30′Nin Zackenberg, that a continuous CALM data series existssince 1996 (Christiansen et al., in press). CircumpolarActive Layer Monitoring (CALM) data are collected as partof the Zackenberg Ecological Research Operations (ZERO)monitoring programme GeoBasis, which is maintainedby the National Environmental Research Institute andthe University of Copenhagen. As this programme hasbeen manned during entire summer seasons since 1996,progressive summer thaw data have been collected to allowfull season annual thaw progression data collection.The CALM monitoring at Zackenberg was designed toinvestigate, at site scales, the effects of interannual temporaland spatial changes in snow cover, as determined by airtemperature, wind speed, dominant wind direction andsnow precipitation on thaw progression and active layerthickness. This is done by operating two different so-calledZEROCALM sites. The ZEROCALM-1 (ZC1) site consistsof 121 grid points, covering a 100 m x 100 m area, with 10m grid size. It is located on a flat marine abraded groundmoraine (Christiansen 2004). The ZEROCALM-2 (ZC2)site has 208 grid points, covering 120 m x 150 m and alsohas a 10 m grid size. ZC2 is located in and around a naturalsnowdrift site (Christiansen 2004). The two sites are located750 m apart in the Zackenberg lowlands, and are locatedbetween 20 and 37 m a.s.l. They are numbered G1 and G2,respectively, in the CALM database (Brown et al. 2000).Thaw ProgressionProbing of ZC1 and ZC2 from early June to the end ofAugust for the last 12 years, including from 6 to 11 sets ofmeasurements in ZC1 and from 6 to 13 sets in ZC2, hasenabled continuous summer thaw progression curves (Fig.1). In the ZC1 grid, thawing was quick down to around 50cm, and below was significantly slower. In the end of thesummer, the curves show very little to no increase in thawthickness, which is when the active layer was established,typically in mid to late August. Some refreezing wasregistered when measurements were done in September.In the ZC2 site, the thaw rate was relatively homogeneousfor most of the summer, with average rates below ZC1.In the end of summer, only little or no additional thawinghappened, like in ZC1, when the active layer was reached.Also in the ZC2 site, some refreezing was registered in yearswhen measurement proceeded into early September.The difference in thaw progression between the ZC1 andZC2 sites is mainly controlled by the different rate of annualFigure 1. Thaw progression calculated as the mean of 121 pointsin ZEROCALM-1 site and as the mean of 208 points in theZEROCALM-2 site.snow depletion in the two sites (Christiansen 2004, Fig. 4).Ground thawing only starts when the snow has melted. Asthe ZC2 site is completely containing the snowpatch, itsgradual backmelting during summer allows still more gridpoints to start thawing, and, therefore, a less steep thawingcurve is established for the entire grid. The year 1999 wasspecial in that the snowpatch stayed through the summer,which caused a much-reduced active layer of only 44 cm inZC2, as several grid points did not melt at all.Active Layer ThicknessThe active layer thickness (ALT) in the ZC1 and ZC2sites varied, respectively, 20 cm and 26 cm in the 12-yearperiod, with the deepest thaw in 2005 (ZC1 80 cm, ZC270 cm) and the thinnest in 1999 (ZC1 60 cm, ZC2 44cm) for both sites (Fig. 1). Generally, the interannual ALT45