| Summary: | Since 2005 quasi-annual jökulhlaups, or glacial lake outburst floods, are registered at the Zackenberg research station (74°28′N, 20°34′W), NE-Greenland. The jökulhlaups typically happen during the sommer months, but were also observed during winter and spring. The Zackenberg jökulhlaups exhibit rapid-rising outburst type characteristics with durations of 12-24 hours, and maximum discharges of 120 to 380 m³/s. The source is a lake dammed by an outlet glacier of the A.P. Olsen ice cap about 35 km inland of the research station. The recorded maximum ice-dammed lake volumes are varying in between 5.7 to 15.6 x10⁶ m³.The year 2018 showed no drainage event and the lake remained filled until the 2019 melting season, accumulating the largest lake volume so far. The 2019 lake outburst happened with beginning of July rather early, but just drained slowly for more than a week to then switch to a rapid-type outburst emptying the lake in about half a day. The subsequent 2020 and 2021 floods continued to show characteristics of slower and mixed-type drainage events. The overall evolution of the 2019-2021 lake drainage patterns suggest a transition back to rapid-type jökulhlaup events. To our knowledge, the observed mixed-type drainage patterns within a single jökulhlaup event has been never reported before, and is challenging the current understanding of subglacial floods. We present all the available hydrometric data for the Zackenberg river and the ice-dammed lake to discuss potential causes for the observed jökulhlaup perturbations.
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