Winter warming events (WWE) are abrupt and short-lasting (hours-to-days) events of extraordinarily warm weather occurring during wintertime, sometimes accompanied by rainfall (rain on snow events; ROS). Through direct heat transfer and changes in the snowpack properties, these events cause changes in the below-ground thermal regime that, in turn, controls a suite of ecosystem processes ranging from microbial activity to permafrost and vegetation dynamics. In addition, the overall impacts of WWE on ground temperatures may also depend on the timing of the events and the preceding snowpack characteristics. The frequency and intensity of these events in the Arctic, including the Swedish subarctic, has increased remarkably during the recent decades and is expected to increase even further during the 21st Century. In addition, snow depth (not necessarily snow duration) is projected to increase in many parts of the Arctic. In 2005, a manipulation experiment was set up on a lowland permafrost mire in the Swedish subarctic, to simulate projected future increases in winter precipitation. In this study, we analyse this 15-year record of ground temperature, AL thickness, and meteorological variables, to evaluate the influence of different types and timings of WWE on permafrost under different snowpack conditions. We found that all WWE types were strongly linked to permafrost warming in both the ambient and the manipulated plots during the winter months, but that significantly warmer summer ground temperatures and, consequently, thicker active layer (AL) were only linked to ROS events in manipulated plots. Additional long-term AL thickness data from nearby permafrost sites indicate an increasingly stronger influence of WWE over time since the mid-1990s, to the detriment of the influence of summer air temperatures.

冬季变暖事件 (WWE) 是在冬季发生的异常温暖天气的突然且持续时间短（数小时到数天）的事件，有时伴有降雨（雨雪事件；ROS）。通过直接传热和积雪特性的变化，这些事件会导致地下热状态的变化，进而控制从微生物活动到永久冻土和植被动态的一系列生态系统过程。此外，WWE 对地面温度的整体影响也可能取决于事件发生的时间和之前的积雪特征。近几十年来，北极地区（包括瑞典亚北极地区）发生这些事件的频率和强度显着增加，预计在 21 世纪还会进一步增加。此外，预计北极许多地区的积雪深度（不一定是积雪持续时间）会增加。2005 年，在瑞典亚北极的低地永久冻土泥沼上进行了一项操作实验，以模拟预计未来冬季降水量的增加。在这项研究中，我们分析了这 15 年的地温、AL 厚度和气象变量记录，以评估不同类型和时间的 WWE 在不同积雪条件下对永久冻土的影响。我们发现，所有 WWE 类型都与冬季月份环境和操纵地块中的永久冻土变暖密切相关，但夏季地面温度显着升高，因此较厚的活动层 (AL) 仅与操纵地块中的 ROS 事件有关情节。