Several studies based on the longest remotely sensed record of Northern Hemisphere seasonal snow cover have indicated that its extent has increased over large areas of Eurasia and North America during the transition from summer to winter. Given current understanding of widespread warming trends, this finding is somewhat surprising. It has been suggested that these increases are artifacts of technological improvements in the data set's production. Alternatively, if such trends do reflect actual changes in the timing and location of snow onset, it follows that associations should exist with altered spatiotemporal patterns of atmospheric activity likely to influence the probability of snowfall. This study places significant onset trends during September to December between 1972 and 2017 within the context of means and trends of a range of relevant metrics of middle‐to‐lower tropospheric activity between 1972 and 2014, summarized from monthly reanalysis data. The results suggest that clear explanatory links exist between earlier (and later) onset, and patterns of trends in 500 hPa geopotential heights and sea level pressure, airflows at 500 and 850 hPa, atmospheric humidity, and near‐surface temperature. These findings suggest that most incidences of progressively earlier Eurasian snow cover result from northward advection of moisture by stronger southerly winds, driven by altered zonal gradients in geopotential height north of the Himalayan ranges. Over North America, moisture has been supplied primarily from maritime sources along zonal airflows induced by meridional gradients.

中文翻译：

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1972–2017年北半球季节性积雪形成趋势的气候背景

根据北半球最长的季节性积雪遥感记录进行的几项研究表明，从夏季到冬季的过渡期间，其范围在欧亚大陆和北美的大部分地区都在增加。根据目前对普遍变暖趋势的了解，这一发现有些令人惊讶。已经提出，这些增加是数据集生产中技术改进的产物。或者，如果这样的趋势确实反映了降雪发生时间和地点的实际变化，那么就应该与可能影响降雪概率的大气活动的时空变化联系起来。根据每月再分析数据总结，本研究在1972年至2014年9月至12月的1972年至2014年9月至12月期间发现了重要的发病趋势，这些数据均来自1972年至2014年对流层中低层活动的一系列相关指标。结果表明，在早期（和晚期）发病与500 hPa地势高度和海平面压力，500和850 hPa处的气流，大气湿度以及近地表温度的趋势模式之间存在明确的解释联系。这些发现表明，大多数较早的欧亚积雪发生是由于喜马拉雅山脉以北的地势高度纬向梯度变化而导致的更强的南风使湿气向北平流。在北美，