Change and attribution of frost days and frost-free periods in China

Frost events are a climatic phenomenon of great severity, exerting profound impacts on agricultural production and human endeavours. From 1961 to 2017, China has experienced a reduction in FDs and elongation of FFPs across most regions. Many factors contribute to the variation of frost events, which highlights the urgent need to scrutinise the intrinsic factors driving these climatic frost events' inter-annual and interdecadal fluctuations. This work analysed the temporal and spatial attributes of FDs and FFPs, using statistical methods to dissect daily minimum temperature data collected from ground observation stations from 1961 to 2017. The results reveal a declining trajectory in annual FDs and an ascending trend in FFPs. However, the interannual and interdecadal variations do not consistently align with the climatological course.  The spatial distribution of the first four modes of EOF exhibits a persistent variation, north-south, monopole, and east-west pattern for FDs and FFPs. In comparison, the modes of FFPs exhibit many complexities and diversities. Moreover, this study considers geographical location, altitude, and proximity to the ocean, as well as atmospheric circulation factors, including circulation indices, ocean indices, and other relevant indices, to elucidate their influence on frost events. In conjunction with the role of greenhouse gases in propelling global warming, we elucidate that the intensity of the Atlantic-European polar vortex, the area of the Indian Ocean warm pool, and the spatial positioning and extent of the subtropical high emerge as preeminent internal oscillations instigating interdecadal shifts in frost events. This study provides valuable scientific insights into the formation mechanisms and evolving patterns of frost events.