Interannual Variabilities of the Cold-Warm Transition Between Early and Late Winter in Shandong and Associated Large-Scale Circulation Anomalies

Based on meteorological stations observational data in Shandong Province and reanalysis data from 1981 to 2023, we apply empirical orthogonal function of the temporal-temporal matrix to extract the modes of temperature intraseasonal variations in winter and analyze the associated atmospheric circulation anomalies. The results show that, in addition to the mode of consistent warming or cooling in winter, Shandong′s winter temperature exhibits mode of intraseasonal transition from cold to warm (or warm to cold) in winter. The variance contributions of these modes are 15.2% and 10.1%, respectively. Under the negative phases of the Arctic oscillation and Northern hemisphere annular mode, the Arctic polar vortex weakens. The stratospheric vortex shifts eccentrically toward Eurasia, while the tropospheric vortex exhibits an "L"-shaped multi-vortex structure. By late-winter, the concurrent weakening of the polar vortex, the Siberian High, and the Aleutian Low attenuates the East Asian winter monsoon. This pattern results in a transition from a cold early-winter to a warm late-winter. Conversely, under positive Arctic oscillation and Northern hemisphere annular mode phases, the Arctic polar vortex strengthens with its center near the Arctic. It forms a spindle-shaped structure extending from North America to Eurasia. In late winter, the enhanced polar vortex, alongside intensified Siberian High and Aleutian Low systems, strengthens the winter monsoon circulation. This pattern facilitates a transition from a warm early-winter to a cold late-winter in Shandong.