Influences of the 11-yr Sunspot Cycle and Polar Vortex Oscillation on Observed Winter Temperature Variations in China

Using the NCEP-2 reanalysis data in 1979-2015, we analyze variations in the coupled stratosphere-troposphere system and attribute them to the polar vortex oscillation(PVO) and the 11-yr sunspot cycle(SC). Subsequently, influences of PVO and SC on the near-ground temperature and extreme temperatures are diagnosed based on observations at 2419 surface stations in China over the same period. Empirical Orthogonal Function(EOF) analysis of geopotential height(GH) anomalies indicates that the first and second EOF modes together can explain nearly 50% of the total variance and they have different driving sources, active periods, and regions. The first EOF mode mainly represents variation characteristics of the polar vortex, and its active periods appear in late winter. It is found that a weakened polar vortex(larger amplitude in the positive time series of the first mode) corresponds to lower daily mean, minimum, and maximum temperatures and more frequent cold nights and days. This cooling effect mainly occur in northeastern China. The second EOF mode is closely related to the SC, and its major active periods are late autumn and early winter. The results reveal that strong solar activity(larger amplitude in the positive time series of the second mode) leads to cooling effects in northern China through accelerating seasonal transformation of the stratospheric circulation and enhancing intensity of the subtropical westerly jet in the upper troposphere and lower stratosphere. The near-ground temperature is lower than usual, especially for daily mean and minimum temperatures. The number of warm nights and days is significantly reduced, and cold nights and days become more frequent. Therefore,the first and second EOF mode time series of GH anomalies can be used as indices of PVO and solar activity, respectively; and can provide indications of winter cooling processes in China.