Abstract:

The characteristics of low-frequency oscillation of the cold air activity in the Beijing-Tianjin-Hebei (BTH) region in boreal winter and the impacts of the low-frequency characteristics of the Siberian High (SH) on them are investigated, using daily temperature observational data and NCEP/NCAR reanalysis data with a time span covering 1981-2015, based on power spectrum and composite analysis methods. The results show that both the daily temperature in the BTH region and the SH have a dominant period of 10-30 days in both daily temperature in the BTH region and the SH, and there exist significant lead-lag correlations between the low frequency oscillations of them. The cold surges in the BTH region mainly occur during the descending phase from the zero position to the trough of the low-frequency oscillation of temperature and the descending phase from the peak to the zero position of the low-frequency oscillation of the SH. A low-frequency cycle of abnormal atmospheric circulation propagating from Northwest Asia to Southeast Asia appears near the surface and in the middle layer, along with the low-frequency oscillations of the temperature in the BTH region and the SH. The abnormally low temperature, which affects the BTH region, originates near the Kara Sea and accumulates at the high latitudes of Asia, where a cooling effect is produced continuously. Meanwhile, marked convergent subsidence occurs in the middle layer, owing to the atmospheric convergence caused by the abnormal cyclone at the high latitudes of Asia. The combined effect of the convergent subsidence in the middle layer and the persistent cooling near surface results in an abnormal high, which later enhances and moves southward with the abnormally low temperature near surface. The East Asian Trough in the middle layer intensifies as this abnormal high moves southward, which brings deep-layer northerly wind to the BTH region. As a result, the cold air activity in the BTH region is intensified.

Key words: cold air in the Beijing-Tianjin-Hebei region, Siberian High, low-frequency oscillation, atmospheric convergence