The two important modes of standardized wintertime zonal-mean zonal wind in Northern extratropics are extracted using the method of empirical orthogonal function (EOF).The leading mode (explaining 34% of the variance)is characterized by anomalous westerly (easterly) in high-middle latitudes and anomalous easterly (westerly) in mid-latitudes,and the antiphase variations of SLP anomalies between polar regions and high-middle latitudes.The leading mode is Arctic Oscillation (AO).The second mode defined as sub-tropical oscillation (STO) (explaining 21% of the variance) emerges as anomalous westerly (easterly) in mid-latitude regions and easterly (westerly) in middle-low latitude regions and shows opposite variations of SLP anomalies between sub-polar regions and sub-tropics.The spatial structure of STO is studied.The results indicate that STO shows a prominent zonally symmetric structure in the horizontal and a quasi-barotropic structure in the vertical.The zonal-mean temperature field associated with a positive phase of STO shows negative anomalies in high-middle latitudes and positive anomalies in sub-tropics,and the enhanced Ferrel moves equator-ward.We still concluded that STO can both be found in winter or the whole winter half year,but not in the summer half year.