LAUSR

Based on daily data from NCEP/NCAR reanalysis for 1948/1949–2015/2016 winters (November through March), this study examines the physical nature of the Arctic Oscillation (AO). Results show that the AO events derived from the daily AO index consist basically of two types of events—the in-phase (IP) and out-of-phase (OP) events. For IP events, their Pacific and Arctic centers are of same signs, while for OP events, the two centers are of opposite signs, as in the traditional AO pattern. This implies that the AO pattern obtained from the empirical orthogonal function analysis is actually a statistics of these two types of events. It turns out that it is the strong IP and OP events with the Pacific centers of amplitude larger than 1.0 which play a crucial role in shaping the nature (the signs) and strength of the Pacific center. The formation features and roles of these strong IP and OP events in the seasonal and decadal variation and the stratospheric control of the Pacific center of the AO are also investigated. This study also suggests paying attention to strong IP and OP events when making weather prediction for the weather anomalies associated with these two types of events may have big differences in some regions.