This study found a positive correlation between the August rainfall in North Korea and Antarctic Oscillation (AAO) in August. Causes of increasing rainfall in the positive AAO phase are (1) the increasing frequency of tropical cyclones that land in or affect the Korean Peninsula, (2) the reinforcement of Australian high (AH) in the Southern Hemisphere, and (3) atmospheric instability at all levels in August in North Korea. The reinforcement of AH forms an anomalous cross-equatorial flow in the western Pacific and plays a decisive role in the northward development of the subtropical western North Pacific high (SWNPH). Furthermore, large volumes of warm and humid air are supplied to North Korea owing to this development. As a result, atmosphere in North Korea becomes unstable, and it is found that the reinforcement of anomalous warm sea surface temperature (SST) in the middle latitude of East Asia is another cause of the instability.