Possible Relationship between Korea Affecting Tropical Cyclone Activity and Pacific Decadal Oscillation in Summer

This study discovered a strong negative correlation between the frequency of Korea affecting tropical cyclones (TCs) in summer and Pacific Decadal Oscillation (PDO). To investigate the cause of such a strong negative correlation between the two variables, the mean difference between the two phases was analyzed by selecting years with the highest PDO index (positive PDO years) and years with the lowest PDO index (negative PDO index) among the PDO index including El Nino-Southern Oscillation (ENSO) years and the PDO index excluding ENSO years. All the analysis results of the mean difference between the two phases were similar in both cases including and excluding the ENSO years. In the positive PDO years, TCs mainly formed in the northwestern region of the western North Pacific (WNP) and a lower TC passage frequency appeared in most areas of the WNP. Furthermore, the TC intensity was somewhat weaker than it was in the negative PDO years. To investigate the cause of such TC activity in positive PDO years, 850 hPa and 500 hPa stream flows were analyzed first. East Asia showed the typical anomalous pressure system pattern of the positive PDO years such as the typical high-west-low-east pressure system of winter and anomalous huge cyclonic circulations in the North Pacific. As a result, Korea was under the influence of anomalous northerlies, which played the role of preventing TCs from moving northward to Korea. In all the analysis results of precipitable water, 600 hPa relative humidity, and sea surface temperature (SST), the positive (negative for vertical wind shear) anomalies were strengthened in the northwestern region of the WNP and the negative (positive for vertical wind shear) anomalies in the southeastern region. Such atmospheric and oceanic environments were associated with the higher genesis frequency of TCs in the northwestern region of the WNP in the positive PDO years. In the regions near Korea, on the other hand, air temperature, precipitable water, 600 hPa relative humidity, and SST all showed negative (positive for vertical wind shear) anomalies, and atmospheric and oceanic environments were formed which can rapidly weaken the TC intensity even if TCs moved northward to Korea in the positive PDO years.