Negative relationship between Korea landfalling tropical cyclone activity and Pacific Decadal Oscillation

The present study discovered a strong negative correlation between Korea-landfalling tropical cyclone (TC) frequency and Pacific Decadal Oscillation (PDO) in the summer. Thus, the present study selected years that had the highest PDO index (positive PDO years) and years that had the lowest PDO index (negative PDO years) to analyze a mean difference between the two phases in order to determine the reason for the strong negative correlation between the two variables. In the positive PDO years, TCs were mainly generated in the southeastern part of the western North Pacific, and lower TC passage frequency was found in most regions in the mid-latitude in East Asia. Moreover, a slightly weaker TC intensity than that in the negative PDO years was revealed. In order to determine the cause of the TC activity revealed in the positive PDO years, 850 hPa and 500 hPa stream flows were analyzed first. In the mid-latitude region in East Asia, anomalous huge cyclonic circulations were strengthened, while anomalous anticyclonic circulations were strengthened in the low-latitude region. Accordingly, Korea was being influenced by anomalous northwesterlies, which played a role in blocking TCs from moving northward to Korea. The results of analysis on 850 hPa air temperature, precipitation, 600 hPa relative humidity, and sea surface temperature (SST) showed that negative anomalies were strengthened in the northwest region in the western North Pacific while positive anomalies were strengthened in the southeast region. The atmospheric and oceanic environments were related to frequent occurrences of TCs in the southeast region in the western North Pacific during the positive PDO years. All factors of air temperature, precipitation, 600 hPa relative humidity, and SST revealed negative (positive for vertical wind shear) anomalies near Korea, so that atmospheric and oceanic environments were formed that could rapidly weaken TC intensity, even if the TCs moved northward to Korea in the positive PDO years.