A possible relationship between Changma and tropical cyclone genesis frequency in the western North Pacific linked to the North Atlantic Oscillation

This study analysed the relationship between Changma and tropical cyclone (TC) genesis in the western North Pacific (WNP). The correlations between Changma onset, withdrawal, and duration in Jeju and the southern and central regions of Korea and TC genesis frequencies in June, July, and June-July in the WNP were analysed. The results showed that Changma duration in the southern region and June-July TC genesis frequency exhibited the highest negative correlation. To investigate the mechanism underlying the correlations, the differences in large-scale environments between the 14 years with the longest Changma duration and the 14 years with the shortest Changma duration were analysed. For 850-hPa stream flows, anomalous anticyclones are found in most WNP regions, while anomalous cyclones are located in the East Asian mid-latitude region. In the long Changma duration years, the WNP subtropical high was strengthened in the north-south and east-west directions, the monsoon trough was weakened, and anomalous downward flows are formed at the latitudinal (longitudinal) band of 10 degrees-27 degrees N (100 degrees-180 degrees E) where TCs mainly occur. All these environments provide unfavourable conditions for TCs to occur. The difference in sea surface temperature revealed triple anomaly patterns, such as warm, cold, and warm, in the north-south direction in the North Atlantic. This spatial distribution was similar to the negative North Atlantic Oscillation (NAO) pattern. From the analysis of 500-hPa stream flow, we confirmed that the NAO acts as a bridge connecting the TC and Changma in the WNP.