A Dynamic Relationship between Typhoon Activity, the North Pacific High and the Tropical Intraseasonal Oscillation in Boreal Summer

We investigated the two-way relationship between typhoons and the North Pacific high and their remote effect on the stationary frontal zone (Baiu/Meiyu front) in the vicinity of Japan at the mature stage of the early summer rainy season (July) using the JRA-25/JCDAS, APHRO_JP and NOAA interpolated OLR. We extracted the two major tracks of the typhoons in July, i.e., Hainan Island course (HC), and Okinawa Island course (OC). Since most of the typhoons have similar tracks and reach their peak in similar locations at each course, we defined the day in which a typhoon reached its maximum as key day and made composite analyses. The two primary tracks of the typhoons depend significantly on the difference in the background atmospheric circulation states in association with the North Pacific high. Such changes in the background states are attributed to the phase change of the tropical intraseasonal oscillation in boreal summer. On the other hand, the typhoons can enhance locally the western ridge of the North Pacific high by inducing negative absolute vorticity advection along their eastern periphery. A combination of a typhoon and its induced anticyclonic circulation anomaly increases water vapor transport into the Baiu/Meiyu front in the vicinity of Japan. At the HC, northward moisture flux over the East China Sea is enhanced along the western periphery of the typhoon-induced anomalous high, triggering heavy rainfall around central Japan. The OC typhoons contribute to cause heavy rainfall in the western part of Japan through reinforcement of east-west pressure gradient. The OC typhoons, which approach the upper-level subtropical jet, can also excite a stationary wave pattern from the western North Pacific to the Pacific coast of the North American continent along the jet, affecting downstream circulation fields. This study highlights the possible dynamic interactions between typhoons and the North Pacific high in early summer season. Understanding of these interactions between the typhoons and the North Pacific high is very useful not only in predicting the track and development of tropical cyclones but also in predicting heavy rainfall in Japan during early summer rainy season and anomalous weather around the Pacific coast of North America.