Relationship between the Boreal Summer Intraseasonal Oscillation and the Pacific-Japan pattern and its Interannual Modulations associated with Synoptic-Scale Disturbances

The relationship between the boreal summer intraseasonal oscillation (BSISO) in the tropics and the summertime tropical-extratropical teleconnection called the Pacific-Japan (PJ) pattern is investigated. The positive correlation between the BSISO and intraseasonal PJ pattern peaks during BSISO phase 8 when the convective center of the BSISO reaches the western North Pacific (WNP). A composite analysis based on the BSISO events shows that intraseasonal responses to the migration of the BSISO extend to the midlatitudes and form circulation anomalies reminiscent of the PJ pattern. During phases 7−8, cyclonic wind anomalies with low-pressure signals drastically intensify north of the Philippines and southeasterly wind anomalies blow into midlatitude East Asia, influencing the summer climate. The intraseasonal variability in the PJ pattern associated with the BSISO undergoes strong interannual modulations. Intraseasonal convection is enhanced widely over the WNP for BSISO phases 5 – 7 in summers of positive seasonal-mean PJ pattern when seasonal-mean convective activity over the tropical WNP is enhanced, defined as the positive PJ years. By contrast, developed convection is confined over the South China Sea in the negative PJ years. Similar features are also found in the horizontal distributions of eddy kinetic energy (K’) representing the activity of synoptic-scale disturbances and of tropical cyclone occurrences. Wave train signals of circulation anomalies extending from north of the Philippines toward North America are apparent during BSISO phase 7 only in the positive PJ years. The differences in location of intraseasonal convection and the activity of synoptic-scale disturbances between the positive and negative PJ years likely lead to different teleconnections to midlatitude East Asia. There are two factors that can cause the differences described above. One is intraseasonal sea surface warming during convectively suppressed phases of the BSISO preceding the active phases. The other is convergence or shear of seasonal-mean horizontal winds associated with interannual fluctuations of monsoon westerlies over the WNP. These results highlight the importance of cross-scale interactions for a better understanding of summer climate in East Asia.