Relationship between the Boreal Summer Intraseasonal Oscillation and the Pacific-Japan Pattern and Its Interannual Modulations

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 vicinity of the Philippines. 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. Other apparent intraseasonal signals in pressure and surface air temperature are found over the midlatitude central Pacific and eastern Eurasia, respectively. The intraseasonal variability in the PJ pattern associated with the BSISO undergoes strong interannual modulations, with enhanced intraseasonal signals in summers of the positive seasonal-mean PJ pattern and suppressed signals in those of the negative PJ pattern. This asymmetry between the positive and negative PJ summers highlights the importance of cross-scale interactions for a better understanding of summer climate in East Asia.