A New Perspective of Pacific–Japan Pattern: Estimated Percentage of the Cases Triggered by Rossby Wave Breaking

Abstract

This study quantitatively examined the relative importance of Rossby wave breaking (RWB) east of Japan to a formation of the Pacific–Japan (PJ) pattern compared with that of tropical atmospheric and oceanographic variabilities. First, cases of the positive and negative PJ patterns are classified into those with and without the RWB occurrence. The result of the classification indicates that the cases of the positive PJ pattern triggered by the RWB account for approximately 20 % of the whole cases of the positive PJ pattern. The number of positive PJ cases with the RWB further accounts for approximately 80 % of those in the cases associated with the RWB. Results of a lag composite analysis and the related Q-vector diagnosis for the cases of the positive PJ pattern with the presence of RWB show that the RWB east of Japan promotes the formation of the PJ pattern through the southwestward intrusion of high potential vorticity air mass toward the subtropical western North Pacific (WNP) and the consequent dynamically induced enhanced convection over the region, consistent with results of previous studies. By contrast, the composite for the cases the negative PJ pattern accompanied by the RWB indicates that the RWB-related upper-tropospheric zonally elongated anomalous circulation and basin-wide sea surface temperature (SST) warming over the Indian Ocean can contribute suppressed convective activities over the subtropical WNP and the consequent formation of the negative PJ pattern although the RWB occurs. The composite for the cases of positive and negative PJ patterns with the absence of RWB further indicates that the convective activities over the subtropical WNP move northwestward with time, causing the formation of the PJ pattern. The formation of the PJ pattern with the absence of RWB is also closely associated with tropical SST and phase of the boreal summer intraseasonal oscillation, illuminating “pure” tropical impacts on the formation of a PJ pattern.