The characteristics and possible growth mechanisms of the quasi-biweekly Pacific-Japan teleconnection in Boreal Summer

摘要

The summertime Pacific-Japan (PJ) pattern exhibits notable quasi-biweekly oscillation on the intraseasonal timescale. This study investigates the characteristics and possible mechanisms associated with the development of the quasi-biweekly PJ pattern using daily data from 1958 to 2016. The results show that the quasi-biweekly PJ pattern distributes northeastward along the great circle route from the northern Philippine Sea to the southern Gulf of Alaska, which is distinct from the meridional structure of PJ pattern on the monthly and seasonal timescales. The tropical anomaly of the PJ pattern presents a baroclinic structure, while the midlatitude anomalies present a barotropic structure, suggesting the barotropic-baroclinic coupling. The PJ pattern shifts westward during its life cycle. The convective activities over the western North Pacific (WNP) are critical in triggering the PJ pattern. The Rossby wave source decomposition analyses indicate that the convective activities over the WNP could excite the anomalous lower-tropospheric convergence and induce a strong lower-tropospheric Rossby wave source through the vortex stretching effect, which could stimulate the poleward wave energy from the Philippine Sea. The northeastward-propagating wave energy further injects upward when it reaches the latitude where the westerly jet stream is located, and eventually a well-organized barotropic-baroclinic coupling PJ pattern is built up. Furthermore, the possible sources of quasi-biweekly convective activities over the WNP are explored. It is suggested that the local air-sea interaction, the northwestward propagation of the quasi-biweekly oscillation mode of the tropical atmosphere and the typhoon activities might jointly contribute to the convection over the WNP and the subsequent development of the PJ pattern.