Structure and dynamics of the summertime Pacific-Japan teleconnection pattern

Convective activity over the tropical western Pacific is known to influence the extratropical circulation over East Asia in the boreal summer in the form of teleconnection, called the 'Pacific-Japan (PJ) pattern', but its structure and dynamics have not yet been studied in depth. In this study, a composite analysis is performed for 32 monthly events of enhanced convection observed to the east of the Philippines. The composited monthly mean vorticity anomalies associated with the PJ pattern are elongated zonally with a distinct poleward tilt with height. This structure differs fundamentally from a combination of the first baroclinic mode in the tropics and the barotropic structure in midlatitudes, as has widely been accepted as a conceptual model of the PJ pattern. A wave-activity flux points polewards only in the lower troposphere, indicating that Rossby wave teleconnection occurs primarily through a low-level south-westerly jet. Those tilted anomalies over the western Pacific can effectively gain kinetic energy in the exits of the mean jet streams in the upper and lower troposphere and available potential energy (APE) in the presence of the vertically sheared jets. The enhanced convection can generate APE effectively, and the associated low-level anomalous circulation acts to increase moisture supply into the convective region while enhancing evaporation from the pre-warmed ocean surface. It is thus hypothesized that the PJ pattern may be regarded as a dynamical mode that can be effectively excited in the zonally asymmetric baroclinic mean flow associated with the Asian summer monsoon with an efficient self-sustaining mechanism through moist processes.