Northward and northwestward propagation of 30-60 day oscillation in the tropical and extratropical western North Pacific

This study investigates the northward, and northwestward propagation of 30-60 day oscillation over the western North Pacific (WNP) at upper, and low levels with a three-dimensional streamfunction tendency equation. In the tropical WNP, the surface frictional effect associated with the cyclonic circulation enhances the low-level convergence at the cyclonic vorticity center to the northwest of the convection, causing the 30-60 day convection to develop northwestward. The vorticity advection induces the 30-60 day circulation at upper and low levels to propagate northwestward, with a baroclinic structure. The combined effect of surface frictional-diabatic heating, and vorticity advection, causes the 30-60 day convection and circulation to develop and propagate simultaneously northwestward. After the convection fully develops, increased static stability, associated with adiabatic cooling, reduced solar radiation due to the cloud-radiation effect, and negative land-surface feedbacks on moisture availability, restrict any further development of the 30-60 day convection. A wave train emanating from the South China Sea/western North Pacific (SCS/WNP) into the extratropical North Pacific, is well established 15-days after the convection reached maximum intensity over the SCS/WNP. The main process and mechanism responsible for the northwestward propagation of this 30-60 day oscillation in the extratropical WNP is similar to the process proposed for the tropical WNP, except that in the mid-latitudes where the coriolis parameter becomes large, the influence of upper-level vorticity advection extends down to the low levels.