Propagation and Maintenance Mechanism of the TC/submonthly Wave Pattern and TC feedback in the Western North Pacific

The propagation and maintenance mechanisms of the tropical cyclone (TC)/submonthly wave pattern in the Western North Pacific in the intraseasonal oscillation (ISO) westerly and easterly phases were explored by examining the vorticity and kinetic energy budget. The wave pattern exhibited an equivalent barotropic structure with the maximum vorticity and kinetic energy in the lower troposphere and propagated northwestward in the Philippine Sea in the westerly phase and north-northeastward near the East Asian coast in the easterly phase. The mean flow advection played a dominant role in the propagation from 1000 hPa to 400 hPa in both phases. Stronger mean flow in the westerly phase resulted in stronger propagation tendency than in the easterly phase.

Barotropic energy conversion is the dominant process in maintaining the kinetic energy of the pattern. As a result, the TC/submonthly wave pattern tended to occur in the confluent zone between the monsoon trough and the anticyclonic ridge, where the kinetic energy could be efficiently extracted from the westerly mean flow associated with the monsoon trough. The individual circulation circuit embedded in the pattern was oriented in the northeast-southwest (east-west) orientation to have optimal growth and propagation during the ISO westerly (easterly) phase. It is suggestive that the amplitude, shape and propagation of eddies in East Asia and the Western North Pacific were strongly constrained by the configuration of the monsoon trough and anticyclonic ridge.

Although TCs had little influence on the propagation mechanism and speed, they had significant effects on the amplitude and energy conversion of the large-scale submonthly wave pattern. When TCs developed in a development-favorable background flow provided by the submonthly wave pattern, they in turn enhanced the amplitudes of the vorticity and kinetic energy of the submonthly wave pattern by as much as 50% and helped extract significantly more (greater than 50%) energy from the background ISO circulation. This TC feedback was much more significant in the ISO westerly phase because of the more wide-spread clustering effect of an enhanced monsoon trough on the TCs.