Relative Roles of ISO Circulation and Moisture Impacts in Tropical Cyclone Development in Western North Pacific

The effects of intraseasonal oscillation (ISO) of the western North Pacific (WNP) monsoon trough (MT) on the tropical cyclone (TC) formation are investigated using the ARW-WRF model. The model is triply nested with the movable finest mesh at a resolution of 3km. A weak bogus vortex is initially superposed on either an active or inactive ISO phase. Numerical experiments show that the MT ISO has a strong control on the vortex development. An active ISO phase favors TC formation, whereas an inactive phase prohibits the vortex development.

The diagnosis of the model outputs shows that the MT ISO can modulate the TC development through both dynamic and thermodynamic processes. The dynamic impact is attributed to the generation of perturbation inflow or outflow in middle troposphere through nonlinear vortex-ISO flow interaction. A cyclonic vortex interacting with ISO cyclonic (anticyclonic) flow during an active (inactive) phase leads to the generation of perturbation outflow (inflow) at 700hPa, which enhances (suppresses) friction induced upward motion above the planetary boundary layer. Meanwhile low-level convergent (divergent) flow associated with an active (inactive) ISO strengthens (weakens) upward moisture transport in the vortex core region. The thermodynamic impact arises from greater (smaller) background specific humidity associated with an active (inactive) ISO, which allows a stronger (weaker) condensational heating.

Further experiments with the separation of ISO dynamic and thermodynamic impacts show that their impacts are nonlinear. Compared to a control experiment with a resting environment, active ISO moisture field has a greater contribution to TC final strength than active ISO circulation field. On the other hand, in the presence of inactive ISO dynamic field, no TC development is observed no matter whether active or inactive ISO moisture field is used. In the presence of active ISO dynamic field, a TC development can be achieved even in the presence of inactive ISO moisture field.