|dc.description.abstract||Record-breaking five tropical cyclones (TCs) formed in June 2004 in the western North Pacific (WNP) where June is normally a transition month to the typhoon season and therefore sensitive to climate oscillations. This special month (June 2004) was an unusual period in the developing stage of a warm (El Nino) episode and a strong convective phase of the Madden-Julian oscillation (MJO). Such climate background is shown to provide large-scale favorable circulations for TC formation: the warm sea surface temperature anomalies (SSTAs) associated with developing El Nino and convective heating of the MJO to jointly induce weaker easterly trade winds and a large-scale cyclonic circulation anomaly in the WNP. A space-time filtering of the outgoing longwave radiation (OLR) and 850-hPa wind fields is performed to identify the MJO, Rossby waves and mixed Rossby-gravity (MRG) waves (or tropical depression (TD)-type disturbances). From the evolution and structure of these high-frequency waves in relation to that of the MJO and the climate background, the heating and enhanced low-level cyclonic flow in the WNP associated with the MJO and climate background are attributed to the initiation, propagation and energy dispersion of tropical Rossby and MRG-TD waves, interacting with convection.
In order to examine the relationship between multi-scale waves and TC formation, the vorticity and wind fields were separated into low-frequency and high-frequency variations by space-time filtering. The low-frequency variation included five scale variables: climatology (Cli), interannual variability (IAV), intraseasonal variability (ISV), equatorial Rossby wave (ER), and mixed Rossby-gravity wave and TD-type disturbance (MRGTD). The remaining component was classified to the high-frequency variation. The scale separation help us to clarify the roles of low-frequency and high-frequency variations on TC formation. Then, the vorticity budget on TC-following coordinate was calculated to examine the interaction between waves and TC. Therefore, the generation terms in vorticity budget can be separated into the balanced dynamics (BD) and nonlinear dynamics (ND). The balanced dynamics included the low-frequency waves interaction and the low- and high-frequency waves interaction. Nonlinear dynamics indicated the nonlinear processes of the convective scale and sub-scale variations. The dominated terms in vorticity budget were the balanced dynamics for four TCs in June 2004, which means that the contributions of low-frequency waves and the low- and high-frequency waves interaction created large-scale favorable environmental conditions for TC genesis. The large contributions of low-frequency waves in TC C, D, and E corresponded to the the active waves in mid- and late June. The vorticity budget can identify the relation between low- and high-frequency waves and TC, which can help us to understand the interaction between waves and TC genesis.