A 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. The relative importance of these large-scale waves to the five TC formations (A-E) is quantified by examining the normalized vorticity at 850 hPa and OLR at the genesis location of each TC. TCs A and C (TCs B and D) were related to the Rossby wave (the MJO), and the MRG-TD was the most related to TC E.
