Air-sea interaction over the eastern side of the Pacific warm pool and its relationship to onset of El Niño have been investigated in previous studies, however the mechanisms of air-sea interaction over the western part of the Pacific warm pool (WPWP) during onset of El Niño are still unclear. This study highlights an overlooked mechanism for SST cooling in the WPWP prior to El Niño onset. We analyze TRITON moored buoy data, CTD data and satellite data, and we focus on the 2002/03 El Niño event, which occurred during a time of good data coverage. It is shown that relatively cold water upwelled along the north coast of Papua New Guinea (PNG) during December 2001, accompanying strong northwesterly sea surface winds along the north coast. The cold water area spread toward the western equatorial Pacific during the late of December 2001. To understand the initial SST cooling mechanism, TRITON buoy data in the WPWP are analyzed. Negative zonal heat advection due to eastward current is the main factor to cool the mixed layer temperature in the WPWP, explaining roughly 50% of the cooling in contrast to the small contribution of net surface heat flux. This cooling requires a source of colder water to the west. Therefore, it is indicated that the upwelled cold water along PNG north coast and its northeastward extension to the equatorial region had an important role in SST cooling over the western equatorial Pacific before the 2002/03 El Niño onset. It is also shown that strong cold SST appeared in the WPWP between 140?E and 150?E after late of December 2001, whilst warm SST distributed in the east of 150?E. Both cold and warm SST generated a positive zonal SST gradient west of the highest positive SST region, which may have enhanced the westerly sea surface wind occurring in this region. The positive SST anomaly region started moving eastward together with the westerly sea surface wind at that time, and then reached the central equatorial Pacific during boreal-spring of 2002, finally leading to the 2002/03 El Niño.