Orbital resolution thermal histories over the last 4 Ma at ODP Site 1143 in the tropical western Pacific are reconstructed using alkenone paleothermometry. The temperature profile is characterized by a steady state of approximately 29 degrees C with fluctuations <1 degrees C before 2.7 Ma and by a strong oscillating state from 2.7 Ma, largely due to cooling by up to 4 degrees C from approximately 29 degrees C in interglacial to 26 degrees C in glacial intervals. This implies a relatively warm and stable surface hydrography during the early and mid Pliocene in this region, influenced by the warm pool before temperature decreases in responding to global cooling and the formation of the distinct glacial stages since the late Pliocene. Therefore, the smaller SST gradient between tropical eastern and western Pacific and between southern and northern South China Sea before the late Pliocene indicates a super-sized Pliocene Pacific warm pool, while the larger SST gradient since then marks progressive intensification of the zonal Walker circulation and meridional Hadley circulation, representing the monsoon circulations in the region. The intensification of the Walker and Hadley circulations over the tropical Pacific may also have helped the onset of glaciations and subsequent deglaciations during the late Pliocene and Pleistocene.